摘要:

The Royal Society of Chemistry Chemical Society Reviews Editorial Board Professor H. W. Kroto FRS (Chairman) (University of Sussex) Professor M. J. Blandamer (University of Leicester) Dr. A. R. Butler (University of St. Andrews) Professor E. C. (3,onstable (University of Basel, Switzerland) Professor T. C. Gallagher (University of Bristol) Professor D. M. P. Mingos FRS (Imperial College Land on) Consulting Editors Dr. G. G. Balint-Kurti (University of Bristol) Dr. J. M. Brown (University of Oxford) Dr. J. Burgess (University of Leicester) Dr. N. Cape (Institute of Terrestrial Ecology, Lothian) Professor B. T. Golding (University of Newcastle upon Tyne) Professor M. Green (University of Bath) Professor A. Hamnett (University of Newcastle upon Tyne) Dr.T. M. Herrington (University of Reading) Professor R. Hillman (University of Leicester) Professor R. Keese (University of Bern, Switzerland) Dr. T. H. Lilley (University of Sheffield) Dr. H. Maskill (University of Newcastle upon Tyne) Professor A. de Meijere (University of Gottingen, Germany) Professor J. N. Miller (Loughborough University of Tech n oIog y ) Professor S. M. Roberts (University of Liverpool) Professor B. H. Robinson (University of East Anglia) Professor M. R. Smyth (Dublin City University, Republic of Ireland) Professor A. J. Stace (University of Sussex) Chemical Society Reviews aims to foster current progress in the chemical sciences and related disciplines. The journal has the broad appeal necessary to enable scientists to benefit from recent advances made in research outside their immediate interests.In particular, students embarking on a research career should find Chemical Society Reviews a particularly Chemical Society Reviews (ISSN 0306-0012) is published bimonthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, UK CB4 4WF. All orders accompanied by payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts., UK SG6 1HN. N.6. Turpin Distribution Services Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1996 annual subscription rate: EEA f120.00; Rest of World f123.00; USA $225.00.Customers in Canada will be charged the Rest of World price plus a surcharge to cover GST. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Second-class postage is paid at Jamaica, NY 1141-9998. Airfreight and mailing in the USA by Publications Editorial Staff Managing Editor Martin Sugden Editorial Production Peter Whittington Editorial Secretary Debbie Halls Editorial Office The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge UK CB4 4WF Telephone +44 (0)1223 420066 Facsimile +44 (0)1223 420247 Electronic Mail (Internet) rscl@rsc.org or sugdenml@rsc.org http://c hemist ry. rsc.org/rsc/ Advertisement sales Telephone +44 (0)171 287 3091 Facsimile +44 (0) 171 494 1134 Typeset by Servis Filmsetting Ltd.Printed in Great Britain by Black Bear Press Ltd. stimulating and instructive springboard to further reading. The Editorial Board encourages an international and interdisciplinary approach to science, which is reflected in the succinct, authoritative articles commissioned. The Board members welcome comments and suggestions; these should be directed to the Managing Editor Expediting Services Inc., 200 Meacham Avenue, Elmont, NY 11003, and at additional mailing offices. US Postmaster: send address changes to Chemical Society Reviews, c/o Publications Expediting Services Inc., 200 Meacham Avenue, Elmont, NY 11003. All despatches outside the UK by Bulk airmail within Europe and Accelerated Surface Post outside Europe. PRINTED IN THE UK. 0 The Royal Society of Chemistry, 1996. All rights reserved. No parts of this publication may be repro- duced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, recording, or other- wise, without the prior permission of the publishers.

ISSN:0306-0012    

DOI:10.1039/CS99625FX001

出版商:RSC    

年代:1996

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS99625BP003

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Calixarene-based Sensing Agents Dermot Diamond School of Chemical Sciences, Dublin City University, Ireland M. Anthony McKervey School of Chemistry, The Queen‘s University, Belfast BT9 5AG, N. Ireland 1 Introduction The development of new and more efficient means of performing real-time monitoring of chemical and biochemical species through the use of sensors is among the most significant chal- lenges facing modern science. The problems involved are multi- faceted requiring a broad understanding of many areas ranging from synthesis to thin layer deposition and surface analysis tech- nologies, and involving computer-based data acquisition and signal processing. The nature of the component used to generate the diagnostic signal is central to determining the overall per- formance of any chemical sensor as this will largely, though not absolutely, define the critical characteristics of the device, namely its selectivity, lifetime and response time.However, despite much effort over the past 30 or so years, the number of really efficient individual sensors remains disappointingly small, probably reflecting the ad hoc nature of the design and synthesis of poten- tial sensing agents. To be fair, the difference between a really effi- cient sensing agent and a hopeless one is very difficult to predict since, on a molecular basis, the processes which together define the overall preference for a target substance, in preference to all interferents, interact in subtle ways. However, recent improve- ments in the power of computer systems and refinements in the algorithms used to minimise molecular energies in solution have enabled more accurate predictions of structures and conforma- tions to be made, and it is now possible in advance to probe how a sensor might interact in a dynamic sense with certain target species in different solvents.In addition, the large amount of information now available should enable statistical tools and pattern recognition techniques to provide more insight into the factors which determine selectivity. 2 Transduction Modes The role of the sensing agent in a chemical sensor is to provide a transduction mechanism which enables an analytical signal to be Dermot Diamond received his BSc, MSc and PhD from the Queen’s University of Belfast.In 1987 he moved to the School of Chemical Sciences, Dublin City University (DCU), where he is currently a Senior Lecturer in Analytical Chemistry. He has recently been seconded as research director of the Biomedical and Environmental Sensor Technology (BEST) Centre at DCU, which is a mul- tidisciplinary unit comprising over 30 sta8. His current inter- ests include sensor arrays, optical and electrochemical sensors, solid-state sensors, flow-injection analysis, and playing the fiddle. obtained. The vast majority of calixarenes investigated as potential chemical sensors have employed an electrochemical transduction mechanism, either potentiometric or voltammetric-amperometric, although recently there has been a strong movements towards optical transduction.These studies are the subject of this brief review. 3 Elect rochem ical Transduct ion There are two principal methods of electrochemical transduction, potentiometric and voltammetric. Both involve the use of electrodes to probe the sample and return an analytical signal. In the potentio- metric method spontaneous processes occur at both electrodes in the electrochemical cell leading to the creation of a cell potential which reaches a steady state when the net current flowing in the cell and measurement circuitry is zero, i.e. the processes occurring at the electrodes are at equilibrium. In the voltammetric method, in con- trast, electrochemical reactions are forced to happen at the working electrode under the influence of an externally poised potential, con- trolled by a potentiostat.Potentiometric Sensors: Ion-selective Electrodes Prior to the development of calixarenes as selective sensing agents for potentiometric sensors, more popularly known now as ion- selective electrodes (ISE), pioneering work by Simon and his coworkers1 over a period of about 20 years had identified groups of compounds whose complexation characteristics, particularly towards alkali and alkaline earth cations, made them suitable can- didates for use as ionophores in ISEs aimed at cation analysis. These compounds included several acyclic and macrocyclic antibiotics, the most prominent being valinomycin for K+ analysis, a number of crown ethers and a family of acyclic amides.2 Simon’s studies identified a set of criteria which any ionophore should meet if it is to function as an efficient sensor.Although Professor McKervey received his BSc, PhD and DSc degrees from Queen’s University, Belfast and, following aperiod at M.I.T.,joined the academic staff in Queen’s. In 1976 he was appointed to the Chair of Organic Chemistry in University College Cork, Irish Republic. He returned to Belfast in 1990 as Professor of Organic Chemistry and Head of the Research Division of the School of Chemistry. Apart from cal- ixarenes, his research interests include catalysed asymmetric synthesis with diazocarbonyl compounds, synthesis of protease inhibitors, the synthetic chem- istry of furans, and clean synthe- sis in chemical development.15 most modem ISEs utilise polyvinyl chloride (PVC), or some similar support material, to give a ‘pseudo-solid’ sensing mem- brane, the signal generation process still involves partitioning the primary ion into a non-polar sensing liquid membrane phase. Hence, the criteria for a successful ionophore can be summarised as follows:* It should be capable of selective complex formation with the primary or target ion. It should be unresponsive to other cations and anions. The ionophore must be retained within the membrane phase. The complex must be able to diffuse freely in the direction of the potential gradient. The stability constant of the complex, p,defined by equation 1, should not be too large, or too small.The kinetics of ion transfer between the aqueous and membrane phases, and of complexation with the ligand (equation 2) should be fast and reversible, where L is the ionophore or ligand, M+ the cation, LM+ the complex, (m) denotes the sensor membrane phase, and (aq) the aqueous phase. Criteria 1 and 2 are vital in determining the selectivity of the sensor, whereas criterion 3 ensures an adequate lifetime. Criterion 4 is necessary to provide a mechanism for charge transfer through the membrane, while criterion 5 is a requirement for ensuring a con- stant concentration of free ion in the membrane over the measuring range, a prerequisite for Nernstian behaviour. Criterion 6 is neces- sary to ensure an acceptable response time to fluctuations in the primary ion concentration during continuous monitoring situations, and to ensure that the signal obtained is reversible.These criteria, in turn, influence the structural and functional- ity features of ionophores required to make them effective in ISE membranes. These features include the presence of polar ligat- ing groups arranged spatially in such a way that they can inter- act strongly with selected ions. The alkali and alkaline earth cations have been particularly attractive targets over the past 30 years with the development of sensing technology for blood analysis as the commercial driving force. For cations suitable ligating groups include ethereal and/or carbonyl (in the form of amide, ester, ketone or carboxylic acid) oxygen atoms, arranged so as to define a polar cavity of sufficient rigidity to maximise selectivity and provide a level of ion-dipole attraction and sol- vation consistent with the stability requirements of the complex formed. These considerations notwithstanding, in order to comply with criterion 3 the ionophores, and the resulting positively charged complexes, will have to be retained in the non-polar membrane phase of the sensor.This may be achieved by adding large non- polar groups around the ligating binding sites so as to shield the effect of the polar groups and the charged complex from the non- membrane environment. However, the molecule should not become too bulky or diffusion of the complex through the mem- brane will be hampered (criterion 4).The processes occurring in our idealised ISE membrane for cation detection are summarised in Figure 1. The aqueous analyte ions [M+ (aq)] in the sample phase are in equilibrium with the electrode membrane phase under the control of the ion-ionophore complexation reaction, generating a boundary potential E:. Likewise, the same ions present in the internal electrolyte generate an internal boundary potential EL. A diffusion potential (Ed)also occurs across the membrane, and complexes (LM+) are able to diffuse in the direc- tion of the concentration gradient until an equilibrium is estab- lished. Of these three potentials, Ek is fixed and Ed is normally constant or zero.Hence, any changes in the overall membrane potential arise solely from fluctuations in EL, which is related to the activity of the analyte ions via the well-known Nernst equa- tion. CHEMICAL SOCIETY REVIEWS, 1996 membrane phase M++L -LM’ sample phase M+ Figure 1 Processes occurring in an idealised ion-selective electrode mem-brane Synthetic Ionophores for ISEs Based on Calixarenes Simon and his coworkers’ led the way in the design of synthetic receptors, primarily based on amides, for metal ions suitable for incorporation into ISEs. Later, other workers looked for alternatives among the various groups of neutral ligating families such as crowns and cryptands which were being developed during the 1970s and 1980~.~Given this activity, it was inevitable that cal- ixarenes would eventually be examined as their structural features met many of the requirements outlined above.The first publication in the area appeared in the 19864and in the intervening years there have been numerous papers highlighting this aspect of the use of calixarnes, not just in ISE devices but in analytical science as a whole. Soon after the discovery in 1985that several calixarene ester derivatives possessed ionophoric activity towards metallic cations, notably the alkali metals, projects were initiated by Svehla and McKervey to incorporate these esters into sensing devices, mainly ISEs. It is appropriate, however, before reviewing these investiga- tions, to summarise briefly those features of calixarenes that make them attractive as potential sensing components in analytical devices.The calixarenes5 are a family of oligophenols linked in macro- cyclic arrays by methylene bridges. They are formed by base-pro- moted condensation of para-alkylphenols with formaldehyde and are available on a multi-gram scale from ‘one-pot’ procedures. The most accessible calixarenes are tetramers, hexamers and octamers, 1, TI =4,6 and 8, respectively. Pentamers are rather less accessible, though preliminary indications of useful ionophoric activity of some pentamer derivatives are beginning to emerge. One of the most attractive features of calixarenes is the ease of chemical mod- ification, making possible changes in ion-complexing selectivity, simply by switching from one ligating functional group to an~ther.~ The name ‘calixarene’ was originally coined by Gutsche to evoke the potential of these molecules to function as molecular cups or baskets for guest molecules or R R =H,dkyl n=4-8 1 Calixarenes Calixarenes possess an upper rim, defined by the para-sub- stituents of the phenolic rings, and a lower rim defined by the phe- nolic hydroxy groups.Between the two lies a hydrophobic cavity whose boundaries are the inside n-surfaces of the constituent aro- matic rings. By appropriate substitution at the hydroxy groups it is possible to create a second actual or potential cavity on the lower rim. Similarly, by attaching additional substituents to the para- positions a further cavity can be constructed on the upper rim.However, this description, while helpful in visualising the recep- tor potential of these molecules, disguises the fact that all the parent calixarenes, i.e. those with free hydroxy groups, are conformationally mobile at ordinary temperatures in solution. By a series of ring flips about the CH,-Ar-CH, bonds the phenolic rings have the freedom to rotate through the annulus of the macro- cyclic making numerous conformations acces~ible.~~ At suffi- ciently low temperatures, however, ring mobility can be 17CALIXARENE-BASED SENSING AGENTS-D. DIAMOND AND M. A. McKERVEY Table 1 Calixarene derivatives used in sensors T Compound No. R R' n 2 But CH,CO,Me 4 3 But CH,CO,Et 4 4 5 But But CH,CO,PhCH,CO,C, 2% 4 4 6 But CH,COMe 4 7 But CH,COAd 4 8 Bu1 CH,COBut 4 9 But CH,COPh 4 11 But CH,CO,Et 6 12 H CH,CO,Et 6 13 But CH,CO,CH,N[CH,],C=O 4 14 15 But But CH,C(S)NEt, CH,CO,CH,CH,SMe 4 4 16 But CH,CH,SMe 4 17 But CH,CH,C( S)NMe, 4 suppressed to the point where NMR spectroscopy reveals the pres- ence of distinct conformations. Notwithstanding the mobility of the parent calixarenes, it is pos- sible by appropriate substitution at the hydroxy groups to produce conformationally fixed derivatives.Two such compounds are the tetramethyl calix[4]arene ester 2 and its ethyl analogue 36(see Table 1). Originally these compounds were synthesised in stable cone conformations.The intention in preparing esters of type 2 and 3 was to explore the possibility of using calix[4]arenes as semi-rigid plat- forms or substructures on which to assemble convergent or poten- tially convergent ligating functional groups, in this case ester carbonyls, so as to form flexible hydrophilic cavities suitable for encapsulating guest cations. The crystal structure of 3 (Figure 2) does indeed reveal such a cavity on the lower rim with an apprecia- ble degree of preorganisationof the ester podands6 Physicochemical measurements6 quickly confirmed that esters 2 and 3, and later many other ester derivatives, do indeed possess sig- nificant affinity for alkali metal salts in biphase extraction from water into dichloromethane, complexation in single solvents, and transport through liquid membranes.The most significant conclu- sions from these early studies with 2 and 3 were: (a) the medium complexing power towards Na+, and (b) the preferences for this cation over the other alkali cations.6 In complexation in methanol the stability constant, P(Na+), has a value log P =5.18 and SNa+,the selectivity over K+, expressed as the ratio P(Na+)/P(K+), approxi- mately 400. This selectivity compares very favourably with that of cryptand 22 1 (SNa+= 1.2), the member of that series best adapted for Na+. Tetraester 3 was also more selective than the naturally occumng ionophore monensin (SNa+=2-6, depending on the liter- ature source). These studies later revealed that Na+ selectivity could be modulated quite dramatically simply by changing the alkyl residue in the ester groups of 3.This is shown graphically for a series of eleven esters in Figure 3 where SNa+reaches a maximum of 2500 for the phenacyl deri~ative.~ The combination of high selectivity for Na+ and medium complexing power proved to be important features of the ionophore profiles of these calixarene esters apropos of their potential as selective sensing agents for this cation, since the stabil- ity constant for the 3 (Na+) complex lies in the optimum range, i.e. log p =ca. 5 in methanol, expressed in equation 1. The optimum range was quantified by the work of Lehn and KirchX who estab- lished the existence of a correlation between the value of p and the transport rate and found that maximum transport rate of alkali picrates by cryptands occurred when log P (methanol) was about 5 units. Thus, it is perhaps not surprising that the efficient natural K+ camer valinomycin shows just about these same properties.Figure 2 X-Ray structure of tetraester 3 showing the cone conformation and the disposition of the ester podands about the hydrophilic cavity.6 3000 I Figure 3 Variation of selectivity Na+/K+ within the tetraester series calix- [OCH2C02R],7 R =(i) Et, (ii) But, (iii) Me, (iv) Bun, (v) Bn, (vi)Ph, (vii) CH,COPh, (viii) [CH,],OMe, (ix) [CH,],SMe, (x) CH,CF,, (xi) CH,C=CH. Initial screening experiments carried out with liquid membrane sensors confirmed that excellent sensors for sodium could be pro- duced with esters 2 and 3 (Figures 4a and 4h respectively). These results were published in 1986as part of the proceedings of an inter- national conference held in D~blin.~ They represent the first use of calixarenes as sensing agents.A second publication in 1987 by Diamond and Svehla9 again highlighted the excellent selectivity of ester 2 for Na+ against K+ and a range of other interferents which can affect the estimation of Na+ in blood, the most important com- mercial application for Na+ measurements. Detailed studies on the properties of PVC membrane ISEs based on esters 2 and 3 con-firmed their usefulness as Na+ sen~ors.~ The related calix[4]arene derivatives, the dodecyl and phenyl esters 4 and 5, show compar- able behaviour.I0 Studies recently completed in which a range of twelve or more tetraesters were screened in PVC membrane elec- trodes indicate that the 2-methoxyethyl ester analogue of 2 pro-duces the most selective Na+ electrode." An obvious application of these sodium sensors is in the clinical analysis of sodium in body fluids.Although sodium is present in blood at elevated levels (typically 120-150 mmol 1-l), the range over which the sodium concentration extends is relatively limited, compared to potassium (1-4mmol I-I). If follows that the signal obtained will have a limited range of a few mV over which the entire normal sodium distribution will occur. Hence, careful experimental design and attention to sampling and signal processing is required in order to obtain acceptable accuracy and precision in the analyt- ical results.Initial studies on the performance of mini-PVC membrane 654321 6 5 4 3 2 1 E L, , I , ( I I , , 1 1 1 6 51 3 2 16 5 4 3 2 1 -log aj Figure 4 Response of liquid membrane electrodes based on (a) 2, (b) 3, (c) 11and (d) 12. These were the first results demonstrating the selectivity of ISEs based on calixarene esters. The tetramers 2 and 3 are clearly Na+- selective while the hexamers 11and 12 are Cs+-~elective.~.~~ electrodes for blood analysis were encouraging. In this study, 44 plasma samples were analysed for sodium with the PVC electrodes based on tetramethyl ester 2, and the results compared with those obtained with a SMAC-Technicon Analyser.Good correlation was found (r = 0.95), but a systematic bias was apparent due to the calibration regime used in the study. A more detailed report of these investigations published the following year confirmed the utility of applying the sensors based on 2 to the analysis of sodium in blood. In parallel with these studies, other ligands were assessed for use in sodium-selective electrodes, including the p-tert-butylcalix[4]arene alkyl ketones 6-4. However, only the methyl ketone derivative 6 produced satisfactory PVC membrane elec- trodes." These were subsequently applied to the analysis of sodium in plasma samples.13 Excellent correlations (r = 0.979, 0.987 and 0.95I, n = 10)were found in comparative tests with three reference instruments (Hitachi 704 Analyser, Flame Photometer, SMAC Technicon Analyser, respectively).However, as before, a bias in the results was apparent in each case. Interestingly, in a paper by Kimura and coworkers, PVC electrodes based on 4 were also applied to the determination of blood sodium. Although only five samples were processed, a positive bias of around 2-3 mmol 1-i was evident in all but one sam~1e.I~ Obviously, when trying to establish a new analytical device in the face of existing technology, any bias in the results is unaccept- able. Bias in analytical determinations commonly arises from systematic errors in calibration. In the above investigations, drift during the calibration and analytical measurements was problem- atic.Its effect was further magnified by the very restricted range found in blood sodium samples, which leads to a narrow voltage range over which the measurements must be made, and, perhaps more importantly, significant 'bunching' of the concentration dis- tribution in the samples. Hence most concentrations focused in a very narrow range (135-140 mmoll-I) with a few outliers on either side which extend the range to perhaps 120-150 mmol 1-sodium. These outliers have a significant influence on the slope of the regression line, and must therefore be determined with partic- ular care. One way to reduce the effect of drift and give very reproducible sample handling is to use flow-injection analysis (FIA). PVC membranes incorporating the methyl ketone 6 and methyl ester 2 CHEMICAL SOCIETY REVIEWS, 1996 derivatives were assessed as detectors in an FIA system for blood sodium analysis and the results demonstrated that the bias described above could be greatly reduced while still maintaining excellent correlation.l5 However, the best results were obtained when the tetramethyl ester 2 was used as an element in an ISE array both in conventional dip-type measurements and in a flow-injection analysis system.16 Using sophisticated calibration and sensor modelling techniques, these papers rigorously demonstrated that 2 could be applied to blood sodium analysis with excellent results (Figure 5). Furthermore, the same ISE was shown to be suitable for the analy- sis of sodium in mineral water samples.125 135 145 (Na+)/mmolr'(Technicon Smac 3) Figure 5 Plasma sodium analysis results obtained with a PVC membrane electrode based on tetraester compound 2 with the results obtained with a SMAC analyser. More recently, sodium-selective PVC membrane electrodes incorporating the ester 2 have been assessed using batch injection analysis (BIA). This technique differs from FIA in that the sample is injected directly onto the sensor surface, and a dilution/mixing effect sweeps the sample quickly away, resulting in high-speed tran- sient signals which can be used for analytical measurements. Initially, a single sodium electrode was investigated and shown to have excellent characteristics for this technique. Subsequently, the electrode was used in a 3 X ISE array (Na, K, Ca) and successfully applied to the analysis of these ions in mineral water ~amp1es.I~ In the array study, the excellent selectivity of the calixarene-PVC membrane was apparent in carryover studies performed during the evaluation of the array, as virtually no response to the interfering ions was indicated.From the above, it is clear that calixarene tetraesters and related derivatives can form the basis of excellent sodium ISEs. Studies on device lifetime showed that the sensors can be expected to be used for months at a time1* and are able to analyse several thousand blood samples before the signal becomes unacceptably affected by membrane coating or leaching of membrane components. Significantly higher sodium selectivity has been claimed with calix[4]arene ionophores other than esters and ketones.Yamamoto and Shinkai combined a crown ether with the lower rim of a calix[4]arene dialkyl ether to produce an electrode showing a Na+ selectivity of lo5 (relative to K+).I9 Membranes containing this ionophore have recently been assessed as the detector in a flow- analysis system and successfully applied to the determination of sodium in blood samples. Solid-state Sodium-selective Sensors In addition to the traditional ISE configuration discussed above, researchers are interested in solid-state designs of these sensors, such as ISFETs (ion-selective field-effect transistors) or coated wire electrodes (CWEs), as these are expected to be easier to mass produce and will be more compatible with the planar fabrication technologies used in the semiconductor and related industries.It is not surprising, therefore, that studies on the performance of ISFETs incorporating calix[4]arene derivatives have recently appeared in the literature. One paper20 describes the characteristics of ISFETs based on the ketones 8 and 9 (Table 1). These gave Nernstian slopes and good selectivity against other group 1 and group 2 cations. A well known problem with these devices is the lack of a well-defined CALIXARENE-BASED SENSING AGENTS-D. DIAMOND AND M. A. McKERVEY internal boundary potential (i.e.Ek in Figure 1) due to the absence of an internal filling solution or compensating mechanism by which charge can be exchanged across the internal boundary. The same problem occurs with CWEs, which differ from ISEs in that the sensing membrane is deposited directly onto a metallic conductor.This leads to a blocked internal interface between the membrane and the metal, as the former conducts only by means of ion move- ment, while the latter is an electronic conductor. Hence CWEs, while simpler in make up than equivalent ISEs, are generally much less stable, and exhibit greatly reduced effective lifetimes. The design proposed by Brunink et aE.20 involved using a poly(2- hydroxyethyl methacrylate) (polyHEMA) hydrogel layer to help anchor the PVC membrane on the gate region of the device and simultaneously reduce the effect of interferents such as CO, which can diffuse through the PVC layer and affect the internal boundary potential.An alternative proposed by Tsujimura et aI.,l was to use calix[4]arenes bearing oligosiloxane moieties in the esters in sili- cone rubber membrane ISFETs. These groups promoted the disper- sibility of the ligands within the rubber membrane leading to more stable responses compared to similar devices based on the ethyl ester tetramer 3. However, no data on the performance of the device in real samples such as plasma are given. One strategy which might overcome this limitation is to sub- stitute a conductor of mixed character which is capable of trans- ferring charge by means of either ion or electron movement. With this in mind, PVC membranes incorporating ligand 3 have been deposited on polypyrrole which was electrochemically formed on platinum substrates.22 The resulting sodium-selective solid-state sensors were been shown to be much more stable than CWE equiv- alents, and were unaffected by the presence or absence of redox- active species in the sample solution which react on polypyrrole surfaces.Impedance studies confirmed a dramatic reduction in the charge transfer resistance through the device compared to CWE devices which had no polypyrrole layer between the Pt layer and the PVC. Potentiometric Sensors for Other Ions One of the main reasons for the great interest in calixarenes as syn- thetic ionophores is the scope for structural modification and elaboration, not just of the calix itself, but of the pendant binding sites.Since the complexation selectivity rests largely on a best- match relationship between receptor, substrate and solvent, which ideally should maximise the complexation free energy of the primary ion compared with that of interfering ions, the ability to vary the cavity size offers the prospect of developing ligands suit- able for use in sensors for ions other than sodium. A calixarene with a cavity intermediate in size between that of a tetramer and a hexamer has been used in an electrode with K+ ~electivity.,~Although this ionophore 10 is a tetraester, the dioxa- A A 10 calix[4]arene substructure in which two of the four bridging methylene units are expanded by additional oxygen atoms, has a cavity size larger than that of a normal tetramer.The resulting electrode has good sensitivity, though over a narrow working range with a somewhat limited selectivity. The selectivity is infe- rior to that of the well known K+ electrode based on valino-mycin. Calix[6]arene derivatives show selectivity towards the larger alkali cations in complexation and extraction and this is reflected in their suitability as ionophores for caesium ion in ISEs. Four PVC membrane electrodes based on hexaesters 11 and 12 have been found to be caesium-selective against a wide range of possible interfering ions24 (see Figures 4c and 44. X-Ray diffraction studies confirm that these ligands are more open and define much larger cavities than sodium-selective tetramers.Initial studies with 12 have indicated that nitrophenyl/octyl ether is the most effective plasticizer to employ for long-life electrodes. The use of calixarenes with soft donor atoms as binding sites in ISEs for heavy metal cations provides another illustration of their versatility in sensing devices. This was first demonstrated by O'Connor et who quantified the performance of calix[4]arene derivatives 13, 14 and 15 with sulfur and nitrogen groups in ISEs sensitive to silver(r), copper(I1) and lead(rI), these ionophores having previously been found to be efficient extrac- tants for heavy metals from aqueous solution into dichloromethane. With the appropriate number and disposition of soft donor atoms selectivity for heavy metal ions over alkali cations can be realised.Calixarenes 13 and 14 show sensitivity for silver(1) but still have some response to alkali cations. Of the three, the thioalkyl ester 15 displayed the best performance in selectivity against sodium with log KPotAgNa = -1.16. A glassy carbon electrode coated with PVC containing 15 was sub-sequently used to follow potentiometric titrations of mixtures of I-, Br-, and C1-. Later similar studies by Malinowska et a1.26 using sulfur-functionalised calix[4]arenes including thioamide 14 confirmed these results with heavy metal ions. However, mercury(1r) interference is a problem with these thio-calixarene- based ISEs. Cobben et al.27have described calixarene-based ISFETs target- ted at silver(r), copper(II), cadmium(I1) and lead(r1) using 29 cal-ixarene derivatives including those described above.ISFETs employing thioethers 16 and 17 exhibited excellent silver(1) sensitivity and good selectivity, though no mention is made of selectivity over sodium ions. Ligand 17 resulted in devices more selective for copper(Ir), although limited data are given, and signif- icantly more for the selectivity against silver(I1) or mercury(rI), both of which would be probable interferents for this type of membrane sensor. Preliminary data are also reported for cadmium(I1) and lead(r1)-selective ISFETs. The behaviour of the latter is curious, showing a slope of nearly 60 mV/decade change in [Pb"] (twice the theoretical value) and the calibration curve shows a response lin- early decreasing below mol l-', in contrast to that observed with most conventional PVC membrane ISEs (and other ISFETs reported by Cobbin).Unfortunately, no data regarding the lifetimes of the devices are given. Voltammetric Sensors for Ions Chemically modified electrodes (CMEs) can be made by immobil- isation of organic molecules at the electrode surface. Although CMEs containing calixarenes are much less well developed than their ISE counterparts, they do offer benefits in electroanalysis because the analytical reagent (the sensor) is confined to the elec- trode surface where a reaction of chemical interest occurs. Among the properties of CMEs that render them attractive is the ability to accumulate trace analytes from solution into the modifying layer, resulting in an increased analyte concentration at the electrode surface.This accumulation process is similar to the electrolytic accumulation employed in anodic and cathodic stripping voltamm- etry, except that in a CME accumulation is achieved by the immobilised receptor, with a selectivity for the target analyte, at the electrode surface. Arrigan et a1.** have examined the use of the polymeric cal- ixarene ester 18 as a modifier of CMEs for voltammetric analysis of lead@), copper(I1) and mercury(I1) ions in dilute aqueous solution. In this study, the calixarene was incorporated into a carbon paste, prepared from carbon powder and a suitable binding agent such as Nujol, and then packed into the electrode body.The electrode was left on open circuit in the presence of an aqueous solution of the analyte for various times to allow encapsulation of the metal ions at the electrode surface (accumulation cycle). The collected ions were then stripped off reductively and determined by anodic differential pulse voltammetry. The overall analytical cycle consisted of encapsulation of the ion by the calixarene, electrochemical I (Bur), I M"' 18 reduction, and anodic stripping measurement. Typically, peak cur- rents for metal anodic stripping increased with accumulation time up to about 5 minutes, after which a plateau region indicated either the attainment of equilibrium or saturation of the calixarene binding sites.This CME showed good selectivity for lead in particular, with LODs of 0.2, 1.0 and 5.0 mmol 1-1 for lead(Ir), copper(1r) and mercury(n), respectively. However, a predictable limitation of the electrode, which stems from the use of a calix[4]arene tetraester as the sensing agent, was interference from alkali cations, notably Na+ (videsupra). The presence of Na+ or K+ caused competition for the binding sites, thus reducing the lead(I1) signal. Clearly, for optimum behaviour the calixarene binding sites need to be matched with target analyte ions. Preliminary attempts to achieve better selectiv- ity matching, through the use of softer binding sites of thioamide 14 in a CME for Ag+, were not successful. The presence of 14 did not significantly enhance Ag+ ion uptake.The use of calixarenes as electrode-surface coatings in amperometric detectors has been described by Wang et ~1.~~who found enhanced selectivity towards neurotransmitters such as dopamine and epinephrine while exclud- ing common electroactive interferences such as ascorbic acid, uric acid and amphetamine^.^^ 4 Optical Transduction An important trend in sensor research over the past several years has been the development of optical methods of transduction for detec- tion and estimation of clinically important species. The objective here is to transduce a chemical signal, e.g.resulting from complexa- tion of a cation, into an optical response, e.g. a colour change, ideally in a completely reversible and reproducible way.Optical based sensing is attractive for several reasons which include inher- ent safety, less noise pickup in signal transmission over long dis- tances, and the possibility of obtaining much more comprehensive information from a single probe (full spectrum vs. one channel of electrochemical information). While there have been several recent publications describing calixarene derivatives capable of signalling the presence of metal ions optically, there are as yet no functioning optodes or optical equivalents of the electrochemical sensors described above. Nevetheless, there are a number of chromogenic and fluorogenic calixarene-based receptors which show promise for metal ion detection. These systems contain a chromophore/fluorophore which can be CHEMICAL SOCIETY REVIEWS, 1996 either appended to the calixarene at the lower or upper rim or else form an integral part of the molecular substructure.On complexa- tion, the environment of the light-responsive probe may be suffi- ciently perturbed so as to produce a significant change in the UV-VIS absorption spectrum or the fluorescence emission spec- trum. For the former, this can be achieved conveniently by using a calixarene with a pH-dependent chromophore, e.g. a phenol, in the presence of a base which alone is insufficiently strong to deproto-nate the phenol in the uncomplexed receptor. Complexation of a cation may trigger the release of a proton to the base which in turn is revealed in the bathochromic shift accompanying phenoxide formation.Such spectral shifts are easily monitored and, through judicious choice of substituent on the phenol, will register as a colour change. In practice, however, there are additional considera- tions which may influence the ability of the system to function as a selective chromoionophore. It is necessary, for example, to estab- lish that the very presence of the probe, possibly in close proxim- ity to the binding sites of the calixarene, does not adversely affect the binding power of the receptor, its ion selectivity, or the stabil- ity of the resulting deprotonated complex. A variety of chromo- phores have been investigated including nitrophenol and azophenol derivatives and in most cases selective transduction of cation complexation has been observed, although the selectivity is somewhat inferior to that realised with equivalent electrochemical devices.N I 19 Shinkai and his coworkers3' have synthesised calix[4]arene 19 with a 4-(4-nitrophenyl) azophenol unit and three ethyl ester residues on the lower rim and found that on cation complexation in the presence of triethylamine the compound exhibits a new absorption maximum at 600 nm which is lithium-selective. Tri- ethylamine alone does not cause any spectral changes, confirming that deprotonation and complexation are integral events in the chromogenic response. Other nitrophenol-based chromogenic cal- ixarenes which show a selective colour response on complexation with a cation in the presence of base include compounds 20, 21 and 22.The former two are tetraesters with one and four nitro- phenol residues, respectively, incorporated into podands on the lower rim. The chromogenic response of both derivatives in tetra- hydrofuran (THF) in the presence of morpholine reveals a Li+ selectivity with a 1WO-fold response over Na+ (Figure 6).32 Compound 22, designed by Sutherland's group,33 consists of a calix[4]arene with a triethyleneoxy bridge across two distal phe- nolic functions to provide binding sites for alkali cations. The chromogenic response is provided by one free phenolic unit with a dinitrophenyl substituent at the para-position with the remaining phenolic ring present as its methyl ether. Compound 22 in chloro- form extracts K+ in preference to Na+ in aqueous solution in the pH range 7-9 with a selectivity of ca.1000 in extraction coeffi- cient and a shift in the absorption spectrum from 437 to 628 nm. Extraction of Mg2+ or Ca2+ under these conditions was not observed. Preliminary experiments suggest that 22 may be suitable for use in optical fibre sensors for measuring K+ concentrations in biological fluids without any significant loss of K+/Na+ selectiv- ity. Kubo and his have designed a chromogenic Ca2+ sensor, again with the objective of developing an optical fibre for clinical analysis, by incorporating an indoaniline chromophore into a calix[4]arene ester as in 23. The optical properties of the CALIXARENE-BASED SENSING AGENTS-D. DIAMOND AND M.A. McKERVEY --N--L o 20 22 L . 400 500 600 700 800 A Inm Figure 6 Typical shifts in UV-VIS absorbance accompanying complexa- tion of a metal cation (in this case Li +) by calixarene ionophore 20 in the presence of base. The results show shifts brought about by addition of LiClO, to a 5 X mol 1-’ solution of 20 in THF containing 20 mm3 morpholine to give final Lit concentrations (moll-’ )of (1) 0.1,(2) (3) 4 X lo-”, (4)8 X lop4,(5) 2 X Curves (6) and (7) illustrate absense of either Li+ salt or base.32 indoaniline probe are easily perturbed by chemical stimuli other than pH change, in this case by interaction of the quinone carbonyl group with divalent cations. Compound 23 is blue; addition of calcium thiocyanate in ethanol causes a large bathochromic shift (ca.100 nm) with a large increase in absorption intensity. Addition of NaSCN, KSCN or Mg(ClO,), causes only minor changes in the absorption spectrum of 23 suggesting a significant selectivity for Ca2+. IR absorption changes for the C=O (ester) and C=O (quinone) groups in the presence of Ca2+ suggest that 23 forms an encapsulated complex on the lower rim of the calixarene. The use of indoaniline-derived sensors has been extended to calix[6]arenes to produce a chromoionophore with a selective U022+ion-induced pronounced colour change in ethanol. An alternative approach that has been applied successfully to a chromogenic sensor for Na+ is to couple a calix[4]arene tetraester ionophore with a Simon-type lipophilic pH-sensitive dye such as ETH 5294? Selective optical transduction is also possible through the use of fluorescent calixarene receptors.The conformational changes which invariably accompany complex formation can be exploited to advantage to perturb a suitable fluorophore attached to either the upper or lower rim of the calixarene. Alternatively, it may be 23 possible to use a combination of a fluorophore with a quenching agent judiciously positioned on the calixarene so that any confor&ational adjustments resulting from complex formation will lead to a change in the extent of fluorescence quenching. Once the effect of complexation on the emission spectrum is known, monitoring a responsive emission wavelength as a func- tion of time enables transient changes in the concentration of the analyte to be detected.Calixarene fluorionophores for alkali cations have attracted most attention. Jin and coworkers attached two pyrenemethyl acetate residues to a calix[4]arene diester as in 24 to produce an intra- molecular excimer-forming sensor which shows a change in fluo-rescent characteristics specifically on complexation of Na+ (Figure 7).36Shinkai’s group, in contrast, used benzothiazole as the fluo- rophore to construct the calix[4]arene sensor 25 which has been described as having ‘perfect’ Li ~electivity.~’+ Na+ selectivity has also been observed with calix[4]arene amides and esters containing anthracene residues, e.g. 26,on the lower rim.3x An Na+ sensory system has been devised in which a calix[4]arene carries a pyrene unit (the fluorophore) and a nitrobenzene unit (the fluorescent quencher) on the periphery of the molecular cavity.39 The conformational changes which accompany complexation of the cation are such that the pyrene and nitrobenzene rings are moved further apart resulting in a dramatic enhancement of fluorescence intensity.A luminescent pH sensor based on the y-tert-butylcalix[4]arene-linked ruthenium(I1) trisbipyridyl complex 27 has been devised by Grigg et a/.40The trisbipyridylruthenium(I1) moiety was chosen as the luminophore with the three free phenolic units of the cal- ixarene acting as acid-base sites. Formation of the phenolate anion(s) causes photoinduced intramolecular electron transfer to occur from the phenoxide ion to the trisbipyridyl ruthenium(rI), thus quenching the luminescence.Once the phenolate ions are protonated, electron transfer is prevented and luminescence is thus restored. The luminescence properties of lanthanide ions have been of much interest because of their potential use as probes and labels for a variety of chemical and biochemical applications. Although working sensors have yet to be constructed, it is known that calix[4Jarene amides form strong complexes with EulI1, TblI1 and GdlI1 ions.,’ The TblI1 complex shows a high luminescence quantum yield and a long luminescence lifetime suggesting that it may be useful for time -resolved fluoroimmuno- assay. CHEMICAL SOCIETY REVIEWS. 1996 k/nm 480 nm-I -----390 nm Na' 30 pmol dm" -1 I I 0.15 0.75 1.5i L K+/ mmol dm" .----..a 4 Na' 15 umol dms t H 1 min Figure 7 (a) Changes in fluorescence emission spectra obtained with fluo- rescent calixarene derivative 24 on addition of NaSCN.(b) Selectivity of fluorescent response measured at 480 and 390 nm on addition of Na+ and K+ions.36 25 27 5 Sensors for Organic Guests The design of receptors for use as sensors for organic guest mole- cules or ions poses new challenges. Unlike many inorganic ions, organic guests are likely to be non-spherical and thus the geometric and stereochemical features essential to effective mutual recogni- tion will need to be accommodated. Furthermore, when neutral ana- lytes are the targets, attractive forces other than purely electrostatic will have to be exploited to ensure adequate binding.Nevertheless, the calixarenes do offer opportunities and some calixarene-based sensors for organic compounds are beginning to appear with cases of potentiometric, voltammetric and optical transduction. Chan and Odashima and their respective coworkers have inde- pendently developed ISEs for organic amines, in protonated form, using calix[6]arene esters 11 and 12. These derivatives function as ionophores in organic potentiometric sensors.3o The electrodes are selective for primary amines against secondary or tertiary amines. Best responses with primary amines are observed for compounds not having a chain branch adjacent to the amino group as, for example, in hexylamine, octylamine and dopamine.The electrode response is presumed to result from formation of an inclusion complex with hydrogen bonding between the protonated amino group of the analyte and the carbonyl groups of the calixarene ester. Chan et al.42 have extended this investigation to include the coupling of a lipophilic hexaester with a pH-sensitive chromoionophore devel- oped earlier by Simon (ETH 5294) to produce an amine sensor. The guanidinium ion is an important analyte target for sensing devices because of its relevance to biological systems. A good receptor for guanidinium should possess multiple hydrogen-bond acceptor sites located in a single plane. A PVC membrane CHEMFET incorporating calixarene receptor 28 capable of multi-ple hydrogen-bond formation has been constructed by Kremer et This device displays an excellent response to guanidinium ions and although potassium, sodium and ammonium are the strongest interfering ions, there is good selectivity for the guanidinium ion even in the presence of these ions.Cyclic voltammetry has been employed by Gokel and his coworkers to study complex formation between the water-soluble sulfonated calix[6]arene derivative 29 and protonated amines and 26 29 CALIXARENE-BASED SENSING AGENTS-D DIAMOND AND M A McKERVEY neutral ferrocene derivatives 3o The voltammetric response is pre- sented in terms of shifts in half-wave potential and current vana- tion, with the protonated analytes exhibiting the larger effects due to electrostatic effects in the binding Although a voltammetnc sensor for neutral organics has yet to emerge, on the basis of the binding mechanism llkely to operate here, one might anticipate that the binding of suitable electroactive amines to a calixarene host could be detected voltammetrically and form the basis of such a sensor Voltammetric studies with calix[4]arenequinone-hydro-quinone systems indicate that these systems may serve as redox- switchable metal-ion binding sensors for a chemical sensor Beer et a1 have described a system based on a calix[4]arene diquinone which is capable of complexation and electrochemical recognition of ammonium and alkylammonium ions 4s A gas sensor for colorimetric determination of tnmethylamine has been devised by McCarrick et a146 using a calix[4]arene bearing four nitrophenylazophenol residues similar to those present in 19 This material when complexed to lithium and immobilised onto filter paper undergoes a dramatic colour change from yellow to red in the presence of gaseous trimethylamine at concentration above 20 ppb within minutes The intensity of the colour change is amine dependent and the device may be applicable to the monitor- ing of fish freshness More recently, Chawla and Srin~vas~~ have described an entire series of doubly bridged calix[4]arenes with azophenol moieties which provide visual detection of amines including ethylenediamine In situ formation of an ionic lipophilic hydrazone forms the basis of a calixarene ISE for determination of formaldehyde4x 6 Patents A search of the patent literature has revealed almost 100 patents filed which include the use of the keyword ‘calixarene ’ These patents cover methods of preparation/synthesis of certain calixarene derivatives, or their use in a wide variety of applications including corrosion inhibitors, fuel additives, hair dyes, charge-controlling agents for developing electrostatic images, additives in epoxy resins and adhesives, developer solution for photographic negatives, extraction of uranium ions, deodorant additive, and stabilisers in rubbers Very recently, the first chemical sensor-related patents have begun to appear, and this trend will probably continue, given the obvious commercial importance of these devices Of the four sensor patents, three relate to electrochemical sensors for alkai metal ions, and one to the use of cation complexing dyes (chromoionophores) in optical sensors 7 TheFuture Sensing applications of calixarene derivatives are only beginning to develop The potentiometric and other electrochemical sensors for metal ions can be regarded as the first generation of these sensors Calixarenes capable of optically signalling complexation with metal ions, while valuable in their own right, could be the pre- cursors of much more interesting sensing materials (see for example the fluorescence signalling of encapsulation of a flavin, pteridine, by calix[4]arene host capable of changing from a ‘closed’ to an ‘open’ It is now recognised that calixarenes can be used as building blocks of much more substantial structures which could be used to sense a huge number of potential hosts, ranging from neutral gaseous molecules (eg toxic solent vapours) to amino acids or more complex biological molecules Working sensors for anions have also yet to emerge 8 References 1 For an excellent review of Simon’s contribution to the development of chemical sensors see H M Widner, Analytical Methods and Instrumentation, 1993, 1, 3 2 D Ammann, W E Morf, P Anker, P C Meier, E Pretsch and W Simon, ISE Rev 1983,5,3 3 E Linder, K Toth, M Horrath, E Pungor, B Agai, I Bitter, L Toke and Z Hell, Fresenius Z Anal Chem ,1985,322, 157 4 D Diamond, Anal Chem Symp Ser, 1986,25, 155 5 General reviews of calixarenes and their properties include (a) C D Gutsche, Calixarenes, vol 1, in Monographs in Supramolecular Chemistry ed J F Stoddart, Royal Society of Chemistry, Cambndge, 1989, (b) V Bohmer and J Vicens, ed ,Topics in Inclusion Phenomena Calixarenes A Versatile Class of Macrocyclic Compounds, Kluwer, 1990, (c) M A McKervey and V Bohmer, Chem Brit,1992,28,724, (d)S Shinkai, Tetrahedron, 1993,49, 8933 6 F Arnaud-Neu, E M Collins, M Deasy, G Ferguson, S J Hams, B Kaitner, A J Lough, M A McKervey, E Marques, B L Ruhl, M -J Schwing-Weill and E M Seward, J Am Chem Soc , 1989,111,8681, S K Chang and I Cho, J Chem Soc Perkin Trans 1, 1986,211, M J Schwing-Weill and M A McKervey, ref 1 pp 149-172 7 F Arnaud-Neu, G Barrett, S Cremin, M Deasy, G Ferguson, S J Hams, A J Lough, L Guerra, M A McKervey, M J Schwing-Weill and P Schwinte, J Chem Soc Perkin Trans 2, 1992, 11 19 8 M Kirch and J M Lehn, Angew Chem Int Ed Engl , 1975,14,555, J P Behr, M Kirch and J M Lehn,J Am Chem Soc ,1985,107,241 9 D Diamond and G Svehla, Trends Anal Chem, 1987, 6, 46, D Diamond, G Svehla, E M Seward and M A McKervey, Anal Chim Acta, 1988,204, 223 10 K Kimura, T Matsuo and T Shono, Chem Lett, 1988,615 11 K M O’Connor, M Cherry, G Svehla, S J Hams and M A McKervey, Talanta, 1994, 41, 1207, A Cadogan, D Diamond, M R Smyth, M Deasy, M A McKervey, E M Seward and S J Hams, Analyst, 1989, 114, 1551, K Cunningham, G Svehla, S J Hams and M A McKervey, Analyst, 1993,111,341 12 M Telting Diaz, F Regan, D Diamond and M R Smyth, J Pharm Bzomed Anal, 1990,8,695 13 M Telting Diaz, D Diamond, M R Smyth, E M Seward and M A McKervey, Electroanalysis, 1991, 3, 37 1 14 K Kimura, T Miura, M Matsuo and T Shono, Anal Chem , 1990,62, 1510 15 M Telting Diaz, F Regan, D Diamond and M R Smyth, Anal Chim Acta, 1991,251, 149 16 D Diamond and R J Forster, Anal Chim Acta, 1993,276,75 17 D Diamond, J Lu, Q Chen and J Wang, Anal Chim Acta, 1993,281, 629 18 D Diamond and F Regan, Electroanalysis, 1990,2, 1 13 19 H Yamamoto and S Shinkai, Chem Lett , 1994, 11 15 20 J A J Brunink B J R Haak, J G Bomer, D N Reinhoudt, M A McKervey and S J Harris, Anal Chim Acta, 1991, 254, 75, D N Reinhoudt, Sens Actuators, 1992, B6, 179 21 Y Tsujimura, M Yokoyama and K Kimura, Electroanulysis, 1993, 5, 803 22 A Cadogan, Z Gao, A Lewenstam, A Ivaska and D Diamond, Anal Chem , 1992,64,2496 23 A Cadogan, D Diamond, S Cremin, M A McKervey and S J Harris, Anal Proc , 1991,28, 13 24 A Cadogan,D Diamond,M R Smyth,G Svehla,M A McKervey,E M Seward and S J Hams, Analyst, 1990,115, 1207 25 K O’Connor, G Svehla, S J Hams and M A McKervey, Talanta, 1992,39,325 26 E Malinowska, Z Brzozka, K Kasiura, R J M Egbennk and D N Reinhoudt, Anal Chim Acta, 1994,298,245, 1994,298,253 27 P L H M Cobben, R J M Egbenck, J G Bomer, P Bergveld, W Verboom and D N Reinhoudt, J Am Chem Soc ,1992,114, 10573 28 D W M Arrigan, G Svehla, S J Hams and M A McKervey, Electroanalysis, 1994,6,97 29 J Wang and J Liu, Anal Chim Acta, 1994,249,201 30 For a more detailed account of the use of calixarenes in electroanalysis see K M O’Connor, D W M Arrigan and G Svehla, Electroanalyszs, 1995,7,205 31 H Shimizu, K Iwamoto, K Fujimoto and S Shinkai, Chem Lett ,1991, 2147 32 M McCarrick, S J Hams, D Diamond, G Barrett and M A McKervey, J Chem Soc Perkin Trans 2, 1993, 1963 33 M A King, C P Moore, K R A Samankumara Sandanayaka and I 0 Sutherland, J Chem Soc Chem Commun , 1992,582 34 Y Kubo, S Hamaguchi, A Niimi, K Yoshida and S Tokita, J Chem Soc Chem Commun ,1993,305 35 K Toth B T T Lan, J Jeney, M Horvath, I Bitter, A Grun, B Agai and L Toke, Talanta, 1994,41, 1041 36 J Jin, K Ichikawa and T Koyama, J Chem Soc Chem Commun , 1992,499 37 K Iwamoto, K Araki, H Fujimoto and S Shinkai, J Chem Soc Perkin Trans I, 1992, 1885 38 C Perez Jimenez, S J Hams and D Diamond, J Chem Soc Chem Commun ,1993,480 39 I Aoki, T Sakaki and S Shinkai, .I Chem Sue Chem Commun ,1992, 730 40 R.Grigg, J. M. Holmes, S. K. Jones and W. D. J. Amilaprasadh Norbert, J. Chem Soc., Chem. Commun., 1994, 185. 41 N. Sabbatini, M. Guardigli, A. Mecati, V. Balzani, R. Ungaro, E. Ghidini, A. Casnati and A. Pochini, .I.Chem. SOC., Chem. Commun., 1990,878. 42 W. H. Chan, A. W. M. Lee and K. Wang, Analyst, 1994,2809. 43 F. J. B. Kremer, G. Chiosis, J. F. J. Engbersen and D. N. Reinhoudt, J. Chem. SOC.,Perkin Trans. 2, 1994,677. 44 K. Suga, M. Fujihira, Y. Morita and T. Agawa, J.Chem. SOC.,Faraday Trans., 1991,87, 1575. CHEMICAL SOCIETY REVIEWS, 1996 45 P. D. Beer, 2. Chen and P. A. Gale, Tetrahedron, 1994,50,931. 46 M. McCarrick, S. J. Harris and D. Diamond, J. Mater. Chem., 1994,4, 217. 47 H. M. Chawla and K. Srinivas, J. Chem. SOC.,Chem. Commun., 1994, 2593. 48 W. H. Chan and R. Yuan, Analyst, 1995,120,1055. 49 H. Murakami and S. Shinkai, J. Chem. SOC.,Chem. Commun., 1993, 1537.

ISSN:0306-0012    

DOI:10.1039/CS9962500015

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

The Structure and Mode of Action of the Cofactor of the Oxomolybdoenzymes D. Collison C. D. Garner and J. A. Joule Chemistry Department University of Manchester Manchester M 13 9PL UK 1 Introduction The concept that a small molybdenum-containing unit might act as a cofactor for the molybdoenzymes was first suggested by Pateman et al. 30 years ago; as a result of work on a series of mutant cells lacking both nitrate reductase and xanthine oxidase activity it was proposed that the two enzymes share a common cofactor.’ Support for this idea came from work by Nason et a1.* who showed that a molybdenum-deficient nitrate reductase from a mutant strain of Neurospora crassa Nit-1 could be reactivated by an acid-denatured extract from ‘any molybdoenzyme.’ Subsequent studies principally by Brill et al.achieved the isolation3 of a cofactor from Component I of nitrogenase containing iron molybdenum and acid-labile sulfide. However although this cofactor FeMoco was capable of reconstituting molybdenum- deficient nitrogenase Component I from the Azotobacter vinelandii mutant strain UW45 it did not reconstitute the nitrate reductase mutant Nit-1. On the other hand a cofactor isolated from xanthine oxidase did reactivate the Nit-1 strain -but not the nitrogenase mutant UW45 -this cofactor is now termed Moco and is the subject of this re vie^.^-^ Subsequently activity has been returned to Nit- 1 nitrate reductase using cofactor produced from other oxo- molybdoenzymes aldehyde oxidase sulfite oxidase and nitrate reductase.5 Moco or structural variants thereof is also the cofac- tor for xanthine dehydrogenase pyridoxal oxidase nicotinate oxidase carbon monoxide oxidase formate dehydrogenase tetrathionate reductase chlorate reductase biotin sulfoxide reduc- tase purine hydroxylase and dimethyl sulfoxide (DMSO) reduc-tase all enzymes involved in redox-xygen-transfer processes. In man hepatic sulfite oxidase is an essential enzyme serving a detoxification purpose converting sulfite into sulfate; lack of the enzyme leads in most cases to death soon after birth.6 The major- DK Joule graduated (University of Manchester) in 1958 and after a PhD (Manchester with G.F. Smith} and post-doctoral work (Princeton R. K. Hill Stanford C. Djei-assi) returned to Manchester as a lecturer with subsequent sabbatical periods at the University of Ibadan Nigeria Johns Hopkins Hospital Baltimore and the University of Maryland (UMBC) USA.He is now a Reader. Dr Joule’s reseurch L oncentrates on nitrogen heterocyclic chem- istry and synthesis of heterocyclic natural products Professor Garner graduated (University of Nottingham) in 1963 and after a PhD (Nottingham with Professor C. C. Addison Drs. J. A. Joule C. D. Garner ity of these enzymes are large and complex containing haem Fe- S and/or flavin centres in addition to MOCO.’ 2 Moco Structural Characterisation Moco is unstable when released from its associated protein and has only been characterised by conversion into derivatives all of which lack the molybdenum. Therefore ideas for the exact location of the metal have been developed using a combination of the evidence for the structure of the organic moiety termed molybdopterin spectro-scopic information as to the oxidation state of the metal and its primary coordination sphere in the enzyme (Section 2.2) and indications from the structures of pteridine-containing model com- pounds (Section 5).2.1 The Organic Ligand From experiments on denatured enzyme extracts Rajagopalan et al.x isolated two pteridines,g ‘Form A 1 (after exposure to H+/I,/KI) and ‘Form B’ 2a (K,[Fe(CN),]/HO-). They noted that the fluorescence of these compounds was not present in the whole enzyme and concluded that molybdopterin in Moco does not contain a fully aromatic pteridine and that 1 and 2a therefore must be formed by oxidative degradations. The phosphate in Form B could be removed by treatment with phosphatase the resulting material 2b being shown to contain a 1,2-diol by its positive reaction with periodic acid.The absolute configuration of the alcohol-bearing side-chain carbon in 1 was later established by comparison of the CD spectrum of the corresponding diol with that of synthetic material (the synthesis is summarised in Section 4).1° D. Sutton and S. C. Wallwork) and post-doctoral research (Caltech H. B. Gray; Nottingham ICI Research Fellow) was appointed as a lecturer at the University of Manchester in 1968. He has been vis- iting Professor at the Universities of Lausanne Strasbourg Melbourne and Florence and was awarded the Tilden Medal of the Royal Society of Chemistry in 198516.He is now Professor of Inorganic Chemistry and Head of Department. His research is con- centrated upon coordination chemistry relevant to the biological chemistry of the d-transition metals. Dr. Collison graduated (University of Manchester) in 1976 and after a PhD (Manchester with C. D. Garner and I. H. Hillier} and post-doctoral work (Manchester F E. Mabbs; Manchester C. D. Garner; UMIST N. J. Blackburn) became an SRC Fellow (Manchester) and subse- quently a Royal Society University Research Fellow (Manchester) and was appointed to his current position of Senior Lecturer at the University of Manchester in 1994.His research concentrates on the electronic structure of d-transition metal compounds.D. Collison 25 The stereochemistry shown for 2 is based on the assumption that it is the same as in 1. 1 2 R' R2 a H bH PO,H H c SMe H Urothione 2c is present in normal urine and is believed to be a metabolite of Moco;" as such it gives a clue to the locations of two sulfur atoms in molybdopterin. Rajagopalan and Johnson subjected urothione to Raney nickel hydrogenolysis; four compounds were produced one of which presumed to have a dihydrothiophene ring was dehydrogenated with selenium dioxide producing material identical with 'dephospho Form B' 2b. Key information'* on the structure of molybdopterin was gained when Rajagopalan and coworkers working with chicken liver sulfite oxidase and cows' milk xanthine oxidase produced a compound which they believed to represent a trapped molybdopterin.These and other data led to a formulation for Moco as 3 with uncertainties as to (i) the oxidation level of the pyrazine ring; (ii) whether the sulfur atoms are attached only (cf.urothione and Form B) at C-1' and C-2' of the side-chain; (iii) the oxidation levels of the side-chain carbons carrying the sulfur atoms; (iv) the tautomeric form of the (reduced) pteridine moiety and (v) the substituent on the phosphate. Other studies have demonstrated that in carbon monoxide dehy- drogenase the pterin unit is linked via the side-chain phosphate to a guanosine-5'-phosphate and in DMSO reductase to a cytosine-5 '-phosphate in each case via a pyrophosphate link; latterly hypo- xanthine and adenine have been shown to be associated with the molybdopterin in the same way.13 Very recently two X-ray crystal structure determinations have shed new light on molybdopterin-cofactors. A hyperthermophilic tungstopterin enzyme ferredoxin aldehyde oxidoreductase from Pyrococcus furiosus contains14 a cofactor which is fascinatingly similar yet subtly different from that proposed by Rajagopalan and other previous workers.The first difference is that in the tungsten cofactor 4 the metal has two pterin ene-dithiolate ligands. The second significant difference is the presence of a dihydropyran ring formed by the formal cyclisation of a side-chain hydroxy oxygen onto the pteridine 7-position of a 5,6-dihydropteridine. Repre- sentation 4 includes the two 0x0-groups indicated by a tungsten edge EXAFS study;IS the phosphates link to the same Mg2+ ion.I4 The crystal structure determination of the aldehyde oxidase from 4 Desulfovibrio gigas,I6 shows a molybdenum cofactor again in a tri- cyclic form comparable to that in 4 but in this case with a cytosine linked at the terminal phosphate and with only one ene-dithiolate ligating the metal centre 5. Considering the quantities of material available for the earlier chemical degradative and spectroscopic work on Moco the close- ness of the deductions to the structures now determined is CHEMICAL SOCIETY REVIEWS 1996 5 Hd \OH commendable.A caveat must be included regarding the accuracy with which the protein crystal structure determinations can delin- eate the state of oxidation at the pyrazine ring carbon atoms or at the side-chain sulfur-bearing carbon atoms. 2.2 The Metal Centre 2.2.1 Introduction Protein crystallographic studies on oxomolybdoenzymes and tung- sten-containing enzymes (tunzymes) have only recently become available (see above). However these studies do not unambigu- ously define the coordination at the metal. The clearest evidence concerning the coordination at the metal centre has been derived from a variety of spectroscopic studies. The complementary use of X-ray absorption spectroscopy (XAS) notably the extended X-ray absorption fine structure (EXAFS) of the molybdenum K-edge and electron paramagnetic resonance (EPR) spectroscopy have domi- nated these investigations because both are able to probe the metal's environment selectively.Interpretations of the data obtained from the enzyme studies have been significantly strengthened by recording corresponding XAS and EPR information for fully characterised chemical analogues. The prefix 0x0 for this group of enzymes is appropriate; not only does each enzyme catalyse a conversion the net result of which can be represented as oxygen atom transfer but also XAS studies have indicated the presence of at least one terminal 0x0 ligand (Mo=O) in (virtually) every system and state examined. Six-coordination and an octahedral geometry dominate the chemistry of molybde- num(vI) molybdenum(v) and molybdenum(1v).l7 For molybde- num(v1) the cis-dioxo moiety { MoO2l2+ achieves the pseudo-octahedral geometry by binding four donor atoms. Each bond trans to an Mo=O group is generally longer than an equiva- lent bond cis; neutral ligands are often found in the former positions and anionic ligands in the latter. The cis-dioxo geometry maximises the Mo(d,)-O(p,) overlaps. Binuclear complexes of molybdenum(v1) exist but the ten- dency to dimerisation via p2-OH linkages becomes dominant on reduction to molybdenum(v). This aspect of the chemistry means that the synthesis of most monomeric analogue complexes and studies of their spectroscopy and reactivity are performed in non- aqueous media.One terminal 0x0 (or sulfido) group is generally found for monomeric molybdenum(v) complexes and hence there is a single trans-site at which easy substitution chemistry can take place. Thus both five-and six-coordinate geometries are common. Both mono- and di-oxo complexes of molybdenum(1v) are found the latter possessing a mutually trans-dioxo geometry which places the d2 electrons in the same metal (d,) orbital leaving the two remaining d orbitals for Mo(d,)-O(p,) overlaps. 2.2.2 Spectroscopic characterisations 2.2.2.1 X-Ray absorption spectroscopy XAS has played a vital role in defining the chemical nature of molybdenum centres in enzymes and how they respond to changes in the oxidation level of Moco and/or to the presence of substrates substrate analogues or inhibitors of enzymic activity.l8 The molybdenum K-edge EXAFS results achieved19 for chicken liver sulfite oxidase are the clearest such data and the interpretation achieved represents a prototype for other oxomolybdoenzymes. The molybdenum site has been investigated in each of its three accessible oxidation levels [(VI) (v) and (rv)] as a function of pH and chloride concentration. The molybdenum(v1) is coordinated by two 0x0-groups at ca. 1.70 A one oxygen (or nitrogen) and three THE STRUCTURE AND MODE OF ACTION OF THE COFACTOR OF THE OXOMOLYBDOENZYMES-D. COLLISON ET AL. sulfur-donor ligands at ca. 2.06 and 2.42 A respectively; two of these sulfur atoms presumably derived from the molybdopterin.8 The molybdenum(vr) centre is not affected by changing the pH from 6 to 9 or by a variation in the chloride concentration.The molybdenum-(v) and -(Iv) centres each possess a single oxo- ligand at ca. 1.69 A one oxygen (or nitrogen) and three sulfur- donor ligands at ca. 2.00 and 2.37 A respectively. Both of these centres appear to bind chloride at pH 6 in 0.3 mol 1-I KCl. EPR spectroscopy showed that the centre can exist in two different forms which are in a pH- and anion-dependent equilibrium. George et al." concluded that the molybdenum K-edge EXAFS data were consistent with one chloride ligand binding to the low pH form and that the number of 0x0-groups remains the same upon transition from the high-pH to the low-pH molybdenum(v) form. Thus reduc- tion of molybdenum(vr) results in the loss of one 0x0-group pre- sumably due to protonation and the generation of an anion binding site.This behaviour is consistent with the chemistry of molybde- num in its higher oxidation states since a cis-{MO~IO,}~+centre is +generally converted into a { MoVO} or { MoIVO} 2+ centre upon reduction. Xanthine oxidase is the most accessible of the oxomolybdo- enzymes and is readily extracted from cows' milk. This enzyme exists in two forms an active and an inactive form caused by loss of a sulfur atom (desulfo). Molybdenum K-edge EXAFS studies20 have shown that the environments of molybdenum(vr) and molybdenum(rv) in desulfo xanthine oxidase closely resemble that l9 of the corresponding oxidation state for chicken liver sulfite oxidase.The principal differ- ence between the centre of the oxidized active form as compared to the oxidized desulfo form is the presence of one sulfido-group (at ca. 2.18 A) plus one 0x0-group rather than two oxo-groups.21 The molybdenum centre of xanthine oxidase is very reactive and both molybdenum K-edge EXAFS and EPR data indicate that the centre of this reactivity is the Mo=S bond. The terminal sulfido-group is lost upon reduction presumably being protonated to form an Mo-SH moiety. Arsenite is a potent inhibitor of xanthine oxidase clear evi- dence for an Mo- S-As interaction and an interbond angle of ca. 80" has been obtained from combined Mo and As K-edge EXAFS studies. 2.2.2.2 Electron paramagnetic resonance spectroscopy EPR spectroscopy selectively probes the molybdenum(v) d1 centres of the oxomolybdoenzymes. The EPR parameters (g-and A-values) of the centre are extremely sensitive to the nature of the coordination sphere.The role of the molybdenum(v) state in the enzymatic reactions has been questioned but nonetheless this state is important since it can be generated within biological samples and it is the only state (other than possibly MolIr) which can be detected by EPR spectroscopy. The work of Bray et ~l.,~,using EPR spectroscopy has profoundly influenced views as to the nature of the active sites of the oxomolybdoenzymes. Indeed the initial experiments of Bray et a1.z3 in 1959 and Meriwether et in 1966 were indicative of sulfur-donor ligands bound to molyb- denum. The presence of nuclei such as IH I3C I4N I7O,31P or 33Sin or near to the coordination sphere of molybdenum(v) can be revealed by EPR spectroscopy as super-hyperfine splittings of resonances.More recently it has become technically feasible to use lH or 31P electron nuclear double resonance (ENDOR) on enzyme samples containing molybden~m(v).2~ 2.2.2.3 Magnetic circular dichroism spectroscopy (MCD) DMSO-reductase from Rhodobacter capsulatus26 and Rhodobacter sphaeroidesZ7 is a soluble periplasmic enzyme (M,= ca. 82000) which contains only Moco as a prosthetic group. This simplicity greatly facilitates spectroscopic studies of the molybdenum centre and this has been exploited in an MCD spectroscopic study of the molybdenum(v) state of these enzymes.,* The spectrum shows six oppositely signed bands ranging in wavelength from 701 to 358 nm which are assigned as dithiolene S to MoVcharge-transfer transitions.2.2.2.4 Resonance Raman spectroscopy Resonance Raman spectroscopy has been used to probe the metal coordination in a variety of metal lop rote in^.^^ However for most pterin-containing molybdenum enzymes other strongly absorbing prosthetic groups dominate the electronic and resonance Raman spectra and to date only DMSO reductase has been studied by this technique. The oxidized and reduced forms of DMSO reductase show vibrations in the 335-385 cm-I region that shift upon enrichment of the enzyme with 34S and therefore have been assigned to Mo-S vibrations.The most prominent feature is the band at 350 cm-' in oxidized DMSO-reductase which shifts to 343 cm-l upon 34S enrichment and has been assigned to a Mo-S (dithiolene) vibration by comparison with the bands at 351 and 348 cm-' in [C,H,MoIV{S,C,[C(O)Me]-quinoxalino}] and [(C,H,),MorV( S,C,[C(O)Me]-pterin J 1 respectively which shift by the same amount.30 3 Synthesis and Properties of Compounds which model the Bioactivities of Moco-containing Enzymes 3.1 Oxygen Atom Transfer 'Molybdenum . . . lies at the epicentre of 0x0 transfer chemistry. More 0x0 compounds have been prepared and characterized more 0x0 transfer reactions are known and more catalytic systems based on these reactions have been devised than for any other and these have been comprehensively re~iewed.~ Molybdenum enzymes catalyse the overall reaction shown by equation 1 where X is the enzyme substrate.The electrons and protons produced by the oxidation of the substrate (or consumed in the reduction of the substrate) may be involved with the molybdenum as the (formal) { MO~IO,}~+/{MoIVO(OH,)J2+ couple. Alternatively the molyb- denum centre may be involved in direct oxygen atom transfer (equation 2.). X +H,O,'XO + 2H+ + 2e (1) Both routes can be employed depending on the enzyme and the operating conditions. Recent work by Holm et al. has demonstrated that DMSO-reductase from R. sphaeroides is an oxotransferase. Thus the overall transfer of an oxygen atom ('*O) from DMSO to 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane (PTA) is catalysed by the enzyme and the labelling of the substrate demonstrated that the oxygen atom transferred did not arise from the solvent? Mechanistic versatility for oxomolybdenum complexes has been demonstrated by the reaction sequence summarised in Scheme 1.33 This system based on the tris(pyrazoly1)borate ligand (L-N,) models some aspects of the reaction chemistry of sulfite oxidase. [(L-N,)MoV102(SPh)] reacts with PPh to produce [(L-N3)MorVO(SPh)] and is capable of catalysing oxygen atom transfer from Me,SO to PPh,.In the presence of H,O a one-electron reduction takes place to yield [(L-N,)MovO(OH)(SPh)] which can be oxidised by 0 to regenerate the starting material. Oxygen isotope labelling reveals that H,O is the source of the 0x0 ligand not 0 and the oxidation state of the molybdenum is suggested to control the level of protonation of the water-derived ligand.LMovO(OSiMe,)(SPh) 'A [CoCpz][LMovOz(SPh)] COCP2. PY \ LMoJVO(SPh)(py) LMO* 'U2(Sk'h) Scheme 1 Holm et al. have developed some elegant ligand~~~ capable of sustaining molybdenum as a monomeric centre during oxygen atom transfer. The compound 6 synthesised by the reaction of [MoO,(acac),] with the dithiol pro-ligand oxidises phosphines and the resultant molybdenum(1v) form 7 reduces N-and S-oxides. CHEMICAL SOCIETY REVIEWS 1996 4 Synthesis of Degradation Products of Moco Following of deoxyurothione the total synthesis39 of (2)-urothione is a triumph for Taylor's strategy for the regioselec- tive synthesis of 4-substituted pteridines; Scheme 4 summarises the key steps.Scheme 2 Reagents i MeOH CH,Cl room temp. (92%); ii DMF room temp. (65%). The redox pair 8 and 9 oxidise/reduce a variety of substrates such as P- Se- and N-oxides some of which are substrates for oxo-molybdoenzymes. Ar,,Ar Ar ,Ar Ar = 4-But C,H 3.2 Dithiolene Complexes The coordination of molybdenum by sulfur demonstrated by spectroscopic studies of the oxomolybdoenzymes together with the constitution of Moco has usually been interpreted to indicate dithiolene (or ene- 1,2-dithiolate) ligation (see 5). The results of the crystallographic studies of the aldehyde oxidoreductase from D. gigasI6 and P.~U~~OSUS'~are consistent with the coordination of one dithiolene to molybdenum and two dithiolenes to tungsten respectively. 2-Scheme 3 Reagents i Na,MoO,. 2H,O H,O pH 6 room temp. (6I %). Dithiolene complexes generally display reversible redox proper- ties. This behaviour is vital for any chemical analogue of the molyb- denum centre of the oxomolybdoenzymes. The maleonitrile dithiolate (mnt) ligand has been to afford [MoO2(mnt),I2- and [M~O(mnt),]~- (Scheme 3) as well as [M~OCl(mnt),]~-; a set of complexes which further reinforce the comparison between these systems and the molybdenum centres of the oxomolybdoenzymes. However the redox potentials reported for oxomolybdoenzymes two couples corresponding to MoV1/MoVand MoV/MoIV processes separated by only ca.200 mV contrast with the observation of two one-electron processes for [Mo(dithiolene),]"- and [MoO(dithio- lene),]"-(n = 0 1 2) complexes which are separated by 3 500 mV.35.36 An intriguing aspect of the properties of Moco is the extent to which the molybdenum and the pterin jointly participate in the redox changes of the centre. Chemical support for this postulate has been demonstrated by studies of [Mo(qdt),I2- (qdt = quinoxaline-2,3-dithiolate) and [(q5-C5H,)Co{S2C,H(quinoxalin-2-y1) }] systems.37 Thus not only may the extent and nature of this cooperation vary from enzyme to enzyme but also it may be meaningless to attempt to separate the metal and ligand redox contributions. Also such behaviour provides an attractive mechanism for modulating the redox potential of Moco by a protein via a control of the state and extent of protonation of the pyrazine ring and the stabilisation of par- ticular tautomers of the partially reduced forms of the pteridine. 0 SMe Scheme 4 Reagents i Et,N EtOH room temp.(92%); ii LiBF aq. MeCN room temp. (98%); iii NaOAc Bu'OH reflux (99%); iv Bu'ONO/CuBr MeCN reflux (56%); v NaSMe THF room temp. (93%); vi NaBH EtOH THF room temp. (90%);vii HC(OMe) p-MeC,H,SO,H room temp. (88%);viii p-MeC6H,S0,H MeOH reflux (86%);ix guanidine hydrochloride NaOMe reflux; x CF,CO,H room temp.; xi 3 mol 1-' H,SO reflux (7996 two steps). If in molybdopterin the sulfur at C-2' is not also linked to the pteridine C-7 then both the formation of urothione as a metabolite and the production of Form B during degradation must involve a cyclisation to produce the thiophene ring this has been inadver- tently m~delled.~~.~' Reaction of the phenyl quinoxalin-2-yl alkyne 10 with [Mo(S4),SI2- (see Section 5 below for fuller discussion of such additions) gave a tris(dithio1ene) complex of molybdenum.Oxidation of this complex produced as well as higher oxidation states of the molybdenum complex a small amount of a metal-free substance shown to be 11 (Scheme 5). 2-11 Scheme 5 Reagents i MeCN reflux (68%);ii e.g.I -50 "C (7%). Reaction of the quinoxaline-dibromoalkene12 with dipotassium trithiocarbonate produced as the main product a tricyclic thiophene then hydrolysed to give 13.It was suggested that the mechanism pro- posed (Scheme 6) to rationalise this unexpected product may well have a bearing on the formation of urothione and of Form B.s Taylor was the first to prove unequivocally the structure of 'Form A' (dephospho) by total synthesis (Scheme 7) in racemic4 and then later in homochiral In the key step a homochiral alkyne 14 obtained from D-mannitol was coupled with a pivaloyl-protected 6-chloropteridine using palladium(0) methodology. Taylor's use of an N-pivaloyl group as a lipophilic protecting group for pteridines considerably facilitates their handling -they are otherwise often very insoluble. THE STRUCTURE AND MODE OF ACTION OF THE COFACTOR OF THE OXOMOLYBDOENZYMES-D. COLLISON ET AL.-HBr then ii l3 f Scheme 6 Reagents i aq. K,CS MeOH room temp. (72%);ii 48% HBr MeOH CH,Cl room temp. (70%). 15 \ vi\ 0 a 16 Scheme7 Reagents:i,Pd(OAc) (0-Tol),P,CuI,Et,N,MeCN 1OOoC(20%); ii 0.5 mol 1-' HCl reflux; iii K,CO MeOH (42%); iv Et,N/MeSO,Cl,CH,Cl,,O "C (72%); v Ph,P MeCN 90 "C (76%); vi BunLi,THE -78 "C then room temp. in solution (equilibration to all E-alkene) (72%);vii Br CH,Cl 0 "C (67%);viii DBU dioxane 100"C (40%). An alternative route to alkyne-acetal 15 starting from ester-amide 16 itself availableM either viadegradation of folic acid or from syn- thetic 6-hydroxymethylpteri11 is also shown in Scheme 7.45 5 Towards a Total Synthesis of Moco Any strategy aimed at a total synthesis of Moco must have several components; one of these is a means for the generation of a molyb- denum-complexed 1,2-dithiolate (probably an ene- 1,2-dithiolate -a 'dithiolene').Several methods have been described for the pro- duction of such units around molybdenum and other metals. We summarise below those routes which we believe to be of greatest relevance for such synthetic endeavours. Coucouvanis' studies46 of the reactions of the polythiomolybde- num anions generated by reaction of [MoS,I2- or [MoS,O,-,]~- (x = 4 3 or 2) with sulfur may be relevant to the biosynthesis of the dithiolene unit in Moco and could be of value in a laboratory synthesis of molybdenum-dithiolene complexes (see also below). Mechanistic sequences were suggested for the reaction of such anions with dimethyl acetylenedicarboxylate; illustrated in Scheme 8 is the reaction of [(S4)2M~=S]2- under anaerobic conditions giving 17. Shown below are two possible interpretations for the key C-S bonding step the process is (i) initiated by nucleophilic attack by a terminal sulfur on the acetylene with the electron reorganisation shown by the curly arrows on 18,or (ii) viewed as a [2 + 21 cyclo-addition 19.Taylor and Stiefel elegantly utilised similar additions in their syn- theses of pteridinyl- and quinoxalinyl-dithiolenes 20 and 21.47The E 17 E=C02Me Scheme 8 19 initial complexes 22 and 23 could be transformed into the ene-dithi- olate systems believed to exist in molybdopterin by treatment with a phosphine (Scheme 9). Hartzler had demonstrated4* that alkynes carrying at least one electron-withdrawing substituent (ester) react with the betaine (Ph,P+ -CS,-) produced from tributylphosphine and carbon dis- ulfide generating an ylide which can be trapped in a Wittig process.Taylor capitalized on this idea showing that addition to the pteridinyl ketone 25 occurred in the same fashion (Scheme It is important to note that an attempt to utilize (protected) alcohol 24 yielded 'only a minute yield' of addition product -it seems that the electron-withdrawal provided by the heterocycle does not acti- vate the alkyne sufficiently for addition to occur. de Mayo showed that diphenyldithiolene-thione26 reacts with Mo(CO) in the presence of light generating complex 27 though in low yield; a somewhat better yield was obtained in a thermal process using the same metal complex but starting from diphenyl- acetylene and sulfur (Scheme 1 l).so An alternative for the further processing of tri- or di-thiocarbon- ates could rest on Rauchfuss' work on tetrathiapentalenedione 28. 0 n /? \Iqylis-s 22 23 20 21 Scheme 9 Reagents i DMF 70 "C (62%22,36% 23); ii Ph,P DMF 70 "C (95%).CHEMICAL SOCIETY REVIEWS 1996 OTBDMS 0 24 25 Scheme 10 Reagents 1 Bu,NF THF room temp (93%) 11 CrO aq H,SO butan-2-one room temp 111 THF -40 OC iv (EtO,C),C=O -50 "C (69% two steps) Ph Scheme 11 Reagents 1 hv CHCl (9%) 11 PhMe 120 "C (25%) 2-28 L J Scheme 12 This compound served as a precursor to 1,2-dithiolene complexes of molybdenum by reacting with tetrathiomolybdate (Scheme 12) 5' Early studies in Manchester established a route to unsymmetrically substituted trithiolenes hydrolysis of the immediate precursors for the tnthiolenes generated ene-dithiolates in solution which could be trapped by reaction with for example [MoO,(acac),] 52 In the light of remaining uncertainties regarding the oxidation level of sulfur- beanng carbons in molybdopterin the option of producing 1,2-dithiolane precursors was addressed in a model system as shown in Scheme 13 53 It was possible to convert the model dithiolane into the dithiolene vza peracid oxidation Scheme 13 Reagents 1 Br AcOH 90 "C (85%) 11 NaS,CNMe EtOH reflux(67%) 111 conc H,SO room temp (58%) then NaHS aq AcOH room temp (64%),iv [Co(C,H,>(cyclooctadiene)] xylene reflux (51%) v NaBH MeOH 0 "C (87%) vi MeSO,Cl pyndine 0 "C (67%) vii m-ClC,H,CO,H CHCl 0 "C then (CF,CO),O to room temp (84%) It has been known for more than 100 years that ortho-phenylene- diamine reacts with glucose giving 2-(~-urubzno-tetrahydroxy-butyl)quinoxaline 29 Manchester studies41 s4 have utilised this readily available substance as the starting point for studies aimed at modelling methods for the elaboration of the C,-side-chain of molybdopterin (Scheme 14) An extrapolation of the strategy illustrated above into pteridine chemistry required the development of a practical route to a 6-tetrahydroxybutylpterin Although the condensation of 29 Me$' I1 Vlll 1x Me Me Scheme 14 Reagents 1 Me,CO conc H,SO room temp (51%) 11 HC(OEt) p-MeC,H,SO,H CH,Cl room temp (93%) 111 Ac,O reflux (76%) IV Br CH,Cl 0 "C (67%) v NaS,CNMe MeOH reflux (40%) vi Br CH,Cl 0 "C (86%) vii NaOMe MeOH room temp (34%),viii MeSO,Cl pyndine room temp (53% plus 30% dimesylate) ix NaS,CNMe EtOH reflux (73%) x (CF,CO),O pyndine reflux xi H,S room temp (30 9% 31 25% 32 ca 50%) xii BnO,CCl NaB(CN)H MeOH room temp (65%) 5,6-diaminopyrimidines with 1,2-dicarbonyl compounds leading to pteridines is a well-known approach to pteridines -the Isay syn- thesis -it usually produces mixtures of 6- and 7-substituted pterins -early reports of the regioselective reactions of sugars with 4,5-diaminopyrimidines had been shown to be in error However a very careful examination of reaction conditions produced a recipe for the synthesis of (D-aruhzno)-tetrahydroxybutylpterin,as its tetraacetate-acetamide 33 in acceptable yields on a multigram scale as shown in Scheme 15 5s The pterin 33 has been processed following the quinoxaline model sequence to afford alkene 34 HO HH OH % 33 Me 34 Scheme 15 Reagents 1 N,H H,O AcOH 100 "C (38%) 11 Ac,O pyn dine 100 "C then recrystallise (70%) 111 K,CO MeOH room temp (55%) iv Me,CO p-MeC,H,SO,H room temp v HC(OEt) p MeC,H,SO,H pyndme CH,Cl room temp (44% two steps) vi 160 "C (67%) The oxidation level and the precise tautomeric form adopted by the pteridine in Moco are still not certain because of this any synthetic strategy must allow vanation in the oxidation level of the pyrazine nng 56 To this end it was shown that reductions of quinoxalinyl-trithiolene35 and quinoxaline-trithiolane 36 produced tetrahydr0-N- protected derivatives 37 and 38," in each of which importantly the sulfur-containing units were untouched It has subsequently been shown that 37 can be selectively oxidized to the mono-protected- dihydroquinoxaline 39 which can then be transferred to a metal centre to give a dithiolene complex 40 (Scheme 16) 58 THE STRUCTURE AND MODE OF ACTION OF THE COFACTOR OF THE OXOMOLYBDOENZYMES-D COLLISON ET AL 10 11 12 13 36 Scheme 16 Reagents I BnO,CCl NaB(CN)H (97%) 11 MnO CH,CI 14 room temp (72%) 111 [Co(C,H,)cyclooctadiene)] (13%) iv BnO,CCl NaB(CN)H (42%) 15 A comparable reduction of 6-substituted-pteridines of which the 16 most relevant to molybdopterin synthesis is 33 produced interest- ingly tetrahydro-derivatives (40) but with the protecting group on 17 N-5 adjacent to C-6-substituents (Scheme 17) 55) 18 BnOC 19 33 41 ti 20Scheme 17 Reagents 1 BnO,CCl NaB(CN)H (65%) 21 22 236 Conclusion This is an exciting time in the development of the understanding of 24 the nature and mode of action of the molybdenum cofactors of the oxomolybdoenzymes and their tungsten counterparts Thus protein 25 cystallography is now providing vital information which comple- ments earlier spectroscopic and chemical studies the total synthesis of Moco is in prospect and with this and the preparation of variants 26 and close analogues studies of the natural systems and the mode of action will be considerably augmented 27 Acknowledgements Work in Manchester has been supported by the 28 SERC and now by the EPSRC and the Royal Society (D C ) the assistance from which we gratefully acknowledge 29 307 References 1 J A Pateman,D J Cove,B M ReverandD B Roberts,Nature 1964 31 201,58 2 P A Ketchum H Y Cambrier W A Frazier C H Madansky and A Nason Proc Natl Acad Sci USA 1970,66,1016,A Nason K Y Lee S S Pan P A Ketchum A Lamberti and J Devnes Proc Natl Acad 32 Sci USA 1971,68,3242 33 3 V K Shah and W J Brill Proc Natl Acad Sci USA 1977,74,3249 P T Pienkos V K Shah and W Brill Proc Natl Acad Sci USA 1977 34 74,5468 4 In the light of several previous reviews on Moco and related topics (refs 5,6a and S Goswami Heterocycles 1993,35 155 1) we reference here 35 only key original papers concerned with the development of the subject 5 J L Johnson in Molybdenum and Molybdenum Containing Enzymes 36 ed M P Coughlan Pergamon Press Oxford 1980 pp 345-383 J C Wootton R E Nicholson J M Cock D E Walters J F Burke W A Doyle and R C Bray Biochem Biophys Acta 199 1,1057 I57 37 6 (a)J L Johnson in The Metabolic Basis of Inherited Disease ed C R Scriver A L Beaudet W S Sly and D Valle McCraw Hill New York 1989 p 1463 (b) G K Brown R D Scholeum H B Croll J E Wraith and J J McGill Neurology 1989,39,252 7 Molybdenum Enzymes ed T G Spiro Wiley New York 1985 38 Molybdenum Enzymes Cofactors and Model Systems ed E I Stiefel D Coucouvanis and W E Newton ACS Symposium Series 535 1993 39 J H Enernark and C S Young Ad\ Inorg Chem 1994,40 1 40 8 J L Johnson B E Hainline and K V Rajagopalan J Biol Chem 1980 255 1783 J L Johnson B E Hainline K V Rajagopalan and 41 B H Arison .I Bid Chem 1984,259 5414 9 The name ‘pteridine’ for pyrazino[2,3-d]pynmidine was coined by 42 Wieland and refers to the first isolations of such compounds in nature -43 the pigments of butterfly wings The term ‘ptenn’ is used for 2-amino 4 hydroxyptendine For a review of pteridine chemistry see W 44 Pfleiderer I Heterocycl Chem 1992,29 583 E C Taylor P S Ray I S Darwish J L Johnson and K V Rajagopalan J Am Chem Soc 1989,111,7664 J L Johnson and K V Rajagopalan Proc Natl Acad Sci USA 1982 79,6856 S P Kramer J L Johnson A A Ribeiro D S Millington and K V Rajagopalan J Bid Chem 1987,262 16357 B Kruger and 0 Meyer Eur J Biochem 1986,157 12 1 J L Johnson K V Rajagopalan and 0 Meyer Arch Biochem Biophys 1990,283 542 J L Johnson N R Bastian and K V Rajagopalan Ptoc Natl Acad Sci USA 1990 87 3190 G Bomer M Karrasch and R K Thauer FEBS Lett 199 1,290,3 1 M K Chan S Mukund,A Kletzin,M W W Adams andD C Rees Science 1995,267 1463 G N George R C Prince S Mukund and M W W Adams I Am Chem Soc 1992,114,3521 M J Romiio M Archer I Moura J J G Moura J LeGall E Engh M Schneider P Hof and R Huber Science 1995,270 1170 C D Gamer and S Bristow in Metal inns in Bioloqy vol 7 ed T G Spiro Wiley New York 1985,pp 343-410 S P Cramer in Adkances in inorganic and Bioinoi yanic Mechanism.ed A G Sykes Academic Press London 1983 p 259 B Hedman K 0 Hodgson and C D Gamer Synchrotron Radiation and Biophlsic v ed S S Hasnain Ellis Horwood Chichester 1990,p 43 C D Garner Ad1 Inorg Chem 199 1,36,703 G N George C A Kipke R C Prince R A Sunde J H Enemark and S P Cramer Biochemistry 1989,28 2075 N A Turner R C Bray and G P Diakun Biochem I 1989,260,563 S P Cramer and R Hille J Am Chem Soc 1985,107 8164 R C Bray Quart Re\ Biophys 1988,21,299 R C Bray B G Malmstrom and T VanngArd Blot hem I 1959 73 193 L S Menwether W F Marzluff and W G Hodgson Name 1966,212 465 B D Howes N M Pinhal N A Turner R C Bray G Anger A Ehrenberg J B Raynor and D J Lowe Bloc hemistry 1990,29,6 120 B D Howes B Bennett A Koppenhofer D J Lowe and R C Bray Biochemistry 1991 30 3969 A G McEwan S J Ferguson and J B Jackson Biochem I 1991 274 305 N R Bastian C J Kay N J Barbara and K V Rajagopalan I Brol Chem 199 1,266,45 N Benson J A Farrar A G McEwan and A J Thomson FEBS Left 1992,307 169 M G Finnegan J Hilton K V Rajagopalan and M K Johnson Inorg Chem 1993,32,2616 Biological Applications of Raman Spectroscopy ed T G Spiro Wiley New York 1988 vol 3 L Kilpatrick K V Rajagopalan J Hilton N R Bastian E I Stiefel R S Pilato and T G Spiro Biochemistry 1995,34 3032 (a) R H Holm and J P Donahue Polyhedron 1993 12 571 R H Holm Coord Chem Re\ 1990,110 183 (b)R H Holm Chem Rer 1987,87 1401 (c)R H HolmandJ M Berg,Acc Chem Res 1986 19,363,Pure Appl Chem 1984,56 1645 B E Schultz R Hille and R H Holm J Am Chem Soc 1995 117,827 Z Xiao C G Young J H Enemark and A G Wedd J Am Chem Soc 1992,114,9194 J A Craig and R H Holm J Am Chem Soc 1989 111,2 1 11 and references therein B E Schultz S F Gheller M C Muettertiey M J Scott and R H Holm J Am Chem Soc 1993,115,2714 S K Das P K Chaudhury D BiswasandS Sarkar J Am Chem Soc 1994,116,9061 E I Stiefel R Eisenberg R C Rosenberg and H B Gray,J Am Chem Soc 1966,88,2956,S L Soong V Chebola S A Koch T O’Sullivan and M Millar Inorg Chem 1986,25,4068 C D Gamer,E M Armstrong,M J Ashcroft M S Austerberry J H Birks D Collison A J Goodwin J A Joule L Larsen D J Rowe and J R Russell in Molybdenum Enzymes Cofactors and Model Systems ed E Stiefel D Coucouvanis and W E Newton ACS Symposium Series 535 1993 pp 98-1 13 A Sakurai and M Goto,J Biochem (Tokyo) 1969,65,755,E C Taylor and L A Reiter J Org Chem 1982,47,528 E C Taylor and L A Reiter .IAm Chem Soc 1989,111,285 C L Soncelli V A Szalai and S J N Burgmayer J Am Chem Soc 1991,113,9877 L Larsen D J Rowe C D Garner and J A Joule J Chem Soc Perkin Trans I 1989 2317 E C Taylor and S Goswami Tetrahedron Lett 1991,32 7357 E C Taylor P S Ray I S Darwish J L Johnson and K V Rajagopalan J Am Chem Soc 1989,111,7664 J R Russell C D Gamer and J A Joule J Chem Soc Peikin Trans I 1992 1245 45 J R Russell C D Garner and J A Joule Tetrahedron Lett 1992,33 337 1 46 D Coucouvanis A Hadjikynacou A Toupadakis S -M Koo 0 Ileperuma M Draganjac and A Salifoglou Znorg Chem 1991 30 754 D Coucouvanis A Toupadakis S -M Koo and A Hadjikyriacou Polyhedron 1989,8 1705 and references cited therein 47 R S Pilato K E Enksen M A Greaney E I Stiefel S Goswami L Kilpatrick T G Spiro E C Taylor and A R Rheingold .I Am Chem Soc 1991,113,9372 48 H D Hartzler J Am Chem Soc 1971,93,4961 49 E C Taylor and R Dotzer J Org Chem 1991,56 1816 50 W Kusters and P de Mayo J Am Chem Soc 1974,96 3502 G N Schrauzer and V P Mayweg Z Nuturforsch Tell B 1964 19 192 51 X Yang G K W Freeman T B Rauchfuss and S R Wilson Znorg Chem 199 1,30,3034 52 S Boyde C D Garner J A Joule and D J Rowe .I Chem Soc Chem Commun 1987,800 CHEMICAL SOCIETY REVIEWS 1996 53 L Larsen C D Garner and J A Joule .I Chem Soc Perkin Trans I 1989,2311 54 L Larsen D J Rowe C D Gamer and J A Joule Tetruhedron Left 1988,29 1453 55 J R Russell C D Gamer and J A Joule Synlett 1992 71 1 R L Beddoes J R Russell C D Gamer and J A Joule Acta Crystulloqr Sect C 1993,49 1649 56 J R Russell C D Gamer and J A Joule J Chem Soc Prrkin Ttuns I 1992 1245 57 R L Beddoes J R Russell C D Gamer and J A Joule Acts Crystalloqr Sect C 1992,48 2075 58 J H Birks PhD Thesis University of Manchester 1994 59 R L Beddoes J R Russell C D Gamer and J A Joule Atru Ctystallogt Sect C 1993,49 1649

ISSN:0306-0012    

DOI:10.1039/CS9962500025

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

The Excited State in Atmospheric Chemistry George Marston Department of Chemistry, University of Reading, Whiteknights, PO Box 224, Reading RG6 6AD, UK 1 Introduction In recent years, the chemistry of the Earth's atmosphere has pro- vided a focus for much scientific activity. Interest has been stimu- lated by concerns about the environmental consequences of Man's activities in general, and phenomena such as Global Warming, Acid Rain and the Antarctic Ozone Hole in particular. Of course, any attempt to understand the causes of these effects necessarily requires a detailed knowledge of the chemistry of the unpolluted atmosphere, and the result is that significant advances in our knowl- edge of atmospheric chemistry have been made in the last two decades. To a reasonable extent, it is possible to discuss the chem- istry of the atmosphere in terms of the behaviour of its components in thermal equilibrium with their surroundings; i.e.for a given chemical species, relative populations of a particular energy level can be calculated using the Boltzmann equation: where nl and n,, are the concentrations of the ith energy level and ground energy level, g, is the degeneracy of the ith level, E, is its energy, k is the Boltzmann constant and T is the temperature. However, just as the atmosphere is not in chemical equilibrium, there are many examples of excited atoms, molecules, radicals and ions that are present in the atmosphere in concentrations far greater than would be expected on the basis of equation (I).It is towards the chemistry of these excited states that this review addresses itself. Much of the discussion will centre on vibrationally and electroni- cally excited species; at typical atmospheric temperatures, the energy required to access these states is, as a general rule, too great to be achieved in thermal processes. For translational energy and rotational energy, the separations of the energy levels are small compared to kT and at thermal equilibrium, a range of levels is excited. However, we shall see that circumstances exist where translational and rotational temperatures of a particular species may be very much greater than the thermodynamic temperature of the surroundings. Ultimately, the energy for excitation within the atmosphere comes from the Sun, either in the form of solar radiation or charged particles (the solar wind).Excitation mechanisms include direct George Marston wlas born in Newcastle upon Tyne, England in 1961. He studied chemistry at the University of Oxford where he received his BA (1984) and, under the superi*ision of Richard Wayne, his DPhil (1987). He then spent two years at NASAIGoddurd Space Flight Center with Lou Stief before returning to Oxford. In 1993 he was appointed Lecturer in Earth Observation Science and Chemistry at the University of Leicester and in October 1995 took up a Lectureship in Chemistry at the University of Reading. His research work involves the application of lab- oratory measurements in spec- troscopy and kinetics to problems in atmospheric chem- istry.absorption of radiation, photodissociation, product formation in exothermic reactions and the interaction of molecules with charged particles. Once formed, these species can influence atmospheric chemistry in many ways. As we shall see in Section 2,O(lD) plays a fundamentally important role in the chemistry of both the tropo- sphere and the stratosphere. The reactivity of O(lD) is significantly different from that of the ground state, O('P), and other examples of this behaviour also exist. Excited atomic oxygen is also impor- tant in the aurorae and airglow and these phenomena are discussed. Radiation emitted from vibrationally excited molecules can be detected in space, allowing remote sensing of many species, while chemical reactions leading to vibrationally excited products can lead to significant problems in the retrieval of concentrations from such measurements.Infrared raditation emitted from the Earth's surface can excite vibrations in a number of trace gases in the atmosphere leading to radiation trapping and the greenhouse effect; in the middle atmosphere, emission from vibrationally excited molecules provides a mechanism for atmospheric cooling. The role of nitric oxide in the thermosphere is a very interesting one; mech- anisms for its formation seem to involve translational, rotational, vibrational and electronic excitation. It is not possible in a review of this length to cover all aspects of the chemistry of excited states in the atmosphere with the detail that they deserve.In particular, it should be noted that almost all atmos- pheric chemistry is driven by photochemical processes and that such processes involve the promotion of molecules to excited states. The dynamics of the reorganisation of atoms following these excita- tions are not considered in this article. Similarly, there IS not room to discuss the notation used to describe electronically excited states [e.g.O(lD) and 02(a1A,)] and the interested reader is referred to an introductory text on spectroscopy.' I do not intend to give a detailed description of the chemistry of the atmosphere or its structure; some of the standard chemistry2 is dealt with in Section 2, and the atmos- phere is divided into regions according to temperature gradient.These regions are (with very rough altitude ranges): the troposphere (< 18 km); the stratosphere (18-50 km); the mesophere (SO-90 km); and the thermosphere (>90 km).2 2 The Importance of O(lD) The first excited state of atomic oxygen, O(ID), is one of the most important trace constituents in the Earth's atmosphere, playing crit- ical roles in many areas of atmospheric chemistry. In this section, its importance in the troposphere and stratosphere is discussed. In chemical terms, the troposphere can be thought of as a low-temper- ature combustion system. One of the most important overall pro- cesses in this region is the conversion of methane, a by-product of many natural processes, to carbon dioxide and water by a reaction that, stoichiometrically, can be represented very simply: CH, + 20, -CO, + 2H,O (1) However, just as in the case of a methane flame, the mechanism for this process is complex and involves a number of radical species.In a combustion system, the energy for the formation of radicals is thermal, while in the troposphere, the energy source for radical formation is solar radiation and the oxidation is photochemical. The critical process is the photolysis of ozone at wavelengths between 290 and 3 10 nm: 33 The atomic fragment, O(ID), then goes on to generate the important radical species OH in the reaction O(lD) + H,O -OH + OH (3) The OH radicals thus formed can then abstract a hydrogen atom from methane molecules OH + CH, -H,O + CH, (4) Once this reaction occurs, the rest of the oxidation can proceed It is outside the scope of this article to discuss in detail this mech- anism, but the important point is that the oxidation of methane (and other hydrocarbons) in the troposphere only occurs after the forma- tion of an excited oxygen atom If excited atomic oxygen were not formed in reaction (2a), hydrocarbon oxidation in the troposphere would occur by a very different mechanism What should also be clear is that the oxidising capacity of the troposphere is strongly dependent on the rate at which photolysis of ozone occurs The rate of photolysis depends on the absorption cross-section, quantum yield and the solar flux in the troposphere Because of the absorp- tion of solar UV radiation by ozone in the stratosphere, the absorp- tion cross-section for ozone photolysis at the wavelengths that reach the troposphere is very small The solar flux increases as wave- length increases, but above h = 310 nm, energy restrictions do not allow formation of O(lD) in a spin-allowed process Nevertheless, experiments carried out by Hancock and coworkers3 indicate that in this region of the spectrum, O(lD) may be generated with a signif- icant quantum yield and the information obtained from these experiments may have important implications for our under-standing of tropospheric chemistry The chemistry of the stratosphere is also strongly influenced by O(lD) In this region of the atmosphere, the dominant chemical species is ozone, the formation and destruction of this molecule can be described by a set of oxygen-only reactions known as the Chapman scheme 0, + Izv-0 + 0 O+ 0, + M -0, + M o,+hv-.O,+O 0 + 0,-0, + 0, Qualitatively, this scheme describes the layered structure of ozone in this region quite well, but predicts ozone concentrations that are greater than those actually observed The origin of the discrepancy is that catalytic cycles of the type x +o,-xo+o, xo+o-x +o, allow trace constituents to control the concentration of ozone because the net effect of reactions (8) and (9) is to convert one oxygen atom and one ozone molecule into two oxygen molcules, while regenerating the species X These trace species are reactive radicals such as C1, OH and NO Chlorine atoms have a direct photochemical source, being formed in the photolysis of chlori- nated organic compounds that reach the stratosphere CF,Cl, + hv+CF,Cl + Cl (10) However, another minor source involves reaction with O(ID) CF,Cl, + O(lD) -C10 + CF,CI (1 1) followed by reaction of C10 with 0 atoms Furthermore, the strong- est source of stratospheric NO is the reaction O(lD) + N,O +NO + NO (12) and the only sources of OH also involve O( D) CHEMICAL SOCIETY REVIEWS.1996 O(lD) + H,O -OH + OH (3) O(lD) + CH, -OH + CH, (13) The effect of these active chlorine, nitrogen and hydrogen species in the natural atmosphere is to reduce stratospheric 0, levels by a factor of two and we see that, just as in the troposphere, O(ID) plays a central role in the chemistry of the region 3 The Troposphere Aside from the chemical change brought about by O(ID) described in the previous section, excited states do not play a very important part in the chemistry of troposphere, although they are of critical importance in determining the temperature of the atmosphere and chemical reactions can be strongly temperature-dependent Quenching of excited species by 0, and N, competes effectively with unimolecular processes such as radiative decay in this region More importantly, the high-energy photons and charged particles that cause excitation in the upper atmosphere have been absorbed and there are few processes with sufficient energy to generate excited states in the troposphere One of the reasons that excited states are important in the atmosphere is that those that are access- ible from the ground state in an allowed transition determine the radiation field throughout the atmosphere In the optical region of the spectrum, excited states can be located using conventional tech- niques,' while in the far-ultraviolet, electron energy loss spec- troscopy4 ? has proved useful The main excitation mechanism in the troposphere is, as we have seen, the photolysis of ozone to generate O(lD) and O,(alA,) The excited atomic fragment is rapidly quenched, but generates ground- state 0 atoms that in turn react with molecular oxygen to form ozone and thus start the cycle again, i e quenching removes O(lD) from the atmosphere only temporarily The molecular product of the dissociation, O,(alA,), is quenched only slowly dt atmospheric pressure, and is present in the troposphere at relatively high concentrations It was thought6 that this species could contribute to the oxidation of hydrocarbons, particularly alkenes, but quantitative studies have shown it to be very unreactive O,(alAJ in solution is known to be important in, for example, biological system^,^ and recent evidence suggests that it may play a minor role in atmos- pheric oxidation in cloud droplets 5, Ozone may also lead to excited state formation 1zu reaction with alkenes These reactions are 1,3-dipolar cycloadditions and the adduct falls apart to give a carbonyl and a carbonyl oxide known as a Criegee intermediate, RIRT-0-0 In the gas phase, the inter- mediate is believed to be generated with significant vibrational excitation and may either decompose or be stabilised and take part in bimolecular reactions Atkinson and coworkers'() have measured OH yields from these reactions and it seems that the OH is gener- ated in the decomposition of the Criegee intermediate A correlation exists between the fractional yield of OH and the standard enthalpy changes for the reactions" of ozone with a series of alkenes The more energy released, the greater the OH yield, as might be expected if the OH is generated from a Criegee intermediate with vibrational excitation However, Hatakeyama et ul l2 have shown that, although the yield of stabilised Criegee intermediates is pres- sure dependent, the yield at the high-pressure limit for (E)-but-2- ene is significantly less than unity They have suggested that the Criegee intermediate is generated not only with vibrational excita- tion, but that some is formed on an unbound electronically excited surface It is clear that much experimental and theoretical work is still required to understand the detailed mechanism of these reac- tions 4 The Aurorae and Airglow The most spectacular manifestation of the existence of excited states in the Earth's atmosphere comes in the form of the Aurora Borealis (The Northern Lights) and the Aurora Australis (The Southern Lights) These phenomena appear in the polar night as shimmering flames that light up the sky, with electronically excited THE EXCITED STATE IN ATMOSPHERIC CHEMISTRY-G.MARSTON states of atomic oxygen playing key roles. These dramatic features result from the interaction of the solar wind with atmospheric constituents at altitudes greater than 100 km, the solar wind having been deflected by the magnetic field of the Earth. Indeed, the Earth's magnetosphere has been described as a gigantic cathode-ray tube that focuses solar electrons onto the Earth's poles, with the atmos- phere behaving as a fluorescent screen.I3 It might be better, in this context, to describe the atmosphere as a phosphorescent screen, because, as will be described, the major transitions giving rise to the emitted radiation are forbidden by the electric dipole selection rules.Incoming solar electrons with energies in the region of 10000eV interact with molecules in the upper atmosphere of the polar regions, leading to processes such as electron impact ionization, dissociative electron attachment and energy transfer. I4 The products of these interactions may emit radiation themselves, or take part in reactions leading to excited entities. Although a number of excited species are generated during aurorae, the most prominent visible features arise from atomic oxygen. The strongest observed emission comes from the O(lS) +O(ID) transition, the so-called 'auroral green line' at A = 557 nm, but a weaker emission in the red occurs from the O(ID)-O('P) transition at A = 630 nm. These excited states are believed to be generated in the dissociative recombination process: l4 Many of the transitions that give rise to the aurorae are formally for-bidden by the electric dipole selection rules, and the extent to which they are forbidden can have a significant effect on the location of auroral features.For example, the origin of the auroral green line is at an altitude of approximately 100 km, while emission from O(ID) occurs at altitudes up to about 400 km. Part of the reason for this difference lies in the different radiative lifetimes of the two states. In order for emission to be observed, the emission rate must compete with the rate of quenching by atmospheric molecules.Emission from O(lS), with a radiative lifetime of 0.7 seconds, can compete with quenching at much lower altitudes (higher pressures) than can O(lD), with a radiative lifetime of 200 seconds. The quenching rates of the two species are also important, O(lS) being quenched much more slowly than O(ID). Emission from O(IS) is also a strong feature of the airglow, vari- ously called the dayglow, twilightglow or nightglow depending on when it is observed. The airglow and the aurorae share many common emission features, but the aurorae are sporadic, intense and concentrated in the polar regions, while the airglow is continu- ous, very weak and can be observed at all latitudes. Excitation mechanisms for particular species in the airglow often differ from those in the aurora.For example, the auroral green line is excited by energy transfer from electronically excited molecular oxygen in the nightglow. A much-simplified mechanism for its formation is: where 0; is an electronically excited state of molecular oxygen. This two-step process is known as the Barth mechanism15 for excitation of the green line. Other emitters in the airglow are primarily electronically excited species, although the hydroxyl radical, OH, with vibrational excita- tion up to 1' = 9 makes a significant contribution to the nightglow.2 The reaction between H atoms and ozone: H + 0, +OH + 0, (17) has just sufficient energy to populate the 9th vibrational levelI6 and is the dominant source of OH(i*) at night.The reaction: 0 + HO, +OH + 0, (18) which has enough energy to populate OH (v =6), has been consid- ered and rejected by KayeI7 as a secondary source of vibrationally Figure 1 Discharge flow-Fourier transform spectrometer apparatus. B, beamsplitter; D,, germanium detector; D,, laser detector; F, flow tube; I, observation region; L, laser; MI, moving mirror; M,, fixed mirror; S, stepper motor. The orange glow emanating from the flow tube is the Lewis-Rayleigh afterglow produced from discharged N,. The major con- tnbutor to the glow is from the N,(B3rIg-A%:) transition, which is observed in aurorae. excited OH. However, laboratory evidence has shown that the reac- tion can give rise to emission from OH (v d 6). Figure 1 shows a discharge-flow apparatus coupled to a Fourier transform infrared spectrometer that has been used to study airglow mechanisms in the laboratory in the near-infrared region of the spectrum.I8 Using this apparatus, Lunt et al.detected emission from vibrationally excited OH formed in reaction 18,and an emission spectrum recorded from the products of reaction 18 is illustrated in Figure 2. A quantitative laboratory study, comparing the strength of this emission to that produced by reaction 17, is required to assess the importance of reaction 18 to the nightglow. Not only does vibrationally excited OH contribute to the airglow, but it can also take part in chemical reactions that are not possible for ground-state hydroxyl radicals. The reaction between 0 atoms and OH to give 02(alAg) as a product is only energetically possible if the OH radical possesses at least one quantum of vibrational energy.0 + OH(v 3 1) -H + 02(a1Ag) (19) 02(aIAg)is one of the strongest airglow emitters, despite the fact that the transition to the ground state 02(X3Z;) is forbidden by three electric dipole selection rules; its emission spectrum'' (measured in the laboratory) is illustrated in Figure 3 at medium resolution (ca.8 cm-I). Evidence that reaction 19occurs comes from field, modelling and laboratory investigations. Gattinger and Vallance-Jones2" have observed correlations in rocket-borne experiments between 02(alA,) emissions and emissions from vibrationally excited OH, while, on the basis of a modelling study, Krasnopolsky2I suggested that if 20% of the overall process generated 02(a1Ag), the observed altitudexoncentration profiles for O,(aIA,) could be explained.In the laboratory, Lunt et al.18have shown that reaction 19does occur, but with an efficiency of only ca. 2.5%. Their results seem to indi-cate that reaction 2 can make only a small contribution to the night time emission, but uncertainties in calibration and the difficulties of modelling a reaction system with OH in a range of vibrational levels mean that further experiments are needed on this reaction. Current interest in O,(aIA,) nightglow is centred on 0-atom recombination followed by energy transfer,22 in a mechanism that is closely related to the Barth mechanism for the formation of O(lS) referred to above.It is worth mentioning at this point a recent study by Miller et al.,23following from work by Slange~-,~~ that illustrates again how vibrational excitation can lead to enhanced reactivity. CHEMICAL SOCIETY REVIEWS, 1996 r 0-OH <o8 *)X It 0)a.-ro v -1 (r" '4 I 1 I 1 I I 1800 1600 1400 1200 1000 Unm Figure 2 Vibrational emission from OH formed in the reaction of 0atoms with HO,. The numbers identify the upper and lower vibrational quantum numbers involved in the transition and the arrows locate the positions of the Q branches. The laser lines are from scattered light from the HeNe laser shown in Figure 1. Resolution, 8 cm-I. (Reproduced with permission from J. Chem.Soc., Faraday Trans. 2, 1988,84, 899) z7800 8000 v/cm-' Figure 3 Emission from the transition O,(a'A, -X-1C;). Resolution, 8 cm-I. Approximately 10% of the photolysis of ozone between 200 and 310 nm occurs to give 0 and 0, in their electronic ground states: 0, + hv -02(X3Z;) + O(3P) (2b) the remaining 90%leading to excited singlets (reaction 2a). In reac- tion 2b, a significant fraction of the residual energy is channelled into vibrational excitation in the molecular fragment. Vibrationally excited 0, cannot contribute to the airglow as 0, does not possess a dipole moment, but reaction with 0, to give 0, is energetically feasible: O,(v 2 26) + 0, -0 + 0, (20) As yet, evidence is circumstantial, but the reaction may affect ozone concentrations by as much as 10% at some altitudes, and further lab- oratory investigation is urgently required. Continuing for the moment with the subject of ozone photolysis, the major daytime contributor to the airglow is O,(a'A,), formed in reaction 2a: 0, + hv-, O,(a'A,) + O(lD) (2a) As discussed in Section 2, there are some uncertainties in the wave- length dependence of the quantum yield for this process., Nevertheless, the broad features of O,(alA,j formation and quench- ing rates are sufficiently well understood in the daytime atmosphere that measurements of emission from O,(a'A,) made by rocket- borne instruments have been used successfully to generate [O,]-altitude profiles.2s These measurements are a slightly unusual example of how emission from excited states can be used to detect species in the atmosphere and this topic makes up the next section of this review.5 Remote Sensing A number of satellite-borne instruments have measured the concentrations of atmospheric constituents by observing emission from their vibrationally excited states. For example, the Limb Monitor of the Stratosphere (LIMS j instrument onboard the Nimbus 7 satellite obtained vertically-resolved concentration pro- files with almost global coverage for O,, NO,, H,O and HNO,. More recently, the Improved Stratospheric and Mesopheric Sounder (ISAMS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES j on the Upper Atmosphere Research Satellite (UARS) made observations on a range of molecules using this method.Concentrations are retrieved from measured radiances in appropriate regions of the spectrum and it is assumed that the species of interest is in thermal equilibrium, i.e. that Boltzmann's THE EXCITED STATE IN ATMOSPHERIC CHEMISTRY-G MARSTON H-0 filter 105 10-~ 1300 1400 1500 1600 1700 vJcm ’ Figure 4 Spectral responses of LIMS 6 2 pm (NO,) and 6 9 pm (H,O) filters and absorption coefficients of H,O v, and NO, v3 fundamental bands (Reproduced with permission from B J Kerridge and E E Remsberg, J Geophys Res 1989,94, 16 323) equation is obeyed This assumption is known as the Local Thermodynamic Equilibrium (LTE) approximation Absolute concentrations can be obtained for the vibrationally excited mole- cule if the instrument is accurately calibrated in the spectral region of interest and the Einstein A factor is known for the vibrational transition Unfortunately, the LTE approximation can break down if mech- anisms other than thermal excitation exist for the formation of the vibrationally excited state that is being observed For example, Solomon et ul 26 have considered whether Nonlocal Thermodynamic Equilibrium (NLTE) effects could influence the retneval of ozone concentrations from radiance measurements by the LIMS instrument at 9 6 Fm The v, fundamental band of ozone emits in this region of the spectrum, but laboratory studies showed that an appreciable fraction of the reaction 0 + 0, + M-0, + M (21) leads to excitation of ozone in this mode In addition, the i31= vibrational level can be excited directly by absorption of infrared radiation emitted from the Earth’s surface From a detailed model- ling study using laboratory data on the kinetics of the formation and quenching of vibrationally excited ozone, Solomon et ul came to the conclusion that reaction 21 had a substantial effect on ozone retrievals at altitudes above 50 km While these interferences cer- tainly increase the difficulty of determining ozone concentrations, these authors point out that the measurements also provide informa- tion about nonthermal processes in the atmosphere Kerridge and Remsberg,’ have considered the possibility that NLTE in the t3 mode of NO, may affect both NO, and H,O retrievals from the LIMS instrument Figure 4 shows the spectral responses of the LIMS filters for H,O and NO, along with spectra for these species between 1300 and 1700 cm I The figure shows (\v3that direct excitation to NO, = 1) by absorption of infrared radiation followed by re-radiation might affect the NO, measure-ments, but could only have a minimal effect on the water retrievals, because the overlap between the NO, (v3 = 1) emission spectrum and the H,O filter is very small However, processes leading to the excitation of higher vibrational levels of NO, could lead to emission in the region of the water filter For example the 1 ,= 6 to \ = 5 transition occurs at the peak transmission of the H,O filter Kerridge and Remsberg considered two possible mech- anisms for the formation of highly vibrationally-excited NO, Absorption of visible solar radiation at wavelengths greater than the dissociation threshold for NO, (400 nm) leads to excitation to the first excited state of NO, Very strong coupling exists between the ground (X2Al)and first excited state (A2B2) leading to vibra-tional excitation in the molecular ground state The reaction of NO with 0, NO + 0, -NO, + 0, (22) is known to form both electronically and vibrationally excited NO, Clough and Thrush published emission spectra of the Av, = A1 ,= -1 combination bands at 3 7 pm, and detected some emission at 6 2 pm, but did not publish the spectrum 28 Laboratory evidence supporting the role of reaction 22 comes from work carried out at the Rutherford Appleton Laboratory, where emission resulting from products of the reaction was recorded using a high-resolution Fourier transform spectrometer 2o Figure 5 shows a spectrum of the emission observed on reaction of NO with 0, The position and structure of the band reveal that it cor- responds to the 1, = 1 to I?, = 0 transition of NO,, showing that reaction 22 could well affect the retrieval of NO, from the LIMS data Furthermore, at lower resolution, emission can be detected at slightly lower energy, as is illustrated in Figure 6 This emiwon appears to come from Av, = -I transitions of NO, with significant amounts of vibrational energy, providing evidence for Kerridge and Remsberg’s suggestion that reaction 22 could influence the retrieval of H,O data There is one further point regarding the formation of excited state products in the reaction of NO with 0, that is of some importance The reaction was once considered as a possible source of nighttime O,(alAg) in the mesosphere, thus contributing to the nightglow However, laboratory evidencelY indicates an upper limit of 3 X 10 for the fraction of reaction 22 that forms O,(aIA,) At the same time, electronically excited NO,(A*B,) appears to be formed in the reaction These two observations are, at first sight, contradictory The reaction of NO with 0,can, as a result of the degeneracy of NO, proceed on two reaction surfaces and the two lowest-lying sets of products are N02(X2Al) + O,(XiC,) and N0,(X2A,) + O,(aIA,) Even if we restrict the transition state to a planar configuration, these two sets of products correlate with reactants, and it is difficult to see how N0,(A2B,) can be accessed Redpath et ul w have tried to reconcile the problem by suggesting that the reaction proceeds through a transition state where the NO approaches the central 0 atom of the ozone molecule, maintaining C, symmetry Under these circumstances, O,(alA,) does not correlate with reactants, wherea5 an excited state of NO, (but not the A2B, state) does However, this explanation suggests that the reaction proceeds through a transition state with a very restricted geometry AdIer-Golden3I has suggested that NO, is actually generated in the ground electronic state, but with high vibrational excitation The vibrationally excited NO, can then cross onto the A2B, state, thus explaining how this state can be formed while 02(alAg) is not Adler-Golden’s description of the reaction mechanism also explains some work carried out by Clough and Thrush,28 who dis- covered that the temperature dependences of the emission from vibrationally and electronically excited NO, formed in reaction 22 were identical These authors proposed that vibrationally excited NO, was formed I-la the electronically excited state, rather than the other way around The difficulty with this mechanism is that it implies that none of the excess energy in the ground-state channel CHEMICAL SOCIETY REVIEWS, 1996 1500 1550 1600 1650 17 10 G/cm-’ Figure 5 Spectrum showing emission from vibrationally excited NO, generated in the reaction of NO with 0,.Resolution, 1 cm-I.I I I 1400 1800 2200 2600 3000 tr/cm-’ Figure 6 Spectrum showing emission from vibrationally excited NO, generated in the reaction of NO with 0,.Resolution, 8 cm-I. is released into the vibrational modes of NO,; because NO, contains the newly formed bond in the reaction, this description of the reac- tion is unconvincing. From the point of view of the LIMS retrievals, the details of the mechanism are very important; only a small frac- tion of the reaction generates electronically excited NO, and Clough and Thrush’s mechanism would suggest that reaction 22 could not significantly affect vibrational populations in the atmos- phere.In Adler-Golden’s interpretation, the experimental evidence allows a significant fraction of the reaction to generate vibrationally excited NO, in the atmosphere. 6 Heat Balance in the Atmosphere In order that satellite instruments such as LIMS and ISAMS are able to measure concentrations of atmospheric constituents, energy, in the form of infrared radiation, must be transferred from the atmos- phere to space. Excited states are clearly linked to this process and have an important part to play in both the heating and the cooling of the atmosphere. In the lower atmosphere, absorption of infrared radiation emitted from the Earth’s surface excites vibrations and leads to warming of the atmosphere. At higher altitudes, vibration- ally or electronically excited species may be generated by a number of mechanisms, and the subsequently-emitted radiation can escape this optically-thin region of the atmosphere and lead to cooling or reduced heating efficiency.It is possible to calculate the temperature of the Earth assuming that it is in thermal equilibrium with the Sun and that both behave as black bodies., The calculated temperature is approximately 256 K, in reasonable agreement with the temperature measured from space on the basis of the total thermal radiation emitted by the THE EXCITED STATE IN ATMOSPHERIC CHEMISTRY-G. MARSTON Earth. However, surface temperatures on Earth are on average around 288 K, significantly higher than both the calculation and measurement from space.The origin of this discrepancy is that while the Earth may be treated, at least approximately, as a black body, the Earth and the atmosphere combined may not. Viewed from space, the total amount of radiation emitted by the Earth gives a temperature of 256 K according to Stefan's Law, but the frequency distribution of radiation is closer to that of a black body with a tem- perature of 288 K with radiation at certain wavelengths not present. These wavelength regions with no radiation result from the absorp- tion of radiation by infrared-active molecules such as CO, and H,O, the greenhouse gases. These molecules are excited into one of their vibrational modes and this energy is ultimately converted into heat, thus increasing the temperature of the lower atmosphere.At high altitudes, collisions can excite vibrations in infrared-active mole- cules (mainly CO,), which can then emit radiation. If the atmos- phere is optically thin, trapping will not occur and the radiation can escape to space, causing atmospheric cooling. The ultimate source of heat for the Earth and its atmosphere is the Sun. While in the lower atmosphere, atmospheric heating occurs mainly indirectly, following warming of the Earth's surface, at higher altitudes heating occurs directly. A good example of the results of direct heating is the temperature inversion observed in the stratosphere., This inversion occurs because ozone is present in rel- atively high concentrations in this part of the atmosphere, and absorbs solar UV radiation which causes a warming effect.The resultant temperature gradient, with warm air above cold, is extremely stable to vertical motion and thus has a very important effect on the atmospheric dynamics of the region. At altitudes below the stratosphere, the efficiency of conversion of solar radiation to heat is very close to unity, but it should be borne in mind that the mechanism for the conversion is complex and at higher altitudes, the efficiency may be very much reduced. Absorption converts solar radiation into atomic and molecular internal energy, and, if bonds are broken, chemical potential energy. Mlynczak and Solomon32 have considered the mechanism of middle atmosphere heating following the photolysis of ozone.As we have already seen, this process occurs through more than one channel: 0,+ hu-O,(aIAJ +O(lD) 0,+ hv-O,(XYZ;) + o(3~) Rate constants for quenching of O,(alA,) by atmospheric gases are very small (ca. 10-20 cm3 molecule-* s-l) and emission at h = 1270 nm can transfer energy from the atmosphere; above 40 km, virtually all photons emitted in the upward direction escape and many photons emitted in the opposite direction will heat parts of the atmosphere far removed from the photolysis event. Mlynczak and Solomon make the point that this process does not constitute atmos- pheric cooling because the energy has not been converted to heat when it is released; rather, the escape leads to a reduction in heating efficiency of the absorbed radiation.Quenching of O(lD) by N, is very rapid and emission from atomic oxygen does not affect heating efficiencies in the lower thermosphere. However, vibrationally excited N, formed in the quenching collision can transfer its energy to CO, in the v3mode which can then emit at h = 4.3 pm. Mlynczak and Solomon used a detailed model to conclude that below about 50 km the heating efficiency of the singlet channel was virtually loo%, but may drop to as low as 65% at 100km. The triplet channel could also have a reduced efficiency if the vibrationally excited 0, were not completely converted to heat. These authors concluded that quenching was efficient, but did not consider the possibility of the reaction of vibrationally excited 0, (Section 3, reaction 20).The effect of chemical potential energy on the heating of the atmosphere was also considered', by Mlynczak and Solomon. This energy is released following exothermic chemical reaction and such reactions may occur a long way removed from where the photoly- sis happened, thus providing a way of transferring energy through the atmosphere; warm polar winters in the mesosphere have been attributed to this behaviour. Mlynczak and Solomon presented evi- dence to show that the reaction of H atoms with 0, is an important source of heat in the middle atmosphere, although their analysis was hampered by uncertainties in the radiative and quenching lifetimes of vibrationally excited OH. It has been assumed that the reaction of 0 atoms with HO, efficiently converts chemical potential energy into heat. However, as we saw in Section 4,this reaction appears to generate vibrationally excited OH and its heating efficiency may be reduced by radiative emission from these species.Cooling by emission from vibrationally excited CO, is the most important mechanism for disposing of heat from the middle atmos- phere. However, at very high altitudes, this process becomes increasingly inefficient and a more important mechanism is via transitions between the fine-structure components of ground-state atomic oxygen, O(3P).O('P) is made up of three components with different values of J, the quantum number for total (orbital plus spin) electronic angular momentum; the components are, in increas- ing energy, 'P,, ?P,and 3P,, with degeneracies of 5,3 and 1, respec-tively. The transition: is believed to make a significant contribution to the cooling of the thermosphere, at least for as long as the fine-structure components exist in thermal equilibrium.The sparsity of collisions in the upper thermosphere is such that local thermodynamic equilibrium is not maintained3, (for z >400-600 km depending on solar activity), and this cooling mechanism is lost as the J = 2 level becomes over- populated relative to the other components. This effect is an inter- esting example as it shows that the excited state can be important because it is not occupied when, on the basis of the Boltzmann dis- tribution, it should be.7 Nitric Oxide in the Thermosphere Another species that contributes to the dissipation of energy from the thermosphere is nitric oxide, NO, which appears to moderate the temperature during periods of high solar and geomagnetic activity. A discussion of the behaviour of this molecule in the thermosphere is particularly pertinent to this review because we must consider translational, rotational, vibrational and electronic excitation! Thermospheric nitric oxide is of great importance because during the long polar winter it is transported down through the atmosphere and affects stratospheric ozone concentration^.^^ The molecule is formed in the reaction of ground-state N(4S) atoms with 0,: N(4S) + 0, -NO + 0 (24) This reaction is slow at room temperature,35 and even at thermo- spheric temperatures is not a major source because NO can be destroyed in the 'cannibalistic' reaction: N(4S) + NO -N, + 0 (25) A more important source of NOJ4involves the reaction of the first electronically excited state of N atoms, N(,D), with 0,: This species is generated in a number of processes, the main lower thermospheric source34 being the dissociative electron attachment of NO+: NO+ + e -N(2D) + 0 (27) while electron impact dissociation of N, is also important: N, +e -N(4S) +N(,D) + e (28) The electrons involved in reaction 28 must possess a significant amount of energy in order to effect the dissociation and are either photoelectrons or secondary auroral electrons.A second excited state of atomic nitrogen, N(,P), is also formed36 in the electron- impact dissociation of N,, but is quickly quenched to N(2D) Barth’4 concluded in a modelling study that NO concentrations in the lower thermosphere were highly sensitive to the branching ratio for N(2D) in reactions 27 and 28 However, there are other uncertainties associated with thermo- spheric NO production It has been suggested that one channel of reaction 26 may lead to the formation of O(‘D) and thus contrib- ute to auroral or dayglow emission at 630 nm GCrard36 suggests that there is no evidence one way or the other for this supposition and that the problem will not be resolved until direct laboratory evidence is obtained Recent measurements indicate that NO is present in the thermosphere in high vibrational states, but also with rotational energy significantly greater than expected from thermal processes 77 Reaction 26 is energetically capable of accessing the observed levels, but angular momentum constraints seem to require significant translational energy in the reactants 37 Theoretical calculations on the dynamics of reaction 24 suggest that translationally hot ground-state N(4S) atoms may react with 0, significantly faster than N atoms with a thermal velocity distri- bution and be an important source of NO, particularly in the daytime thermosphere M Emirsion from electronically excited NO is also detected in the thermosphere,39 one source being the recombination of N and 0 atoms The Sand y bands of NO have been detected by rocket-borne spec- trorneterc3’ and used to derive nighttime N(5) altitude+oncentra- tion profiles (during the day, fluorescent emission from electronically excited NO interferes with such measurements, but has been used to determine NO concentrations) Emission from NO(b4C ) generated in reaction 29 has been detected in the near- infrared,40 and it can be assumed that a considerable fraction of the reaction leads to NO(a4n) formation, as only this state and the ground state correlate with the reactant atoms It is likely that these 5tates have some role to play in the heating efficiency of reaction 29, doublets formed in the reaction will emit radiation to space, ulti- mately giving ground-state NO(X2n), emission from NO(a4n) is forbidden by the electronic dipole selection rules, and so energy rtored in this state should be efficiently converted to heat 8 Concluding Remarks In this paper the role of the excited state in the atmosphere has been reviewed It is clear that excited states are important in many aspects of atmospheric chemistry and physics Some parts of the science are quite well understood, the chemistry of O(ID) in the troposphere is a good example, although it should be remembered that there are uncertainties in the quantum yield for its formation from ozone photolysis at wavelengths that are important in the lower atmos- phere Advances in satellite technology allow excited species to be observed in the remotest regions of the atmosphere and enormously fast digital computers can be used to interpret the measurements through modelling studies These models require a detailed knowl- edge of the fundamental physical and chemical processes that occur in the atmosphere and such knowledge comes from laboratory experiments Many questions concerning these fundamental pro- cesses remain unanswered and even details from well-studied systems (e g ozone photolysis) can prove to have important conse- quences Determining quantitative details for the mechanisms of formation of excited species and their losses through quenching, radiative decay and reaction require sophisticated experimental methods and still provide a significant challenge for laboratory sci- entists CHEMICAL SOCIETY REVIEWS, 1996 9 References 1 J M Hollas, Mode? n Spectroscopy, 2nd edn ,Wiley, Chichester, 1992 2 R P Wayne, Chemistry of Atmospheres, 2nd edn , Clarendon Press, Oxford, 1991 3 S M Ball and G Hancock, Geophys Res Lett, 1995,22, 1213 4 W Johnstone, N J Mason, W R Newell, P Biggs, G Marston and R P Wayne, J Phys B At Mol Opt Phys , 1992,253873 5 J E Davies, N J Mason, G Marston and R P Wayne, .I Phys B At Mol Opt Phys , 1995,28,4179 6 A U Khan, J N Pitts, Jr ,and E B Smith, En\ iron Sci Techno1 ,1967, 1,656 7 R V Benasson, E J Land and T G Truscott, Excited States and Free Radicals in Bioloqy and Medicine, Oxford University Press, Oxford, 1993 8 F Bohm, G Marston, T G Truscott and R P Wayne, .I Chem Soc Faraday Trans , 1994,90,2453 9 B C Faust and J M Allen, J Geophys Res , 1992,97, 12 913 10 R Atkinson, S M Aschmann, J Arey and B Shorees, J Geophys Res , 1992,97,6065 11 G Marston, unpublished results 12 S Hatakeyama, H Kobayashi and H Akimoto, J Phys Chem , 1984, 88,4736 13 S I Akasofu, Am Scientist, 1981,69,492 14 M H Rees, Physics and Chemistrq of the Upper Atmosphere, Cambndge University Press, Cambndge, 1989 15 C A Barth,Ann Geophys 1964,20, 182 16 G E Streit and H S Johnston, J Chem Phys , 1976,64,95 17 J A Kaye, J Geophys Res , 1988,93,285 18 S T Lunt, G Marston and R P Wayne, .I Chem Soc Faraday Trans 2, 1988,84,899 19 G Marston, D Phil Thesis, Oxford, 1987 20 R L Gattinger and A Vallance-Jones, in Physics and Chemistry of Upper Atmospheres, ed B M McCormac, D Reidel, Dordrecht, 1973 21 V A Krasnopolsky, Planet Space Sci , 1985,34,511 22 R P Wayne, Res Chem Intermed , 1994,20, 395 23 R L Miller,A G Suits,P L Houston, R Toumi, J A MackandA M Wodtke, Science, 1994,265, 183 1 24 K 0 Patten, Jr, P S Connell, D E Kinnison, D J Wuebbles, T G Slanger and L Froidevaux, J Geophys Res, 1994,99, 12 1 1 25 J F Noxon, Planet Space Sci , 1982,30,545 26 S Solomon, J T Kiehl, B J Kerndge, E E Remsberg snd J M Russell 111,J Geophys Res , 1986,91,9865 27 B J Kerridge and E E Remsberg, J Geophys Res , 1989,94, 16 323 28 P N Clough and B A Thrush, Trans Faraday Soc , 1969,65,23 29 G Marston, J Ballard, B J Kerndge and R P Wayne, manuscnpt in preparation 30 A Redpath, M Menzinger and T Carnngton, Chem Phys , 1978, 27, 409 31 S M Adler-Golden, J Phys Chem ,1989,93,684 S M Adler-Golden, J Phys Chem , 1989,93,691 32 M G Mlynczak and S Solomon, J Geophys Res -Atmospheres, 1993, 98, 10517 33 R Sharma, B Zygelman, F von Esse and A Dalgarno, Geophys Res Lett, 1994,21, 1731 34 C A Barth, Planet Space Sci , 1992,40,315 35 A J Bamett, G Marston and R P Wayne, J Chem Soc Faiaday Ttaizs 2, 1987,83, 1453 36 J C Gerard, Planet Space Sci , 1992,40,337 37 P S Armstrong, S J Lipson, J A Dodd, J R Lowell, W A M Blumberg and R M Nadile, Geophys Res Lett, 1994, 21,2425 38 J W Duff, F Bien and D E Paulson, Geophys Res Lett, 1994, 21, 2043 39 P D Tennyson, P D Feldman, J F Hartig and R C Henry, J Geophys Res , 1986,29,595 40 P Biggs, G Marston and R P Wayne, J Mol Spect , 188,129,236

ISSN:0306-0012    

DOI:10.1039/CS9962500033

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

A Strategy for Constructing Photosynthetic Models Porphyrin-containing Modules Assembled Around Transition Metals Anthony Harriman Center for Fast Kinetics Research University of Texas at Austin Austin Texas 78712 USA Jean-Pierre Sauvage Laboratoire de Chimie Organo-minerale Faculte de Chimie Universite Louis Pasteur I rue Blaise PascaI 67008 Strasbo urg France 1 Introduction The photosynthetic process and especially the rapid electron-trans- fer reactions occurring within protein-bound pigment complexes has both fascinated and inspired scientists for many decades Following from elegant biophysical investigations including mutagenesis and allowing for X-ray crystallography much is now known about the intimate structure and reaction pathways of bacte- rial photosynthetic reaction centre (RC) complexes * The main function of the RC is to generate an energy gradient using succes- sive electron-transfer (ET) processes to span the cytoplasmic mem- brane During photosynthesis extremely long-lived charge-reparated states Jt1,2>1 s) are formed the redox sites being widely spaced (ta 70 A) with the unitary quantum efficiency for charge reparation (CS) being achieved by virtue of a large expenditure of (photonic) energy The cascade of ET steps that provides for long- range CS includes many individual reactions each of which is suf- ficiently exergonic so as to be capable of competing with the inherent reverse ET process This unique feature of the RC has been an important stimulant in the motivation for designing artificial molecular systems aimed at mimicking the structure and/or func- tion of the bacterial photosynthetic RC We note with interest that the initial (rapid) ET reaction occurs between large macrocyclic ringrl * and ir followed by slower reactions that involve quinones and haems In the latter case ET occurs at the metal centre not on the tetrapyrrolic ring with the fastest reaction requiring 270 ns Synthetic models of the RC complex have advanced consider- ably in recent years especially those based on covalently-linked porphyrin-quinone entities,3-6 and many are able to mimic some of the essential features of the natural system We have concentrated on modelling the primary ET step between tetrapyrrolic nuclei that occurs within 3 ps in the RC complex,8 despite the 17 8 centre-to-centre separation (Figure 1) In particular we have shown howjudi- cious incorporation of transition metals into the structure can influence the subsequent reactivity Our basic strategy is illustrated in Figure 2 where it appears that transition metal centres can be used either to control the electronic properties of the bridge between donor and acceptor and thereby modulate the rate of ET or to assemble well-oriented structures containing terminal donor and acceptor porphyrinic subunits Zinc(rr) and gold(nr) porphyrins have been selected as donor and acceptor respectively because of their highly favourable spectroscopic and electrochemical proper- Jean-Pier re Sauvage was born in I944 in Paris After studying at Strasbourg University he did his PhD with J M Lehn on rryptates and spent some time as a post-doc with M L H Green at Oxford He is nou a research direc tor (CNRS)at the Faculty of Chemistry His main scientific interests range from chemical topology (cutananes and knots) to models of the photosynthetic reaction centre complex and one dimensional multicomponent transition metal complexes z l\ QI3 Q Figure 1 A fragment of the RC as isolated from Rhodopsrudomonas 1 iridrs with labels provided for the special pair of bacteriochlorophyll molecules (P) which acts as the primary electron donor the accewory bacteriochlorophyll (B) which mediates ET the bacteriopheophytin (H) which is the primary electron acceptor the menaquinone (Q,) which i$ an electron relay and the ubiquinone (QB)which is the hnal electron accep tor Note the presence of a non haem iron atom and that ET involves only the labelled pigments (L branch) The unmarked bacteriochlorophyll and bacteriopheophytin molecules (M branch) do not appear as redox inter mediates See refs 1 and 2 for original descriptions of the RC ties They represent acceptable albeit crude models for the bacte- riochlorophyll special pair (primary donor) and bacteriopheophytin Anthony Hari-iman was born in 1949 in the West Midlands After attending Wolverhampton Polytechnic he joined Sir George Portei s research group at the Royal Institution of GI eat Britain in I974 before moving to the University of Texas at Austin in 1988 His major research interest is cont erned u ith the development oj' artlfi cia1 photosynthetic systems mostly based on metallopor phyrins and their exploration by transient spectroscopic tech niques 41 CHEMICAL SOCIETY REVIEWS I996 oc *@ c Figure 2 The principle whereby a tranwion metal centre (black dot) that coordinates to appropriate chelating ligands (solid arcs) can gather redox and/or photo active porphynns (lozenges) into a single assemblage Note the two porphynn rings which can differ by virtue of their substitution pattern and/or central metal cation may be attached to the same ligand or to separate ligands In both cases formation of the central complex is essential In the upper line it will govern the electronic properties of the spacer connecting the two porphyrinic chromophores whereas in the second case (bottom line) the metal will be the assembly point necessary for formation of the multicomponent molecular set I M=2H energy transfer 2 M = Au(lll) electron transfer Figure 3 Structure of the 2,9-diphenyl- 1,lO phenanthroline bndged bis porphyrin illustrating the oblique arrangement of the porphynn rings Note the bis( 3,5-ter t-buty1)phenyl groups have been omitted from each of the vacant meso-positions for clarity of presentation These substituents provide an important label for 'H NMR studies and increase the solubil- ity of the bis porphyrin in organic solvents At least one of the porphyrin rings can be selectively excited with light of an appropriate wavelength while the nature of the second cation (M) controls the photochemistry that follows absorption of a photon by the zinc(rr) porphyrin subunit (primary acceptor) present in the RC It is important to realise that for this pair of porphyrins redox chemistry occurs exclusively at the macrocyclic ring and not at the metal centres As such bis-por- phyrins formed from these modules differ from the well-studied analogues containing iron(rr1) or manganese(rI1) porphyrins Two types of asymmetric bis-porphyrins can be envisaged namely (z) oblique structures having both porphyrin subunits attache; to a single Iigand,1° giving a centre-to-centre separation of cu 14 A and (II) linear structures having only one porphyrin p&r Iigandll and in which the centre-to-centre separation is cu 30 A The synthetic approaches differ markedly for the two classes of bis- porphyrin but both involve isolation of a chelating ligand bearing the necessary functionality that permits subsequent construction of the porphyrin ring The ligand has to remain sufficiently accessible for complexation with appropriate metal cations so as to form well- defined metal complexes of correct stoichiometry 2 Oblique Bis-porphyrins Oblique bis-porphyrins such as 1 and 2 (Figure 3) are obtained in multi-step procedures from 1,IO-phenanthroline with controlled metallation of the corresponding free-base porphyrin precursor I2 The X-ray structure obtained for the corresponding bis-zinc (11) ana-loguel indicates a Znfl-Zntr separation of cu l 3 6 8 (Figure 4) the shortest porphyrin edge-to-edge separation being ca 8 5 8 Of course the shortest through-bond (edge-to-edge) pathway exceeds even the centre-to-centre separation and is estimated to be ca 22 Figure 4 X Ray crystal structure obtained for the symmetrical bis zinc complex having the same organic backbone as 1and 2 It is assumed that the general geometncal features of the bis-porphynn are only slightly dependent on the nature of the metals located in the coordination sites The Zn Zn interatomic distance is 13 7 A in the present structure A The subsequent photochemistry of the bis-porphyrin IS con-trolled by the choice of the second cation M such that rapid energy- (M = 2H') or electron-transfer (M = Au"+) processes dominate upon excitation of the appended zinc porphyrin Energy transfer most likely proceeds vzu Forster-type dipole-dipole transfer,I4 as has been observed for many other covalently-linked bis-porphyrins having either alkyl or aryl bridges With respect to photoinduced electron transfer the oblique bis-porphyrin 2 is unique among model systems in that both porphyrin rings are photoactive thereby permitting facile comparison of the photochemistry occurring 1. za singlet and triplet excited states l5 I6 Laser flash photolysis studies carried out with 2 in acetonitrile solution at 25 "C have been used to elucidate the reaction pathway together with rate constants for individual steps following selective excitation of either porphynnic subunit (Figure 5) I The excited states of both porphyrins (I e singlet and triplet for the zinc por- phyrin and triplet for the gold porphyrin) undergo intramolecular electron transfer to form a common radical pair which persists for A STRATEGY FOR CONSTRUCTING PHOTOSYNTHETIC MODELS -A HARRIMAN AND J -P SAUVAGE + hv h = 532 nm/ 600 ps P Figure 5 A pictorial representation of photoinduced electron transfer in the oblique zinc(ir)-gold(1rr) bis-porphyrin 2 following selective excitation into either porphyrin subunit giving the lifetime of each intermediate species as measured by transient spectroscopic studies made in acetonitrile at 25 "C 4 3 Figure 6 (a) Template synthesis of porphyrin stoppered 121-and [3] rotdxanes The macrocycle (A) incorporating a coordinating fragment (thick line) inter acts with a metal centre (black dot) and an asymmetrical open chain chelate (B) beanng a single porphynn and a precursor function (X) which is small enough to pass through the nng After the threaded intermediate (C) IS formed the second porphynn nng is subsequently constructed giving ri5e to the transition metal complex-containing rotaxanes (D) and (E) Demetallation leads to the free-ligand rotaxanes (F) and (G) from (D) and (E) respectively By applying the strategy described above [the hatched lozenge now represents a gold(rI1) porphyrin and the templating metal is a copper(1) cation] an asym metrical bis porphyrin of type (F) is obtained (white lozenge symbolizing a free-base porphyrin) (b) This free coordination site can be metallated with zinc(ii) to afford the [2] rotaxane 3 Owing to the synthetic procedure necessary for assemblage of these rotaxanes the porphyrinic subunits bear different substitution patterns Note the substituents on the porphyrin rings (see Figure 7) have been omitted for clanty of presentation while the overall charge on [2]-rotaxane 3 is +2 about 600 ps before undergoing charge recombination (CR) to reform the original species The radical pair is formed from the excited singlet state of the zinc porphyrin in ca 55 ps (AGO = -0 75 eV edge-to-edge distance = 8 5 A)compared to ca 3 ps for the corresponding process in tbe RC complex (AGO = -0 39 eV edge-to-edge distance = 9 5 A) l7 Furthermore whereas the rate of the primary ET reaction occurring in the RC complex is essentially independent of ternperature,lx 2 exhibits both a significant activation energy in solution (Ed = 0 009 eV)I9 and a 1 10-fold decrease in rate of ET on moving from fluid solution at 25 "C to a frozen glass at 77 K 20 Under the latter conditions the radical pair could be detected by EPR spectroscopy Detailed mechanistic studies19 were found consistent with photo- induced ET between the terminal porphyrin subunits in 2 occurring via orbitals localized on the bridging 1,lO-phenanthroline spacer At first glance we might find the rate of ET in 2 surprisingly slow if the process involves superexchange with the aromatic bridge since ET through such moieties can be rapid 21 In contrast the protein medium surrounding the reactants in the RC may involve a substantial attenuation factor," causing the rate of ET to depend critically on separation distance For 2 there was no indication that CR within the radical pair involved a superexchange mechanism and in fact this process might occur through the intervening space occupied by solvent molecules This being so it was considered that the rate of the forward ET step but not that of CR could be modu- lated by varying the energy of the LUMOs and/or HOMOS on the spacer moiety The latter situation can be realised by incorporating the bridging 1,lO-phenanthroline ligand into a [2]-rotaxane (Figure 6) 22 Synthesis of the [2]-rotaxane 3 relies critically on the templat- ing role of Cul ions a strategy used successfully to prepare molec- ular knots23 and other such exotica,24 with the same reaction favouring formation of the corresponding [3]-rotaxane 4 (Figure 7) 25 Following selective excitationz6 into the zinc(i1) porphyrinic subunit of the [2]-rotaxane 3 the corresponding radical pair was formed in ca 1 ps a 55-fold increase in the rate of ET relative to the corresponding bis-porphyrin 2 In competition with CR the zinc porphyrin 7r-radical cation oxidizes the central Cu' complex (Figure 8) before the original ground-state species is restored By removal of the copper(1) cation from [2]-rotaxane 3 giving rise to 5 and CHEMICAL SOCIETY REVIEWS 1996 Figure 7 Structure of the copper(1) containing 131 rotaxanes having a central octaalkyl diphenyl porphyrin and terminal gold(rrr) tetraaryl porphynns Note subrtituents on the zinc(1r) and gold(ir1) porphyrins are the same as for the corresponding [2] rotaxane illustrated in Figure 6(b) The [3]-rotaxane 4 is obtained in 32% yield while the same reaction affords the corresponding [2] rotaxane 3 in 25% yield The overall charge on compound 4 is +4 . 20 ps L +' h = 586 nm Figure 8 A pictorial representation of photoinduced electron transfer in the copper(1) containing [2]-rotaxane 3 having terminal zinc(i1) and gold(1Ii) por-phyrins following selective excitation into the zinc(l1) porphyrin subunit giving the lifetime of each intermediate species as measured by transient spectro- scopic studies made in acetonitrile at 25°C Note the central copper(1) cation is represented by a black dot and carnes a positive charge subsequent insertion of a zinc(i1) cation to form 6 (Figure 9),z7 it was possible to establish a good correlation between the rate of the forward ET process and the reciprocal of the energy gap between the relevant orbitals on the porphyrin excited state and on the bridg- ing ligand Noteworthy 'H NMR investigationsz5 provided unambiguous evidence that [2]-rotaxanes 3 and 6 possess the same geometry thus facilitating comparison of the two systems solely in terms of their electronic properties This finding is entirely consis- tent with a superexchange mechanism involving molecular orbital5 resident on the bridging 1,lO-phenanthroline moiety The rate of CR was hardly affected by these changes in molecular architecture As such this strategy permits design of systems capable of rapid forward ET but relatively slow CR It should be noted that for syn- thetic convenience the nature of the zinc porphyrin donor present in the [2]-rotaxanes differs somewhat from that used in the simple bis-porphyrin This change affects the driving force for the various ET steps and because of adjacent alkyl substituents on the pyrrole rings forces the connecting phenyl ring to become essentially orthogonal to the plane of a porphyrin ring Despite the slightly larger edge-to-edge separation and smaller driving force (AGO) the rate of the primary ET step in the RC complex is much faster than that found for 2 at 25 "C The elec- tronic coupling matrix element estimated for the RC complex (V = 25 cm-is also significantly smaller than the correspond- ing value (V = 85 crn-[)19 measured for 2 However the high rate of ET characteristic of the RC complex arises because the driving force is comparable to the total reorganization energy (A) associ-ated with ET such that the activation energy ir close to zero and reaction occurs at the apex of a Marcus-type energy-gap profile [Note in its simplest form the activation free energy change may be expressed as AC* = [(A + AGo)2/4A] where A is the reorganiza- tion energy accompanying electron transfer and AGO is the thermo- dynamic driving force for electron transfer This expression holds well for ET in the normal (z e ,-AGO < A) region but needs to be modified to allow for high frequency quantum modes usually represented by a single (average) vibrational frequency when applied to ET in the inverted region ] For 2 this is not the case (-AGO = 0 5A) and ET occurs well within the 'normal' region of the Marcus-type parabola Indeed the activationless rate of ET (I e -AGO = A) for 2 extrapolated from temperature and solvent dependence studies,I9 corresponds to a lifetime for the excited state of the zinc porphyrin of ca 6ps For the copper(1) [2]-rotaxane 3 we note that -AGO = A so that as for the RC complex the rate of ET is at a maximum It should be noted however that although formation of the radical pair in the [2]-rotaxane 3 occurs on the same time scale as forward ET in the RC complex the inherent rate of CR (k = 2 X lo9s I AGO == -1 2 eV) in these various bis-por- phynnsz7 is much faster than that found in natural RC complexes (k = 2 X lo7 s-' AGO = -0 9 eV) This situation most probably anses because CR for the RC complex (-AGO = 3 6A) occurs well A STRATEGY FOR CONSTRUCTING PHOTOSYNTHETIC MODELS -A HARRIMAN AND J -P SAUVAGE 5 63 (I PS) (36 PS) Figure 9 Sequential removal of copper([) from [2]-rotaxane 3 with cyanide ions giving rise to 5,and insertion of zinc(n) into the vacant coordination we \o as to form the zinc(i1) containing porphyrin stoppered [2]-rotaxane 6 Lifetimes for the zinc porphyrin excited singlet state as quenched by intramolecular electron transfer to the gold(rrr) porphyrin are provided for each of the [2]-rotaxanes Note substituents on the porphyrin rings which are the same a\ shown on Figure 7 have been omitted for clarity of presentation while the overall charge on 6 is +3 Figure 10 Strategy used to assemble asyrnmetncal bis porphyrins around a central transition metal cation (black dot) The basic module consists of a por phyrin (lozenge) covalently attached to a chelate (arc of a circle) Provided the transition metal complex is resistant to Iigand-exchange processes stepwise ET can be envisaged after selective illumination of a porphyrin ring within the Marcus inverted region whereas that for the model \ystems (-AGO == 1 3h) is much closer to the apex {Note when considering ET reactions occurring deep within the inverted region it I\ necessary to allow for the magnitude of the electron-vibra- tional coupling strength [S = 4/2 where A is the change in equilibrium configuration for the particular (z e averaged) vibra- tional mode expresced in units of the root-mean-square displace- ment at the zero-point energy] For the RC S is small so that the rate of ET should decrease rapidly with increasing driving force For isolated porphyrinic subunits however S 3 0 5-1 such that the inverted effect is less pronounced and the rate of CR will decrease more gradually with increasing driving force } 3 Linear Bis-porphyrins Having demonstrated by virtue of [2]-rotaxane 3 being a reason- able model for primary ET in the RC complex the efficacy of transi- tion metal ions like Cul towards tuning the electronic properties of the bridge it is opportune to expand the role of the metal complex Thus the transition metal cation used to modulate electronic cou- pling between donor and acceptor can be employed also as a tem- plate capable of gathering and orienting the two porphyrinic components 25 The Iigand to be attached to each porphyrin and the binding mode used to assemble the two modules into the final con- jugate were selected so as to facilitate strict stereochemical control especially with regard to the distance between the various electro- and photo-active components It was further considered that uni- dimensional structures were particularly well adapted to accommo- date multistep ET reactions eventually leading to long-lived and spatially-remote CS states Although there have been several report^^()-^^ describing porphyrin-metal complex dyads it was decided that linear rigid structures could be achieved most readily using 2,2’ 6’,2’’-terpyridine as the chelating ligand This required access to novel materials bearing appropriate functional groups The general strategy for making these one-dimensional multi- component molecular systems is illustrated in Figure 10 In princi- ple a single complexation step should facilitate assembly of a bis- porphyrin by gathering two relatively simple (porphyrin-containing) molecular modules around the central transition metal In addition to serving as a bridge between the terminal porphyrins the metal complex so formed could also act as an electron relay Indeed in the corresponding mono-porphyrinic dyads 7 and 8 (Figure 11) it was observed3S that fluorescence from the porphyrin was extensively quenched due to rapid electron transfer to the appended metal complex The presence of a phenyl ring between porphyrin and terpyridyl ligand plays a crucial role since by decou- pling the reactants it slows down the rate of CR from fs to ns time- scales For example with ruthenium(1r) bis-(4’-phenylterpyridyl)d\ spacer CR is sufficiently slow (I e 2 ns) to permit engineering of triads in which successive ET steps can compete with CR (Figure 10) In these latter triads excitation into the terminal zinc por- phyrin results in ET to the central ruthenium(I1) bis-terpyridyl complex followed by secondary ET to the appended gold(m) complex (Figure 12) 36 Consequently ET occurs by two consecu- tive steps over a ZnlI-AuIil separation of ca 30 A rate constants for individual steps as determined from laser flash photolysi studies are indicated in Figure 13 It is interesting to compare the lifetime of 33 ns measured for the ultimate charge-separated state in the triad with the value of 2 ns found for that in the corresponding ruthe- nium(I1)-containing dyad since this illustrates the advantage of using a multistep ET pathway Light absorbed by the central ruthe- nium(ii) complex in 9 is transferred to the appended porphyrin most likely via a Dexter-type electron-exchange mechanism In triad 9 the primary ET step leading to reduction of the central Ru” complex (AGO = -0 25 eV edge-to-edge separation = 7 A) requires 50 ps at 25 “C and is essentially quantitative 36 This rela- tively slow rate of ET arises because of an almost orthogonal align- ment of the bridging phenyl ring which restricts electronic coupling between the reactants (V = 12 cm I) and from the fact that ET occurs well within the ‘normal’ region (-AGO = 0 5h) However CHEMICAL SOCIETY REVIEWS 1996 7 M = Rh(lll) 8 M = R~(ll) Figure 11 Structures of the photoactive dyads 7 and 8 containing zinc(1r) porphyrin and rhodium(1n) bis-terpyridyl subunits A bridging phenyl ring can be used electronically to decouple the redox-active subunits as can be seen by examination of the relevant rate constants for charge separation (k,J and charge recombination (kJ measured in acetonitnle solution for 7 (kcs = 3 X 10" s-' k = 8 X lo9 s I) and the analogue not having the bridging phenyl nng (kcT= 2 X 10'2 s-I k >4X lo1*s-I) The tripartite compound 7 is classified as a dyad rather than a tnad since it contains only two different redox centres Note that rhodium(r1r) can be replaced by ruthenium(I1) with only a slight modification in the observed rates of electron transfer hv 9 Figure 12 Structure of the bis-porphynn-containing triad 9 with the time scales of important ET steps being indicated The central ruthenium(l1) bis-ter pyridyl complex serves to gather and onent the terminal porphyrins and to act as a redox intermediate in the ET pathway leading to formation of an inter- porphyrin radical pair This complex is not involved as a redox intermediate in interporphyrin charge recombination (2 10 II 88 eV)II 331 ns I ground state Figure 13 Overall reaction scheme for the ET pathway that follows upon illumination of the zinc(I1) porphynn subunit in tnad 9 The various energy levels are obtained from spectroscopic and electrochemical measurements while the lifetime for each ET step has been determined by transient spectroscopy primary CR IS also slow becduse reaction occurs deep in the inverted region (-AGO = 4h) and this permits secondary ET resulting in reduction of the gold(ir1) porphyrin to occur with a yield of ca 60% Ipterporphyrin CR occurs over an edge-to-edge separation of ca 20 A and takes place on the same time scale (k = 3 X lo7s AGO = -1 2 eV) as primary CR in the RC complex despite the much larger separation that prevails for the model system Again this situation can be explained in terms of a Marcus-type energy gap profile since CR in the tnad (-AGO = 17A) is expected to occur near to the apex It is interesting to note that in the corresponding triad having a free- base porphyrin in place of the zinc porphyrin the initial light-induced ET reaction still takes place despite an unfavourable driving force (AGO=0eV) In this case interporphyrin CR is slower (k = 1 X lo7 s-1 AGO = -1 4eV) than found for 9 It is also noteworthy that for 9 the interporphyrin radical pair survives for several microseconds in an ethanol glass at 77 K 4 Conclusion We have indicated that photoinduced ET between terminal por- phyrins can occur across an interspersed metal complex the Iatter species appearing as a virtual or real redox intermediate over dis- tances relevant to the RC complex (Figure 14) Our model systems developed to date although capable of high rates of ET do not possess optimal thermodynamic properties with respect to the mutual relationship between AGO and h which is an integral feature A STRATEGY FOR CONSTRUCTING PHOTOSYNTHETIC MODELS -A HARRIMAN AND J -P SAUVAGE )=\ +a@-' \I -'% Figure 14 Companson of natural and artificial porphyrin assemblages (a) Fragment of the RC showing the primary electron donor (P) and acceptor (H) together with the bndging bactenochlorophyll (B) (b) Corresponding porphyrin stoppered [2]-rotaxane 3 where the donor IS a zinc porphynn the accep-tor is a gold(m) porphynn and the copper(1) bis 1,lO-phenanthrolinecomplex serves as the bndge (c) Bis-porphynn based triad 9 where the donor is a zinc porphyrin the acceptor is a gold(ir1) porphynn and the interspersed ruthenium(1i) bis-terpyndyl complex provides the bndge Only in the latter case does the bridge also act as a redox intermediate in the forward ET process of the RC Future model systems will need to take better advantage of this relationship in order to maximize rates of CS and at the same time minimize rates of CR It is also apparent that close attention must be given to the preferred orientation of any bridging phenyl rings since this can influence the extent of electronic coupling between appended redox-active subunits Indeed the inadvertent positioning of alkyl substituents on the tetrapyrrolic macrocycle may make a major contribution to this effect we have however demonstrated the necessity for such bridges Even so triads such as 9 can be considered as belonging to a family of molecules that represents a genuine modular approach to construction of multi-component molecular arrays having a logical and well-defined arrangement of the subunits as in the RC and which allows for a cascade of ET events The availabilityof a gradient of redox centres is an important if not essential aspect of designing molecular systems capable of circumventing CR and thereby realizing long-lived widely-spaced CS states 37 3x The modular approach advo-cated here represents a viable and highly versatile alternative to the common and successful,practice of using only covalent bonding to assemble giant arrays It provides new opportunities for the construction of elaborate molecular architectures that closely resemble the RC complex In particular the methodology can be readily adapted to synthesize higher-order oligomers possessing quinoid and/or haem-like redox centres that could serve to extend the ET sequence so as to cover many redox partners Acknowledgement We are deeply indebted to our many colleagues whose efforts made possible the work described herein and in par-ticular we appreciate the significant contributions made by Anne Brun Jean-Claude Chambron Sylvie Chardon-Noblat Jean-Paul Collin ValCrie Heitz and Fabrice Odobel Financial support from CNRS NSF NATO and NIH IS gratefully acknowledged 5 References I J Deisenhofer and H Michel Angew Chem Int Ed Engl 1989,28 829 2 R Huber Angew Chem Int Ed Engl 1989,28,848 3 J S Connolly and J R Bolton in Photoinduced Electron Transfer,ed M A Fox and M Chanon Elsevier Amsterdam 1988 Part D p 303 4 D Gust and T A Moore Science 1989,244 35 5 K Maruyama and A Osuka Pure Appl Chem 1990,62 15 1 1 6 M R Wasielewski Chem Re1 1992,92,435 7 W W Parson in New Comprehensive Biochemistry Photosynthesis,ed J Amesz Elsevier Amsterdam 1987,p 43 8 J L Martin,J Breton,A J Hoff,A MigusandA Antonetti Proc Nut1 Acad Sci USA 1986,83,957 9 J C Chambron S Chardon-Noblat A Hamman V Heitz and J P Sauvage Pure Appl Chem 1993,65,2343 10 V Heitz S Chardon-Noblat and J P Sauvage Tetrahedron Lett 1991 32 197 11 F Odobel and J P Sauvage New J Chem 1994,18 1 139 12 S Chardon Noblat and J P Sauvage Tetrahedron 199 1,47,5123 13 C Pascard J Guilhem S Chardon Noblat and J P Sauvage New J Chem 1993,17,331 14 S Chardon-Noblat J P Sauvage and P Mathis Angew Chem Znt Ed Engl 1989,28,593 15 A M Brun A Hamman V Heitz and J P Sauvage,.I Am Chem Soc 1991,113,8657 16 A M Brun S J Atherton A Harriman V Heitz and J P Sauvage J Am Chem Soc 1992,114,4632 17 S Franzen R F Goldstein and S G Boxer J Phys Chem 1993 97 3040 18 G R Fleming,J L MartinandJ Breton,Nature(London) 1988,333,190 19 A Harriman V Heitz and J P Sauvage,J Phys Chem 1993,97,5940 20 A Harriman V Heitz M Ebersole and H van Willigen,J Phys Chem 1994,98,4982 21 A Osuka K Maruyama N Mataga TAsahi I Yamazaki and N Tamai,J Am Chem Soc 1990,112,4958 22 J C Chambron V Heitz and J P Sauvage J Chem Soc Chem Commun 1992 1131 23 C 0 Dietrich-Buchecker and J P Sauvage Angew Chem Inr Ed Engl 1989,28 189 24 A Livoreil C 0 Dietnch-Buchecker and J P Sauvage J Am Chem Soc 1994,116,9399 25 J C Chambron V Heitz and J P Sauvage J Am Chem Soc 1993 115 12378 26 J C Chambron A Hamman V Heitz and J P Sauvage,J Am Chem Soc 1993,115,6109 27 J C Chambron A Hamman V Heitz and J P Sauvage J Am Chem Soc 1993,115,7419 28 M Plato K Mobius M E Michel-Beyerle M Bixon and J Jortner .I Am Chem Soc 1988,110,7279 29 F Odobel J P Sauvage and A Harriman Tetrahedron Lett 1993,34 8113 30 A D Hamilton H D Rubin and A B Bocarsly J Am Chem Soc 1984,106,7255 31 E S Schmidt T S Calderwood and T C Bruice Inorg Chem 1986 25,37 18 32 M Gubelmann A Hamman J M Lehn and J L Sessler J Phys Chem 1990 94 308 33 N M Rowley S S Kurek M W George S M Hubig P D Beer C J Jones J M Kelly and J A McCleverty J Chem Soc Chem Commun 1992.497 CHEMICAL SOCIETY REVIEWS 1996 34 J. L. Sessler V. Capuano and A. K. Burrell Znorg. Chim. Ada 1993 37 D. Gust T. A. Moore P. A. Liddell G. A. Nemath L. R. Makings A. L. 204,93. Moore D. Barrett P. J. Pessiki R. V. Bensasson M. RougCe C. 35 J. P. Collin A. Harriman V. Heitz F. Odobel and J. P. Sauvage J.Am. Chachaty F. C. De Schryver M. Van der Auweraer A. R. Holzwarth and Chem. SOC. 1994,116,5679. J. S. Connolly J.Am. Chem. SOC.,1987 109,846. 36 A. Harriman F. Odobel and J. P. Sauvage J.Am. Chem. Soc. 1994,116 38 M. R. Wasielewski G. L. Gaines M. P. O’Neil W. A. Svec and M. P. 548 1. Niemczyk J. Am. Chem. SOC.,1990,112,4560.

ISSN:0306-0012    

DOI:10.1039/CS9962500041

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Probing the Intermolecular Potential Spectroscopy or Molecular Beam Scattering? Anthony J. McCaffery School of Molecular Sciences University of Sussex Brighton BN 1 9QJ UK 7 Introduction Many of the most significant developments in chemistry this century have relied heavily on the contributions spectroscopy has made to the subject. Often this has been in the form of molecular identification or of structure determination. On the other hand at a more fundamental level spectroscopy has provided most of the remarkably precise detail that we posses on individual molecules and which underpins our understanding of macroscopic chemical behaviour. Still to be revealed is the nature of the dynamical forces that give rise to chemical or physical change the motivation for the work described in this review. The origin of this all-pervading influence of spectroscopy is the unique capability of light to act as a quantum state probe a characteristic that results from the very specific properties carried by the photon. As a result the interaction of light with atoms and molecules is reasonably predictable from rigorous or approximate selection rules. Using spectroscopic methods we are now in a position to investigate say the forces within mole- cules the dynamics of deactivation of excited species and many other intrumolecular processes that are of chemical signifi- cance. When we come to enquire about the forces between molecules in the past spectroscopy has been less evident and the main tech- niques of investigation have been (atomic and) molecular beam scattering. This is partly a consequence of the transitory nature of the interaction between species in the gas phase which makes the design of an appropriate spectroscopic probe more difficult. However the success of early atomic beam scattering experiments in unravelling details of the interatomic potential appears also to have been influential in the design of probes of the intermolecular interaction. A significant new factor that enters in experiments involving molecules is that the kinetic energy of collision may be con- verted into internal (or potential) energy of the molecule and thus not be observable as recoil velocity and deflection angle of the colliding species. The need for detailed knowledge of the complete (kinetic and potential) energy balance before a picture A.J.McCaffery u'as an undergraduate and postgraduate student at Exeter University. His PhD completed in 1963 wws directed by S. F. Mason on optical activity in metal complexes. Following one year as a CIBA Fellow in Copenhagen and three with P. N. Schutz at the University of Virginia working on magnetic optical activity he was uppointed to University of Sussex as Lecturer then Reader and Professor in 1984. He was Dean of the School of Molecular Sciences 1984-1 989 and is currently a Pro Vice-Chancellor. His programme investigating collision dynam- ics using laser spectroscopy begun in 1978,has led to signif- icant experimental and theoret- ical advances. He was a recipient of the 1992 RSC Spectroscopy award. of the intermolecular potential could emerge has led to a re-eval-uation of the scattering method for interactions involving mole- cules. A complete picture of energy disposal in collisional (reactive or non-reactive) events requires knowledge of internal energies of col-liding species and their velocities before an interaction and the new values after. One component of this complex requirement may be simplified since it is the relative velocity before and after the colli- sion that is significant. The need to know internal molecular ener- gies however means that the distribution of population over molecular rotational vibrational and electronic states must be deter- mined before and after collision. This latter requirement is so plainly suited to spectroscopic methods that the interfacing of lasers to molecular beam scattering experiments was a natural development. At this point it is worth noting that internal state selection prior to interaction may be achieved with some degree of effectiveness using a supersonic jet expansion and thus the problem often reduces to quantum state detection. However despite this the full determination of the energy budget in collisional interactions of molecular species remains dauntingly complex and few experimentalists have achieved this level of specification. 1.1 Spectroscopic Approaches Parallel to the development of molecular beam methods for studying intermolecular interactions has been one which focused on quantum state changes within the interacting species with no attempt to define precisely the kinetic energy component of the energy balance equation. This arose in part from technological developments in the form of lasers capable of transferring substantial numbers of molecules into a single molecular quantum state. Transfer out of this to some other quantum state by an intermolecular interaction may then be monitored in one of a variety of ways to give state-to-state transition rates or cross-sections. A motivation of this was the search for selection (or propensity) rules analogous to those found in spectroscopy which would give insights into the nature of the process.2,3 Figure 1 illustrates the complexity of the colli- sional problem by comparison to that involving the photon-molecule interaction. One of the advantages of spectroscopic experiments is the rela- tive ease with which they may be performed. For example much useful work has been done in simple glass or metal collision cells. In addition the ease with which samples may be changed and experiments performed had led to a sizeable database of rates and cross-sectionsin which many molecules and collision partners have been investigated. This contrasts molecular beam scattering studies which require a longer term commitment to a particular system and consequently the database is smaller. The combination of spectroscopy and molecular beam scattering raises the possibility that all components of the energy equation may be determined; kinetic energies from scattering and potential energy from internal state distributions determined by spectroscopy. The second major development in reaction and collision dynamics experiments involved the interfacing of lasers to the molecular beam apparatus to facilitate state-to-state measurements. Much important new data has emerged from these demanding experi- ments. However each increase in sophistication represents a quantum leap in the cost of an experiment since two-laser molecu- lar beam experiments are now commonplace and those involving 49 CHEMICAL SOCIETY REVIEWS 1996 f AV =n Aj =n A rn = v. small Figure 1 This figure schematically contrasts the photon-molecule interaction for which there are known rules of engagement with that for the atom-mole- cule encounter where few rules appear to control the outcome. The curious restriction on m that is frequently found in atomiliatom elastic and inelastic collisions is the subject of detailed discussion in ref. 1. three or four lasers are not unknown. Despite such ingenious experi- ments true insight into the controlling forces in the most funda- mental of chemical processes still remains elusive. In this review the focus is on experiments that to an extent represent a de-sophistication of the way in which intermolecular interactions are studied. Thus spectroscopy with its inbuilt capa- bility to determine potential energy disposal becomes the starting point and to this is added also spectroscopically the facility to select and determine the relative kinetic energy of the interacting species. The result is an all spectroscopic method of obtaining the state-to-state differential scattering cross-section for non-reactive or reactive collisions. Although the methods described will gener- ally require two lasers the experiments are simple collision cell spectroscopic ones with the many advantages such an approach brings. It will be apparent from the preceding paragraph that the very active field of van der Waals spectroscopy in which the long-range intermolecular forces are observed directly through bound-state spectroscopic methods on species at ultra-low temperatures will not be part of this review. This spectroscopic method continues to be very powerful in the experimental determination of intermolecular force^.^ Here we address the question of how velocity determination may be added to the inherent quantum state selectivity of radiation how selectively may it be employed and how diagnostic of changes in relative velocity such a method might be. The mechanism through which velocity selection and detection is achieved is the Doppler effect a phenomenon that has long frustrated the search by spec- troscopists for higher and higher resolution in atomic and molecu- lar spectroscopy. Sub-Doppler laser spectroscopy under the right conditions can specify or determine the complete energy budget of a molecule kinetic and potential. In addition the presence of a unique axis the light propagation direction to which all measure- ments are referenced allows velocity magnitudes and directions to be ascertained. 1.2 Molecular Velocities; Directions Magnitudes and Distributions Scattering experiments are fundamentally about changes in veloc-ity; more precisely they are about changes in velocity distributions. It may at first sight seem paradoxical that in order to investigate the potential energy surface experiments that focus on accurate kinetic energy determination are devised. It will be apparent from the fore- going discussion that changes in kinetic energy are a reflection of processes on the potential energy surface and that in molecules kinetic-potential energy interconversion is a major complicating factor. In order to know what has happened to the initial relative kinetic energy of the reactants it is essential to map the destinations of prod- ucts. Initial and final kinetic energies are in the form of relative velocity distributions. The angular distribution of product relative velocities from a known reactant distribution is the quantity known as the differential scattering cross section (DCS). Clearly the DCS is all-important in any attempt to balance the kinetic energy budget. Discussions of velocity distributions in any context tend rapidly to become very mathematical since the distribution functions of unselected and selected velocities are quite complex. As a result it is easy to lose sight of the physical processes involved. This next section will attempt to illustrate velocity distributions in a pictorial fashion in order to develop a physical picture of the scattering process and the distributions of velocitites that result from using molecular beam or laser methods. The distribution of molecular velocities in a gas cell depends as is well known on the temperature of the cell and the masses of the individual molecules. It is important to distinguish distributions of molecular velocities from those of molecular speeds. The Maxwell-Boltzmann distribution of velocities is given by the expressionf(v,) = (m/2mV)i/2exp(-rnv~12kT)for the x compo-nent. Probability distributions of molecular velocities peak at zero PROBING THE INTERMOLECULAR POTENTIAL A. J. McCAFFERY velocity. The distribution function of molecular speeds has the more familiar form that of the Maxwellian distribution f(v) = 4.rr(m/2~rkT)~/~v~exp(-mv2/2kT) (1) It is the presence of the term v2 weighting the probability distribu- tion that gives the speed distribution function its characteristic shape the peak of which shifts with temperature and with mass. In processes that are initiated by collisions it is the relative speed distribution that is most relevant. The motion of two independent particles may be decomposed into that of the relative motion and the motion of the centre-of-mass. Only the former can induce a colli- sion. The relative speed in effect is a velocity since in each individ- ual collision event the direction is specified as that between the colliding centres. Like molecular speeds these directions are ran- domly oriented in space. The distribution of relative velocities has the form of a speed distribution. This particular point is rarely emphasised in textbooks and is often the source of confusion. A useful way of visualising this is to consider an extreme example such as that of collisions between a gas and a stationary partner a solid surface for example. The distribution of relative velocities in a gas has Maxwellian form with the collision system reduced mass substituted for molec- ular mass. The mean relative velocity is given by 8kT/7rp1/*where p is the reduced mass of the colliding pair. We can see that the mean velocity will be high when one of the colliding partners is a light atom or molecule and that the mean velocity will be low when both species are heavy. This represents the ‘natural’ distribution of velocities that the molecular dynamics experimenter must manipu- late in order to achieve a distribution of known directions and mag- nitudes in the experimental frame. Although this is not a central part of this review a brief discussion of molecular beams experiments is given here in order that the Doppler selection technique may be set in context. In molecular beam scattering experiments the colliding species enter the scattering chamber initially through a small hole in the sample-containing section of the apparatus. Two main regimes exist the first when the hole is small compared to the mean free path of the colliding species and the second when the hole is large com- pared to the mean free path. The former condition causes the sample species to effuse into the chamber with a velocity distribution very similar to that inside the container. The latter condition particularly when combined with high pressure conditions in the sample con- tainer gives rise to the supersonic expansion. In this internal motions of the molecules undergoing expansion are converted into highly directional flow as a result of large numbers of collisions generally with a carrier gas such as He or Ar which form part of the expansion. Figures 2(a) and 2(b) show the velocity distributions that characterise the effusive and the supersonic nozzle. In many respects the supersonic nozzle produces an ideal velocity distribu- tion for molecular dynamics experiments since the distribution of velocities may be very narrow and the molecules have generally been cooled to their lowest quantum states. However the effusive expansion does give the experimenter the opportunity to vary both selected velocity and initial quantum state by means of additional selective devices. This is less straightforward in the case of the supersonic expansion although the mean velocity may be changed by changing the mass of the species undergoing expansion. Velocity selection of reactants beginning with distributions as described above has been a central theme in molecular beam studies. Detection of reaction product velocity distributions gener- ally utilises angularly variable time-of-flight detectors. These methods give product velocity distributions in the coordinate frame defined by the experiment. Transforming these distributions to give the equivalent function in the collision frame is less straightforward and laboratory frame-to-collision frame transformations often cause loss of specificity of the detail of an experiment. Mechanical methods of velocity selection do not represent the sole techniques available. In principle any physical phenomenon that is affected by molecular velocities represents potentially a method of velocity selection. The Doppler effect is one such and as is well known the absorption frequency apparently exhibited by an atom or molecule is a function of the speed of that molecule in the direction of the light beam. To the spectroscopist the Doppler effect represents an unwelcome intrusion into the serious business of greater and greater spectral resolution since the distribution of mol- ecular velocities causes line-broadening. True to the principle that each problem constitutes an opportunity in disguise a growing number of researchers have utilised the Doppler effect either to select or to detect molecular velocities in molecular dynamics experiments. 1.3 Molecular Velocity Selection via the Doppler Shift Spectral lines are not infinitely narrow in the gas phase and among the many broadening mechanisms are the lifetime of the excited species with consequent uncertainty in the absorption frequency and that due to random motion of the species. Transition frequen- cies of moving molecules are shifted from the unperturbed reso- nance frequency by an amount proportional to the speed of the molecule relative to that of the probing light beam. For light species at room temperature and for heavier molecules at elevated temper- atures molecular speeds of several thousand metres per second are not uncommon leading to linewidths broadened to several GHz. The basic relation between observed absorption frequency ad unperturbed resonant frequency a. and molecular velocity along the laser propagation direction vz is In equation 2 c is the speed of light. The Doppler shift is a form of inhomogeneous line broadening in contrast to that due to the uncertainty in lifetime and excitation into a specific region of the line profile will result in the selection of only a sub-set of the molecules that undergo the transition. In certain circumstances therefore a narrow line laser may be used to select a sub-group of molecules of definable velocity by tuning the laser into a specific region of the Doppler profile (this process is frequently referred to as &tuning since perfect tuning i.e. line centre excites only v = 0 components). Using commercially available cw dye lasers this selection process may be very precise indeed in the selection of v,. Conversely the shift of resonant absorption from line centre may be used to determine the distribution of molecular f -?\ Effusive nozzle 11 III Supersonic nozzle Figure 2 Pictorial view of the velocity distributions from (a) an effusive source and (b) a supersonic nozzle. Arrow length is a representation of speed and thickness a measure of probability. CHEMICAL SOCIETY REVIEWS 1996 differential cross-section (DCS) for the fine structure transition in excited sodium in collisions with Ar Further experimental develop- ments led to determination of velocity distribution of products in Dopplerprofile(a)/ detuning (i' ([\ positions etuning ax6 z Af velocity vectors have cylindrical symmetry about z I vz Figure 3 (a) A typical gas phase rovibronic Doppler profile with three detuning positions highlighted (b) The range of molecular velocities accessed by each of these laser-selected wavelengths It is apparent that large detunings select velocity distnbutions similar in form to those of the supersonic nozzle (c) Each detuning wavelength selects z components of velocity and hence there is cylindncal symmetry about the z axis vz values Figure 3 illustrates the process of molecular velocity selection by wavelength selection in different regions of the Doppler profile The suggestion that Doppler shift measurements could be used to define or to determine velocity distributions of molecules in scat- tering expenments was first made by Kinsey s He demonstrated that the velocity distribution of products from a scattering experiment could be determined from Doppler scan lineshape measurements made as a function of the direction of the incident light The Fourier transform of the Doppler profile obtained with incident light along 8 4,say is the same as the three-dimensional Fourier transform of thefull velocity distribution evaluated on a line in Fourier space par- allel to the 8 4 direction By scanning at a number of angles it is possible to reconstruct the entire Fourier map which may then be inverted to the velocity distribution In addition to providing a new method for tackling the complex problem of simultaneous quantum state and velocity distnbutions of products there are additional advantages inherent in the method Kinsey drew attention to a signal-to-noise gain equivalent to the Fellgett advantage that make Fourier transform methods the most favoured for IR and NMR instrumentation He calculated' a gain of up to 1 O4 for Fourier transform Doppler methods over conventional techniques A further advantage is the simplicity of transformation from laboratory to collision frame coordinates discussed in more detail below The Doppler shift method was demonstrated experimentally by Phillips Serri Ely Pritchard Way and Kinsey6 who determined the rotationally inelastic scattenng for Na,-rare gas collisions 1.4 Relative Velocity Determination Using Spectroscopy A particularly significant development in the context of this review was by Smith Scott and Pntchard8 who demonstrated that the Doppler selection method could be used to specify relative velocities using a straightforward transformation of laser frequency within the Doppler profile (the laser detuning) This it should be emphasised is not a special spectroscopic tnck but consists mainly in appropn-ate choice of molecule for excitation and collision partner An example illustrates this If our target molecule whose velocity has been selected by a Doppler shift technique collides with an infinitely massive partner (a surface say,) the relative velocity is then very well defined since it is identical with the molecular velocity As the collision partner becomes less massive the precision of the trans- formation from molecular to relative velocity is reduced owing to target motion Smith et a1 used sub-Doppler selection of relative velocity to obtain for the first time the dependence of state-to-state rotational transfer cross-section on initial relative velocity The key equation in this selectivity was derived by Smith et a1 which relates laser detuning via the z-component of selected mole- cular velocity to the distribution function of relative velocities This is written in equation 3 in the form of a probability distribution (or density) which gives the probability of finding a relative velocity of magnitude v whose angle of inclination to the z-axis (the direction of propagation of the light beam) is a,once the z-component of mol-ecular velocity v,,~has been specified This equation is In equation 3 s = mJm Distnbutions of the form given in equa-tion 3 are best illustrated pictorially Figure 4presents the distnbu- tions of relative velocity as a function of 'qser detuning for the collision pair Liz-Xe It can be seen that for frequencies close to the centre of the Doppler profile values of a range widely about the perpendicular Selectivity in magnitude from line centre excitation also lacks high precision Excitation in the wing of the Doppler profile leads to greatest selectivity of both angle and magnitude In the extreme wing rela- tive velocities become closely locked to the laboratory axis with the high value of molecular velocity translated directly into relative motion It is of course one of the ironies of experimental work that this ideal distribution should be that which is least accessible in practice in this case because signal strength is much reduced With the relationship of equation 3 we have the basis of a new use for spectroscopy namely as an alternative to the molecular beam apparatus It should also be borne in mind that any method based on high resolution spectroscopy has quantum state selectivity naturally built in This in principle may be very specific indeed A feature to note is that equation 3 relates relative velocity vectors to the labo- ratory frame directly This contrasts the situation in a crossed mol- ecular beam experiment in which the collision-laboratory frame transformation may be a difficult and error-prone process There is an additional advantage that follows from tying the velocity vector to the laser propagation direction which stems from the fact that internal molecular vectors may also be aligned in the excitation process either parallel or perpendicular to this same axis These might for example be the transition dipole moment or the rotational angular momentum vector The ability to specify the directions of these vectors relative to one another before a collision and to determine them after is of great significance for the field of collision and reaction dynamics as we discuss later in this review 2 Effect of Collisions If we can go from laser detuning via the molecular velocity distnb- utions to those of relative velocity then clearly we can reverse this process to obtain relative velocity distributions from spectroscopic PROBING THE INTERMOLECULAR POTENTIAL A J McCAFFERY 0. B 0.6 0. 4 0.2 0.0 5. o 0'0 00 \Q Qe' *ac cLo Figure 4 Three-dimensional plot of probability of selecting molecular velocity vm as a function of laser detuning I e of increasing v The example shown is for Li,-Xe at lo00 K in a thermal cell r-PI\ \I \ I \ b I I I I probe '\ 4 fluorescence I a-Figure 5 Two-laser double resonance configuration for probing the result of a collision taking molecules from state (b) to state (c) lineshapes This might be done as part of the analysis of a traditional molecular beam expenment in the mode of Kinsey or alternatively following a velocity selection process (see below) in which transla- tionally hot atoms produced by photolysis have kinetic energies suf- ficiently large that they dominate the relative velocity equation However were this to follow the kind of velocity determination by Doppler selection described above then we would have a wholly spectroscopic technique for determining state-to-state differential scattering cross-sections A double resonance spectroscopic configuration would be needed for this process of the kind illus- trated in Figure 5 but two-laser experiments are now relatively common and relatively cheap compared to the most sophisticated of molecular beam methods This then is the basis of the method descnbed in this review We have from the earlier section a relation between laser wavelength and relative velocity magnitude and direction We now need to con- sider the effect of collisions on velocity distributions and on the expenmental observable the double resonance lineshape This is discussed below again using a mix of pictorial representation and of mathematical description We begin with a qualitative considera- tion of the velocity changes induced by a non-reactive collision and their effect on the lineshape To simplify this discussion we assume that the initial distribution of relative velocities may be replaced by a single vector v,. repre-senting the most probable value selected by the laser From earlier discussions on velocity selection it will be apparent that detuning the laser into the wings of the Doppler profile increasingly narrows Figure 6 An illustration of the effect of angle a (that between z axis and velocity direction) on the velocity selected double resonance (VSDR) linewidth Initial vr is the same in each case as is the scattenng angle 0 The double resonance linewidth is given by the projection of the base of the scattering cone on the frequency axis As a increases for a given scat tenng angle so does the projection on the zaxis the distribution towards this ideal representation First we assume the system undergoes an elastic collision i e no change in internal quantum state of the molecule and hence no change in relative velocity (though of course atom and molecule velocities may change individually) Some interaction takes place and this is reflected in a change in direction of v or scattering through the scat- tering angle 8 This can be shown diagrammatically as in Figure 6 Note that the length of v is unchanged and that scattering through angle 8may take place over a range of angles (x)to the initial plane The effect of this on an experimental double resonance lineshape is shown qualitatively in Figure 6 Clearly for a given elastic (or inelastic) process the more closely parallel the initial vector is to the z-axis the narrower will be the final lineshape given by the pro- jection of the base of the scattering cone on the z-axis This is shown in Figure 6 When the process is inelastic changes in internal energy are at the expense of magnitude of relative velocity Thus for the case say of rotational excitation v would be shortened and deflected through angles 8 xas shown in Figure 7 Two instances are shown in this figure Initial I? is kept contant in each case but two different values of scattering angle 8 are shown together with a qualitative representation of the expected lineshapes Again the lineshape is a function of the size of the scattering cone base and its angle of inclination to the z-axis The experiment therefore reveals the origins of collision broadening in gas phase spectroscopy and fur- Figure 7 This shows the effect of different scattering angles (0) on the VSDR linewidth In the two cases shown initial relative velocity is iden- tical but different scattenng angles produce cones of different base diam eter and hence different linewidth thermore it is apparent that the width of the double resonance line contains valuable information on the distribution of the final v vectors which may be related to the scattering angle From this simple pictorial example it is apparent that the kind of information normally associated with molecular beam experiments i e the angular distribution of the relative velocity vectors may be obtained from a purely spectroscopic method The two examples given above are useful in that they form the basis of a simple test of the experiment and its ability to measure velocity magnitudes and angular distributions Thus we anticipate that for a given selected initial v the double resonance linewidth should increase markedly with rotational inelasticity (I e as 8 increases) and furthermore we would expect that for a given inelasticity (i e fixed 0) the linewidth should vary predictably with laser detuning Thus lines should be narrow when v; is selected in the Doppler wing and broad when tuned close to line centre The foregoing discussion took place in terms of an idealised dis- tribution of relative velocity vectors whereas in fact it is the molec-ular velocity distribution that gives us optical access to velocity selection through the Doppler effect This is treated fully in the theory but can be represented pictorially using Newton diagrams to represent molecular atomic and relative velocities in a molecular dynamics experiment Figure 8 shows such a diagram with initial and final velocity combinations representing an idealised scattering experiment in which initial atom and molecular velocities and directions are known The outcome is a scattering event of the kind described in detail above but now we show explicitly the fate of the atomic velocity vector In each case the relative velocity is shown as the resultant of the atomic and molecular velocities These may also be displayed on the same diagram as mass- weighted relative motions of atom and molecule in the centre-of- mass (com ) frame with the motion of c om shown separately The broken line in Figure 8 represents this c o m motion (which has no influence on the dynamics) Atomic molecular and relative --b Vr final Figure 8 Representation of the velocities of atom (v,) molecule (I~)and relative motion (vr) before (heavy arrows) and after (light arrows) a colli- sion (b) The cone of final atomic molecular and relative vectors result- ing from scattenng CHEMICAL SOCIETY REVIEWS 1996 Figure 9 Illustration of a simple geometric relationship between linewidth and most probable scattenng angle for the ideal case of a single vector velocity scattering cones are shown together with the projection of molecular and relative velocities on the quantisation axis Although this is a more complex picture than those used above to describe the principle of the double resonance method it illustrates all atomic and molecular motions and shows that the detection of the spread of moleculur velocities vzu the double resonance lineshape is also a measure of the spread of the final relative velocity vector In this section qualitative arguments have been presented and used to describe how relative velocity magnitudes and angles may be determined spectroscopically A simple relationship exists between the scattering angle and double resonance linewidth This is illus- trated in Figure 9 which shows a planar cut through the scattering cone to include v’ and the z-(detuning) axis The geometric relation- ships are evident from the figure as is the direct relationship between scattering angle (0) and linewidth (Av;)of the spectroscopic signal I) To derive an expression we begin with the relations sin 8 =x/v{ sina = AvYk cosa = v,Jv From these it is straightforward to obtain equation 4 This qualitative view of velocity selection and of the scattering process is valuable in visualising the experiment and in making a rapid analysis of the double resonance data A full treatment of this lineshape is more complex and has been addressed by more than one author One approach has been in the context of velocity chang- ing collisions (VCC) and dephasing collisions in Na(*P)-rare gas atom interactions lo These elastic interactions cause subtle changes to the lineshape in a double resonance experiment as Gallagher and colleagues have demonstrated” and contribute to the lineshape of the ‘parent’ transition Inelastic processes cause a more substantial redistribution of the initial velocity vectors and these authors have studied fine-structure changing collisions using the velocity selected double resonance (VSDR) method *I To analyse data they utilised a theoretical treatment which expresses scattering ampli- tudes in terms of the kernels and frequencies of the collision inte- grals through a series of velocity averaging steps A density matrix formalism was employed by Liu and Dickinson’*in a more general treatment that includes both atomic PROBING THE INTERMOLECULAR POTENTIAL A J McCAFFERY and molecular systems The density matrix permits rn-degeneracy to be included via its irreducible components and these authors have chosen to express the two-step double resonance lineshape in terms of the collision integral This may then be related to the generalised cross-section Liu and Dickinson give full expressions for the atomic and the molecular inelastic transition cases There is a pragmatic approach to the analysis of double resonance lineshapes l4 It is the DCS that we wish to extract from experi- ment which measures the redistribution of velocity vectors causing the lineshape to broaden on collision A simple approach would be to assume some functional (parameterised) form for the DCS and to vary the parameters until the shape of the double resonance line can be duplicated This is the basis of the method we have used and is described in more detail below For a full description of the process however it is necessary to give the theoretically derived expression for the double resonance lineshape This can be found in recent publicationsi3 and was derived using probability density expres- sions for the variables that are involved in the selection the colli- sion and the detection processes The linewidth expression is given as a probability density of the final velocity component distribution P(~~,Jvrn) P(v ,lV ) = SSSSSP(v Iv 1p<qIvr) X P(v,,cyIvm )P(X)sincuszn~~dXdedcvdv (5)di There are kinematic constraints on this equation represented by 6 functions in the full expression These are not given here and the reader is referred to the above mentioned publications for complete expressions The quantities on the right of equation 5 represent the following probabilities the distribution of z-axis velocity projec- tions given a specified laser detuning (vz),the distribution of in- plane scattering angles I e the DCS (note this is velocity dependent) the distribution of relative velocity magnitudes and their angle of inclination to the z-axis for a specified z-component of initial relative velocity and finally the distribution of out-of- plane scattering x Note that this is generally assumed to be iso- tropic Thus in equation 5 the DCS is the only unknown function and by using an assumed form for this on a trial and error basis a best fit to the lineshape may be used to refine the parameters of the DCS function This is usually done with a series of nested integrations until reproduction of the experimental lineshape is satisfactory l4 Note that the linewidth in the absence of collisions will be deter- mined by a number of factors The natural lifetimes of all states involved I e levels (I b c‘ and d of Figure 5 will determine the unperturbed double resonance linewidths and additional broad- ening factors such as power broadening etc must be eliminated In this we assume such effects to be small compared to inelastic colli- sion velocity redistribution effects 3 Experimental Realisation In science it is rare for a field of research to have no prehistory and VSDR is not thus marked Some twenty years ago Berman15 out- lined the theoretical basis of the measurement of the DCS in atom-atom collisions using double resonance methods of the kind discussed here He also pointed out some of the advantages of spec- troscopy over scattering methods in particular the direct linking of laboratory and collision frames referred to earlier The theoretical development by Berman was somewhat different from that outlined above Berman derived expression? that relate line broadening to the collision kernels quantities utilised for many years in theories of line broadening and representing the probability of a (state-to- state) collision induced velocity change Generally this velocity is that of the probed species In this review we have focused on the reorientation of the relative velocity vector as a result of the collisions with explicit expressions given for the angular distnbution This leads to a ready visualisation of the result of a collisional interaction although the net result is the same as the theoretical treatment first outlined by Berman The first systematic measurements of molecular lineshapes following colli- sion-induced state change in a sub-Doppler double resonance expenment were by Gottscho Field Bacis and Silvers I6 BaO was the molecule chosen and this represented the first wholly spectro- scopic state-to-state velocity selection and detection expenment Elastic and rotationally inelastic processes were studied with some variation of initially selected velocity The unfavourable mass ratio in the systems studied (BaO Ar CO,) prevented these authors from making the molecule-relative velocity transformation that is the feature of the treatment described in this review By assuming negligible velocity change consequent on small A] changes Gottscho et a1 were able to obtain scattering angle data The most striking feature of their results was the dramatic line narrowing that occurred on changing collision partner from Ar to CO This was explained by the authors in terms of the long-range forces that are anticipated to be important in the case of BaO-CO collisions The development outlined in this review describes a process by which the relative velocity of the collision is selected prior to colli- sion and then detected after the interaction The first experimental lineshapes in a VSDR experiment were reported by Reid McCaffery and Whitaker,17 in which the physical basis of the experiment was described Further refinement of the experiment was necessary before fully reliable lineshapes and DCS functions were obtained and a description of experiment and results are given in recent publications I4 Figure 10 shows the experimental arrangement used by Collins et al l4 Two single frequency cw dye lasers form the principal optical elements but this level of sophistication is not essential The requirement is that the laser linewidth be narrow compared to the Doppler width of the probe species and for very light molecules numerous laser systems fulfil this criterion One laser (the pump laser) excites a narrow velocity distribution of molecules to a spe- cific electronic vibrational and rotational (E v,J)state This creates a population ‘spike’ in a specific level of the upper state as shown diagrammatically in Figure 11 The narrowness of the spike reflects the velocity selection process as discussed in detail above It is also affected by collisional interactions some of which are energetic enough to change the quantum state of the molecule and others which are less violent In this second category are elastic collisions M 3 Lens 1 coded PMT 699 29 Figure 10 Experimental set up for the VSDR expenment CHEMICAL SOCIETY REVIEWS 1996 I += I I b C I I Figure 11 Schematic diagram to illustrate the production of velocity- selected excited states from the Doppler profile and the broadening that would occur as a result of quantum state change in the excited state. which cause velocity redistribution and hence line broadening and phase-changing collisions that interrupt the wavetrain of the radi- ating molecule also broadening the observed linewidth. The last two categories of collision are found experimentally to have only a very small effect compared to those which cause a quantum state change. This is shown schematically in Figure 11. The major line broadening effects are the result of the inelastic colli- sion causing the quantum state change and may be analysed in terms of the theory discussed above. Double resonance experiments are capable of giving spurious results and it is important to use experimental methods that dis- criminate against unwanted signals and also to know well the spec- troscopy of the system under investigation. Two-photon processes with pump and with probe lasers must be eliminated for example and checks made for (linear) dependence of signal on both lasers. In the experimental set up in the author's laboratory,14 pump and probe lasers are modulated at different frequencies and signal detected at the sum. This intermodulation method eliminates many spurious signals. Sharp cut-off filters eliminate spontaneous emis- sion from the first excited state and background from the heated cell. The experimental method may be varied quite widely according to the demands of the investigation and equipment availability. For example the pump laser might be an IR source and ground state pro- cesses investigated. The method would then have similarities to a technique used in photofragmentation experiments known as over- tone mediated photodissociation.I9 This would eliminate problems arising from spontaneous emission from the initial level. For enhanced sensitivity the probe laser(s) might be chosen to excite directly or indirectly the ionisation continuum. Again this would help overcome some difficulties described above which occur with optical detection of two signals from lasers of similar frequency. Pulsed or cw lasers may be used in principle. This is probably a good point at which to emphasise the advan- tage of optical methods of velocity selection and detection some of which were described by BermanIS many years ago and are briefly referred to above. Most obvious is the high degree of quantum state selectivity that results from using narrow line lasers. Individual electronic vibrational and rotational states may be accessed rela- tively easily and a process investigated systematically as a function of each of these. The selection and detection process may be of fine structure (spin-orbit) and hyperfine (nuclear spin) states. Detuning of the laser from line centre gives molecular velocities directly and these may be transformed into relative velocities with the laser propagation direction as the axis of quantisation. It is this last fact that gives the technique particular strength since the transformation from laboratory to collision frame coordinates is a source of consid- erable uncertainty in molecular beam experiments. In addition to these features the use of polarised light in selection and detection adds a major new dimension which results from the directionality of the electric vector of the radiation field. The nature Ili excited state L=m=i &m=-j I I JI'r I f j -vector directions I II (+) c.p. lightI I I I ii ground state $ Figure 12 Effect of circularly polarised excitation on ground and excited state rn-populations. The resulting distribution of ]-vectors for each state is shown on the right. and directional properties in the molecular frame of particular transitions are well known from spectroscopy. For instance elec- tronic transition moments in diatomic molecules may be polarised along or perpendicular to the bond and it is straightforward to deter- mine which is the case for a particular transition. Optical excitation in an isotropic medium is a process ofphotoselection and only those molecules whose transition moments have projection along the direction of the electric vector of the light beam will be excited. Again this is with reference to the direction defined by the laser beam. More precise identification of molecular direction comes from the use of linearly or circularly polarised light when rotational resolution is achieved. These two forms of polarisation achieve molecular directionality through the different distributions of mag- netic sub-strates that they excite.*O Here the m states represent pro- jections of the rotational angular momentum vector on the propagation direction. For a given value of j the rotational angular momentum (2j + I) m states are possible ranging from j to -j. Excitation using circularly polarised light generally biases the m state distribution towards j or -j in a dipolar array of populations whilst use of linearly polarised light gives a quadrupolar distribu- tion that peaks either around m = 0 or simultaneously at j and -j This is shown pictorially in Figure 12 which also displays how knowledge of m state distribution gives molecular directionality. Much of this is well known and has been widely used in molec- ular dynamics experiments for a number of years. What is novel in the application described here lies in the fact that the radiation used for state selection and molecular directionality via say the m dependence is that used also for relative velocity selection. Thus the possibility exists of building in directly a correlation between the vectors representing the relative velocity direction and those describing the molecule's orientation with both being referenced to the propagation direction of the lasers. Some years ago Herschbach and coworkers*' realised the importance of detailed correlated knowledge of directionality of velocity angular momentum and transition moment vectors in collisional processes. They introduced the term vector correlation to molecular dynamics together with an extensive theoretical formalism much of which has its origin in nuclear physics. It will be evident from the foregoing that as more vectors are cor- related in an experiment the fewer are the averaging assumptions that must be made leading to greater detail in the interpretation of results. A typical two-vector correlation experiments is an older- style molecular beam measurement with well defined initial relative PROBING THE INTERMOLECULAR POTENTIAL A J McCAFFERY velocity before collision and distribution of velocities detected after When laser detection of product angular momentum direction is added a three-vector correlation may be obtained Most infor- mative is the four-vector correlation experiment and the VSDR expenment descnbed here when used with full polarisation of light in both selection and detection is the basis of such an experiment This was first demonstrated by Collins et a1 ** who described the first four-vector correlation molecular dynamics experiment 3.1Some Results The VSDR method of obtaining the DCS is very new and only one molecule atom collision system has been investigated using this technique to date However it is worth emphasising that the measurement of the state-to-state DCS is sufficiently difficult that these quantities have been evaluated (by combined laser and mole- cular beam methods) on very few occasions indeed Afull descrip- tion of the results obtained in this work is given in the original publications but here a bnef summary is given to demonstrate that the theory outlined in earlier sections does indeed appear to work and to give results that do reliably represent the state-to-state DCS and its velocity dependence Figure 13 shows some of the VSDR lineshapes obtained in the experiment of Collins et a1 l4 displayed in a way that shows the dependence upon AJ (upper plots) and on selected velocity (lower plots) Note that these are normalised The signal intensity drops considerably as the pump laser begins to probe the outer regions of the Doppler profile and as inelasticity increases The variation of VSDR linewidth as AJ increases is particularly marked Initial rela- 09 08 07 06 05 04 03 02 0 Intens1ty 1 A09 08 07 06 05 04 03 0 tive velocity is approximately constant for each experiment and the linewidth can be seen to increase from around 50 ms I to close to ten times this value for AJ = 10 (Table 1) The signal-to-noise ratio detenorates somewhat as inelasticity increases nevertheless the trend is apparent and rotational inelasticities of up to 20 angular momentum units in Li were observed The examples shown are for AJ increasing Similar though sig- nificantly not identical results are found for AJ decreasing It is worth reflecting again on the process involved that yields these line- shapes Note that intially a narrow distribution of velocities is selected and is maintained through elastic collisions This is appar- ent from the AJ = 0 lineshape As collisions occur the experiment monitors in turn those in which relative velocity has been converted into rotational angular momentum The velocity vectors will shorten therefore and be distnbuted about the most probable scat- tering angle in a manner that reflects the shape of the intermolecular potential and the distribution of available velocities The projection of this distribution of final velocities on the z-axis (the wavelength axis) is what IS measured in the experiment and displayed in Figure 13 In this context therefore the steady increase of linewidth with inelasticity is what would be expected on simple physical grounds A pictorial representation of this process is given in Figure 7 A second significant trend can be seen in the lineshapes dis- played in Figure 13 which plots VSDR shapes for the AJ = 4 process as the relative velocity of collision increases A marked narrowing of the lineshape is seen as the pump laser moves into the wings of the Doppler profile to select higher-velocity encoun ters A simple physical explanation of this process is illustrated in Figure 6 As the selected velocities become more parallel to the 000 (b) Figure 13 Some VSDR lineshapes for the case of Li,(A'Cu)-Xe (a) The dependence of lineshape on rotational inelasticity for the same detuning The steadily increasing linewidth is a manifestation of increasing scattenng angle shown schematically in Figure 7 (b) The effect of velocity selection on a given inelas tic process The linewidths narrow appreciably as detuning increases owing to the change in the projection of the scattering cone base on the frequency axis as indicated in Figure 6 CHEMICAL SOCIETY REVIEWS 1996 Table 1 Most probable scattering angles for the Aj = 10 transition calculated from full fitting analysis (La) and the single vector model (s.v.). The double resonance linework is taken as the fwhm value. 77Li,-Xe Aj= 10 T4vz Linewidth Linewidth v, %Po m s-I m s-l m s-' m s-' (+ 1") Aj = 10 Aj = 0 S.V. La. 3 0 756 67 780 32 -184 732 66 800 30 -331 726 65 850 28 30 -534 676 63 960 25 26 -699 645 61 1070 25 17 1 f -902 58 1 57 1230 20 14 -1067 506 54 1370 16 12 -1251 449 53 1520 14 10 1-Figure 14 Double resonance configurations suitable for VSDR. -1399 455 52 1650 14 -1601 398 50 1840 11 laser axis they project a narrower distribution on that axis. For the same inelasticity and hence the same cone of scattering the z-axis projection will be smaller and the VSDR linewidth narrower. The observed linewidth trend follows this intuitive argument in respect of its main features and this provides useful evidence that the experiment is indeed sensitive to the quantities that it purports to measure. There is more detail to be extracted from these lineshapes concerning change in collision trajectories as velocity increases which not relevant to this survey but is discussed in the original paper. An analysis of the data shown in Figure 13 and of other results is given by Collins et al.I4in terms of an impulsive collision at the hard wall of the intermolecular potential with torque generated through the anisotropic component of the repulsive potential. A hard ellipse provides a useful basic model for this and the trends described in the previous paragraphs are well accounted for using these simple concepts. 3.2 Further Developments The determination of scattering quantities particularly the state-to- state DCS is very new and the full range of contexts in which it may be adopted is as yet unexplored. Alternatives to the method described above do exist which will be described briefly and consist mainly of variations on the double resonance theme. The VSDR experiment is basically a four-level double resonance three of which are coupled by radiation and the fourth by collision. Several configurations are possible two of which are shown in Figure 13. The first of these will be recognisable as the basis of the VSDR experiment described above. It highlights a potential dis-advantage of the method in that the probe laser explores the second (or higher) electronic excited state of the system. The experimenter may have to identify and characterise this higher lying level since knowledge of the high-lying regions of many molecules is often sparse. The main problems would emerge as the results of line-broadening perturbations predissociation for example though it should be emphasised that knowledge of the identity of the final state is not essential. A more useful configuration may turn out to be that of Figure 14(h) or (c). In these only the first excited electronic state is involved and molecular constants and occurrence of perturbations are more often known for this level. Special experimental tech- niques are needed for these experiments since of course level 3 [and level 2 in case (c)] will generally be populated in Boltzmann fashion and it is the population due exclusively to collisional trans- fer from level 1 that we wish to identify. One advantage of the VSDR configuration (a) is that population of level 3 (under single collision conditions) may only be via level 2. The use of a high power modulated IR pump laser would provide the basis of a configuration (c) experiment. A sensitive method based on configuration(b)is one in which the level 1 molecules are labelled using polarised light and the probe searches for this polarisation label in the molecules of level 3. This particular experiment known as laser polarisation spec- troscopy (LPS) is well known as the basis of a highly selective spectroscopic technique.23 Its physical basis is as follows. The pump laser polarises a specific rotational level of the molecule by excitation with circularly polarised light. This selectively depletes m-states of the lower level by transferring them to the excited state.20 The probe laser is linearly polarised using the first of two polarising prisms and a second polariser crossed with the first is placed ufter the sample but before the detector. For most wave- lengths therefore no light passes to the detector since the polarisers are crossed. If the probe laser comes into coincidence with a transi- tion that originates on the pumped ground state however this situa- tion changes. Transitions sharing a common ground state with the pumped level appear to be optically active and such transitions become circularly dichroic and birefringent. Under the right experi- mental circumstance the former of these dominates and a Lorentzian double resonance lineshape results. A detailed descrip- tion of LPS may be found in the comprehensive text on laser spec- troscopy by Demtroder.24 The major use of the LPS method has been a means of simplify- ing complex molecular spectra for the purpose of identification since only transitions originating on the pumped level are seen strongly. However as noted in the original work by Schawlow and known from specific studies of m-transfer rates,25 the polarisation label survives elastic and inelastic collisions and thus LPS is poten- tially a method via which collisional population may be detected. Apart from the instances cited below the method has not been used to study collisions systematically. This is surprising since the tech- nique is potentially very sensitive by virtue of the null background away from resonance. Furthermore the existence of a collisional LPS signal contains important dynamical information. It tells us that some fraction of the original m-state distribution must have been retained. In fact the original Schawlow study demonstrated that the polarisation label is preserved through both vibrationally and rotationally inelastic collisions under conditions representing many collisional interactions. A great advantage of the LPS method is the ability to study ground states of molecules. In this way it should be possible to compare the double resonance method with more conventional molecular beam techniques. Only two studies so far have been made. Note that in order to utilise the methods described above it is essential to excite with narrow line laser radiation preferably to utilise a heavy collider in order to determine relative velocities and most important of all to resolve pure dichroism signals (Lorentzian lineshapes) in order to identify the broadening arising from veloc- ity changes. The study of collisional LPS lineshapes in NaK-K collisions by Kasahara et a1.26represents a very significant develop- ment since they demonstrated that high quality lineshapes could be PROBING THE INTERMOLECULAR POTENTIAL A J McCAFFERY obtained for highly inelastic processes They also showed that colli- sions energetic enough completely to randomise velocity distribu- tions still were accompanied by a high degree of polmsation More recently Wilson and M~Caffery,~ have obtained velocity-selected polarisation double-resonance (VSPDR) lineshapes for Li and Na in collision with Xe from which most probable scattenng angles may be extracted Li proved to be unusual in that the smallest inelasticity was accompanied by a very wide velocity distribution quite unlike the behaviour of this molecule in the excited state A possible explanation for this behaviour is the presence of reactive atom-molecule collisions Technological advances can be expected to play a role in the development of spectroscopic methods of determining velocity distributions Lasers with wide tuning range based on parametric amplification in solids will play a significant role in investigating molecules that cannot be accessed using cw tunable devices based on organic dye solutions High power pulsed lasers bnng new problems to a spectroscopic investigation e g low duty cycle nonlinear effects and others However they offer great advantages in terms of wavelength range and of course the possibility of time resolution This latter property would be valu- able in extending the LPS technique Using cw lasers the effec- tive sampling time is that which the molecule spends in the probing beam 3.3 Other Spectroscopic Strategies A very significant development in recent years has been the use of lineshape analysis to probe the products of photodissociation expenments using polarised laser-induced fluorescence (LIF) Dixon2x has shown that careful lineshape analysis of individual rotational transitions allows the experimenter to correlate product state velocities and rotational angular momentum and to link these to laboratory coordinates via the transition dipole moment This has now become an essential component of modern fragmentation experiments and yields insights into the mode through which dis- sociation occurs A recent and novel development utilises ‘hot’ atoms produced with very high velocities by photodissociation Such suprathermal atoms are in effect velocity polarised and in subsequent bimolecu- lar reactions29 can have sharply defined relative velocities Knowledge of the photodissociation asymmetry (p parameter),* allows us to link the distribution of initial relative velocities to the laboratory frame and Doppler-resolved polarised LIF investigation of the products of the reaction yields data on translational and rota- tional anisotropies relative to the laboratory axis In the best of cir-cumstances therefore it is possible to correlate initial and final relative velocities and product rotational alignment with both initial and final relative velocities There are strong similarities here to the VSDR experiment and hence sub-Doppler probing of the products would be expected to yield the DCS as well as other vector correlations Recently this has been achieved and reactive scattering expenments have been carried out in thermal cells using spectroscopic methods and have yielded data of quality normally associated with the best of molec- ular beam experiments An impressive range of systems has already been investigated using these basic principles emphasising perhaps one of the advantages of spectroscopic methods cited earlier in this review namely the ease of switching the molecular species involved in the study Wolfrum30 reported rotational alignment of OH produced by the reaction of velocity polansed H atoms with 0 and Bersohn and coworkers3’ utilised suprathermal H atoms in collision with poly- atomic deuterides followed by Doppler analysis of displaced D atoms to investigate the mechanism of substitution in tetrahedral analogues of methane Hancock and coworkers32 measured vector correlations in product species CO following the reaction of photo- lytically produced O(3P)with CS Simons and coworkers29 have studied the reaction of hot O(l0) atoms with N,O to produce NO interpreting the results as evidence of direct stripping dynamics and collinear collision geometry The method clearly has considerable potential in a diverse range of systems as Zare and colleagues dem~nstrate,~~in the measurement of state-to-state DCS for the reaction H + D -HD + H using photolytically produced hot H atoms as the reaction initiator 4 Summary The use of purely spectroscopic methods in the study of collision- induced change has taken a major step forward very recently Of particularly significance is the selection and/or detection of molec- ular and relative velocities by constructive use of the Doppler effect In this way all components of the energy exchange kinetic and potential may be determined expenmentally In this review methods by which this may be accomplished are described and some results of recent experiments shown Some of the advantages of a purely spectroscopic approach are discussed and contrasted with the molecular beam approach New and very detailed informa- tion on collisions and reactions could result from a more accessible technique and it is conceivable that widespread use of spectroscopic methods will reveal the underlying principles governing the basic processes of chemistry Acknowledgements Numerous coworkers have made major contnbutions to the work described in this review over a period of several years and the author wishes to thank them Particular thanks are due to Dr K Reid for her initial work on this project References 1 A J McCaffery Z TAlWahabi M A Osbome and C J Williams J Chem Phys 1993,98,4586 2 TOka Adv At Mol Phys 1973 9 127 3 C Ottinger and R N Zare J Chem Phys 1969,51,5222 4 D J Nesbitt Furuduy Disc Chem Soc 1994,97 1 5 J L Kinsey J Chem Phys 1977,66,2560 6 W D Phillips J A Sem,D J Ely,D E Pntchard,K R Way andJ L Kinsey Phys Rev Lett 1978,41,937 7 J A Sern J L Kinsey andD E Pritchard J Chem Phys ,1981,75,663 8 N Smith T P Scott and D E Pritchard J Chem Phys 1984,73,599 9 A J McCaffery J P Richardson,R J WilsonandM J Wynn,J Phys B At Mol Opt Phys 1993,26,705 10 M J O’Callaghan and J Cooper Phys Rev A 1988,39,6026 11 M J O’Callaghan and A Gallagher Phys Rev A 1989,30,6193 K E Gibble and A Gallagher Phys Rev A 1991,43 1366 12 W K Liu and A Dickinson J Phys B 1991,24 1259 13 K L Reid and A J McCaffery J Chem Phys 1992,96,5789 14 TL D Collins A J McCaffery J P Richardson R J Wilson and M J Wynn J Chem Phys 1995,102,4419 15 P R Berman Adv At Mol Phys 1977,13,57 16 R A Gottscho R Field R J Bacis and S J Silvers J Chem Phys 1980,73,599 17 K L Reid A J McCaffery and B J Whitaker Phys Rev Lett 1988 61,2085 18 T L D Collins A J McCaffery J P RichardsonandM J Wynn Phys Rev Lett 1991,66 137 19 F F Crim Annu Rev Phys Chem ,1993,44,397 20 See for example A J McCaffery in Gas Kinetics and Energy Transfer (Specialist Penodical Report) vol 4 senior reporters P G Ashmore and R J Donovan The Royal Society of Chemistry London 1981 p 47 for a pictonal descnption Other sources include R N Zare Angular Momentum Wiley New York 1988 A J Bain and A J McCaffery J Chem Phys 1985,83,2632 21 D A Case,G M McClelland and D R Herschbach,Mol Phys 1978 35,541 22 T L D Collins A J McCaffery and M J Wynn Phys Rev Lett ,1991 66 137 23 R Teets R Feinberg T W Hansch and A W Schawlow Phys Rev Lett 1976,37,683 24 W Demtroder Laser Spectroscopy 4 Spnnger Verlag Berlin 1982 ch 10 25 A J McCaffery M J Proctor and B J Whitaker Annu Rev Phys Chem ,1986,37,223 26 S Kasahara and H Kato Act Phys Hung 1994,74,329 27 R J Wilson and A J McCaffery to be published 28 R N Dixon J Chem Phys 1986,85 1866 29 M Brouard S P Duxon P A Enriquez R Sayos and J P Simons J Phys Chem 1991,95,8169 M Brouard S P Duxon P A Enriquez and J P Simons J Chem Phys 1992,97,7414 CHEMICAL SOCIETY REVIEWS 1996 30 J. Wolfrum in Selectivity in Chemical Reactions ed. J. C. Whitehead 32 F. Green G. Hancock and A. J. Orr-Ewing Faraday Disc. Chem. SOC. Kluwer Dordrecht 1988. 1991 91 79; M. Costen G. Hancock A. J. Orr-Ewing and D. 31 G. W. Johnson S. Satyapal R. Bersohn and B. Katz J. Chem. Phys. Summerfield J.Chem. Phys. 1994,100,2754. 1990 92 206; A. Chattopadhyay S. Tasaki R. Bersohn and M. 33 A. Orr-Ewing and R. N. Zare Annu. Rev. Phys. Chem. 1994,45,315; Kawasaki,J. Chem. Phys. 1991,95 1033; B. Katz J. Park S. Satyapal H. Xu N. E. Shafer-Ray F. Merkt D. J. Hughes M. Springer R. N. S. Tasaki A. Chattopadhyay W. Yi and R. Bersohn Faraday Disc. Tuckett and R. N. Zare to be published. Chem. Soc. 1991,91,73.

ISSN:0306-0012    

DOI:10.1039/CS9962500049

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Surface Chemistry of Titania (Anatase) and Titania-supported Catalysts Konstantin 1. Hadjiivanov and Dimitar G. Klissurski Institute of General and Inorganic Chemistry Bulgarian Academy of Sciences 1113 Sofia Bulgaria 1 Introduction Titania is the principal white pigment in the world and has wide applications in various branches of industry such as the production of plastics enamels artificial fibres electronic materials and rubber.' In the field of catalysis the interest in TiO is mainly due to its application as catalyst or support. In many catalytic reactions titania demonstrates neither a high activity nor a high selectivity,2 but in some cases it is among the most efficient of catalysts. For instance TiO is a classical photocatalyst for water decomposition and for other photocatalytic It shows very good prop- erties in the Claus process (SO + 2H,S +3s + 2H,0).6 Sulfate- modified titania is one of the few superacids that are at present the only known heterogeneous catalysts for Friedel-Crafts acylations and it is active in a number of other organic synthetic reaction^.^ However titania is mainly used as a support. This is of course due to some extent to its good mechanical properties its inertness and its low price. In contrast to silica which is one of the most widely used supports and whose normal role is to ensure that the supported phase has a large surface area titania interacts in many cases with the supported active phase. This behaviour determines the unique catalytic properties of the latter. Thus vanadia-titania catalysts are the best ones for a number of selective oxidation reactions. In par- ticular they have found industrial application in the oxidation of 0-xyleneX and in the DeNO process6 (NO + NH * N + H,O). Other titania-supported oxides such as Fe,O MOO WO MnO and CuO also exhibit a high activity and selectivity in the latter reac- tion. Noble metals are of special interest. Very often their activity with respect to CO hydrogenation strongly increases when they are sup- ported on titania.9 This is associated with the so-called strong metal-support interaction (SMSI) which will be considered below. Despite the fact that such catalysts have not yet found industrial applications they have been the subject of numerous investigations and the effective use of titania as a promoter is a result of these studies.'() TiO is also used as an additive for improving the prop- erties of some supports such as alumina. The use of titania in the field of catalysts is schematically presented in Figure 1. The properties of the titania surface are decisive for its catalytic application. On the one hand the catalytic activity and selectivity Konstantin Hadjiivanov was born in Sofia in 1958.He graduated in 1981 from the University of Sofia and started work as a Researcher at the Institute of General and Inorganic Chemistry of the Bulgarian Academy of Sciences where he received his PhD degree in chem- istry in 1990. During his scien- tific career he has served as a Visiting Scientist at the labora- tories of Professors A. Davydov (Novosibirsk Russia) G. Busca (Genova Italy) and H Knozinger (Munich Germany). He also joined the laboratories of Professors M. Che (Paris France) and J.-C. Lavalley (Caen France) as a Postdoctoral Fellow. He is cur- rently continuing his research on IR spectroscopic character- ization of catalyst surfaces. 61 of TiO itself depend directly on the kind and concentration of the different active sites and on the other the surface affects the forma- tion of definite structures of the active phase of titania-supported catalysts. Other important characteristics of TiO such as its adsorption ability and its pigmentation also depend on the surface properties. The breaking of the crystal lattice at the surface leads to the appear- ance of ions with a lower coordination number than that in the bulk. They are named coordinatively unsaturated surface (c.u.~) ions and have a tendency for additional coordination. The C.U.S. cations (in this case Tin+) possess an uncompensated positive charge and coor- dinate molecules with a free electron pair i.e. they are Lewis acids while the C.U.S. oxygen anions are Lewis bases and adsorb acidic molecules. Even after evacuation at high temperatures at which most admixtures are desorbed residual hydroxy groups exist on the oxide surfaces. They may have an acidic or basic character. The surface properties also depend on the deviations from stoichiometry e.g. the existence of various forms of excess oxygen possessing different oxidation activities and vice i'ersa formation of a reduced oxide which is responsible for the reduction properties of the samples. Various admixtures may also affect the adsorption sites. The adsorption sites are usually not directly observable (e.g the C.U.S.ions) or the data on them are scarce (surface OH groups). For that reason their interaction with so-called probe molecules is investigated. The properties of the adsorption sites are determined by the characteristics (stability and spectral features) of adsorbed probe molecules. Titania is usually encountered as three different modifications anatase rutile and brookite.' The latter modification has no practi- cal importance owing to its low stability. Anatase is thermodynam- ically stable up to 800 "C and an anatase-rutile transition occurs above this temperature. The rutile obtained however does not change into anatase after cooling owing to the high activation energy of the back transition. Both anatase and rutile crystallize in a tetragonal lattice. The coordination number of titanium is 6 and that of oxygen 3. The two modifications differ in number of common edges of the TiO octahedra 4for anatase and 2 for rutile. It is anatase that is more widely used for catalytic purposes. That is why we shall concentrate on this crystallographic modification. Prof. Dimitar Klissurski obtained his PhD (1968) and DSci (DHabil) (1981) degrees from the Bulgarian Academy of Sciences. He is author and coauthor of over 200 research papers 15 reviews,5 books and 45 patents. His research interests are mainly in the areas of cutalysis and material science. He is currently a member of the Internutionul Council oj the Congress on Catalysis und the Council of the international Mechanoc hemicul Association (MA) and President of the Bulgarian Mec hano- chemical Society. During the period 197G1992 he wm a member of the Editorial Board of the journal Matenals Chemistry and Physics and is currently on the Editorial Bourd of International Journal of Mechanochemistry. e g in alumina support \ nfl e g for CO hydrogenation /I Pr TI0 e g ft,O decomposition e g noble metals on TiO HeterogeneousCatalysis e g Claus reaction Figure 1 Application of TiO in the field of catalysis. 2 Preparation of Titania Different methods for synthesis of titanium dioxide result in prod-ucts with different structures (anatase or rutile) crystallinity and contaminants. As a consequence the surface properties of TiO strongly depend on the preparation technique. There are two main methods for obtaining titania for industrial purposes.'.' The first is the so-called 'sulfate method'. The titanium source (usually ilmenite FeTiO,) is dissolved in sulfuric acid the resulting solution is purified to remove iron and then hydrolysed. The precipitate titania in hydrous form is calcined in order to eliminate water The product obtained has the structure of anatase since the sulfate ions stabilize this modification. TiO prepared via the 'sulfate route' always contains sulfate ions which affect its surface acidity (see Section 4. I .). The other widespread technique for manufacturing titania is the vapour-phase oxidation of TiCl, TiCl + 0 -TiO + 2C1 (1) The products are characterized by a narrow particle size distrib- ution the average diameter being 20 nm. The main contaminant is C1 and sometimes Si and Al. The surface of this type of titania is deeply dehydroxylated. However the phase purity is not high. For instance the typical commercial fumed TiO (anatase) made by Degussa contains 25% of rutile. On a laboratory scale titania is usually prepared from TiCI or titanium alcoh01ates.I~ TiCl is hydrolysed by water at 5-10 "C and after that ammonia is added to obtain hydrous titania. The product formed is pure anatase whereas rutile is produced when the reaction is held above 80 "C. The degree of surface hydroxylation may be controlled by the temperature of the subsequent calcination. The samples prepared in this way usually contain chloride ions but their concentration may be minimized by washing of the final (cal- cined) product. To obtain chlorine-free titania one usually hydroly- ses titanium alcoholates [the preferred compounds are Ti(OC,H,) and Ti(OC,H9) since the lighter alcoholates react very violently]. The products are of high purity the carbon contaminants being eliminated by calcination of the products. Titanium dioxide is also produced by oxidation of metallic tita- nium by oxygen or water as well as by hydrolysis of titanium com- pound in the gas phase. As already noted heating of anatase at 800 "C results in the formation of rutile. Some contaminants and additives decrease the transition temperature which is often a reason for deactivation of anatase-supported catalysts. 3 Surface Chemistry of Anatase The name of anatase originates from the Greek avay7au~(upward tension upwards direction) and is due to the long form of its crys- tals. The cleavage planes mainly exposed on the anatase surface are 100 and 011101 (= means isostructural). However the faces 100=010 110 111 112 and 113 are also observed although to a smaller extent. I2 The same faces are characteristic of the crystallites CHEMICAL SOCIETY REVIEWS 1996 of disperse samples but the high concentration of crystal edges steps and comers should also be taken into account. The best way of studying the titania surface is to investigate the properties of the separate faces using single crystals and look for an anology with disperse samples. Since appropriate anatase single crystals are not available,' information on the surface chemistry of anatase has to be based on studies of disperse samples. Most results have been obtained by IR spectroscopy. Along with the properties reported in the literature which are common many differences have also been observed. The latter are due to (i) sample morphology (mainly quantitative differences) and (ii)the strong influence of impurities (which results in a qualitative change in the properties). 3.1 Surface Hydroxy Groups Hydrated titania obtained by hydrolysis of titanium salts has the crystal structure of anatase or rutile.14 It is assumed that its surface is completely hydroxylated. Drying even at room temperature leads to irreversible dehydroxylation. The degree of dehydroxylation is a function of the heating temperature and titania slowly loses its water when calcined up to 600 "C. Samples prepared by 'dry' methods (eg.fumed anatase) are characterized by a strongly dehydroxylated surface. However even after evacuation at 700 "C the so-called residual hydroxy groups are present on the anatase surface.''-20 They are observed in the 3800-3600 cm-I region of IR spectra. Their surface localization has been proved by isotopic exchange with heavy water' and by interaction with coadsorbed mole- cule~.~~,~~-~~)At least 12 kinds of OH groups have been reported by different authors. It has been established that some of them are due to impurities e.8. the band at 3740 cm-I appears as a result of the presence of ~i1icon.I~~~~) Most authors report the existence of two kinds of residual hydroxy groups on anatase which produce IR absorption bands at 3715 and 3675 cm-1.14-16 These two bands are readily observable after evacuation at about 300-400 "C. An increase in the evacuation temperature (combined with the use of modern evacuation techniques) results in a complex spectruml8.l9 containing a set of low-intensity bands in the 380&3600 cm-I region. The problem of the precise interpretation of the different hydroxy groups of anatase has not been solved. The more complex spectrum of highly dehydroxylated samples is probably due to different spectral behaviours of hydroxy groups which are identical from a crystallographic viewpoint but different with respect to their more distant surroundings (e.g. neighbouring sites which may or may not be occupied by OH groups). Water adsorption on strongly dehydroxylated anatase takes place in two ways part of the water molecules are dissociated forming hydroxy groups again while the major part of the water is adsorbed molec~larly~~[in the IR spectra this is demonstrated by the 6(H20) band at 1600 cm-I]. Repetition of the hydroxylation and dehydroxylation cycles leads to a decreasing concentration of the sites for dissociative adsorption probably owing to surface reconstruction.I There are many differences between the results obtained for the concentration of residual surface hydroxy groups on anatase. The methods which exclude OH groups from adsorbed water show that the concentration of residual hydroxyls is about 0.5 OH nrn-,.,' Thus irrespective of the fact that this concentration depends on the morphology of the samples and their pretreatment it may be stated that only a small part of the anatase surface is covered by hydroxy groups. 3.2 Surface Acidity In order to estimate surface acidity adsorption of weak (CO benzene) and strong (ammonia pyridine) bases is usually studied. l1 A typical probe molecule for fine determination of Lewis acidity is CO. Carbon monoxide is coordinated by a o-bond to metal cations which have no d electrons e.g. Ti4+. The stronger the bond the higher the IR stretching frequency of adsorbed CO. Testing oxi- dized anatase (containing no Ti3+ ions) with CO reveals the exis- tence of two kinds of c.u.s. Ti4+ ions differing in ele~trophility.'~-l~ The stronger site5 (vco at 2208 cm-I) are usually denoted by a SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI t I 1 2250 2200 2150 2100 v/cm" Figure 2 FTIR spectra of CO adsorbed on anatase at 100 K (a+) Increasing amounts of CO and (f)an equilibnum pressure of 2 Torr CO (K Hadjiivanov and J C Lavalley unpublished results) while p is used for the weaker ones (vco at 2190 cm-') The increase in the equilibnum pressure results in saturation of the a sites with CO at about 2 Tom whereas the p sites are not fully occu- pied even at pressures above 100 Torr Both adsorption forms are weak and are easily destroyed by evacuation As a rule the concentration of a sites is lower and some samples with a regular crystal shape may even have no a sites l8 When CO is weakly adsorbed the number of monitored sites could be increased by lowenng the temperature of adsorption Carbon monoxide adsorption on anatase at CQ 100 K (see Figure 2) also leads to detection of both (a and p) types of sites and C-0 stretching modes are observed at 2206 and 2175 cm-l 22 The band at 2175 cm-l is very intense which implies that in addition to the sites detected at room temperature (p'),some sites which are inert at room temperature (p")are involved in the adsorption An addi- tional band at 2165 cm-I is also observable and corresponds to CO adsorbed on c u s Ti4+ sites possessing a very low electrophility (y sites) The increase in CO coverage (at both ambient and low tempera- tures) leads to a shift in maxima of the v(C0) bands to lower fre- quencies owing to two overlapping effects static and dynamic shifts The static shift is produced by interaction of the admolecules through the support while the dynamic interaction is usually of a dipole-dipole type and occurs through-space 22 The dynamic inter- action is observed when the admolecules are parallel are localized in the same neighbourhood and on the same plane and vibrate with the same intrinsic frequency The latter condition allows elimina- tion (and calculation) of the dynamic shift by the use of T0-l ,CO isotopic mixtures A dynamic shift is observed with the bands at 2175 and 2165 crn-I,,* I e the p and the ysites are situated on defi- inite anatase faces whereas with the carbonyls formed on the asites (2208 cm-1 band) at least one of the requirements for dynamic shift is not satisfied The question about the detection of Ti3+ ions using CO adsorp-tion is still open It has been reported that Ti3+-C0 carbonyls on partially reduced anatase give absorption bands at 21 15 cm- I According to other studies,18 Ti3+ ions cannot be observed by testing with CO because they are oxidized by the latter Indeed Ti3+ ions on anatase possess a very strong reduction ability and are oxi- dized even by water to Ti4+ lo Studies on the adsorption of strong bases lead to simultaneous and selective detection of both Lewis and Brgnsted acidities When ammonia is coordinated to c u s cations usually the S,(NH,) band at about 1200 cm-* is analysed This band is sensitive to the strength of the bond formed the stronger the bond the higher the frequency Ammonia protonation I e formation of NH; shifts the symmetnc N-H deformation modes of 1680 cm I However the band typical of ammonium ions is S,(NH;) at about 1450 cm I Ammonia adsorption on anatase leads to the appearance of two bands in the 1500-1000 cm region about 1220 (weak) and at 11 80 (strong) cm l5 l7 lX 2o The latter band is shifted to 1145 cm I at maximum coverage These results have been considered for a long time as evidence for the presence of two kinds (namely a and p) of titanium cations differing in electrophility I5 l7 This is in agreement with both the qualitative detection of a and p titanium cations by CO adsorption and the lower concentration of the a sites However some experimental results contradict this opinion (I) the amount of adsorbed ammonia significantly exceeds the amount of CO adsorbed at room temperature,lX and (21) using another strong base pyridine it is possible to detect one type of Lewis acid sites only l7 Recently it has been established that the band at 1220 cm is produced by dissociated ammonia since a new type of OH groups (3658 cm I) appear at the same time IxThe original OH groups of anatase do not protonate ammonia but those at 3658 cm-I display a weak Brgnsted acidity and form NH; groups at equilibnum ammonia presssures Thus the band at 1180 cm characterizes ammonia coordinated to Lewis acid sites and this mode is not sen- sitive enough to distinguish between a and p sites The data obtained by adsorption and coadsorption of NH and CO show the existence of several types of c u s titanium ions (Table 1) It is seen that in order to perform an efficient surface analysis of titania sup- ported catalysts (I e to establish the location of the active phase and the eventual existence of a bare titania surface) combined testing by CO and NH should be employed Ammonia and pyridine adsorption show that the original OH groups of anatase exhibit no Brgnsted acidity However titania hydroxy groups protonate tnmethylamine which is a stronger base l5 The acidity of hydroxy groups is determined more precisely and quantitatively by the hydrogen bond method Weak bases (e g benzene) form a hydrogen bond with the protons of the hydroxy groups during adsorption The higher the mobility (acidity) of this proton the stronger the bond formed and the more pronounced the weakening of the O-H bond respectively This causes a shift of the OH stretching modes to lower frequencies Benzene adsorption on anatase20 leads to an average shift of the OH stretching modes by about -120 cm I which corresponds to a weak acidity of the surface hydroxy groups It is interesting that preadsorption of ammonia on Lewis acid sites decreases the absolute value of this shift and it then amounts to -60 cm-l I e NH transmits electrons through the substrate and causes a twofold decrease of anatase surface hydroxyl acidity At present no sufficiently good probe molecule for determina- tion of surface basicity is known Most often CO is used for this purpose Adsorption of CO on anatase proceeds (I) on Lewis acid Table 1 Charactenstic IR frequencies of probe molecules (CO NH,) adsorbed on different sites of the anatase surface Characteristic frequencies (v/cm I) CO adsorbed CO adsorbed Sites at 293 K at l00K NH adsorbed at 293 K a 0; -2208 0 -2208 Omdxb-2206 Om -2206 P' 0 -2192 0,,,-2185 0 -2192 0 -1180 F -I Omax -2175 Om -1145 Y -0 -2165 Omax -2164 Sites for dissociative 7 7 1220 adsorption -OH groups -2155 a The value at zero coverage calculated by interpolation The value at saturation A . t ,rr.r I.,300 3600 3400 r 3200 3000 2800 1400 1200 1000 v/cm-' Figure 3 FTIR spectra of methanol adsorbed on (a) rutile and (b) anatase evacuated at room temperature (reproduced with permission from ref. 23). sites where linearly bonded CO is formed,17 and (ii)on basic sites i.e. 02-ions and OH groups. Carbonates are formed with the participation of the oxygen anions while the hydroxy groups form hydrogencarbonates.ls,l These compounds are decomposed during evacuation at room temperature which indicates a weak surface basicity of anatase. One can compare ammonia and water adsorption. In general it is nondissociative in both cases. However these molecules dissociate on particular centres whose concentration is low. This is evidence for the presence of a certain proportion of strong acid-basic pairs on the anatase surface. The centres of dissociative adsorption are more precisely monitored by alcohols. On most oxides alcohol adsorption proceeds with breaking of the 0-H bond. Dissociation of different alcohols also takes place on rutile while on the major- ity of the anatase sites these compounds are adsorbed coordina- ti~ely.~~The IR spectra of methanol adsorbed on both titania modifications are shown in Figure 3. The existence of coordina- tively bound methanol on anatase is shown by the S(C0H) band at 1365 cm-1 (arrowed) whereas this band was not observed with methanol adsorbed on rutile since the 0-H bond is broken during the dissociation. This essential difference in surface chemistry of the two TiO modifications is probably decisive for their different adsorption and catalytic properties. Thus both anatase and rutile are n type semi- conductors with almost the same bandgap. In this respect they meet CHEMICAL SOCIETY REVIEWS 1996 tence of face 001 only. However 001 cannot be the only plane exposed on the surface. Another model,21 focused on the four-co- ordinated Ti4+ sites from the 111 plane alone has the same dis- advantage. Busca et all7 have proposed a model considering the structure of most faces characteristic of the anatase crystallites (011 010 001 and 110). According to this model the four-coordinated titanium cations from face 110 are the strong Q sites the five-co- ordinated cations from the other faces represent the weaker p sites and the hydroxy groups are localized on crystal lattice defects. Although much more advanced this model cannot explain the variety of acid sites either. Evidently the difference in properties of the titanium cations cannot be due to differences in their coordination number alone. Hadjiivanov el al.'8 have proposed a model explaining the high heterogeneity of titanium cations in anatase taking also into consideration the effect of the second coordination sphere. According to this model titanium cations from isolated acidic-basic pairs (c.u.s. Ti4+-02-) are more electro- philic than the titanium ions from acidic-basic rows (c.u.s. -. --Ti4+a2-Ti4+-02-. . . ). In the latter case the electrophility decreases owing to the formation of stronger bonds with two and not with one C.U.S. oxygen ion. Figure 4 presents a scheme for the anatase surface. It is obvious that regardIess of their situation all titanium cations may be divided into the following groups I. Four-coordinated Ti4+ (faces 110 111 and 113 as well as the edges of the 110 face). These ions are referred to Q sites. Owing to the different directions of the oxygen vacancies CO molecules adsorbed on (Y sites are not parallel which is the reason why no dynamic shift is observed with the Q carbonyls. 11. Five-coordinated Ti4+ participating in acidic-basic pairs (on the most characteristic faces such as 101=011 and 100=010). These ions correspond to the p sites. At room temperature only half of the sites are occupied owing to an induced heterogeneity of the surface. 111. Five-coordinated Ti4+ participating in acidic-basic rows (faces 001 and I12 and edges 101 X 011). They are the y sites. IV. Ti3+ ions. Stoichiometric considerations show that the C.U.S. titanium cations situated on the edges of the 001 plane are of a Ti'+ type.'* Since Ti3+ ions are not characteristic of oxidized anatase they are assumed to be the places of localization of the residual hydroxy groups. Thus the titanium cations are stabilized in the fourth valency. The most appropriate centres for dissociative adsorption are situ- ated on the 110 face for two reasons (i) the 110 face contains the most acidic four-coordinated Ti4+ ions; (ii) the cationic vacancies on this plane are bridged i.e. two C.U.S. 02-ions act together as a Lewis base. This point of view is in agreement with the low concentration of the centres for dissociative adsorption but is not supported by direct experimental evidence. It is important to establish the reason for the predominant mole- the requirements for photocatalytic decomposition of However the anatase photoactivity is considerably higher. Obviously the local structure of the catalytic site is also important. It is established that the photocatalytic decomposition of water (as well as the interaction of water with alkanes) on titania involves mainly molecularly adsorbed H,0.3 Thus the lack of sites for dis- sociative adsorption on anatase seems to be an important reason for its photocatalytic properties. 3.3 Models of the Anatase Surface Different models of the anatase surface have been proposed on the basis of its adsorption properties. Primet et af.l5 have considered the 001 face supposing it to be the main face exposed on the surface. These authors are of the opinion that both types of surface hydroxy groups are localized on C.U.S.titanium cations the band at 3715 cm-' being characteristic of isolated hydroxy groups and that at 3675 cm-I of OH groups situated on neighbouring titanium cations. Dehydroxylation leads to surface reconstruction which explains the nondissociative water adsorption. The C.U.S. titanium cations thus obtained represent the /3 sites whereas the dehydroxy- lated titanium cations from unreconstructed centres are a sites. This pioneer model of the anatase surface takes into account the exis- ~ater.~.~cular adsorption of alcohols and water on anatase and for their dis- sociative adsorption on rutile. The cleavage rutile planes contain five-coordinated Ti4+ and two-coordinated 02-ions. Schemes of the 1 10and 100mile faces are presented in Figure 5. It is seen that in contrast with the case of anatase the coordinative vacancies of the Ti4+ and 0,-ions are either not parallel or are situated in differ- ent layers. The same applies to the other faces of rutile. Thus a two- centre adsorption of water (alcohol) molecules is possible on anatase whereas on rutile dissociation is necessary to ensure binding of adsorbed molecules to two surface sites simultaneously. The above model explains well the high heterogeneity of the anatase surface and the relative concentration of the different sites. It is also confirmed by the dependence of the concentration of the different sites on the sample morphology.'* The main problems remaining are (i) a detailed interpretation of the different OH groups; (ii)explanation of the heterogeneity of p sites (now it seems to us that this heterogeneity is induced i.e. an adsorbed CO mole- cule changes the acidity of the cationic sites in the vicinity) and (iii) exact interpretation of the sites for dissociative adsorption. Elaboration of a precise model of the anatase surface explaining all properties is a question for the future. SURFACE CHEMISTRY OF TITANIA-K. I. HADJIIVANOV AND D. G. KLISSURSKI 001 100 110 Figure 4 Scheme of an anatase crystal with different exposed faces. e C.U.S. Ti4+ ions; o saturated Ti4+ ions from the subsurface layers; C.U.S. 02-ions; @ saturated 02-ions from the surface layers 0 saturated 02-ions from the subsurface layers. 110 100 0.2nm Figure 5 Schemes of the ‘unit cells’ of the 110 and 100mile planes. Ti4+ cations 0 02-anions. The ions without vacancies (plane 1 lo) although situated on the surface are coordinatively saturated. 4 Surface Chemistry of Anion-modified Anatase A new type of catalyst may be developed by synthesis of new sub- stances with a new bulk structure or by modification of the surface of known substrates. Anatase is a typical example of the second possibility. It strongly adsorbs various anions and cations which causes a pronounced change in its surface properties. We shall con- sider anatase modified by sulfates phosphates and peroxides. In the two former cases the anions are adsorbed on Lewis acid sites and via ion exchange with surface hydroxy groups while the OH groups alone participate in the formation of peroxide compounds. 4.1 Sulfate-modified Anatase Sulfur is often present in the commercial titanias especially when they are prepared via the so-called ‘sulfate method.’I4 It is also accumulated on titania-based catalysts during the Claus reaction. However titania and zirconia modified by sulfuric acid or sulfates have been the subject of numerous investigations since 1983 when Hino and Arata7 reported their superacidity and their unusual cat- alytic properties (see Introduction). Titania may be sulfated by treat- ing with H,SO impregnation with (NH,),SO or adsorption of sulfur-containing compounds followed by oxidation. Part of the C.U.S.oxygen ions from the TiO surface are replaced by sulfate ions on sulfated titania (ST).24-25These sulfates are stable up to m.600 “C and are characterized by a strong band (typical for them) at 1380 cm-’ which corresponds to S=O stretching modes. In principle ST dehydroxylation is more difficult than that of pure anatase.I9 In cases of strongly dehydroxylated ST samples the sulfate ions attract electron density from a neighbouring titanium cation via an induc- tive effect. As a result the electrophility of the titanium ions increases. This enhanced Lewis acidity is detected by testing with CO; when adsorbed on sulfated a sites CO produces an absorption band at 2215 cm-l while on sulfated p sites absorption is observed at 2200 cm-’.24 Busca et af.17have also reported formation of dicar- bonyls. Ammonia adsorption on dehydroxylated samples shows mainly the presence of Lewis However weakly dehy- droxylated ST exhibits Brgnsted acidity (a strong IR band at 1445 cm-after ammonia adsorption resistant to evacuation) which is assumed to be due to monomeric SO,H groups24 or to polymeric sulfates.2s At present it is not clear whether the enhanced Lewis or the generated Brgnsted acidity are responsible for the unique cata- lystic properties of ST. The first supposition is supported by the absence of superacidity of sulfated silica (which in principle pos- sesses no Lewis acidity). Nevertheless the Lewis acidity of ST is not very high. The low temperature of the catalytic reactions indi- cates that most probably ST catalysts are bifunctional 1.e. both aprotonic and protonic acid sites participate in the catalytic process.24 4.2 Phosphate-modified Anatase Phosphorus is often present as a pollutant in commercial titania samples. It is also used as a promoter in Ti0,-supported catalysts.’ Phosphated anatase may be obtained by adsorption of phosphate ions from acidic solutions or by impregnation. By analogy with ST one may expect a change in surface chemistry of anatase after phosphatation. However in contrast to sulfates the phosphate anions block the Lewis acid sites of anatase.26 Thus in contrast with the pure oxide phosphated anatase does not adsorb C026and has a hydrophobic surface.*’ A hypothesis was proposed26 according to which the role of phosphorus as a promoter in vanadia-titania cat- alysts consists in blocking of the free C.U.S. titanium cations which usually catalyse parasitic reactions (e.g.production of CO and CO during o-xylene o~idation,,~ formation of N,O in the DeNO process,2xetc.). 4.3 Peroxide-modified Anatase Hydrogen peroxide also belongs to the class of anionic modifiers. As a rule univalent anions are not sorbed strongly on anatase but hydrogen peroxide interacts with the surface hydroxy groups according to the reaction:29 Ti-OH + H,O -Ti-OOH + H,O. (2) These surface hydroperoxo groups begin to decompose at about 80 "C evolving oxygen. Hence peroxide-modified anatase is not of interest for catalysis at high temperatures. Surface peroxide species are of importance in low-temperature catalytic and especially in photocatalytic reactions (in the latter case H,O is often a reaction intermediate). Peroxide-modified titania could find important applications in the synthesis of some molecularly deposited cata- lysts owing to its increased adsorption capacity relative to pure titania with respect to ions tending to form peroxides.3o 5 Vanadia-Titania Catalysts Among the titania-supported oxide catalysts V,O,/TiO (anatase) is the most important. It shows better catalytic properties in o-xylene oxidation than does vanadium oxide itself as well as vana- dium oxide deposited on other oxides and in particular on r~tile.~' V,O,/T'iO catalysts are also the most commonly used for the DeNOA process. Vanadia-titania catalysts are usually prepared by impregnation the most widely used vanadium source being vana- dium oxalate. In recent years much attention has focused on the so called 'molecularly deposited' catalysts. They are prepared via ion-exchange or by grafting i.e. by a reaction of vanadium compounds with the anatase surface hydroxy groups. When vanadia is deposited on anatase its surface acidity decreases. As a rule its distribution on anatase is not uniform. Probe molecules (CO NH,) detect bare titanium ions even on catalysts containing a higher amount of V,O than that necessary for mono- layer formation.72 This disadvantage is avoided in the so-called molecularly deposited catalysts where the distribution is more homogeneous. Many theories have been proposed to explain the unique effect of anatase as a support. Vejux and C~urtine~~ have shown the crystal- lographic similarity between faces 001 100 and 010 of anatase and face 010 of vanadium pentoxide. They are of the opinion that during deposition epitaxial growth of vanadium oxide crystals with exposure of the 010 face proceeds. This face contains V=O groups which are responsible for the good catalytic properties. In the case of active molecularly deposited catalysts however no separate V,O phase is formed and it may be assumed that the two-dimen- sional compounds possess a structure similar to that of face 0 10. A series of investigations have shown that depending on the vanadia coverage various surface compounds are formed. Isolated monooxovanadyl groups and two-dimensional vanadium oxide clusters correspond to low coverages whereas a separate V,O phase and polyvanadates are observed at higher coverages.,() The nature of the surface vanadium-oxo species also depends on the calcination temperat~re,,~ as shown in Figure 6. At high tempera- tures an anatase-rutile transition (favoured by vanadium additives) and dissolution of VO in the isostructural rutile crystals with formation of a VkTi -,02(r) phase are among the reasons for cata- lyst deacti~ation.~.~~ In all cases vanadidanatase catalysts contain species with a double V=O bond. This bond is well monitored in the IR spectra by the 2v(V=O) overtone at 2060-2054 cm-I. Part of the surface vanadia species are also characterized by the presence of V-OH groups (bands in the 3680-3660 cm-I region). These hydroxy groups cause the BrGnsted acidity of the catalysts [a ad5(NH+,) band CHEMICAL SOCIETY REVIEWS 1996 110-200 "C 350"c 450-575 "c 650"c 750 "C Figure 6 Model of evolution of V,O,RiO with calcination temperature (reproduced with permission from ref. 27b). at 1460 cm-I after ammonia adsorption]. According to Bond et al.34 it is the joint effect of the V=O and V-OH groups that determines the good catalytic properties of vanadidanatase catalysts in selec- tive oxidation reactions. The Lewis acidity of the V,O,/TiO cata-lysts is weak no VSf-CO carbonyls are formed after CO adsorption. However the stronger base ammonia is coordinated to Vs+ sites [S,(NH,) at 1240 cm-'] causing rearrangement in the VO surface complexes and disappearance of the V=O bond.30 It should be noted that all of the above considerations relate to oxidized cat- alysts which however are reduced and reoxidized during the reac- tions. Many researchers have pointed out the lack of activity of titania in the oxidation of o-xylene but Wachs et al.,' have demonstrated that anatase leads to the full oxidation of some of the reaction inter- mediates. Thus the bare titania surface of vanadidanatase catalysts lessens their selectivity. This point of view could also be extended to other reactions and catalysts. For instance during the DeNO process NH,NO,-like species are formed on the anatase surface.28 These species decompose to the undesired reaction product N,O. Hence one of the reasons for N,O formation over titania-supported oxide catalysts could be the incomplete covering of the titania by the active phase. These examples show that the design of the Ti0,- supported oxide catalysts is of great importance for their catalytic performance. 6 Titania-supported Metal Catalysts One of the main reasons for the numerous studies of titania-sup- ported metal catalysts is the so-called strong metal-support interac- tion (SMSI). However even in the absence of a SMSI effect these catalysts are of interest. Thus platinized titania exhibits a much higher activity in a series of photocatalytic reactions than pure anatase.s Combined Pt-RuO,/TiO catalysts are the most promising ones for photocatalytic water cleavage. 6.1 Preparation of Titania-supported Metal Catalysts In most cases metal salts are precursors for preparation of supported metals. After deposition the metal ions are reduced. Both the pre- reduction treatment and the reduction conditions are found to affect the structure of the metal formed and especially the average parti- cle size. In a number of catalytic reactions (the so-called structure- sensitive reactions) the catalytic properties strongly depend on the metal particle size.35 In addition the catalytic performance of bimetallic catalysts is affected by the homogeneity of distribution SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI M '+ containing Titnnia ph ike ion exchange I M '' containing phase i low tenipernture reduction b reduction with hydrogen I C next ion exchange I d reduction with hydrogen I e Figure 7 Principal scheme of the multi ion-exchange process of the two metals in the separate particles That is why the design of supported metal and bimetal catalysts is crucial The techniques used most often for preparation of supported metals are impregnation and ion exchange 35 Here we shall discuss two methods of deposition of noble metals which are specific for titania TiO is an effective photocatalyst for deposition of noble metals from solutions of their salts ScThis process is used for the synthesis of mono- and bi-metallic catalysts supported on titanium dioxide 36 The amounts of deposited metals are controlled by the concentration of their ions in the solutions The main restriction to the method is that it can be applied to photosensitive systems only Recently Hadjiivanov et a1 37a proposed the multiple ion- exchange method the model system used being Pt-TiO Platinum-containing ions are adsorbed initially on anatase CO adsorption reveals the occupation of Ti4+ Lewis acid sites during the process Testing with CO again after the reduction of Ptn+shows that part of the initially occupied adsorption sites are liberated This phenome- non allows subsequent ion exchange efc (see Figure 7) The increase in platinum content is smaller than 100% owing to phys- ical blocking of part of the exchange sites by metal particles The catalysts obtained by repeated ion exchange are characterized by a high dispersion and a narrow metal particle size distribution It has been established that the process may also be applied to the system Ag-TiO 37h At present the question about whether other supports may be used is still open 6.2 A Strong Metal-Support Interaction (SMSI) The consideration of the surface and catalytic properties of titania- supported noble metals is associated with SMSI which is one of the most interesting and most studied effects in catalysis The term SMSI was introduced by Tauster et a1 38 to denote the effect respon- sible for the drasoc decrease in CO and H chemisorption on titania-supported metals after increasing the reduction temperature from 200 to 500 "C The following peculianties of SMSI have to be noted (1) the effect occurs only when the hydrogen adsorption on rpL1Titnnid high temper,iture redtiction I TiOv Titdnin reduction nhove 800 "ci I Titdnia I Figure 8 Titania supported metal catalysts reduced at different tempera tures (a) Before reduction (b) a metal particle on titania formed after low temperature reduction (c) partial coverage of the metal particle with TiOAmoieties after high temperature reduction and (d) full encapsulation of the metal particle into the support after reduction at a very high tern perature (see text) the supported metals is dissociative (zi) it is not due to change in the metal particle sizes Catalytic systems have been denoted as LTR (low temperature of reduction) and HTR (high temperature of reduction) depending on whether this temperature is below or above 300 "C The present enormous interest in SMSI could not have amen from the low chemisorption capacity of the HTR catalysts only However they have also demonstrated some unusual catalytic properties The TOF (turnover frequency z e the activity per surface metal atom) of LTR and HTR catalysts towards structure- insensitive reactions is almost the same but structure-sensitive reactions proceed at a much lower rate on HTR catalysts I e in the presence of SMSI Many hypotheses have been proposed to explain SMSI Initially electronic effects were considered to be the cause but now the fol- lowing explanation is accepted titania from HTR catalysts is partly reduced and a suboxide phase migrates onto the metal parti- cle (see Figure 8) Thus the part of the metal surface partially covered by TIO-~ is blocked As a result the chemisorption capacity of the metal strongly decreases and structure-sensitive reactions which need a larger ensemble of metal atoms are suppressed The existence of admixtures e g Na+ facilitates the migration of TiOx moieties and SMSI occurs at lower temperatures of reduction 75) At a higher reduction temperature partial or complete encapsulation of the metal particles in titania may occur Other effects that are reported to occur on reduction above 800 "C are the formation of intermetallic compounds and alloys The large interest in SMSI is strengthened by one more phenom- enon With some metals such as platinum supported on titania the TOF of hydrogenation of CO as well as of compounds containing a carbonyl group increases by up to two orders of magnitude after HTR It is supposed that the suboxide phase being a strong reducer favours the dissociation of CO extracting oxygen while the metal serves as catalyst for the hydrogenation of the carbon obtained However there is some evidence that although associated with CHEMICAL SOCIETY REVIEWS 1996 rn TI^+-co a Ru3'(CO) A RuO-CO I I I I 7 Figure 9 FTIR spectra of CO (1 Torr) adsorbed on a LTR Ru/TiO catalyst at 100 K (solid line) and after heating to 373 K in a CO atmosphere fol- lowed by cooling again to 100K (dotted line) (K Hadjiivanov and J C Lavalley unpublished results) SMSI the increase in the CO hydrogenation rate is not directly pro- voked by SMSI because (I) it has been established that water (which is produced in the CO + H reaction) oxidizes the suboxide and SMSI disappears (zz) in some cases even LTR Me/TiO cata- lysts have a higher TOF than does the corresponding metal deposited say on silica It is evident that the suboxide phase is not unchangeable during CO hydrogenation On the contrary reduced titanium ions can probably be oxidized directly not only by water but also by CO It seems that in this case HTR is needed to ensure the initial formation of the suboxide phase on the metal particles During the reaction the titanium cations formed are altervalent z e they are reduced by hydrogen but oxidized by water and CO thus facilitating CO dis-sociation This mechanism also explains the varying effect of HTR on different metals For instance CO is difficult to dissociate on Pt and that is why Pt/TiO catalysts strongly enhance their TOF after HTR The IR spectrum of CO adsorbed on a LTR RuDiO catalyst is shown in Figure 9 At 100 K all of the ruthenium is in the Ruo state but heating in a CO atmosphere up to 373 K is accompanied by formation of Run+ z e the activation of CO in this case is not hindered This explains the small effect of the reduction tempera- ture and the type of support on the TOF of supported ruthenium dunng hydrogenation of the CO bond 39 7 Conclusions 7.1 Heterogeneity The anatase surface is highly heterogeneous Three kinds of Lewis acid sites (differently coordinated Ti4+ ions) and at least two kinds of hydroxy groups are present on the surface This high hetero- geneity is due to the exposure of different planes on the real crys- tallites 7.2 Local Arrangement The local arrangement of the anionic and cationic vacancies on the anatase surface determines the lack of centres for dissociative adsorption of water and alcohols This particular anatase property is relevant to (I) the low hydroxyl coverage of anatase and (zz) its (photo)catalytic behaviour 7.3 Anions Adsorbed anions strongly affect the surface properties of anatase Surface sulfates increase the Lewis acidity whereas phosphates block the Lewis acid sites Both anions induce a Bronsted acidity 7.4 Design The design of titania-supported oxide catalysts is very important since a bare titania surface often leads to parasitic reactions and decreases the selectivity of the catalysts 7.5 SMSI Titania-supported metals reduced above 300 "C are charactenzed by a strong metal-support interaction (SMSI) This effect strongly decreases the chemisorption capacity of the supported metals and is due to their coverage by Ti-suboxide moieties 7.6 Turnover Frequency The TOF of some metals in CO hydrogenation is enhanced when supported on titania This effect is also due to covering of the metal particles with a Ti-containing phase bur the catalysts are not in the SMSI state during the reaction Acknowledgments This work was supported by the Bulgarian National Research Foundation (Project X-486) 8 References 1 J Whitehead Titanium Compounds Inorganic in Kirk Othmer Encyclopaedia of Chemical Technology 3rd edn executive ed M Grayson Wiley New York 1983 vol 23 p 131 2 G I Golodets Heterogeneous Catalytic Reactions Involving Molecular Oxygen (Studies on Surface Science and Catalysis vol 15) Elsevier Amsterdam 1982 3 M Anpo Res Chem Intermed ,1989,11,67 4 A Mills R H Davies and D Worsley Chem Soc Rev 1993,22,417 5 Energy Resources through Photochemistry and Catalysis ed M Gratzel Academic Press Harcourt Brace Jovanovich New York 1983 6 S Matsuda and A Kato Appl Catal 1983,8 149 7 K Arata and M Hino in Proc VII Sov Yaponsku Semin Katal ,ed A A Davydov Irkutsk 1983,Nauka Novosibirsk 1983,p 7 (Chem Abs 1985 102,61889t) 8 V Nikolov D Klissurski and A Anastasov Card Rev Sci Eng ,199 1 33,315 9 M A Vannice J Catal ,1982,74 199 10 G L Haller and D E Resasco Adv Catal ,1989,36 173 11 A A Davydov Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides ed C H Rochester Wiley Chichester 1990 12 I Kostov Mineralogy 3rd edn ,Nauka i Izkustvo Sofia 1973 13 Handbuch der Praparativen Anorganischen Chemie ed G Brauer Ferdinand Enke Verlag Stuttgart 1975 14 R Pletnev A Ivakin D Kleshchev T Denisova and V Bumistrov Hydrated Oxides of Group IV and V Elements Nauka Moscow 1986 (Chem Abs 1987,106,60379~) 15 M Pnmet P Pichat and M V Mathieu J Phys Chem ,1971,75,1216 1221 16 D J C Yates J Phys Chem ,1967,65,746 17 G Busca H Saussey 0 Saur J -C Lavalley and V Lorenzelly Appl Catd 1985,14,245 18 (a)K Hadjiivanov A Davydov and D Klissurski Kinet Katal ,1988 29 161 (6)K Hadjiivanov 0 Saur J Lamotte and J -C Lavalley 2 Phys Chem (Munich) 1994,187,281 19 C Morterra J Chem Soc Faraday Trans I 1988,84,1617 20 K Hadjiivanov D Khssurski G Busca and V Lorenzelli J Chem Soc Faraday Trans 1991,87 175 21 G Munuera F Moreno and J A Prieto Z Phys Chem (Munich) 1972 78 113 22 A A Tsyganenko L A Denisenko S M Zverev and V N Filimonov J Catal 1985,94 10 23 G Ramis G Busca and V Lorenzelli J Chem Soc Faraday Trans I 1987,83 1591 24 K Hadjiivanov and A Davydov Kinet Katal 1988,29,460 25 M Waqif 3 Bachelier 0 Saur and J -C Lavalley J Mol Catal ,1992 72 127 26 K Hadjiivanov D Klissurski and A Davydov J Catal 1989,116,498 27 (a)I E Wachs R Y Saleh S S Chan and C C Chersich Appl Catal 1985 15 339 (b) R Y Saleh I E Wachs S S Chan and C C Chersich J Catal ,1986,98 102 28 K Hadjiivanov V Bushev M Kantcheva and D Khssurski Langmuir 1994,10,464 SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI 69 29 D Klissurski K Hadjiivanov M Kantcheva and L Gyurova J Chem 36 J M Henman J Disdier P Pichat A Gonzalez-Elipe G Munuera and Soc ,Faraday Trans 1990,86,385 C Leclercq J Catal 1991 132,490 30 M Kantcheva K Hadjiivanov and D Klissurski J Catal 1992 134 37 (a)K Hadjiivanov J Saint-Just,M Che J M Tatibouet,J Lamotte and 299 J -C Lavalley J Chem Soc ,Faraday Trans 1994 90 2277 (h) K 3 1 J Haber Oxygen in Catalysis Decker New York 1991 Hadjiivanov E Vassileva M Kantcheva and D Klissurski Marer 32 H Miyata Y Nakagawa T Ono and Y Kubokawa Chem Lett 1983 Chem Phys ,1991,28,367 1141 38 S J Tauster S C Funk and R L Garten,] Am Chem Soc 1978,100 33 A Vejux and P Courtine J Solid State Chem 1978,23,93 170 34 G C Bond J P Zunta S Flamerz P J Gellings H Bosch J G van 39 T Komaya A T 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ISSN:0306-0012    

DOI:10.1039/CS9962500061

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

The Nitro Group as Substituent Otto Exner Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Praha 6, Czech Republic Tadeusz Marek Krygowski Department of Chemistry, University of Warsaw, 02 093 Warsza wa, Pasteura I, Poland 1 Introduction If a structural unit is denoted as a ‘substituent’, this usually has one of the two following meanings: (1) The substituent is a smaller part of a molecule which can be introduced by a simple chemical operation, particularly when it can directly replace a hydrogen atom. (2) The substituent is a smaller and less important part of a mole- cule which influences the properties of the molecule in a quantita- tive sense but does not alter its general chemical character: the latter is controlled by another group present: the functional group’ (or the reaction site).The nitro group is a substituent par excellence, both typical and important, according to either definition. For instance, 4-nitrophe- no1 will be viewed in most circumstances as a substituted phenol, much less often as a substituted nitrobenzene. It can be prepared easily from phenol, but not simply from nitrobenzene. Its chemical and physicochemical properties are more closely related to those of phenol than to those of nitrobenzene. This view, however, may change if an appropriate physicochemical property is studied. For instance in electroreduction the nitro group will act as the functional group. On the other hand, the nitro group is a ‘strong’ substituent since the differences in acidity, reaction rates and spectral shifts between nitrophenol and phenol are large compared to the effects of other subs tituents.In this review the term nitro group is used in a narrow sense as NO, bonded to a carbon atom. We shall deal only with the second aspect of substitution, and particularly with the quantitative mea- sures of the substituent strength estimated by substituent constants u1 -3 as well as by other parameters; the electronic and geometric structure of the nitro group must be also mentioned. We give much attention to nitrobenzene since it has been more extensively inves- tigated than any other nitro compound. Otto Exner, born in Praha, formerly Czechoslovak Republic, received a PhD degree from the Institute of Chemical Technology in I951 and a DSc degree from the Czechoslovak Academy of Sciences in 1961.He has worked in several Institutes of this Academy, and has been forced several times to change his employment on polit- ical grounds, and partly also because his interests lay between organic and physical chemistry. He has lectured at several Universities in Czechoslovakia, since 1969 as Professor of Organic Chemistry, and has had appointments as visiting Professor in Ituly, Sweden and France. His main scientific interest is in correlation analy- sis, particularly the Hammett equation, isokinetic relation-ships, and reactivity-selectivity principles: secondary interests are dipole moments, conforma- tions and hydroxylamine deriv- atives.2 Properties and Electronic Structure of the Nitro Group The nitro group may be derived formally from N,O, which is a strong oxidizing agent. If the nitro group is attached to a hydro- carbon of any kind, it changes the electron affinity of the molecule significantly. Alkanes and benzene are not reduced electrochem- ically, whereas their nitro derivatives are, with rather low values of formal potentials of reduction. The earlier formulated structure 1 of the nitro group has been replaced by the resonance of two degenerate forms 2 *3 which describe in a somewhat cumbersome way the simple fact that the two oxygen atoms are equivalent. In particular also the canonical structure 4 comes into consideration; its significance is that the second positive charge is delocalized over the moiety to which the nitro group is attached.The formally positive charge on the nitro- gen atom explains the strong electron-attracting power of the whole substituent. This is manifested by the high value of the group electronegativity and of the dipole moments (see later sections). Recently also a contribution from the biradical form 5 was consid- ered4 and estimated in the case of p-nitro derivatives of aniline and phenol to be ~ignificant.~ Even the old structure 1 could again be taken into account since in fact the NO bond lengths are required for a double bond.4 For chemical consequences the form 4 is par-ticularly important and instructive; it is postulated in textbooks, but its weight in p-nitrophenol and p-nitroaniline was found recently5 to be only about 1%.This is in line with many other theoretical and experimental studies of nitrobenzene and its derivatives,6--” which all have shown a limited importance of the form 4. c13 C 23 c 33 +,Oe /Om-N@ -N \oe \om c 43 c 5) Tadeusz Marek Krygowski was born in Poznah, Poland and received his PhD degree in 1969 and a DSc degree in 1973, both from the Department of Chemistry of Warsaw University. Since I964 he has been working at this University, from 1983 as a Professor of Chemistry. He has lectured at many universities in many countries, serving as an invited Professor in Canada and France. Elected as presi- dent of the Polish Chemical Society in 1994, his muin inter- ests are: ion-pairing in organic electrochemistry, solvent and substituent efsects, structural organic chemistry, and studies on aromaticity.71 Table 1 Geometry of the nitro group attached to various moieties Bond Molecule Bond length Id/& angle (") CN NO NO ON0 Nitrobenzene,Io EP 1.486(2) 1.2234(4) 125.32(8) Nitrobenzene," XD (low temp.) 1.467(1) 1.227(1) 123.2(1) Corrected for liberation 1.477 1.229 1.233 p-Nitroaniline,12 XD 1.434(2) 1.227(2) 121.6(2) N,N-Diethyl-p-nitroaniline,13XDh 1.437(5) 1.234 1.223 12 1.9(3) 1.429(5) I .221 1.232 12 1.7(4) Corrected for liberation 1.441 1.243 1.232 1.434 1.229 1.241 p-Nitrophenolate anion' XD 1.418(5) 1.241(2) 1.247(2) Nitromethane,I4 MW 1.489 1.224 125.3 XD: X-ray diffraction; ED: electron diffraction; MW: microwave measurements. Two independent molecules in the asymmetric unit.3 Geometry of the Nitro Group The geometry of the nitro group does not depend significantly on the nature of the moiety to which it is attached. Data collected in Table 1 present this clearly. The most sensitive parameter is the length of the C-N bond, which may serve as an approximate measure of the resonance effect of the nitro group, interacting via this bond with the moiety to which it is attached. Thus if a partial CN double bond is induced, it means that the canonical structure 4 contributes significantly to the description of the molecule. The C-N length in nitrobenzene is almost the same as in nitromethane, in spite of the sp3 character of the Catom in the methyl group.In electron diffraction'" or low-temperature X-ray diffraction measurements" on nitrobenzene on the one hand, and microwave determination of the geometry of nitromethane14 on the other, the difference is statistically insignificant. It might be concluded that the resonance effect of the nitro group in nitrobenzene is very low or even practically zero. In other words, the structure is described sufficiently by the canonical structures 6 and 7 while structures 8-10 have low weights. As mentioned this need not apply when the nitro group is attached to strongly electron-donating moieties; then structure 11 may be of importance.C 63 c 73 c 7a) C 83 4 Electronic Substituent Effects and Substituent Constants 4.1 Hammett Constants The electron-attracting character of the NO, group is shown in a simple and convenient way by means of Hammett constant^,^ urn and up.Defined originally as the substituent effect on the acidity of substituted benzoic acids in water, these constants can be viewed as experimental quantities. Subsequently, they have been obtained also from other reactions or as statistical mean values from many reactions.' Table 2 gives some examples and the values agree quite well. The electronic effect of NO, is thus regular and well predict- able. In addition it is strong, approaching almost the end point of the scale. (Only 7% of uncharged substituents, show higher values, and CHEMICAL SOCIETY REVIEWS, 1996 Table 2 Some selected values of substituent constants CT for the nitro group Assumed substituent Representative Kind of effect values determination" I + reduced M 0.7 1 Statistical3 0.7 1 'Preferred*' 0.7 1 pK in water3 I+M 0.81 Statistical' 0.78 'Preferred2' I + enhanced M 0.78 1.23 pK in water3 pK aniline3 1.28 pK p heno13 I or F 0.76 pK acetic acid' 0.68 pK quinuclidine' 0.78 h 0.65 Indirect2 0.64 I9F NMR shift2 0.66 Calculated] M 'normal' 0.17 IR intensity3 0.13 Indirect2 0.0 Indirectt9 0.16 IyFNMR shift2 0.19 Calculatedz0 M enhanced 0.46 Indirect's M reduced 0.0 Empiricalz2 Electronegativity 0.40 Calculated26 0.46 Calculatedz2 Polarizability -0.26 Calculated2* a Secondary literature sources are cited wherever possible.From 4-nitrobicyclo- [2.2.2]octanecarboxylic acid.' most of them are rather exotic.) Since nitro compounds are easily available, this group has been of decisive importance in formulat- ing and verifying the validity of the Hammett equation; in fact an irregular effect of some weaker substituents, may be masked by the strong effect of the nitro group. This group has always been present in any set of substituents recommended for studies of reactivity or other properties, the so-called minimum basis set. '5 The use of meta-and para-substituted derivatives of benzene as model compounds has the rationale that direct steric interaction is eliminated.' An additional constraint for the validity of the Hammett equation is the absence of direct resonance between the substituent and the reaction centre ('through resonance' as in p-nitroaniline). If this constraint is abandoned, the so-called dual con- stants are derived,' denoted @ or u-.In the case of the nitro group only u-values come into consideration.According to Table 2 they are rather different from normal up:the difference is usually explained in terms of resonance described by the canonical stmc- ture 11. Accordingly, the normal constants upare also assumed to be composed of an inductive and a smaller resonance part and enormous effort has been given to their quantitative separation.',2 In the case of the nitro group the inductive effect is evidently much more important and any kind of separation must begin with it.4.2 Inductive Substituent Effect Evaluation of a more or less purely inductive effect requires the replacement of the benzene ring considered for the Hammett equa- tion by a rigid alicyclic system' such as bicyclo[2.2.2]octane or quinuclidine. Remarkably, even simple aliphatic systems, for instance derivatives of acetic acid, give concordant results: the direct steric interactions are negligible for common substituents.16 The resulting constants are denoted as inductive, a,,but some authors2 prefer the term field constants, uF.However the question of how the effect is transmitted is immaterial and in fact ill formu- lated.17 More important is the fact that several model systems give the same relative result, as shown for the NO, group in Table 2.To obtain numerically concordant results each kind of the substituent constant must be multiplied by normalizing factors. Therefore, the agreement in Table 2 depends also on these factors which in turn depend in part also on the nitro group itself. The agreement for one THE NITRO GROUP AS SUBSTITUENT-O. EXNER AND T. M. KRYGOWSKI substituent does not tell much; any disagreement could be observed only if the behaviour of the nitro group were rather different from that of the other substituents. This is not the case: again the effect of NO, is strong and regular. A purely theoretical approach18 to the inductive effect uses pseudo-molecules with a non-bonded substituent at a fixed dis- tance: for instance the enthalpy of the isodesmic reaction, equation (l), can be calculated. Even here the resulting constants must be normalized.Examples of these results are given in Table 2 as 'cal- culated' values. NH? NH, NH: NH,'f4.sA w H+H H+H1 I I IX H H X 4.3 Mesomeric (Resonance) Effect Since the inductive effect is omnipresent, any evaluation of the mesomeric effect always means that two similar systems need to be compared.' When the inductive effects can be assumed to be equal, the mesomeric effect is obtained by subtracting the two values. In the case of the NO, group the inductive effect is strong, and sub- tracting two almost equal values is not dependable, as stated in clas- sical textbooks.However, the pioneering work of Taft15 yielded a resonance constant uR,small compared to uIbut not negligible (Table 2). The calculation was complex; the two systems compared were substituted acetic acid esters and substituted benzoic acids; the inductive effects were not equal and were normalized with refer- ence to bicyclo[2.2.2]octane- 1-carboxylic acids. One of the present authors tried to improve this normalization with the result that the mesomeric effect of the nitro group is near to zero in benzoic acids;I9 it was concluded that even the resonance in nitrobenzene corresponding to the canonical structures 8,9 and 10 has almost zero weight with respect to the accuracy of common experimental approaches.This finding was strongly opposed1s,21 but was later rediscovered by Taft himself and used particularly for gas-phase acidities.,, With benzoic acids as models, it is probably not possible to obtain more accurate results, nor to decide whether the effect is small or 'practically zero'. This picture is changed when substituted phenols or anilines are used as model compounds. From the constants a;one can define constants u;which are certainly not zero (Table 2). These constants describe the substituent effect of the nitro group in several similar systems and can be compared with the effects of other acceptor groups; hence they could be accepted as a measure of the resonance effect. Evidently, the canonical structures 8-11 are much more important for derivatives with a conjugated electron-donating group.These results were independently confirmed by a novel approach, based on determination of the weights of canonical struc- tures from experimental bond lengths,23 the so-called HOSE model. The results obtained for p-nitroaniline, its N,N-diethyl derivative and p-nitrophenolate anion are given in Table 3 and compared with those for nitrobenzene. In order to use nitrobenzene as a reference, for which there is no possibility of forming structure 11 by a through-resonance effect, only structures 6-10 were considered for all the compounds. Since the N-0 bonds are rather insensitive to substituent effects in which the nitro group is involved, the N-0 bonds were not considered in the calculations.Even if the HOSE estimates of the weights of canonical structures are only approxi- mate, it is clear that a considerable change of the resonance effect is observed when comparing nitrobenzene and the p-nitrophenolate anion: an increase from 40 to 51% for the sum of the weights of 8, 9 and 10 is observed. It is significant that structure 10, with a sym- metrical (C,) localization of double bonds, increases most signifi- cantly (from 13 to 21 %). This may be connected with two additional kinds of interactions: (i)a contribution of structure 11 which yields the same changes in geometry of the system under study, and (ii) an important contribution of a structure, in which the field effect of the nitro group causes the lone pairs of electron-donating substituents to interact more strongly with the n-system of the ring.4,5,22 We con- clude that the real structure of these para-derivatives is described Table 3 Weights of canonical structures indicating resonance interactions in nitrobenzene and its derivatives (HOSE Molecules Weights of canonical structures 6+6a 7+7a 8 9 10 NB 29.9 29.9 13.3 13.3 13.5 PNA 26.1 26.9 14.4 14.5 18.2 pNPhA 24.7 24.0 15.2 14.9 21.3 DpNA 3,5-DNXy 2,6-DNXy 26.3 28.6 30.1 26.3 28.6 30.1 14.1 13.6 12.9 14.1 13.6 12.9 19.1 15.8 14.1 a Abbreviations:NB: nitrobenzene; pNA: p-nitroaniline;pNPhA: p-nitrophenolate anion; DpNA: NjV-diethyl-p-nitroaniline;3.5-DNXy: NjV-dimethyl-4-nitro-3,5-xylidine; 2,6-DNXy: N,N-dimethyl-4-nitro-2,6-xylidine.not only by structures 6,6a, 7 and 7a, which indicate no resonance between the nitro group and the ring, but also by relatively large contributions of the canonical structures 8,9,10 and 11. An analy- sis of the geometry of nitrobenzene] 1 reveals negligible resonance, even if the weights of the appropriate canonical structures are not exactly equal to zero (Table 3). The same applies for several other acceptor substituents. Note that some physicochemical values have been used to evalu- ate the mesomeric effect directly. For instanct the IR intensity of the vI6band in mono-substituted benzenes24 was used thus. The value of a"Rfor NO, is not zero (Table 2) but it is not certain whether it is not influenced slightly also by the inductive effect: this could be of importance just for NO,.A theoretical model for the resonance effect may be based on the extent of withdrawal of electronic charge from the ring by substitu- ents in mono-substituted derivatives of benzene. STO-3G calcula- tions2s for the nitro group yield a non-zero value of 0.13 which is lower than that in Table 2 which was obtained from nitroethene as a model.20 As well as depending on the model, these kinds of values must depend also on the basis set used for calculations. For more reliable results it may be necessary to take electron correlation into account. 4.4 Other Substituent Effects Electronegativity constants26 are still somewhat mysterious and have been used very rarely, as have polarizability constants22 also (Table 2).The steric constants of the nitro group have been less extensively studied, the hydrophobic constants a little more: for the values see ref. 27. All these effects are overshadowed by the strong polar effect. 5 Pro and Cons for Resonance Effects in Nitrobenzene Derivatives 5.1 Problem Statement As seen already in previous sections, there is a serious dispute con- cerning the importance of resonance in nitrobenzene derivatives. Many arguments have been brought together from various areas outside the framework of the theory of substituent effects. For this reason we give a brief review in a separate section. Evidently the results will be different for nitrobenzene itself and for its derivatives with an electron-donating substituent in a conjugative position, 5.2 Theoretical Calculations The effect of the nitro group on the rest of the molecule and on global or local properties may be described by the energies of the lowest unoccupied and highest occupied molecular orbitals, LUMO and HOMO, and by the global charge distribution.These may all be obtained from molecular orbital calculations. Ab initio calculations on nitrobenzene with optimization of geometry at the 6-31G gave the following extents of with- drawal of the electronic charge from the ring: 0.521 for u electrons, 0.067 for Telectrons, with a total charge transfer of 0.588. Other calculations using different basis sets25,28-30 have given a similar picture.On considering the m-electron charges, it is immediately clear that typical resonance structures which may be drawn for nitrobenzene 6-10 are realistic but the effect is not too large. However this statement does not fit with the total electron charges: of all carbon atoms only C-1 is positively charged. The point is that all hydrogen atoms are a source of electrons for the carbon atoms to which they are attached. 5.3 Structural Arguments Analysis of the geometry of nitrobenzene'' leads to the conclusion that the mesomeric effect of the nitro group on the ring is very small. This conclusion is essentially in line with the above-mentioned quantum chemical calculations which either oppose the existence of this effect29 or indicate its rather low ~alue.~~.~~ The situation is quite different for p-nitrobenzene derivatives with electron-donating substituents leading to a push-pull effect.Valence bond calculations for p-nitroaniline and p-nitrophenol yielded very low weights for structure 11(<1.0%)5but, on the other hand, an analysis of the geometry of N,N-diethyl-p-nitroaniline 12, N,N-dimethyl-4-nitro-3,5-xylidine13 and NJV-dimethyl-4-nitro- 2,6-xylidine 14 leads to the conclusion8 that the steric effect of methyl groups in positions 3 and 5 disturbs the geometry of the N-Ph-N fragment significantly less than their action from posi- tions 2 and 6. pe2 c12> C133 ci4> Thus, the only conclusion may be that the contribution of the structure 11is important in descriptions of the molecular geometries of 12,13 and 14.This is well illustrated by a significant decrease of the sum of the weights of (8 + 9 + 10) from 47% for 12 to 43% for the 3,Sderivative and down to 40% for the 2,6-derivative, which is comparable to 40.1 % for nitrobenzene itself. Further, indirect support for the through-resonance effect in para-substituted derivatives of nitrobenzene comes from the good linear dependencies of the weights of various canonical structures on upor u;of the conjugated s~bstituents.~ For the canonical struc- ture 11the correlation coefficient r is -0.934 for 14 data points; not much worse are r-values for similar scatter plots with other canon- ical structures. 5.4 Discussion of Dipole Moments A classical argument for resonance originates from the dipole moment of nitrobenzene, which is greater than that of nitromethane or 2-nitro-2-methylpropane (Table 4). The difference was called the mesomeric dipole moment.31 Moreover, in 2,4,6-trimethyl-nitrobenzene possible resonance is suppressed and the dipole moment is actually reduced.32 However convincing this reasoning appears, it can be challenged when a more complete series of deriv- atives is compared33 (Table 4).With increasing size of molecules of aliphatic and alicyclic nitro compounds, their dipole moments increase owing to pure induction; ultimately they exceed the value for nitrobenzene. On the other hand, the dipole moments of 2,4,6- trialkylnitrobenzenes decrease continuously with increasing size of the alkyl group, although a further efficient hindrance to resonance is no longer possible.(At a twisting angle of say 70" the resonance is practically inhibited.) These facts can be understood in terms of induction, either within the alkyl group of RNO, or in the ortho-CHEMICAL SOCIETY REVIEWS, 1996 Table 4 Experimental dipole moments of some nitro compounds (in Debye)' R RNO, 2,4,6-R3C6H2N02 H - 3.97 Me 3.16 3.65 Et 3.21 - Pr* 3.31 3.59 Bur 3.42 3.45 1-Adamantyl 4-Adamanty l 1-Diadamantyl Cyclo-C,H, I 3.53 3.55h 3.76h 4.03 3.46 --- Ph 3.97 3.40 a Benzene solution, 298 K, data from ref. 33 unless otherwise noted. Ref. 34. alkyl groups of C,H,R,NO,, but a quantitative estimation is hardly possible.In any case the simple difference between an aliphatic and an aromatic derivative cannot be taken as a measure of the mesomeric effect: any resonance in nitrobenzene has not been proven from dipole moments. 5.5 Correlation of Reactivity Data This matter was summarized recently,35 and has been a subject of much disp~tation,~~.~~,l~,*~ proceeding mostly in terms of substitu-ent constants and not always in an understandable way. The indis- putable experimental fact is an approximate linear relationship between pK values of substituted benzoic acids, for para- vs. meta-derivatives with the same substituent. It is valid for substituents NO,, CN, S02X, CF,, CCl,, CH2Hal and others (only with slight deviations also for COX), in aqueous systems,19 non-aqueous sol- vent~,~~and the gas phase,35 not only for pK values, but also for rate constants of various reactions.19 Conjugated substituents, like OR, NR, and halogens, deviate very distinctly, The slopes of corre- sponding linear plots show that the substituent effect is somewhat stronger from the para- than from the meta-position.The simplest explanation is that all these substituents act by a single main mech- anism: it is merely a question of terminology whether it is called simply the inductive effect. Within the benzene nucleus the so-called .rr-inductive effect19 may be operative, explaining the greater effect in the para-position; this is in agreement with some quantum chemical calculation^.^^ The opponents of this idea argued that the mesomeric effect must be present but that it is proportional to the inductive effect for all named substituents.This cannot be l53I6 directly disproved: the problem is how such a general proportional- ity can come to exist for such different structures. Essential for the reasoning is the linearity of the plot, not its slope. Attempted proof that the slope is sometimes less than unityI6 has a statistical defect: omitting the point for hydrogen (origin of the coordinates) made it impossible to determine the slope reliably. Concluding this discussion, we offer the opinion that the pres- ence or absence of resonance must be considered with respect to an observable quantity and to its accuracy.With this in mind we find practically no resonance in nitrobenzene but can observe it in deriv- atives like 4-nitroaniline or 4-nitrophenolate anion, the extent of the observed resonance effect being dependent on the electron-donat- ing power of the para-substituent. An evident resonance effect of the nitro group is observed in the case of nucleophilic substitution but particularly in the case of vicarious nucleophilic substitution of hydrogen,38 for which it is a sine qua nun. 6 Conclusions The nitro group is an outstanding substituent which should be included whenever possible in studies of substituent effects: its effects are strong and can be quantitatively estimated with reliabil-ity. On the other hand, the theoretical interpretation of these effects is not always unambiguous, particularly the existence of resonance.THE NITRO GROUP AS SUBSTITUENT-0 EXNER AND T M KRYGOWSKI The idea of separating inductive and resonance effects evidently has its limits and the term resonance should generally be used in a quan-titative sense with respect to certain observable quantities Resonance is certainly not present, or at least not observable, in every single case where resonance formulae have been written in the literature Acknowledgements Both of us extend particular thanks to John Shorter who has kindly read and commented on the manuscript T M K wishes to acknowledge support of the research grant BST 24/94 7 References 1 0 Exner, Correlation Analysis of Chemical Data, Plenum Press, New York, 1988 2 C Hansch, A Leo and R W Taft, Chem Rev, 1991,91, 165 3 0Exner, in Correlation Analysis in Chemistry, ed N B Chapman and J Shorter, Plenum Press, New York, 1978, p 439 4 S D Kahn, W J Hehre and J A Pople, J Am Chem Soc , 1987,109, 1871 5 P C Hiberty and G J Ohanessian, J Am Chem Soc , 1982,104,66 6 K B Lipkowitz, J Am Chem Soc ,1982,104,2647 7 P Politzer, L Abrahamsen and P Sjoberg, J Am Chem Soc ,1984,106, 855 8 T M Krygowski and J J Maunn, J Chem Soc ,Perkin Trans 2,1989, 695 9 T M Krygowski and I Turowska-Tyrk, Coll Czech Chem Commun 1990,55, 165 10 A Domenicano, G Schultz, I Hargittai, M Colapietro, G Portalone, P George and C W Bock, Struct Chem ,1989,1, 107 11 R Boese, D Blaeser, M Nussbaumer and T M Krygowski, Struct Chem , 1992,3,363 12 M Colapietro, A Domenicano, C Marciante and G Portalone, Z Naturforsch Ted B, 1982,37, 1309 13 J Maurin and T M Krygowski, J Mol Struct , 1988,172,413 14 A P Cox and S J Wanng, J Chem Soc ,Faraday Trans 2, 1972,68, 1060 15 S Ehrenson, R T C Brownlee and R W Taft, Progr Phys Org Chem , 1973,10, 1 16 M Charton, Progr Phys Org Chem , 1981,13,119 17 0Exner and Z Fnedl, Progr Phys Org Chem , 1993,19,259 18 S Mamott and R D Topsom, J Am Chem SOC , 1984,106,7 19 0Exner, Collect Czech Chem Commun ,1966,31,65 20 S Mamott and R D Topsom, J Chem Soc, Perkin Trans 2, 1985, I045 21 V A Palm, Osnovy Kolichestvennoi Teorii Organicheskikh Reaktsir, Izd Khimya, Leningrad, 1967 ch IX 2, C J Hine, Structural Effects on Equilibria in Organic Chemistry, Wiley, New York, 1975, sect 3-312, Y Tsuno, M Sawada, T Fuji and Y Yukawa, Bull Chem Soc Jpn , 1979, 52, 3033 W Adcock, M J S Dewar and B D Gupta, .I Am Chem Soc , 1973,95,7353 22 R W Taft and R D Topsom, Progr Phys Org Chem , 1987,16, 1 23 T M Krygowski, J Kruszewski and R Anulewicz, Acta Crystallogr Sect B, 1983,32,732 24 R T C Brownlee, R E Hutchinson, A R Katntzky, T T Tidwell and R D Topsom, J Am Chem Soc , 1968,90, 1757 25 T M Krygowski, G Haefelinger and J Schuele, 2 Naturforsch ,Teil B, 1986,41,895 26 S Mamott, W F Reynolds, R W Taft and R D Topsom, J Org Chem , 1984,49,959 27 L Hansch and A J Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology, J Wiley, New York, 1979 28 C W Bock, M Trachtman and P George, J Mol Struct (Theochem),l985,155,122 29 J P Ritchie, Tetrahedron, 1988,44,7465 30 T M Krygowski, K Wozniak C W Bock and P George, J Chem Res (S), 1989,396 31 L E Sutton, in Determination of Organic Structures hy Physical Methods, ed, E A Braude and F C Nachod, Academic Press, New York, 1955,p 373 32 H Kofod, L E Sutton, P E Verkade and B M Wepstser, Rec Trav Chim Pays Bas, 1959,78,790 33 V VSeteEka and 0 Exner, Collect Czech Chem Commun , 1974,39, 1140 34 0 Exner, V JehliEka, L VodiCka and P Jakoubek, Collect Czech Chem Commun , 1980,45,2400 35 M Decouzon, 0 Exner, J F Gal andP C Maria, J Phys Org Chem 1994,7,615 36 0Exner and K Kalfus, Collect Czech Chem Commun ,1976,41,569 37 E R Vorpagel, A Streitwieser, Jr and S D Alexandratos, J Am Chem Soc , 198 1,103,3777 38 M Makosza and J Winiarski, Acc Chem Res ,1987,20,282

ISSN:0306-0012    

DOI:10.1039/CS9962500071

出版商:RSC    

年代:1996

数据来源: RSC