ISSN:0300-9238    

DOI:10.1039/F298783FX001

出版商:RSC    

年代:1987

数据来源: RSC

ISSN:0300-9238    

DOI:10.1039/F298783BX003

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 29-35 Chemical Reactions on Clusters Part 6.-Reactions of Simple Ketones in Association with Argon and Carbon Dioxide Clusters David M. Bernard and Anthony J. Stace* School of Molecular Sciences, University of Sussex, Falmer, Brighton BNl 9QJ Ion clusters of the type Ar;CH3COC2Hf, Ar, CH3COC3H;, (C02);CH3COC2Hf and (C02);CH3COC3H; for n = 1 to 21 have been formed by electron impact following the adiabatic expansion of an inert gas-ketone mixture. In each case the ketone was observed to undergo unimolecular decomposition to give Ar;R,CO+, Ar,.R$O+, (C02) :RLCO+ and (CO,);KCO+, where RL and Rs represent the larger and smaller aliphatic grouping on either side of the carbonyl group, respectively. Other products, which are present in the mass spectrum of the isolated ion, do not appear when the ketones are clustered with argon and carbon dioxide.These observations are discussed in terms of an integral cluster structure where the oxygen atom is retained in all the reaction products and steric interactions prevent it from participating in any rearrangement processes. In recent e~perimentsl-~ we have studied the unimolecular decomposition of a series of molecular ions in association with inert gas clusters. In general, the observed fragmentation patterns of the cluster-bound ions have differed significantly from those found for the corresponding isolated ions. One of the principal conclusions we have drawn from previous experiments is that reactive ions occupy sites on or close to the surface of the cluster.The reasoning behind this conclusion is twofold. First, many of the observed reactions involve extensive fragmentation of the polyatomic ion concerned and such processes could shatter the cluster if the reactant ion were completely solvated by the inert gas. Secondly, the relative intensities of the fragments from large clusters (more than 20 inert gas atoms) appear to be independent of cluster size. In terms of the influence the inert gas clusters appear to have in determining the fragmentation patterns, we would suggest that this is expressed through steric and possibly caging interactions. A further important aspect of previous e~perimentsl-~ has been concerned with establishing the mechanism responsible for exciting reactant ions above their dissociation limits. Existing chemical evidence'-3 suggests that some form of charge-transfer mechan- ism is in operation, with the molecular ion receiving the energy difference Ip (inert gas) -Ip (molecule), and recent photoionization experiments have provided strong support for this conclusion?-6 Once molecular ions have acquired the above energy difference, there exists within the cluster a competition between the breaking of covalent bonds and vibrational predissociation of the inert gas component.Approximate calcula- tions suggest that in order for the molecular ion to decompose the reaction timescale has to be less than 10-'2s.'-3 In this paper we present the results for the reactions of butan-2-one and pentan-2-one in association with argon and carbon dioxide clusters. Our prime objective in these particular experiments has been to examine the influence the inert gas cluster component has on the fragmentation patterns of the above molecular ions.If we can establish the exact role of the inert gas component, it may be possible to consider some of the cluster reactions as models for surface processes. 29 Chemical Reactions on Clusters Experimental Neutral clusters were generated by the adiabatic expansion of a gas mixture through a 100 pm pulsed nozzle operating at ca. 20 Hz. Following collimation through a 0.5 mm diameter skimmer positioned 2 cm from the nozzle, the modulated cluster beam was ionized by electron impact and mass analysed on a modified A.E.I.MS12 mass spec- trometer. The ion signal was monitored via a lock-in amplifier (Brookdeal 9503SC) which took its reference from the unit driving the pulsed nozzle. The electron impact energy in all the experiments was 70eV. As with previous e~perirnentsl-~ in this series, the optimum ketone concentration required to produce intense ketone-argon and ketone-carbon dioxide clusters was obtained by first introducing ketone vapour into the nozzle source from a reservoir attached to the gas line. The ketone was then progressively diluted with carrier gas until the clusters of interest appeared. The optimum ketone concentration appeared to be of the order of 100ppm. All the experiments were carried out at a nozzle stagnation pressure of 46psi.t The presence of impurities and possible mass coincidences with isotope peaks placed limitations on the accuracy with which measurements could be made.Although all the ketones used were of 99% purity or better and sodium sulphate was placed in the ketone reservoir to remove water it was not possible to remove all traces of the latter. Since Ar-H,O has a nominal mass of 58 and, as will be seen later, one of the possible products from the fragmentation of pentan-2-one also has the same nominal mass, any water present obscured the identification of this fragment. In the past’ isotopes have been used to overcome this problem, but this we have yet to do with these particular ketones. To minimise interference from Ar;N,f and Ar;O; clusters and their CO, counterparts, each ketone was repeatedly degassed.A further source of interference arose from metastable peaks formed from the reaction Ar: --* Ar:-l +Ar, as they coincide with the Ar;CH,CO’ peak for n in the range 8-12 from both butan-2-one and pentan-2-one. Intensity values for these ‘hidden’ peaks were obtained by measuring the ratio of the metastable peak height to the height of the adjacent 36Ar-Ar,-1 isotope peak with and without a ketone present. Since the metastable peak intensity was influenced by the presence of the ketone, the difference between the two ratios allowed us to calculate the ‘hidden’ peak intensity. However, there remains some uncertainty associated with the accuracy of these particular results.To maintain sensitivity the spectra were run with the ion source and collector slits set close to their maximum width. The subsequent loss of resolution made it difficult to observe individual high mass ion clusters where the peak separation was only 1 amu. In particular it was difficult to separate the fragment (C02);CH3CO+ from (C02):+l for n > 15. Throughout all the experiments the pressure in the expansion chamber was ca. 10-4-10-5 Torr,$ in the collimation chamber it was 10-5-10-6 Torr, in the ion source of the mass spectrometer the pressure remained below Torr and in the analyser tube the pressure was always less than lop7Torr. These low values allow us to disregard the possibility that ion-molecule reactions are responsible for our observed results.Results and Discussion The fragmentation patterns of the butan-2-one and pentan-2-one molecular ions in association with argon and carbon dioxide clusters have been studied. As stated in the introduction, we have concerned ourselves primarily with differences in behaviour t 1 psi =6.894 76 x lo3Pa. $1 Torr = 101 325/760 Pa. D. M. Bernard and A. J. Stace 31 30, 28, 26, 24. 22. .$ 20. 18, .- a, 16.*z 14,- !i12,-4 10- 0, 6, 4, 2, 0 I W . . . . ....,. between the cluster-bound and isolated molecular ions. Therefore, we have not examined in detail some of the other factors identified in earlier experiments, ie. the pressure and temperature dependence of relative product ion intensities.1-3 The fragmentation patterns for the isolated ions are given below.79s CH3CO+ CH3COC2Hl--C2H5COf --* C2Hf + C2Hl butan-2-one CH,CO+ CH3COC3H; C3H7COf + C2Hf -+ C2H; CH3C( OH)=CH2 pentan-2-one In our experiments we observed the following ions from the respective ketones: butan-2-one: Ar;CH,CO+, Ar;C,H,CO+, Ar;CH,COC,Hf, (C02),*CH3CO', (CO2),,-C2H5CO+, (CO2),-CH3COC2H;; pentan-2-one: Ar,-CH3CO+, Ar;C,H,CO+, Ar,.CH3COC3HT, (CO2),.CH3CO', (CO2),.C3H7CO', (C02),.CH3COC3H;. The relative intensities of the observed fragments are presented as a function of cluster size in fig. 1-4. Differences between our results and those for the isolated ions Chemical Reactions on Clusters 0 2 V , 4 l .6 . . , 8 , . 10 . , T 12 * . 14 T d 16 18 n Fig. 2. Relative product ion intensities plotted as a function of carbon dioxide cluster size for the decomposition of (CO2);CH3COC2Hf. For each point the intensity of the product ion on (CO,), was divided by the intensity of the parent ion on (CO,),. The open triangles represent the results for C2H5CO+and the solid triangles are for CH,CO+. The error bars indicate *1 standard deviation. All other details are as for fig. 1. can clearly be seen. First, we do not observe the fragments C2Hl (m/e = 29) and C2HT (m/e =27) from any of the molecular ions. Secondly, the McClafferty rearrangement fragment for pentan-2-one, CH,C(OH)=CH: (m/e = 58), is not present in the experi- ments involving carbon dioxide clusters.Unfortunately, it is not possible to confirm that the rearrangement fragment is also absent in the argon cluster experiments because of the mass coincidence between Ar,-,-CH,(OH)=CHl and Ar;H,O+. The fact that we did observe intense (CO,);H,O' peaks leads us to believe that any peaks occurring at Ar,-X+, where X has an m/e of 58, are probably due to the presence of water and not rearrangement products. The observed differences between the fragmentation patterns of the isolated and cluster-bound ions lends further support to earlier conclusions regarding the structure of these mixed clusters.'-3 It has been proposed that the oxygen atom in simple ketones and ethers forms an integral part of the structure of a and that the hydrocarbon component protrudes from the main body of the inert gas component.Hence, formation of the ions Ar;C2H; and Ar;C2H; would have involved the removal of an oxygen atom from its favoured position and this could explain why these ions are not observed. Similarly, steric hindrance would prevent formation of the McClaff erty rearrangement product, (CO2),-CH3C(OH)=CHl, because the labile proton would not have access to an oxygen atom embedded in the inert gas component. Table 1 shows normalised ion intensities for butan-2-one and pentan-2-one that were measured in our experiments, in charge-transfer experiments' and in the 70eV mass spectra of the two isolated parent ions.' In terms of our proposal that the molecular ion in the cluster is excited by charge transfer from the inert gas component, the data for Ar+ and CO; (recombination energy 15.8 and 13.8 eV, respectively) obviously represent upper limits to the energy available to each ketone.Since it has been shown that the ionization potential of a cluster is less than that of a single the D. M. Bernard and A. J. Stace 22. 20, 18, energy available to the molecular ions could be lower than that determined from the recombination energies given above. A factor which could be responsible for lowering the available internal energies still further is translational excitation of the charge-transfer agent.I3 For these reasons charge-transfer data for H,O+ and CO+ (recombination energy 12.4 and 14.0 eV, respectively) are also presented7 in order to provide a com- parison between our data and those from ketone fragmentation at low internal energies.Previous results have suggested that the energy available to the reactant ions on either an argon or a carbon dioxide cluster is lower than that provided by charge transfer from the respective atom or m~lecule.~ Finally, the 70 eV mass spectral data show how each ketone fragments when the isolated parent ions are generated under the same electron- impact conditions as those used in the cluster experiments. The comparison made in table 1 again emphasises the differences in fragmentation behaviour between our results and those obtained from more ‘conventional’ ion excita- tion methods. In particular, the CH3CO+ (m/e=43) fragment ceases to dominate as the most intense peak when the cluster results are compared with either the isolated ion mass spectra or the charge-transfer results; for the most part the relative ion intensities from the last two types of experiment are very similar.Inspection of the cluster experimental results shows that for the reactions of pentan-2-one on argon clusters, the C3H7CO+ peak (rn/e=71) is more intense than the 43+ peak for most values of n. Similarly, for butan-2-one on argon clusters the intensity of C2H5CO+ (m/e= 57) is comparable to that of the 43+ ion at most values of n. In contrast, the relative intensites of the same two fragment ions on C02 clusters more closely resemble those given for the other excitation processes considered in table 1.The results for the argon clusters are, however, consistent with observations from several very low energy electron impact studies of ketone fragmentation patterns. These experiments *,I4 suggest that forma- tion of the 43+ ion from ketones of the type CH3CORL, has a larger critical energy of reaction than the competitive process leading to the formation of RLCO+, where RL is the larger aliphatic unit, i.e. C2H5 and C3H7 in the present experiment^.'^-^^ Taking these facts into consideration we would suggest that in the present series of cluster experiments the molecular ions on argon receive very little internal excitation. In 34 Chemical Reactions on Clusters 20-18, 16, I x Y.-m 1 4, c u 12, .e 10, 8-6, 4, 2, n Fig.4. Relative product ion intensities plotted as a function of carbon dioxide cluster size for the decomposition of (C02);CH3COC3H;. For each point the intensity of the product ion on (CO,), was divided by the intensity of the parent ion on (CO,),. The open triangles represent the results for C3H7CO+ and the solid triangles are for CH3CO+. The error bars indicate *l standard deviation. All other details are as for fig. 1. Table 1. Normalised data for butan-2-one and pentan-2-one this work charge-transfer data" 70 eVb mass (Ar)13 (C02)13 Ar' CO; CO+ H20+ spec. butan-2-one CH~COC~H; 100 58 8 3 0 3 16 C2H,CO+ 75 16 5 5 5 15 7 CH,CO+ 81 100 100 100 100 100 100 -- 35 17 24 15 24 - 1% 4 20 0 20 CH3COC3H; 100 100 17 pentan-2-one 2 0 3 25 C3H7CO+ 96 25 12 4 4 13 9 CH3C(OH)=CHl CH3CO+ 55 -83 - 12 100 4 100 4 100 15 100 9 100 C2Hf - 10 2 4 1 16 C2H,+ - 19 7 20 1 10 " From ref.(7). From ref. (9). contrast, the relative product ion intensities on the C02 clusters suggest that the latter are acting as moderately high energy charge-transfer agents. This reversal in behaviour between argon and carbon dioxide clusters is in complete contrast to any previous observations we have so far made on cluster reactions. It is also evident from fig. 1-4 that the total product ion intensities on C02 clusters (as opposed to the intensities of individual ions) are similar to those found for argon clusters.Again, this behaviour is very different from that observed in previous experiments. 1-3 As yet we cannot offer an explanation for this change in behaviour on the part of C02 clusters. A further interesting observation can be made regarding the absence of the McClafferty rearrangement product (m/e = 58) for pentan-2-one. It can be seen from table 1 that in the charge transfer and electron impact experiments, the relative intensity D. M. Bernard and A. J. Stace of the rearrangement product is almost always comparable to that of the C,H,CO+ ion. However, in spite of the latter ion having a relatively high intensity in the cluster reactions, there is no evidence of the rearrangement product. As in previous experi- ment~'-~such an observation is consistent with the proposai that rearrangement reactions involving an oxygen atom are sterically hindered by the presence of the inert gas component.Conclusion In this paper we have presented the results of a series of experiments in which we have studied the fragmentation patterns of butan-2-one and pentan-2-one in association with argon and carbon dioxide clusters. The presence of an inert gas component appears to have considerable influence on both the nature and relative intensities of the reaction products, A recurrent theme in both these and previous cluster is the central role played by an oxygen atom in dictating the fragmentation processes. Such behaviour supports our conclusion that the oxygen atom in either a ketone or ether molecular ion forms an integral part of the structure of a cluster.The formation of such a unit appears to restrict the range of reaction paths available to the molecular ion, i.e. all the observed product ions exhibit retention of the oxygen atom, and rearrangement processes involving the transfer of an ion or atom to the oxygen are suppressed. References 1 A. J. Stace, J. Am. Chem. SOC.,1984, 106, 4380. 2 A. J. Stace, J. Am. Chem. Soc., 1985, 107, 755. 3 A. J. Stace, J. Phys. Chem., in press. 4 A. Ding, J. H. Futrell, R. A. Cassidy, L. Cordis and J. Hesslich, Surf: Sci., 1985, 156, 282. 5 W. Kamke, B. Kamke, H. U. Kiefl and I. V. Hertel, Chem. Phys. Lett., 1985, 122, 356. 6 P. D. Dao and A. W. Castleman Jr, J. Chem. Phys., 1986, 84, 1435.7 J. Turk and R. H. Shapiro, Org. Muss Spectrosc., 1971, 5, 1373. 8 H. Budzikiewicz, C. Djerassi and D. H. Williams, Muss Spectrometry of Organic Compounds (Holden-Day, Cambridge, Mass., 1967). 9 Eight Peak Index of Muss Spectra (Mass Spectrometry Data Centre, Reading, 2nd edn, 1984). 10 P. M. Dehmer and S. T. Pratt, J. Chem. Phys., 1982, 76, 843. 11 G. G. Jones and J. W. Taylor, J. Chem. Phys., 1978, 68, 1768. 12 S. H. Linn and C. Y. Ng, J. Chem. Phys., 1981, 75, 4921. 13 T. 0.Tiernan and C. Lifshitz, Abstracts, Int. Con$ Mass Spectrometry, Brussels, Belgium, 1970. 14 R. G. Cooks, A. N. H. Yeo and D. H. Williams, Org. Muss. Spectrosc., 1969, 2, 985. 15 E. Murad and M. G. Ingraham, J. Chem. Phys., 1964, 40, 3263. 16 W. Carpenter, A. M. Duffield and C. Djerassi, J. Am. Chem. SOC.,1967, 89, 6167. 17 A. J. Stace, J. Phys. Chem., 1983, 87, 2286. Paper 6/ 1024: Received 23rd May, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300029

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987,83, 37-47 Some Applications of Collision Spectroscopy of Gas-phase Ions John H. Beynon,* A. Gareth Brenton and Robert K. Boyd? Royal Society Research Unit, University College of Swansea, Swansea SA2 8PP Three inelastic processes that can occur during collisions of high-velocity gas-phase ions with gas molecules are considered. These are: charge exchange (sometimes called electron transfer), in which, typically, one or more electrons from the gas are transferred to the ion; charge stripping, in which one or more free electrons leave the ion; and excitation, in which the ion and/or the gas become excited but neither changes its charge state. Deductions that can be made concerning the states populated in these processes and the critical energies involved from measurements of the translational energy loss (or gain) of the fast-moving product are outlined.Novel equipment designed to improve the attainable energy resolution in the third processes is described. For brevity, the discussion is restricted to consideration of positively charged ions. Collisions of high-energy ions with gas molecules can effect a variety of inelastic processes in addition to elastic scatterin of the ions through a range of angles.' The interaction time is short, of the order lO-''to s for 10 keV ions of mass-to-charge ratio ca. 100, and the primary event in an inelastic collision is a purely physical one involving electronic excitation of one or both of the collisi'on partners. If the ion is formed in a repulsive state as a result of the collision, excitation will be followed by the chemical reaction of fragmentation, perhaps in a time of the order s.For polyatomic ions, fragmentation may be delayed by the time needed for the energy to concentrate in a particular bond via a series of radiationless transitions to high-lying rovibronic states of the electronic ground state. In many cases, fragmentation is preceded by isomerisation of the ion to a chemical structure of lower enthalpy and the necessity for isomerisation can further increase the time before fragmentation occurs. The present work will concentrate upon the first (physical) step and generally deal with relatively simple systems. It will consider aspects of three kinds of inelastic collisions resulting, respectively, in charge exchange,2 charge stripping3 or excitation without change of ~harge.~Only in the second of these processes is a free electron produced.The first process can be represented by Mx++G + M(x-y)++ GY+ (1) where Mx+represents the projectile species and G a collision gas molecule. The arrow beneath a symbol indicates that species to be moving with high velocity. The simplest such process is that in which x =y = 1. For a system in which M = G, e.g. for collisions of argon ions on argon atoms, there is an exact energy balance and the process is called 'resonance charge exchange' and has a large cross-section' of the order of lo-'' cm2. When M and G are different species, there is no longer an exact energy balance and the cross-section is very much smaller.It is usual to observe the high-velocity product of the reaction, M(x-y)+,and it will be shown below how, from this product, the states of excitation of both product species can be inferred. Because of the much greater difficulty of making measurements on uncharged products, experiments are usually conducted for product ions carrying one or more charges. The reactions are sometimes referred to as electron-capture reactions. ? Present address: Department of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada. 37 Collision Spectroscopy of Gas-phase Ions m1 4' / ,@-0 1 1 I I-\ \before collision \ after collision Fig. 1. Illustration of the progress of an inelastic collision between a projectile of mass rn, and a stationary target of mass rn2.Charge stripping can be represented by the reaction Mx++G + M(x+y)++Gz++(y+z)e-. (2) The reactions are most easily observable when y = 1. Interest in these reactions usually centres on measuring the ionisation energy of Mx+.For this reason, most experiments have been concerned with determining the minimum energy required to remove an electron from MXf,i.e. y = 1, z =0 in these experiments and the free electron carries with it a negligible amount of translational energy. Excitation without change of charge can be represented by M"'+G -+ MX+*+G*. (3) Either one or both of the collision partners may become excited as a result of the collision.Interest in such reactions has centred either on attaining high energy resolution in the case of small ions or on producing sufficient excitation of Mx+*to cause fragmentation to occur. Inelastic Scattering Processes The collision system of the ion and gas molecule is an isolated one and the energy balance in the reaction can be used to infer information about the overall reaction process by making measurements on only one of the products of the reaction. Consider, for example, a projectile of mass rn, and velocity vy approaching a stationary target of mass m2, as shown in fig. 1. Before the collision at a large separation the interaction potential energy is negligible and the projectile moves in a straight line towards the target, the impact parameter being 6, as shown in the figure.The trajectories of the ion and target then become affected by the interaction potential until they separate to a distance sufficient for the interaction potential to become negligible. When this occurs, the trajectories of both collision partners again become rectilinear. For collisions involving a particular value of overall inelasticity (sum of the additional excitation energies of the projectile and target following collision) valuable information can be deduced even in the absence of any explicit knowledge of the potential function describing the interaction during the collision itself. Let o1 and v2 be the velocities of rn, and m2 after the collision at angles O1 and O2 to the original direction of motion of rn,.Let Ey be the translational energy of rn, J. H. Beynon, A. G. Brenton and R. K. Boyd before collision, El and E2 the translational energies of m,and m, after collision and Q the inelasticity. Coordinates fixed in the laboratory frame of reference have been chosen rather than a centre-of-mass system because, as will be seen below, there exists a real ambiguity in most cases as to which centre-of-mass is appropriate. Five relation- ships definining El, E2, el, 62 and Q are theoretically possible from the laws of conservation of energy and of momentum and the equations of motion. These last two equations can be solved in principle only if the impact parameter is specified (an experimental impossibility) and if the potential-energy surface describing the interaction is known (which it is only in very simple systems).Thus, one must deduce as much as possible about the post-collision situation using only the three conservation equation^.^ In the majority of experiments it is the fast projectile or its fragmentation products that is detected. Thus it is appropriate to choose E2 and O2 for elimination from these three equations. When this is done and approximations are made appropriate for small angle of scattering (el= 0) and small translational energy loss { (E';-El)<< E';}it can be shown that the inelasticity Q is given by 0= (E;-El) -(ml/m2)E';e: where el is in radians. The first term in this equation is the translational energy lost by the projectile; the second term represents the translational energy gained by the target.Under experimental conditions for which (m,/ml) > 10 or so, the correction to Q for the translational energy of the target is negligible. The above can be applied to the three reaction types as follows. For reaction (l), let be the internal energy of the projectile ion and E~ be the sum of the excitation energies of the product ions. Let Ei(M) be the energy required to ionise M(x-y)+to Mx+ and Ei(G) the energy required to ionise G to Gy+.The energy balance for the reaction then requires that the difference of translational energy (Ey-El) between the reactant and product high velocity species is given by +(ET -El) = -~ 1~2 -Ei(M)+ Ei(G). If El is measured and E;, Ei(M) and Ei(G)are known, then the difference (E~-E,) is determined. If the energies of the excited states of the reactant and product ions are known, then an attempt can be made to match the difference ( s2-E,) to a particular set of states.The accuracy with which this can be done depends on the accuracy to which the energies of the excited states above the ground states are known and the accuracy to which measurements can be made. For reaction (2) it is required to find the minimum energy needed to remove an electron from the reactant ion. If a plot is made of the energies of the product charge-stripped ions, ions of maximum translational energy will correspond to this minimum energy process since translational energy is converted into electron energy in the charge-stripping process.It is thus necessary to determine the translational energy corresponding to the high-energy side of the product ion peak and from this the ionisation energy of the Mx+ion can be deduced. In both the above cases the experimental methods are similar; in both, the ion M"' is selected by, say, the magnetic sector of a double-focussing mass spectrometer and the translational energy of the product ion is measured using an electric sector. Because one is measuring a small difference between two large energies Ey and El it is, in both cases, desirable to devise a method for calibrating the energy scale of the electric sector. In the case of electron capture this is done by using a simple system such as ye2' + He -+ ye" + He" in which E~ is necessarily zero and there are few possible values of E~ and these are widely separated.Thus there is no chance of confusion in establishing the energy scale. In the case of charge stripping, an ion of known ionisation energy is used to calibrate the energy scale. The ion Ar'+ has sometimes been used, but it is more usual to use the Collision Spectroscopy of Gas-phase Ions molecular ion of toluene for calibration since it has a much larger cross-section for charge stripping than Ar" and its ionisation energy is well established. In the case of excitation without change in the number of charges carried by the ion, the ion M"' is selected by the magnetic sector and introduced into the gas cell. The ions leaving this cell will consist largely of ions Mx+that have passed through without loss of translational energy and a minor fraction of Mx+*ions that have lost translational energy in a collision.The ions of the two different translational energies are separated using the electric sector. The resolution achievable with a double-focussing mass spectrometer used in this way compares favourably with that of spectrometers specially designed for translational energy loss studies. The magnet spreads the ion beam into a momentum spectrum. Ions undergoing excitation transfer negligible momen- tum to the target gas and are scattered through only a small angle. The product ions Mx+* are thus still spread into the appropriate momentum spectrum to undergo velocity Gcus in the following electric sector.The final image is thus very sharp and the energy resolution is high because of the double-focussing properties of the magnetic sector/elec- tric sector combination. The attainable resolution will be discussed in more detail below. The three types of reaction discussed above can be used to study any ion species that can be made in a mass spectrometer ion source. By fragmentation reactions or by ion/molecule reactions within the source, ions can be synthesised that do not have stable neutral counterparts and which have not therefore been easily amenable to techniques such as photoelectron spectroscopy. For example, the ion CHf has been extensively studied6 and its ionisation energy determined; the ion NO; has also been synthesised in its less stable N-0-0 configuration7 and its collisional behaviour studied.Because the collisional technique is not stringently dependent on optical selection rules it can be used to study 'forbidden' transitions and because it does not depend upon the necessity for optical emission it is well suited to the study of stable or long-lived (metastable) states. Electron Capture For the process of single electron capture by a doubly charged ion colliding with a neutral molecule the potential-energy functions for the initial and final states may be approximated by: V,(r) = -2e2a (G)rW4+ cr-12 and Vf(r) = e2r-l- e2r-4[a(M)+a(G)]/2-AE where r is the internuclear distance, a the polarisability and AE is the energy defect of the reaction. The dominant attractive term in v(r)is the ion-induced dipole interaction which varies as the polarisability of G.The repulsive van der Waals term is approximated by an r-12 dependence, although the exact form is not important, since, at the reaction distances it is the first term that is mainly involved. The Vdr)function is dominated by the Coulombic repulsion term with a small perturbation at intermediate values of r from the ion-induced dipole interactions. Thus, electron capture has been recognised as a reaction that can occur through avoided curve-crossing. The transition from the initial to the final state is supposed to occur near the crossing point of a pair of potential-energy curves that correspond to different states of the separated atoms. For a process that is only slightly exothermic, the crossing point may be at a large value of r where the coupling is very small.In this event, the probability of a transition at the crossing point will be small compared to that at small internuclear distances. A typical energy loss/gain spectrum (for collisions of 02+ions on He)* is shown in fig. 2 and the computed potential-energy curves are shown in fig. 3. The nomenclature used J. H. Beynon, A. G.Brenton and R. K. Boyd 1 I I I I I I I 1 16X I IVXl,P llaX I 11px LX I1ax IIEXI npx II I IIIEX III6X IIIyX IIIPX I I I I -12 -8 -4 0 4 8 12 16 20 AEIeV Fig. 2. Energy loss/gain spectrum for collisions of 02+ions on He. in that due to Kamber et aL9-I, I1 and 111 represent the ground state (2p23P0)and the metastable states 'p2 'Doand 2p2 'Soof the incident 02+ions, a, p, 7,... are the ground state and sequence of excited states of the product 0'' and X, A, B, . . .the ground and sequence of excited states of He". It can be seen that the spectrum of fig. 2 is dominated by the reaction channel IPX that involves capture of an electron to give the first excited state of O", a significant contribution from the IyX reaction and minor contributions from several other processes. From the collisions of such ions as 02' and Ne2+lo on rare-gas and molecular targets and from collisions involving transfer of more than one electron from the collision gas" it is possible to make the following broad generalisations.The reactions observed occur by curve-crossing and by non-curve-crossing processes. The probability of reaction is greatest when there is a crossing of the potential-energy curves at internuclear distances of between 4 and 7 a.u. This corresponds to the reaction being exothermic by ca. 4 eV. The Wigner-Witmer spin conservation rule is observed by heavy rare-gas targets, but some reactions violating spin conservation have been unequivocally identified, e.g. for He, Ar and N2targets." Charge Stripping A review of charge-stripping experiments with ions ranging in size from monatomic to polyatomic has been given recently by AS^.^ Early work using mass spectrometers showed that rare-gas ions produced in an electron impact source contained, in addition to ground-state ions long-lived electronically excited ions." The ions could be characterised by the translational energy loss suffered in collisions with gas molecules in which doubly Collision Spectroscopy of Gas-phase Ions 8 IIIIIIIII 6 4 2 %---.2c 0)E -2 -4 -€ I 1 I I I I I Fig.3. Potential-energy curves for the system 02++ He -D 0" + He". charged ions were formed. A typical ion translational energy spectrum is shown in fig. 4. Later work has extended these studies to a variety of monat~mic,'~"~ diatomic,15 triatomic16 and p~lyatomic'~^" species. It has been possible to measure in this way the vertical ionisation energy of almost any singly charged ion that can be formed in a mass spectrometer, to an accuracy of the order 0.3 eV. Such measurements are possible even if the doubly charged polyatomic ion undergoes fragmentation.The fragments share the translational energy in proportion to their mass and thus carry with them a memory of the translational energy loss. The versatility of the method can be illustrated with two examples. The method was used to measure the ionisation energy of CH';. A value of 17.9 eV was obtained" and the product CHZ+ ions were shown to be stable on the mass spectrometer timescale. The previously accepted value for this ionisation energy was 28.0 eV2' (measured by Auger spectroscopy from neutral CH,). Furthermore, it had been predicted that CH;+ would be unstable and break in a time of the order s.Perhaps the greatest success of the method to date has been the preparation of He;' from To avoid interference, it was necessary to prepare the He*: from a mixture of 3He and ,He and to filter out ions of Niformed in the field-free region from minute traces of air in the instrument. These ions have the same energy-to-charge ratio as 'He4He2+ formed by charge-stripping. After all precautions were taken a current of 6 x A of Heif, the smallest diatomic molecule known (bond length 0.704 A) was observed as shown in fig. 5. The ionisation energy of He': was measured as 37 f2 eV. J. H. Beynon, A. G. Brenton and R. K. Boyd I I I I I I I 50 40 30 20 10 0 energy loss / eV Fig. 4. Charge-stripping spectrum for collisions of Kr" +N2-+Kr2++N2.I 1 I 1 I I I 120 100 80 60 40 20 0 energy loss I eV Fig. 5. Charge-stripping spectrum of (3He4He)'+. Collision Spectroscopy of Gas-phase Ions I I I I I I 1gx IEX 16X IyX IpX' IaX II IIII -8 -4 0 4 8 12 energy 1ossjeV Fig. 6. Energy loss spectrum for collisions of N'*+ on He. Translational Energy Loss Spectroscopy When an ion collides with a neutral target gas without change of charge, as in the case of reaction (3), both the collision partners may become excited as a consequence of conversion of translational energy of the fast ion to internal excitation of the products. For collisions in the keV range there is generally negligible momentum transfer to the target gas for small scattering angles, and thus excitation (or de-excitation) processes can be characterised by measuring directly the translational energy loss spectrum (TELS) of the non-decomposing ions undergoing collisions.Early experiments in which an ion collides with a neutral target gas without change of charge4923 used proton beams, where excitation of the ion does not occur, to study target excitation. Excellent work has been described by Moore and D~ering;~~ for the excitation of molecular nitrogen Moore25 demonstrated an energy resolution of 150 meV, whilst Linder26 improved on this by a factor of three for H+ collisions with CO and H2. Recently, Itoh et dZ7have demon- strated the latent potential of the technique, with an ultimate resolution of ca. 10 meV, at the lower end of the energy range they studied (which was from 50 to 400 eV).They illustrated their attainable performance in a study of the rotational states of H2 by Li+ impact. In virtually all these experimental arrangements, improvements in the energy resolution were made by lowering the collision energy and retarding the product to low energies ( <50 eV) prior to the energy analysis stage. There are, however, many instances in which considerably higher translational energies are needed to drive the reactions in which high energy resolution is also required. In the studies just described, the experimental equipment was purpose-built for translational energy loss spectrometry and lacked the many features and facilities that modern mass spectrometers have, such as a wide choice of ionisation techniques.However, it has recently been shown by Illies and Bowers2* that a high-resolution double-focussing mass spectrometer, used with 8 keV ions, can achieve an ultimate resolution of 100meV. Suppose that AE is the excitation energy imparted in the collision, then it would seem at first sight that a resolution of EY/AE would be needed in order J. H. Beynon, A. G. Brenton and R. K. Boyd 3 FFR A collision cell 2FFR GT4FFR detector Fig. 7. Schematic diagram of the new translational energy loss spectrometer being built in Swansea. to separate the excited product ions from the main beam of unexcited ions. The required performance would, however, depend on the definition of the 'resolution'.Resolution is generally defined in mass spectrometry in terms of a '1O0/o valley' between two peaks of equal height, i.e. to a contribution of the skirt on one peak at the centre of the other peak of a few per cent of the peak height. The probability of excitation in a beam of ions passing through a mass spectrometer collision cell is likely to be of the order 1 in lo4 or so and thus, to resolve the resultant TELS peak the skirt of the much larger peak due to ions that have not undergone collisions would need to be reduced to about 1 in lo4. Resolution would thus need to be defined in terms of a 0.01% valley. This value is obviously much lower than the generally quoted mass resolution figure for a mass spectrometer. Illies and Bowers demonstrated the performance by resolving the vibra- tional transitions from X 2Zgto the A 'nuand B 'E: states of Nl impacting on a helium target gas.Another example of the technique, involves energy loss studies of the doubly charged ion N.'+ colliding on He.29 Fig. 6 shows the translational energy loss spectrum. The peaks are labelled according to the following notation: I represents the ground state (2p 'Pl12)of N.*+incident ions, whilst 11,111and TV represent the first and successive higher excited states of these ions; Y, a and b represent the ground and successive higher excited states of the target atoms, whilst X represents single ionisation of the target atom. The spectrum is dominated by the process I +I11 which is due to excitation of N*2+ from its ground state (2p'Pl12)to the second excited state (2p2205,2).The cross-section for this process decreases rapidly as the collision energy is reduced.An Instrument for Translational Energy Loss Spectrometry In designing a new instrument for translational energy loss measurements, the object was to retain the versatility of the double-focussing mass spectrometer, but to improve the ultimate resolution. The final design chosen consists of a magnetic sector followed by two identical electric sectors. The performance is determined very largely by the electric sectors which provide a double-focussing combination. The gas cell in which the ions are excited is located between these sectors, the arrangement being shown schematically in fig.7. The principles underlying the design have already been de~cribed,~'aberrations produced in the first electric sector being largely cancelled in the second because of the symmetrical arrangement. The entire energy-dispersed beam enters the collision cell; ions will undergo inelastic collisions in all regions across this energy dispersion. The product ions that have lost translational energy will retain the Collision Spectroscopy of Gas-phase Ions correct energy dispersion to be refocussed after the second electric sector to give a double-focussed image. Thus, to plot the spectrum of translational energy loss, it is only necessary to scan the second electric sector voltage over a small range. The TRIO (third-order ion optics) computer program3' has been used to calculate the aberrations in the image of S2 produced at S, for all positions, directions and velocities of ions passing through S2.As a result of the calculations it has been decided to position four hexapoles, as shown in fig. 7, for correcting some of the aberrations and two weak lenses (also shown in the figure), for controlling the position of intermediate direction focus between the electric sectors. The system should operate most efficiently for the smallest values of ion excitation energy because these correspond to the smallest angles of deflection of the ions in the inelastic collisions and thus give minimum degradation of the double focussing. The role of the magnet is to illuminate S2 with ions of the chosen mass-to-charge ratio.Although the magnet produces a momentum (and thus an energy) dispersion across S2, this does not appreciably affect the size of the image at S4, due to the double-focussing properties of the pair of electric sectors. The same momentum disper- sion is retained across the final image. It is hoped that this equipment, which is currently under construction, will match or even exceed the best energy resolution so far achieved in previous studies that were described in the last section. We thank the Royal Society and the University College of Swansea for support of this work. References 1 J. B. Hasted, Physics of Atomic Collisions (Butterworths, London, 2nd edn, 1972). 2 J. H. Beynon, R. K. Boyd and A. G. Brenton, in Advances in Muss Spectrometry, 1985 (John Wiley, Chichester, 1986), part A, p.437. 3 T. Ast, in Advances in Muss Spectrometry, 1985 (John Wiley, Chichester, 1986), part A, p. 471. 4 J. T. Park, Adv. At. Mol. Phys., 1983, 19, 67. 5 R. K. Boyd, E. E. Kingston, A. G. Brenton and J. H. Beynon, Proc. R. SOC. London, Ser. A, 1984,392, 59. 6 C. J. Proctor, C. J. Porter, T. Ast, P. D. Bolton and J. H. Beynon, Org. Muss Spectrom., 1981, 16, 454. 7 M. Guilhaus, A. G. Brenton, J. H. Beynon, A. O'Keefe, M. T. Bowers and J. R. Gilbert, In?. J. Muss Spectrom. Ion Process., 1985, 63, 11 1. 8 E. Y. Kamber, A. G. Brenton, J. H. Beynon and J. B. Hasted, J. Phys. B, 1985, 18, 933. 9 E. Y. Kamber, D. Mathur and J. B. Hasted, J. Phys. B, 1982, 15, 263. 10 E.Y. Kamber, A. G. Brenton and J. H. Beynon, J. Phys. B, 1984, 17, 4919. 11 E. Y. Kamber, W. G. Hormis, A. G. Brenton, J. B. Hasted and J. H. Beynon, J. Phys. B, 1985, 18, 117. 12 T. Ast, J. H. Beynon and R. G. Cooks, J. Am. Chem. SOC.,1972, 94, 6611. 13 C. J. Porter, C. J. Proctor, T. Ast and J. H. Beynon, Int. J. Muss Spectrom. Ion Phys., 1982, 41, 265. 14 M. RabrenoviC, T. Ast and J. H. Beynon, In?. J. Muss Spectrom. Ion Process., 1984, 61, 31. 15 C. J. Proctor, C. J. Porter, T. Ast and J. H. Beynon, Int. J. Muss Spectrom. Ion Phys., 1982,41, 251. 16 C. J. Porter, C. J. Proctor, T. Ast and J. H. Beynon, Croat. Chem. Actu, 1981, 54, 407. 17 T. Ast, C. J. Porter, C. J. Proctor and J. H. Beynon, Bull. SOC. Chim. Beogrud, 1981, 46, 135. 18 M. RabrenoviC, C.J. Proctor, T. Ast, C. G. Herbert, A. G. Brenton and J. H. Beynon, J. Phys. Chem., 1983, 87, 3305. 19 M. RabrenoviC, C. G. Herbert, C. J. Proctor and J. H. Beynon, Inf.J. Mass Spectrom. Ion. Phys., 1983, 47, 125. 20 T. Ast, C. J. Porter, C. J. Proctor and J. H. Beynon, Chem. Phys. Lett., 1981, 78, 439. 21 R. Spohr, T. Bergmark, N. Magnusson, L. 0.Werme, C. Nordling and K. Siegbahn, Phys. Scr., 1970, 2, 31. 22 M. Guilhaus, A. G. Brenton, J. H. Beynon, M. Rabrenovib and P. von Ragui Schleyer, J. Phys. B, 1984, 17, L605. 23 Collision Spectroscopy, ed. R. G. Cooks (Plenum Press, New York, 1978). 24 J. H. Moore and J. P. Doering, J. Chem. Phys., 1970, 52, 1692. 25 J. H. Moore, J. Geophys. Res., 1972, 77, 5567. J. H. Beynon, A. G. Brenton and R. K. Boyd 26 F. Linder, in Electronic and Atomic Collisions, ed. N. Oda and K. Takayanagi (North Holland, Amsterdam, 1980), p. 525. 27 Y. Itoh, N. Kobayashi and Y. Kaneko, J. Phys. B, 1981, 14, 679. 28 A. J. Illies and M. T. Bowers, Chern. Phys., 1982, 65, 281. 29 E. Y. Kamber, A. G. Brenton and J. H. Beynon, Int. J. Mass Spectrorn. Ion Process., 1986, 68, 203. 30 J. H. Beynon, A. G. Brenton and L. C. E. Taylor, Znr. J. Muss Spectrom. Zon Process., 1985, 64, 237. 31 T. Matsuo, H. Matsuda, Y. Fujita and H. Wollnik, Mass Spectrorn., 1976, 24, 19. Paper 6/1201; Received 13th June, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300037

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987,83, 49-59 Some Approaches to Spectroscopic Characterization of Polyatomic Cations John P. Maier Institut fur Physikalische Chemie, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Spectroscopic approaches aimed at vibrational and rotational characteriz- ation of open-shell polyatomic cations by means of their electronic transitions are described. Two of the techniques used rely on the detection of the fluorescence of the electronically excited cations in the gas phase. These are based on emission spectroscopy using electron impact excitation to probe rotationally cooled ions in supersonic free jets and to investigate ions of unstable molecules, and on laser excitation spectroscopy on ions produced by Penning ionisation and collisional relaxation.With the latter method rotational studies of the electronic transitions of some tri- and tetra-atomic ions have also been carried out. In the case of the non-fluorescing ions, the electronic transitions can be followed in absorption in neon matrices at 5 K. Examples of such studies and the information forthcoming are given. Spectroscopic characterization of ions has become the focus of much experimental effort in recent years.'32 In considering open-shell polyatomic cations comprised of more than three atoms, the progress really started about 20 years ago with the advent of photoelec- tron spectroscopy which provided the first extensive set of data on their electronic state^.^ Though it presented a general method yielding important insight into the electronic structure of the ions and their precursor molecules, the finer spectroscopic details remained unravelled owing to the inherent limited resolution of the technique. Neverthe- less, the information gained on the energy disposition of the electronic states of open-shell cations enabled one to devise, or apply, spectroscopic approaches relying on the wavelength measurement of absorbed or emitted photons, and thus to improve drastically the resolution with concomitant gain in information on their vibrational and rotational structure.The first method to be applied systematically was electron impact excitation of the emission from electronically excited ions in a crossed-beam a~~angement.~ This led to the identification, especially by reference to the photoelectron spectra, of the electronic transitions responsible for the emission in over 100 larger open-shell cations.Many of these involve halogeno-substituted benzene and polyacetylene cations or their struc- turally related derivative^.^ The vibrational frequencies of a number of the totally symmetric modes in the ground and lowest excited electronic states could be deduced from such emission spectra, albeit with an accuracy of ca. *lO cm-' determined not by the optical resolution used but by the inhomogeneous broadening of the bands. Consequently, the development in this approach was to replace the effusive source of the molecules by a seeded helium supersonic free jet.5 By this means, rotationally and, to a lesser extent, vibrationally cooled molecular species are produced and then ionised.The ions thus produced retain their low rotational temperature, whereas their vibrational population is spread on ionisation among the vibrational levels accessible according to the selection rules and their Franck-Condon factors. The result is a drastic narrowing of the vibronic bands 49 Spectroscopic Characterization of Polyatomic Cations in the emission spectrum, enabling the band maxima and hence vibrational frequencies to be deduced to within *l cm-', or even less.6 This technique can also be applied to the study of the emission spectra of ions of unstable molecules produced 'in situ' by fast flow approaches, and examples of such studies are given in this article.The detection of the fluorescence from the electronically excited cations opened up the possibility of applying the 'laser-induced fluorescence' approach to probe more extensively the vibrational structure in the excited state and to benefit from the inherent higher resolution of the technique in rotational ~tudies.~ Again, some selected examples are chosen to illustrate these aspects. The other techniques which rely on the radiative decay, and have in recent years been applied to study electronically excited cations, involve the detection of photons in coincidence with either energy-selected electrons or mass-selected ions. Their aim was to investigate the radiationless decay pathways and rates of the excited ions and the reader is referred to recent reviews on this subject8 The third approach discussed here is the vibrational characterization of the cations in the excited electronic states by measuring the absorption spectra in neon matrices at 5 K9 The advantage of the technique is that one does not require the excited ions to fluoresce as in the abovementioned methods; the disadvantage lies in the lower resolution and sometimes in the complications arising due to matrix effects.In these experiments the ions are generated by photoionization of the molecules embedded in the matrix and the absorption is enhanced by the total internal reflection technique." Spectral informa- tion, not only on the lowest, but sometimes also on higher-lying excited electronic states of the cations can be obtained.Vibrational Characterization One of the goals of our studies on open-shell cations of the larger polyatomics has been to obtain their vibrational frequencies in the ground and lowest excited electronic states. To this end, rotationally cooled ions have been investigated in emission and by laser excitation.6 In the former case, the ions are obtained rotationally cooled to 5-10 K by CQ. 200 eV electron-impact excitation of a helium supersonic free jet seeded with the molecular precursor. The narrowing of the vibronic bands in the emission spectra is considerable as compared to the spectra using an effusive source; in addition the otherwise overlapping bands become discernible.In the measurements af the laser excitation spectra, the cations are initially produced by Penning ionization using He or Ar metastables (depending on the first ionization energy of the molecule). The rotational and vibrational cooling is achieved by collisional processes with the unexcited rare gas bath (1-10 mbar). When the rare gas is cooled by a liquid-nitrogen environment, the rotational and vibrational temperatures of the cations produced aie also red~ced.~ A pulsed dye laser, operating with a bandwidth of ca. 0.2 cm-' for the vibrational characterization, transfers part of the cationic population to an optically accessible lowest excited electronic state and the total fluorescence is monitored as a function of the laser wavelength.In practice, optical cut-off filters and gated detection help to improve the signal-to-noise ratio. By this means the vibrational details of the ion in the excited electronic state are mapped out.6 In fig. 1 the principal features of the emission and laser excitation approaches to characterize vibrationally rotationally cooled ions are summarized. The complementary nature of the two experiments is evident. In the emission -measurements radiative relaxation from the initially populated levels in the excitzd, A, electronic state by the ionization process to vibrational levels of the ground, X, cationic state is observed. Thus one sees the excitation_of_the totally symmetric modes (with respect to the common symmetry elements of the X, A states) as well as sequence transitions of the noe-totally symmetric modes which are sufficiently populated in the molecular ground (XI state. J.P. Muier E Trot 10 K Trot z 10 K M 'X emission supersonic free jet Fig. 1. Principal features of the approaches to characterize vibrationally rotationally cooled open-shell polyatomic cations using electron-beam excitation of the emission from supersonic free jets and by laser excitation of fluorescence after Penning ionization and collisional relaxation. Owing to the supersonic expansion, the vibrational temperature is around 50 K and hence only the lowest frequency (<200 cm-') modes need be considered. On the other hand, in the laser excitation experiment the vibrational temperature of the ions in their ground state is expected to be reduced to, say, 100K, and thus again most of the population is concentrated in the zeroth vibrational level and to some extent in level: lying within ca.300 cm-l above it. The population of the_ vibtational levels in the A state-now depends on the Franck-Condon factors for the A +-X transition rather than the A +X ones in the emission experiment (CJ fig. 1). Consequently, differences in the intensity and types of modes excited are often apparent in the two sets of spectra. Comparison of these data can therefore be crucial for the interpretation of the structure in the respective spectra. As an example, the emission and laser excitation spectra of the A2ng**'rIn, transition of rotationally cooled triacetylene cation" are shown in fig.2. The emission spectrum corresponds to T',, =5-10 K and the excitation spectrum to Trot== 100 K. As can be seen, the emission spectrum maps out primarily the vibrations1 structure in the ground state of the ion, whereas the excitation spectrum maps the A2Hg state. There are four totally symmetric, (T;, fundamentals,12 vl to v4, which are excited in the two states in progressions and in combinations between themselves. Also apparent is the excitation of the degenerate vl0 mode in double quanta in the excited state (this gains intensity by Fermi interaction with the near-lying allowed 4; transition) and of the lowest-frequency degenerate ~13mode in the ground state which remains sufficiently populated in the supersonic expansion.The vibrational frequencies can be obtained to within *2 cm-' from these spectra and the values for the cri modes are summarized in table 1. It is seen that frequency changes relative to the molecular values (cf:table 1) reflect the description of the electronic structcre of triacctylene and its cation.13 For example, the v2(CEEC) values are smaller in the X 'rIU and A 2Hgstates compared to the molecule, indicating bonding characteristics in the CrC region in the highest and penultipate occupied molecular orbitals. Conversely, the v4 (C-C) frequency is lgrger in the X state than in the X 'Zf molecular one, but slightly smaller in the A 211g,pointing to Spectroscopic Characterization of Polyatomic Cations 1''"I' ~~~l~lll,.II,~,,,,l,,,,(, ,~ ,,, 20500 20000 19500 19000 18500 18000 17500 ij/ cm-' Fig.2. The A 'IIg f* r? 211uelectronic transition of rotationally cooled triacetylene cation observed in emission [(a)0.04 nm] and as laser excitation spectrum [(b)0.02 nm] [from ref. (ll)]. The emission spectrum was excited by cu. 200 eV electron impact on a seeded helium supersonic free jet (concomitantly excited helium and hydrogen emission lines are labelled with a dot) and for the excitation spectrum the ions were produced by Penning ionization with argon metastables and collisional cooling in a liquid-nitrogen environment. antibonding and almost non-bonding characteristics in the two highest occupied molecular orbitals, respectively. Similar vibrational characterizations have been carried out by these two techniques for many of the open-shell polyatomic cations for which the radiative decay is known.These are listed in table 2, where the references to the studies are given. A further useful feature in the emission spectra using supersonic free jets is the change in the rotational profile of the vibrational transitions as a function of temperature. Although the optical resolution of the emission technique is not high (typically 0.3-1 cm-' f.w.h.m.) and full rotational details are usually not resolved, nevertheless in some J. P, Maier 53 Table 1. Vibrational frequencies (*2 cm-')_of the totally symmetris CT; modes of triacetylene cation in the ground X 211uand first excited A 211g state inferred from the emission and laser excitation spectra (cf: fig.2)" v1 v2 v3 v4 state v(C-H) v(C-C) v(C=C) v(C-C) 5; 3313 2201 2019 625<2nld 2182 1903 632 A2n, (3243) 2053 1880 617 a The molecular ground-state values are from ref. (12). Table 2. Open-shell organic cations for which vibrational frequencies (*1-2 cm-') have been obtained in their ground and excited electronic states from the emission spectra (ES) and laser excitation spectra (LES) at reduced rotational temperatures ref. band system ES LES X-C_N+, X = C1, Br, I 14 -X--C=C-H+,X=Cl 15 16 = Br 17 18 =I 17 19 X-C~EC-X+, X = C1, Br, I 20 21 = CN -22 C1 -CEC-Br+ 23 23 XfCrCj2H+, X = Cl, Br 24 24 XfC=Cj,X+, X = Cl, Br 25 25 HfC=Cj3H+ 11 11 CH,-C-C-X+, X =C1, Br 26 26 CH3+C_Cj2X+, X = C1, Br 27 27 = CN 28 28 29RfC-Cj2H+, R =CH3, CD3 ii2~+d2~30 2~uRfC=Cj2R+, R = CH3, CD3 A C* 22~g 31 30,32 R=CFj 33 33 cases the contour changes are distinct enough to yield important spe_ctroscopi_c informa-tion. An cxample of this is given by the origin bands of the A211, f* X211gand A 211gt*X 211utransitions of di- and tri-acetylene cations, respectively." In the absence of a _rotatiGnal analysis, one cannot decide whether the spin-orbit splitting is larger in the X or A degenerate 211states.In fig. 3 are shown the recordings in emission of the origin bands of these transitions of the di- and tri-acetylene cations at Trot-5-10 K (top traces) and Trot=70-100 K (bottom). Considering first the rotational profile in the case of diacetylene cation, one can see that the (I1 branch becomes clearly apparent on the high-energy part of the band at the low temperature; the Q2branch lying at lower energy is not seen because its intensity is 1/9 of that of the Q1 branch according to the Honl-bondon factors.Because one expects the R = 3/2 and R = 1/2 sub-components of the X 211nand A 211u states to lie in the inverted sequence, i.e. A" and A'<O, and as the observation (fig. 3) shows that the R = 3/2 origin lies to higher energy of the R = 1/2 one, it is concluded that !A"!> !A'].This is, of course, in accord with the rotational analysis of this transition which has been carried out, and the inferred spin-orbit constants are A"= -33 cm-' and Spectroscopic Characterization of Polyatomic Cations I1 I I 16680 16670 16660 16650 19730 19720 19710 c/cm-' Fig.3. Rotational profiles of the origin bands of the A211, +2211u and A 'nu--* 2211gtransi-tions of triacetylene (left, 0.02 nm) and diacetylene (right, 0.007 nm) cations, respectively, observed in emission using seeded helium supersonic free jets [from ref. (ll)]. The top traces correspond to a rotational temperature of 5-10 K and the bottom ones to 70-100 K. A' =-3 1 In contrast, the individual rotational lines of triacetylene cation transi- tion can not be resolved but again in the low rotational temperature emission spectrum the Q1 branch becomes apparent on the high-energy side of the band (fig. 3), though not as clearly as with the diacetylene cation.The separation of the R1and Q1 maxima is ca. 2 cm-', as for the diacetylene cation. Consequently, the conclusion is once more that [A"[>IA'I and, since the bands are red-shaded, that B"> B'. Another area where the initial spectroscopic information is most readily obtained by emission spectroscopy is in the study of cations of unstable, or very reactive, molecular species. The ones of concern to us are cations of such molecules which have been previously studied by microwave and photoelectron spectros~opies.~~ We have adapted the developed methods of their production in order to search for the emission spectra of the cations following electron-impact ionisation. For this purpose the reactor (usually an oven) was incorporated near to the ionisation region of the crossed electron-sample beam apparatus and the emission of thejroducts formed by fast-flow pyrolysis or related reactions was observed.The A 211+ --* X 211electronic transitions of HCP+ and DCP+,36 FCP+,37HBS+ and DBSf,38 FBS+, ClBS+39 we well as the A 2A,--* 22E transition of CH3CP"40have now been observed. The identification of the band systems has greatly been helped by the known photoelectron spzctra of ;heir molecular prec~rsors.~~ In fig. 4 is reproduced the detected A 21S++ X 211 emission spectrum of DBS' obtained by ca. 200 eV electron-impact excitation on the gaseous reaction products after passing D2S over B at ca. 1000°C.38 Two partially overlapping band sub-systems are apparent as a result of the spin-orbit splitting, A"= -322 cm-', in the ground state of the ion.A vibrational analysis can be carried out; the pattern is, however, complicated by Renner-Teller and Fermi interactions, the more detailed examination of which leads to the spectroscopic parameters mimicking th,e effects. yost of the vibrational frequen- cies of the three fundamental modes in the X 211and A 'lS+ states have been obtained for the H1'B32S+, H10B32S+, D' 1B32S+and D'oB32S+ isotopes.38 Having located the spectral features of interest using this electron impact method, one can envisage higher- resolution studies by means of laser excitation. J. P. Muier 55 000 -00 v 0 1 2 I1 I I I 1 001-000 n riHBS+ 000 -10 v 0 1 I 1 I I I 1 I 1 I I I I I 21000 20000 19000 18000 17000 fi/cm-' Fig.4. The A 'Z+ -+ 2 211 emission band system (0.026 nm) of DBS+ obtained by ca. 200 eV electron impact on the products of the reaction between D2S and B chips at ca. 1000°C [from ref. (38)]. Bands of HBS+ are also apparent owing to incomplete deuteration (H/D=0.25). Rotational Characterization In the case of the cations of unstable molecules,-rotationa_l structure has been resolved so far by the emission technique only for the A 2C+-+ X 211transitions of HCP+ and HBS' with a resolution of ca. 0.005nm. A preliminary set of rotational and related constants has been given for HCP' and DCP+ from the analysis of the spectra;41 for the final analysis the application of the laser excitation technique is calLed for to a_ttain the higher-resolution data. For HBS+ the rotational constants in the X 211 and A 'C+ states are known with reasonable accuracy from emission measurement^;^^ however, the spectrum and the constants given were incorrectly attributed to an 2C-211transition of BS.Our studies of the vibrational features in the emission spectra on deuteration confirm the carrier to be FBS'. Thts the published constants42 refer to the respective isotopes of HBS' in the X 211 and A 2C+ states and not to the BS In recent years measurements of the rotational structure have been successfully csrried oui by the laser-excitation technique (0.02 cm-' band-pass) for the A 2113/2 5 2113/2 electronic transitions of OCStM and of X-C=C-H+ (X = C1,16 Br, I ) and of their deuterated derivatives.The case of OCS+ is somewhat special because fluorescence is detectable only from the zeroth vibrational level of the rR, = 3/2 and a= 1/2 components of the A 211state.46 The fluorescence is quenched for the higher levels by the faster (>lo9s-*) predissociation pathway, which already begins to dominate even for the lowest level (cu. 7 x lo6s-I) compared to the fluorescence rate of ca. 2 x lo6s-'.~' Thus the laser excitation of the fluorescence is not the most suitable approash for this ion and in fact the inferred rotational constants for the lowest level of the X 211and A *IIstates44 have in the meantime been confirmed by laser-photodissoci- ation studies whereby the S+ fragment ion produced is m~nitored.~~ The rotational structure of not only the origin but also of bands involving excitation o,f the C--X stretching vibration in the excited state has been studied for the A 2113,Fe* X 2113,2 and iodo-acetylene" cations. In transition of the chloro-,16 br~rno-~' fig.5 is shown the laser excitation spectrum of the origin band of this transition of Spectroscopic Characterization of Polyatomic Cations 20 545 20 540 20 535 ;/ cm-' eg. 5. Higher-resolution laser excitation spectrum (0.02cm-') of the origin band of the A zI13/2-X 2113/2 transition of the deuterobromoacetylene cation. The rotational assignments for the two bromine isotopic species are outlined. Table 3. Rotational constants, Be, (cm-'l for the halo enoacetylene cations in their zeroth vibrational level of the X 2113/2and j2113/2states inferred from the rotational structure in the laser excitation spectra ~~ 35C1-CEC-H+ 0.194 647 (49) 0.170 881 (48) 16 37C1-C=C -H+ 0.191 063 (60) 0.167 680 (60) 16 35C1-C-C-D+ 0.176 968 (38) 0.156 338 (37) 16 37Cl-C=C-D+ 0.173 665 (46) 0.153 342 (45) 16 79Br-C-C-H+ 0.137 794 (37) 0.121 351 (36) 45 "Br-CzC-H+ 0.137 327 (38) 0.121 032 (38) 45 79Br-CEC-D+ 0.125 767 (29) 0.111 312 (29) 45 81Br-CrC-D+ 0.125 152 (33) 0.110 772 (33) 45 I-CZC-H+ 0.109 59 (7) 0.096 69 (7) 19 I-C-C-D+ 0.100 44 (8) 0.088 99 (8) 19 ~ a Figures in parentheses represent one standard deviation.deuterobromoacetylene cation recorded with ca. 0.02 cm-' resolution. The rotational assignment is outlined above the lines.Owing to the presence of the two naturally occurring isotopes of bromine, there are two overlapping R and P branches, the Q branches being too weak to be detected at such source temperatures. As was also the c3se with- the chloro- and iodo-acetylene cations, only the R = 3/2 sub-system of the A 'lI c* X 211 transition could be studied because the a=1/2 levels are insufficiently populated as a result of the collisional relaxation of the ions prioEto laser excitation. For the bromoacetylene cation the spin-orbit separation in the X 211state is 1OOOk 160 cm-'. J. P. Maier The Beffrotational constants for the zeroth levels of the two electronic states, as well as the values in the upper state involving the u3 (C-Br) vibrational levels, have been obtained for the naturally occurring isotopes of the halogenoacetylene cations and of their deuterated analogues.In table 3 the BLffand B,”,constants obtained for these ions are collected. Since not all the isotopic derivatives could be studied, only partial r, geometric structures are available. Absorption Spectra in Neon Matrices In order to obtain spectroscopic information on the cations which do not relax by a radiative transition, approaches other than those outlined above are necessary. One such technique is based on the measurement of the absorption spectra of the electronic transition of the cations held in neon matrices at ca. 5 K9 The molecular precursor of the ion is first embedded in a neon matrix at a ratio in the region 1:2000-5000 after which the matrix is irradiated by either a windowed H(Ly a),or windowless Ne(1) photon source.The cationic absorption intensity usually reaches a plateau after few minutes of photoi~nization.~’ The absorption spectrum is measured in the usual fashion with a ca. 0.1 nm band-pass monochromator, photodiode or photomultiplier and phase- sensitive detection. The absorption path length is, however, enhanced by passing the light in a waveguide manner through the matrix via ca. 100 pm slits parallel to the cold substrate and matrix surface.” The substrate is of rhodium- or aluminium-coated oxygen-free copper. Thus a light pathlength of 2 cm is achieved. This technique was applied initially to ions by Bondybey et a2.: who studied the absorption spectra of some of the fluorinated benzene cations, several of which were known to fluoresce.By comparison of the matrix data with the gas-phase data in such cases, it is found that although the electronic transitions (of 211t*211type) are batho- chromically shifted a few hundred wavenumbers in the matrix relative to the gas phase, the vibrational frequencies themselves tend generally not to differ by more than 5-10cm-’. Thus analysis of the structured absorption spectra of ions in neon matrices can be most useful for the determination of vibrational frequencies in the excited electronic state. Information on the cationic ground state is not obtained because at ca. 5 K the vibrational population is entirely in the lowest level.We have adapted this technique to investigate some of the open-shell cations the emission spectra of which could not be detected. In the first instance these included the structurally related ions, X+CEC+~X+ with n = 1 or 2, characterized also in the gas phase for X = C1 and Br,20*21 but not for X = I, n = 1,50or 2 51 and then the species studied first in matrices, X-C=N+,52 X-CrC-C=N+ 53 with X =C1, Br or I, as an aid to subsequent gas-phase Most recently, the absorption spectra of the cations of cyanoacetylene, methylcyanoacetylene and of cyanogen could be obtained.55 For such ions, the only information hitherto available was from photoelectron spectros- This yielded the location (*0.02 eV) of the accessible doublet states in the photoionization process and one or two vibrational frequencies in the lower electronic states with an accuracy of kt80 cm-’.In fig. 6 are shown the electronic absorption spectra of cyanogen and methyl- cyanoacetylene cations in a 5 K neon matrix.55 Although the vibrational bands are not as sharp as is often the case for ions constrained in neon matrices, an assignment of the peaks is nevertheless possible. This is indicated in the figure. ge transition shown for cyanogen tation is from the zeroth level of the ground state X 2113/2,g to the third excited state, C 2113/2,u, and one infers the vibrational frequencies (k10 cm-’) of the ul and u2, cr: modes and of the u4 degenerate T mode which is excited in double quanta and gains intensity by Fermi interaction with the transitions involving the v2 vibration (cJ: fig.6). InJhe case ef methykyanoacetylene cations, two partly overlapping electronic transitions, B *E3/2, A 2Al +X 2E3/2, are observed. The totally symmetric u2 and u6 58 Spectroscopic Characterization of Polyatomic Cations 0.15-08 (aj 0 0.1 -e8’0.05-0.0 I 5000 5100 5200 5300 5400 5500 5600 5700 5800 5900 wavelength/ 8, (b) A 2A,+22E3,2 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300 wavelength/ A Fig. 6. The electronic absorption spectra (0.1nm resolution) of the indicated transitions of (a) the cyanogen cation and (b) the methylcyanoacetylene cation in neon matrices at cu. 5 K [from ref. (%)I. modes are excited in the A 2A, state and v5 an4 v6 in th,e 52E state.Also, it should be noted that the relatively high intensity of t,he A 2Al +-X 2E3,2band system is a result of vibronic coupling of the B 2E and A 2A1 states which lie merely ca. 0.3 eV apart.57 In conclusion, it is hoped that this method can now be used to obtain the initial data on the electronic transitions of ions that are known only by mass-spectroscopic measurements, such as fragment and isomer ions, and subsequently enable more refined gas-phase experiments to be undertaken. The research studies described in this article have been possible owing to the efforts of the various coworkers whose names are to be found in the references. The projects have been financed throughout the years by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung.References 1 Molecular Ions, ed. J. Berkowitz and K-0. Groeneveld (Plenum Press, London, 1983); Molecular Ions: Spectroscopy, Structure and Chemistry, ed. T. A. Miller and V. E. Bondybey (North-Holland, Amsterdam, 1983); Gas-phase Ion Chemistry, ed. M. T. Bowers (Academic Press, London, 1984), vol. 3. 2 R. J. Saykally and R. C. Woods, Annu. Rev. Phys. Chem., 1981,32, 403; T. A. Miller, Annu. Rev. Phys. Chem., 1982,33,257;T. A. Miller and V. E. Bondybey, Appf. Spectrosc. Rev., 1982, 18, 105; T. A. Miller and V. E. Bondybey, Philos. Trans. R. SOC.London, Ser. A, 1982, 307, 617; C. S. Gudeman and R. J. Saykally, Annu. Rev. Phys. Chem., 1984, 35, 387.3 D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970). 4 J. P. Maier, in Kinetics of Ion-Molecule Reactions, ed. P. Ausloos (Plenum Press, New York, 1979), p. 437; J. P. Maier, Chimia, 1980, 34, 219. 5 A. Camngton and R. P. Tuckett, Chem. Phys. Lett., 1980, 74, 19; T. A. Miller, B. R. Zegarski, T. J. Sears and V. E. Bondybey, J. Phys. Chem., 1980, 84, 3154; D. Klapstein, S. Leutwyler and J. P. Maier, Chem. Phys. Lett., 1981, 84, 534. 6 D. Klapstein, J. P. Maier and L. Misev, in Molecular Ions: Spectroscopy, Structure and Chemistry, ed. T. A. Miller and V. E. Bondybey (North-Holland, New York, 1983), p. 175. J. P. Maier 7 T. A. Miller and V. E. Bondybey, J.Chim. Phys. Phys. Chim. Biol., 1980,77,695;J. P. Maier, Acc. Chem. Res., 1982, 15, 18. 8 J. P. Maier and F. Thommen, in Gas-phase Ion Chemistry, ed. M. T. Bowers (Academic Press, London, 1984), Vol. 3 p. 357; S. Leach, J. Mol. Struct., 1986, 141, 43. 9 V. E. Bondybey, T. A. Miller and J. H. English, J. Chem. Phys., 1980, 72, 2193. 10 R. Rossetti and L. E. Brus, Rev. Sci. Instrum., 1980, 51, 467. 11 D. Klapstein, R. Kuhn, J. P. Maier, M. Ochsner and W. Zambach, J. Phys. Chem., 1984, 88, 5176. 12 E. Bjarnov, D. H. Christensen, 0.F. Nielsen, E. Augdahl, E. Kloster-Jensen and A. Rogstad, Spectrochim. Acta, Part A, 1974, 30, 1255. 13 F. Brogli, E. Heilbronner, V. Homung, and E. Kloster-Jensen, Helu. Chim. Acta, 1973, 56, 2171. 14 J. Fulara, D. Klapstein, R.Kuhn and J. P. Maier, J. Phys. Chem., 1985, 89, 4213. 15 D. Klapstein, R. Kuhn and J. P. Maier, Chem. Phys., 1984,86,285; J. Electron Spectrosc., 1985,35, 171. 16 M. A. King, J. P. Maier and M. Ochsner, J. Chem. Phys., 1985, 83, 3181. 17 J. Fulara, D. Klapstein, R. Kuhn and J. P. Maier, J. Phys. Chem., 1986, 90,2061 18 J. P. Maier and L. Misev, J. Chem. SOC., Faraday Trans. 2, 1984, 80, 43. 19 J. P. Maier and M. Ochsner, J. Chem. SOC., Faraday Trans. 2, 1985, 81, 1587. 20 D. Klapstein, J. P. Maier and W. Zambach, Chem. Phys., 1983, 77, 463. 21 J. P. Maier and L. Misev, In?. J. Mass Spectrom. Ion Processes, 1984, 58, 243. 22 J. P. Maier, L. Misev and F. Thommen, J. Phys. Chem., 1982, 86, 514. 23 D. Klapstein, J. P. Maier, M. Ochsner and W.Zambach, J. Electron Spectrosc., 1984, 34, 161. 24 D. Klapstein, J. P. Maier, L. Misev and W. Zambach, Chem. Phys., 1982, 72, 101. 25 D. Klapstein, J. P. Maier and L. Misev, J. Chem. Phys., 1983, 78, 5393. 26 D. Klapstein, R. Kuhn, J. P. Maier, M. Ochsner and T. Wyttenbach, Chem. Phys., 1986, 101, 133. 27 D. Klapstein, R. Kuhn, J. P. Maier, L. Misev and M. Ochsner, Helu. Chim. Acta, 1984, 67, 1222. 28 D. Klapstein, J. P. Maier, L. Misev, F. Thommen and W. Zambach, J. Electron Spectrosc., 1984,31,283. 29 S. Leutwyler, D. Klapstein and J. P. Maier, Chem. Phys., 1983, 78, 151. 30 J. P. Maier and L. Misev, Chem. Phys., 1980, 51, 311. 31 D. Klapstein, R. Kuhn, S. Leutwyler and J. P. Maier, Chew. Phys., 1983, 78, 167. 32 T. A. Miller, V. E. Bondybey and B.R. Zegarski, J. Chem. Phys., 1979, 70, 4982. 33 D. Klapstein, J. P. Maier, L. Misev, F. Thommen and W. Zambach, J. Chem. SOC., Faraday Trans. 2, 1982, 78, 1765. 34 J. H. Callomon, Can. J. Phys., 1956, 34, 1046. 35 H. W. Kroto, Chem. SOC. Rev., 1982, 11, 435 and references therein. 36 M. A. King, D. Klapstein, H. W. Kroto, J. P. Maier, 0.Marthaler and J. F. Nixon, Chem. Phys. Lett., 1981, 82, 543. 37 M. A. King, D. Klapstein, H. W. Kroto, R. Kuhn, J. P. Maier and J. F. Nixon, J. Chem. Phys., 1984, 80, 2332. 38 M. A. King, D. Klapstein, R. Kuhn, J. P. Maier and H. W. Kroto, Mol. Phys., 1985, 56, 871. 39 M. A. King, R. Kuhn and J. P. Maier, J. Phys. Chem., 1987, in press. 40 J. Lecoultre, M. A. King, R. Kuhn and J. P. Maier, Chem. Phys.Lett., 1985, 120, 524. 41 M. A. King, D. Klapstein, H. W. Kroto, J. P. Maier and J. F. Nixon, J. Mol. Spectrosc., 1982, 80, 23. 42 J. K. McDonald and K. K. Innes, J. Mot Spectrosc., 1969, 29, 251. 43 These states, incorrectly attributed to BS, are labelled F and G in the tables of K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Vol. IV, Constants of Diatomic Molecules (Van Nostrand, New York, 1979). 44 M. Ochsner, M. Tsuji and J. P. Maier, Chem. Phys. Lett., 1985, 115, 373. 45 M. A. King, J. P. Maier, L. Misev and M. Ochsner, Can. J. Phys., 1984, 62, 1437. 46 M. Horani, S. Leach, J. Rostas and G. Berthier, J. Chim. Phys., 1966, 63, 1015. 47 D. Klapstein and J. P. Maier, Chem. Phys. Lett., 1981, 83, 590. 48 R. Kakoschke, U.Boesl, J. Hermann and E. W. Schlag, Chem. Phys. Lett., 1985, 119, 467. 49 V. E. Bondybey and L. E. Brus, Adv. Chem. Phys., 1980, 41, 269; V. E. Bondybey and T. A. Miller, in Molecular Ions: Spectroscopy, Structure and Chemistry, ed. T. A. Miller and V. E. Bondybey (North- Holland, Amsterdam, 1983), p. 125. 50 S. Leutwyler, J. P. Maier and U. Spittel, Mol. Phys., 1984, 71, 437. 51 S. Leutwyler, J. P. Maier and U. Spittel, Chem. Phys. Lett., 1983, 96, 645. 52 S. Leutwyler, J. P. Maier and U. Spittel, J. Chem. Phys., 1985, 83, 506. 53 S. Leutwyler, J. P. Maier and U. Spittel, J. Chem. SOC., Faraday Trans. 2, 1985, 81, 1565. 54 J. Fulara, D. Klapstein, R. Kuhn and J. P. Maier, J. Phys. Chem., 1985,89, 4213; R. Kuhn, J. P. Maier, L. Misev and T. Wyttenbach, J. Electron Spectrosc., 1986, in press. 55 J. Fulara, S. Leutwyler, J. P. Maier and U. Spittel, J. Phys. Chem., 1985, 89, 3190. 56 C. Baker and D. W. Turner, Proc. R SOC. London, Ser. A, 1968, 308, 19. 57 G. Bieri, E. Heilbronner, V. Hornung, E. Kloster-Jensen, J. P. Maier, F. Thommen and W. von Niessen, Chem. Phys., 1979, 36, 1. Paper 611203; Received 13th June, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300049

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 61-68 The Calculation of Potential-energy Surfaces for Molecular Ions David M. Hirst Department of Chemistry, University of Warwick, Coventry CV4 7AL A review is given of the application of the multi-reference configuration interaction method for the ab initio calculation of potential-energy surfaces for molecular ions. The importance of using adequate basis sets and of choosing suitable sets of reference configurations is discussed. Recent calcu- lations for SiH+ and SiHl are outlined in the context of their relevance to recent spectroscopic and dynamical experiments. In recent years there have been several major developments in experimental techniques for the investigation of the spectra of molecular ions and for the study of the dynamics of ion-molecule reactions.The use of ion-beam techniques for the laser spectroscopy of ions has been reviewed by Carrington' and these methods have been used to obtain spectra for species such as CO+, O;, CH+, HD+ and Hr.' Laser photofragment spectroscopy, in which the spectrum is obtained by monitoring the appearance of fragment ions arising from the predissociation of levels in the excited state, has recently provided detailed information for ions such as SH+, SiH; and PH+.2 The laser magnetic resonance technique has been used by Saykally to obtain rotation spectra for hydrogen halide ions (HBr+, HC1+ and HF'), for OH+, SH+ and H20+.3 High-resolution infrared spectra for a variety of molecular ions have been obtained by Saykally using velocity modulation infrared spectro~copy.~ Infrared absorption spectra for molecular ions such as D30+, HN; and H30+ have been obtained by Sears and Davies using diode laser absorption spectro~copy.~ The development of guided beam techniques by Armentrout and his coworkers has enabled accurate measurements to be made of the energy dependence of total reaction cross-sections for a wide variety of ion-molecule reactions such as Ar++H2(D2,HD), Si+ +H2 and C++H2(D2, HD).6 Differential cross-sections can be measured by the conventional crossed-beam technique.' Detailed information about the states of the product ion or molecule in an ion-molecule reaction can be obtained by the detection of chemiluminescence.Ottinger and coworkers have made extensive studies of the chemiluminescence from electronically excited states for reac- tions such as B+ +H2,C++ H2,N++H2, Al+ +H2 and Pt+H2.* Vibrational distributions for products formed in the electronic ground state can be obtained by detection of infrared chemiluminescence in the flowing afterglow experiment' or by laser-induced fluorescence detection in the crossed-beam technique developed by Leone and coworkers.lo In many cases the interpretation of the experimental data can be facilitated by the use of calculated potential-energy surfaces for the systems of interest.There are several levels at which theory can be used. For the interpretation of molecular spectra of a diatomic molecule one requires calculated potential-energy curves for the states of interest.For light molecules it is now possible to make such ab initio calculations using large basis sets and taking electron correlation into account by extensive configuration interaction. Vibration-rotation levels for the state can then be calculated by solving the nuclear Schrodinger equation using the potential obtained by interpolating between the ab initio points. Reviews by Bruna et al" and by Peyerimhoff l2 discuss many applications and recent examples from our own work include BH+? AlH+ and OH+.13 For simple triatomic species it is possible to make ab initio calculations of comparable accuracy".'2 61 Potential-energy Surfaces for Molecular Ions and methods have been developed for the calculation of vibrational 1e~els.l~ Further-more, ab initio calculations enable one to determine the intersections between potential surfaces and such information is very valuable for the interpretation of perturbations in the spectrum.For ion-molecule reactions the ultimate objective is the calculation of a potential-energy surface which is sufficiently extensive to be used in dynamical calculations. This is a very formidable undertaking because calculations have to be made for a very large number of geometries. An additional complication for ion- molecule reactions, as compared with reactions between neutral species, is the prevalence of non-adiabatic effects involving surfaces of the same symmetry in close proximity. Very extensive calculations for the surfaces of the C++H2 system have been made by Sakai et aLi5 However, even if it is not feasible to calculate potential-energy surfaces which are sufficiently extensive for dynamical calculations, theoretical potential surfaces are of great value for the qualitative interpretation of the observed reaction dynamics.In particular, one can locate regions where non-adiabatic transitions may be important. For example, in the reaction of N+ with H2one can understand, on the basis of calculated potential surfaces,16 why the dynamics show a change from involving a collision complex at low relative energies (ca. 1eV) to a direct stripping process at higher energies. In the reaction of B+ ('S) with H2, chemiluminescence is observed from the B' 2C+ and A211 states of BH+.84 These states can only be formed from the reactants via non-adiabatic transitions. Potential-energy surfaces have been calculated for the Bf +H2 system17 and possible mechanisms have been suggested for the formation of the chemiluminescent states of BH+.Ab Initio Configuration Interaction Calculations In this section we review briefly the methods which are appropriate for the calculation of potential-energy surfaces for molecular ions. More detailed discussions can be found elsewhere." The configuration interaction method takes as a starting point a suitable set of molecular orbitals {+i}which are expressed as linear combinations of a set of atomic basis functions {xr}: +i =ZCirXr. (1) C The first prerequisite for a satisfactory description of a potential surface is the appropriate choice of a basis set.Most ab initio calculations for polyatomic species use basis sets of contracted Gaussian functions in which each basis function xr is a linear combination of Gaussian functions. It is generally recognized that in order to treat adequately the changes in electronic distribution that occur as bonds are formed and broken one requires at least two basis functions for each atomic orbital in the valence shell of each atom plus a set of polarization functions (p functions for hydrogen and d functions for first-row atoms). Two commonly used basis sets which fulfil this requirement are the double-zeta plus polarization (DZP) basic sets" and the 6-31G"" basis developed by Pople and coworkers.20 The importance of including polarization functions in calcula- tions for molecular ions was demonstrated quite dramatically by Pearson and Roueff ,21 who showed that for C++H2,with the inclusion of polarization functions, the intersection between the 2Al and 2B2surfaces lies at an energy of 63 kJ mol-' below that of the isolated reactants in contrast to an energy of 43 kJ mol-' above obtained by Liskow et aZ.22using a basis set which did not include such functions. However, the molecular orbital method is not capable, in general, of giving a satisfactory description of a potential-energy surface because the wave function does not usually dissociate correctly.Furthermore, it does not make adequate allowance for electron correlation.These deficiencies can be overcome by the method of configuration interaction. If the basis set consists of m functions, the molecular orbital calculation will yield m molecular orbitals. Let us suppose that in the molecular orbital function D. M. Hirst Qo, n of these orbitals are occupied. Thus there will be m -n unoccupied virtual orbitals. By exciting electrons from the occupied orbitals to the virtual orbitals we can generate a set of configuration state functions, Qi, which have the same spin and symmetry as Qo. In the method of configuration interaction, the wavefunction Q is written as a linear combination of the functions CDi : N @= 1 aiQi. i=O The coefficients ai are obtained by using the variation method.In general, this full CI method will yield a much larger number of functions Qj than can be conveniently handled and it is necessary to place some restriction on the excitations included. It can be shown by perturbation theory that the most important configurations are those obtained by the excitation of one or two electrons from Qo. However, for the calculation of potential-energy surfaces we cannot restrict ourselve5 to this set of configurations. The function Q0 may give a reasonable description of the equilibrium geometry of the molecule in its ground state but will not adequately describe dissociation or excited states. Thus, in order to give a balanced treatment of all regions of the surfaces of interest we have to choose a set of reference configurations {ak}which will include Qo and those configurations appropriate to excited states and the dissociation limits.All single and double excitations are then generated from the set of reference configurations {ak}.The set of reference configurations should include all those which will have a coefficient 30.1 in the final CI wavefunction. Even with these restrictions one may generate a very large number of configurations. The length of the configuration list can be reduced to the order of a few thousand by the use of perturbation theory.23 Electron correlation can be accounted for by the methods of many-body perturbation theory,24 but generally the method is restricted to cases where one reference function is the dominant configuration in the CI wavefunction.It does not seem to be capable of describing parts of the potential surface where several reference configurations are of comparable importance as in the bond-breaking process in the reaction CH4 -+ CH3+ H (3) for which many-body perturbation theory25 gives a potential surface which rises much more steeply than that obtained with the CI method.26 Potential-energy Curves for SiH+ Four electronic states, namely X 'Z+, A 'n, a 311 and 3Z+, of SiH+ correlate with Si+(2P)+H. Analyses of the A 'n --* X 'E+ band system have been given by Douglas and L~tz~~ and by Carlson et al.28and spectroscopic constants for these two states have been obtained. Currently Sarre and coworkers29 are investigating the spectrum of SiH+ by laser photofragment spectroscopy and have observed resonances which they attribute to transitions from the X 'E+ state to quasibound levels of the A 'n state.In order to facilitate the interpretation of their data we have made a set of ab initio calculations for the potential-energy curves of the X 'Z+, A 'n, a 311 and 3Ec+states of SiH+. Bruna and Peyerimhoff 30 have calculated potential-energy curves for these states but have published their results only as a diagram and not in a form such that the potential curves can be used for the calculation of the vibration-rotation levels which will be of importance in the experiments of Sarre et al. High-quality calculations have been made for the ground state by Rosmus and Meyer3' but, again, full details of the potential-energy curve were not given.The multi-reference CI method was used for the calculation of the potential-energy curves and full details of the basis set and configuration interaction are given elsewhere.32 The calculated potential-energy curves are shown in fig, 1 and the tabulated energies can be found in ref. (32). We have used the potential-energy curves obtained by Potential-energy Surfaces for Molecular Ions -289.9-2M-a\ Eh -289.0 -209.2 -289.3 1 2 3 4 RlA Fig. 1. Potential-energy curves for the states of SiH+ correlating with Si+(2P) +H('S). (Repro-duced, with permission, from Gem. Phys. Lett. 1986, 128, 504.) Table 1. Calculated and experimental spectroscopic constants for SiH+ X lZ+ a 3~ calc. expt.calc. oe/cm-' 2155.35 2157.10 1729.79 oexe/cm-' Be/cm-' a,/cm-' 38.82 7.6786 0.2082 34.21 7.6603 0.2096 103.68 7.3553 0.5 179 A GlI2/cm-'AG3/2/cm-' D/eV 2077.71 2000.07 3.097 2088.68 2020.21 3.22 f0.03 1522.42 1315.06 0.893 interpolation between the calculated energies in the calculation of the vibration-rotation levels for the X 'C' and a3n states. In table 1 we report the spectroscopic constants derived from the calculated vibration-rotation levels. In the case of the X 'I1+ state we obtain excellent agreement with experiment and thus the constants calculated for the 311 state should be reasonable predictions. The A 'II state is less well described in that the calculated well depth is 0.091 eV (733 cm-'), whereas the experimental dissociation energy is estimated to be 1230 f210 cm11.28 However, the accurate calculation of small dissociation energies is a severe test for theory. Potential-energy Surfaces for SiHl There is currently interest in the spectroscopy of SiHt2' and in the dynamics of the reaction6b Si++H2 + SiH++H. (4) D.M. Hirst Fig. 2. Bending potential curves for the 22A,and states of SiHl, These curves_have been calculated for bond lengths of r = 1.4828, for the X state and r = 1.465 %, for the A state. A portion of the curve for the 2B2state (r= 1.465 %,) is also shown. (Reproduced, with permission, from MoZ. Phys., 1986, 59, 141.) Curtis et aL2' have observed transitions from the 22A1state to the A 'B1state, which undergoes predissociation to Si+ or SiH+.Elkind and Armentrout6b have measured the energy dependence of the total reaction cross-section for reaction (4) and concluded that the reaction occurs without any activation energy beyond the endothermicity of the reaction. Potential-energy surfaces have bee!! calculate4 for SiHl with the objective of provid- ing bending potential curves for the X 2A1and A 2Bl(21-I)states which can be used for the calculation of vibronic levels and of obtaining potential-energy surfaces which can be used in the interpretation of the cross-section data. Full details of the configuration interaction calculations have been published else- where.33 The calculated geometries for the 2Al (rsiH = 1.482 A, LHSiH = 120") and the 211 ( rSiH= 1.468 A) states are in quite good agreement with the experimental values obtained by Curtis et ~1,~'and the endothermicity of reaction (4) is calculated to be 1.450 eV, whereas the experimental value is 1.245 eV.6b The bending potential curves for the 2Al and 2B,(211)states are shown in fig.2, along with a limited portion of the *B2curve.The calculation of the vibronic levels for the 2A1and 2Bl states is currently in progress29 and it is hoped that the results of these calculations will facilitate the assignment of the observed vibronic levels.2b Contour diagrams for the potential-energy surface of the 2A1 and *B1states, for perpendicular approach of Si+ to the mid-point of the H-H bond, have been obtained from grids of 100 points.The surface for the 2A, staie is shown in fig. 3 and indicates that the equilibrium geometry of the ground state, X 2A1, of SiH: is separated from the asymptote Si+ + H2 by a substantial barrier of ca 4.2 eV. The 2Bl surface is qualita- tivel similar, with a barrier of ca.3.5 eV separating Si'+ H2 from the potential well of the YII state. Thus it would appear that reaction (4) cannot occur without activation Potential-energy Surfaces for Molecular Ions %H/A Fig. 3. The potential-energy surface for the 2 2Al state of SiHt as a function of the perpendicular distance R of Si+ from the mid-point of the HH bond and the HH distance, rHH. Contours are drawn at intervals of 0.02 Eh, contour 1= -289.80 &. (Reproduced with permission, from Mol.Phys., 1986, 59, 141.) energy for C2, geometries. The situation for collinear geometries is quite different in that there is a saddle point on the 'Z+ surface for SiHH+ in the vicinity of rSiH = 1.67 A, rHH = 1.08 A at an energy of ca. 1.60eV relative to Si+ ('P) + H2. The energy of this saddle point relative to SiH+ ('Z+) + H is 0.14 eV and thus there is virtually no activation barrier other than the endothermicity for reaction for collinear geometries. In the reaction of C++ H2, the deep well in the 2A1surface is separated from reactants by a barrier of ca. 3.5 eV (analagous to the Si'+ H2 case). However, it has been suggested that this well can be reached for near C2vgeometries via an avoided intersection involving the two 2A' surfaces correlating with the 2A1and 2B2surfaces.For CHZ the 2B2surface is mildly attractive and the avoided intersection with the 2A,surface is at 0.65 eV (Pearson and Roueff21) or 0.52 eV (Sakai et al.15) below the energy of C++ H2. This avoided intersection gives a route to the 2A, well and ultimately to CH' ('X+)+H which does not involve any barrier other than the endothermicity of the reaction. So the question arises as to whether the intersection beween the 2Al and 2B2surfaces for SiHl occurs at an energy below that of the products SiH+ (lZ+) + H. The 2B2 surface for SiHl is initially very flat with a very shallow well of depth of ca. 0.14 eV relative to Si+ +H2. The portion of the seam of intersection of the 2Al and 2B, surfaces for the range of R values between 1.2 and 1.5 8, has been mapped.For R values between 1.25 and 1 45 A. the energies at the intersection are comparable with the energy of the saddle point (-289.735 81 Eh) in collinear geometries and with the energy of SiH+(2C+)+ H (-289.741 16 Eh). For example, for R = 1.35 A and rHH = 1.7 8, the energies of the 2A1 and 2B2 states are -289.743 41 Eh and -289.745 45 &, respectively. Thus for C, geometries close to C2, symmetry in this region, one would expect to be able to pass from reactants to products on the lower 2A' surface without there being any significant D. M. Hirst energy 01 activation other than the endothermicity of the reaction. We have not performed calculations for C, geometries and cannot, at this point, say over what range of angles reaction is likely to occur with minimal activation energy.The author acknowledges the collaboration with Dr Martyn F. Guest in much of his work in this area and thanks the S.E.R.C. for the provision of computing time. References 1 (a) A. Carrington, Roc. R SOC.London, Ser. A, 1979,367,433; (b) A. Carrington and R. A. Kennedy in Gas Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1984), vol. 3, p. 393; (c) A. Carrington and R. A. Kennedy, J. Chem. Phys., 1984, 81, 91. 2 (a) C. P. Edwards, C. S. McLean and P. J. Sarre, Mol. Phys., 1984, 52, 1453; (b) M. C. Curtis, P. A. Jackson, P. J. Sarre and C. J. Whitham, Mol. Phys., 1985, 56, 485; (c) C. P. Edwards, P. A. Jackson, P. J. Same and D. J.Milton, MoI. Phys., 1986, 57, 595. 3 (a)R. J. Saykally, K. C. Lubic and K. M. Evenson in Molecular Ions, Geometric and Electronic Structures, ed. J. Berkowitz and K-0. Groeneveld (Plenum Press, New York, 1983), p. 33; (b) R. J. Saykally, Chem. Br., 1985, 21, 159. 4 C. S. Gudeman and R. J. Saykally, Annu. Rev. Phys. Chem., 1984, 35, 387. 5 (a) T. J. Sears, P. R. Bunker, P. B. Davies, S. A. Johnson and V. Spirko, J. Chem. Phys., 1985, 83, 2676; (b) T. J. Sears, J. Opt. SOC.Am. I?, 1985, 2, 786; (c) P. B. Davies, P. A. Hamilton and S. A. Johnson, J. Opt. SOC.Am. B, 1985, 2, 794. 6 (a) K. M. Ervin and P. B. Armentrout, J. Chem. Phys., 1985, 83, 166; (b) J. L. Elkind and P. B. Armentrout, J. Phys. Chem., 1984, 88, 5454; (c) K. M. Ervin and P.B. Armentrout, J. Chem. Phys., 1986, 84, 6738; 6750. 7 (a)B. Friedrich and 2.Herman, Chem. Phys., 1982,69,433; (b)B. Friedrich, W. Trafton, A. Rockwood, S. Howard and J. H. Futrell, J. Chem. Phys., 1984,80,2537; (c) R. A. Curtis and J. M. Farrar, J. Chem. Phys., 1986, 84, 127; (d) M. F. Jarrold and D. M. Hirst, Mol. Phys., 1981, 42, 97. 8 (a) Ch. Ottinger. in Gas Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1984), vol. 3, p. 249; (b) Ch. Ottinger, in Gas Phase Chemiluminescence and Chemi-ionization, ed. A. Fontijn (Elsevier, Amsterdam, 1985), p. 117; (c) I. Kusonoki and Ch. Ottinger, J. Chem. Phys., 1984,80, 1872; (d) B. Muller and Ch. Ottinger, J. Chem. Phys., 1986, 85, 232; 243. 9 (a) C. E. Hamilton and S. R. Leone in Gas Phase Cherniluminescence and Chemi-ionization, ed.A. Fontijn (Elsevier, Amsterdam, 1985), p. 139; (b) V. M. Bierbaum, C. B. Ellison and S. R. Leone in Gas Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1984), vol. 3, p. 1. 10 (a) D. R. Guyer, L. Huwel and S. R. Leone, J. Chem. Phys., 1983,79,1259; (b) L. Huwel, D. R. Guyer, G-H. Lin and S. R. Leone, J. Chem. Phys., 1984,81, 3520. 11 P. J. Bruna, G. Hirsch, R. J. Buenker and S. D. Peyerimhoff ,in Molecular Ions, Geometric and Electronic Structures, ed. J. Berkowitz and K-0. Groeneveld (Plenum Press, New York, 1983), p. 309. 12 S. D. Peyerimhoff, Faraday Symp. Chem. Soc., 1984, 19, 63. 13 (a) M. F. Guest and D. M. Hirst, Chem. Phys. Lett., 1981, 80, 131; (b) M. F. Guest and D.M. Hirst, Chem. Phys. Lett., 1981, 84, 167; (c) D. M. Hirst and M. F. Guest, Mol. Phys., 1983,49, 1461. 14 (a) G. D. Carney, L. L. Sprandel and C. W. Kern, Adv. Chem. Phys., 1978, 37, 305; (b) M. PeriC, R. Runau, J. Romelt, S. D. Peyerimhoff and R. J. Buenker, J. Mol. Spectrosc., 1979, 78, 309; (c) S. Carter and N. C. Handy, Mol. Phys., 1982, 47, 1445; (d) S. Carter, N. C. Handy and B. T. Sutcliffe, Mol. Phys., 1983,49, 745; (e) S. Carter and N. C. Handy, Mol. Phys., 1984, 52, 1367. 15 S. Sakai, S. Kato and K. Morokuma, J. Chem. Phys., 1981,75, 5398. 16 (a) M. A. Gittins, D. M. Hirst and M. F. Guest, Faraday Discuss Chem. SOC.,1977, 62, 67; (b) C. F. Bender, J. H. Meadows and H. F. Schaefer, Faraday Discuss Chem. SOC.,1977,62,59; (c) D. M. Hirst, Mol.Phys., 1978, 35, 1559. 17 D. M. Hirst, Chem. Phys. Lett., 1983, 95, 591. 18 (a) D. M. Hirst, in Dynamics of the Excited State, ed. K. P. Lawley (Wiley, Chichester, 1982), p. 517; (b) D. M. Hirst Potential Energy Surfaces: Molecular Structure and Reaction Dyanmics (Taylor and Francis, London, 1985). 19 T. H. Dunning and P. J. Hay, in Methods of Electronic Structure Theory, ed. H. F. Schaefer (Plenum Press, New York, 1977), p. 1. 20 P. C. Hariharan and J. A. Pople, Theor. Chim. Acta (Berlin), 1973, 28, 213. 21 P. K. Pearson and E. Roueff, J. Chem. Phys., 1976, 64, 1240. 22 D. H. Liskow, C. F. Bender and H. F. Schaefer, J. Chem. Phys., 1974,61, 2507. 23 R. J. Buenker and S. D. Peyerimhoff, Theor. Chim. Acta (Berlin), 1974, 35, 33. 24 S. Wilson, Electron Correlation in Molecules (Clarendon Press, Oxford, 1984). 25 R. J. Duchovic, W. L. Hase, H. B. Schlegel, M. J. Frisch and K. Raghavachari, Chem. Phys. Lett., 1982, 89, 120. 68 Potential-energy Surfaces for Molecular Ions 26 D. M. Hirst, Chem. Phys. Lett., 1985, 122, 225. 27 A. E. Douglas and B. L. Lutz, Can.J. Phys., 1970, 48, 247. 28 T. A. Carlson, J. Copley, N. DuriC, N. Elander, P. Errnan, M. Larsson and M. Lyra, Astron. Asrrophys., 1980;83, 238. 29 P. J. Sarre, personal communication. 30 P. J. Bruna and S. D. Peyerimhoff, Bull. SOC.Chim. Belg., 1983, 92, 525. 31 P. Rosmus and W. Meyer, J. Chem. Phys., 1977, 66, 13. 32 D. M. Hirst, Chem. Phys. Letr., 1986, 128, 504. 33 D. M. Hirst and M. F. Guest, MoZ. Phys., 1986, 59, 141. Paper 61 1020; Received 23rd May, '1986

ISSN:0300-9238    

DOI:10.1039/F29878300061

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 69-87 Properties of Gas-phase Ions Information to be obtained from Photoelectron Spectroscopy of Unstable Molecules John M. Dyke Department of Chemistry, The University, Southampton SO9 5NH The information to be obtained from the study of unstable molecules in the vapour phase by photoelectron spectroscopy is reviewed, using examples drawn from high-temperature pyrolysis experiments and rapid atom-molecule reaction. An example is given of a chemi-ionization reaction studied by electron spectroscopy, and the information to be obtained from such studies is outlined. Vacuum ultraviolet photoelectron spectroscopy (p.e.s.) is a technique which may be used to investigate ionic states obtained by removal of one electron from a neutral molecule.Because of the energy of the radiation used (typically ca. 20 eV) only valence electrons can be removed and the resolution of the technique (typically ca. 200 cm-') is such that for small molecules only electronic and vibrational information on ions is obtained. Rotational structure is not normally resolved. As a result, p.e.s. is a relatively low-resolution spectroscopic method. Despite this, however, p.e.s. measurements on short-lived gas-phase molecules have provided a wealth of useful information on molecular ions, some of which has not previously been obtained by any other method. In order to study short-lived molecules in the vapour phase a number of preparative methods have been used. These include: (a) microwave discharge of a flowing gas mixture, (b) use of a suitable gas-phase reaction, typically a rapid atom-molecule reaction and (c) high-temperature pyrolysis. Of these methods, only high-temperature pyrolysis will be described, as the first two methods are well established techniques which have been extensively utilized in other areas such as gas kinetics.When the vapour pressure above a solid sample is low at room temperature, a study of the vapour phase by p.e.s. will require an increase in temperature of the solid. A range of heating methods are possible in principle, but for a number of technical reasons radio-frequency (r.f.) induction heating has been chosen by the Southampton p.e.s. group as being particularly applicable to photoelectron spectroscopy. 1,2 After experimenting with a number of designs and geometries, the heating arrangement shown in fig.1 was chosen. In this diagram, the r.f. induction heating assembly is mounted inside the ionization chamber of a photoelectron spectrometer, which in turn is mounted on a large liquid- nitrogen-trapped diffusion pump. As shown in fig. 1, an induction coil is wound coaxially around the furnace which contains a solid sample. When an r.f. current is passed along these coils eddy currents are induced in the surface of the furnace and this gives rise to a heating effect. In practice, a pulsed form of the radio frequency is used so that interference effects with the detection electronics can be eliminated by using a suitable gating unit and photoelectron signals are recorded during the 'off' pulse.The solid sample is vaporized downwards into the photon beam of the spectrometer and the photoelectrons produced are sampled at 90"to the photon beam by the slits of the spectrometer and the energy of the electrons is analysed by a 150" hemispherical analyser. With a 16 kW induction heater, for a carbon furnace of wall thickness 1 mm, a maximum furnace temperature of 2800 K can be achieved, whereas with a tungsten 69 Photoelectron Spectroscopy of Unstable Molecules 0 5 -cm Fig. 1. Diagram of a radio-frequency inductively heated furnace assembly used in high-temperature p.e.s. experiments.',' A, aluminium alloy; B, brass; C, graphite; D, ceramic; E, insulator; F, tungsten; 0,induction coil; 0,water cooling.furnace of similar dimensions a maximum furnace temperature of 2600K can be obtained. Further details of this heating method as used in photoelectron spectroscopy have been described elsewhere.lY2 A photoelectron spectroscopic study of a molecule produced by high-temperature pyrolysis or a rapid atom-molecule reaction often yields spectroscopic information on a molecular ion which has not previously been obtained by any other method. In order to assess the reliability of the spectroscopic parameters obtained it is useful to consider photoelectron spectra of small molecules, where the ionic states observed have previously been studied by other methods, such as optical emission spectroscopy. If the vibrational frequency in the ground electronic state of the neutral molecule is large with respect to kT, the only vibrational level populated in this state is the lowest vibrational level and vibrational structure is present in the photoelectron band, as shown in fig.2. For the photoelectron band obtained, the separation of the vibrational components can be measured and values of the spectroscopic constants 6,and 0,x, can be derived, provided that the vibrational energy levels in the ion are assumed to fit an anharmonic oscillator expression. A very simple example where this method has been applied occurs in the He1 photoelectron spectrum of N2. This spectrum shows three bands, which all show clearly resolved vibrational structure, and by measuring the vibrational separations in each band values for the ionic constants, 0,and G,x,, can be obtained.Table 1 shows a comparison of the values of 0,obtained for each ionic state with the corresponding values obtained previously3 by the higher resolution method of optical emission spectros- copy. As well as providing separations of vibrational levels in a given state of an ion, the photoelectron spectrum contains further information in that the relative vibrational intensities in a band can be measured and this can be used to calculate the change in equilibrium bond length between the molecule and ion. Hence the equilibrium bond J. M. Dyke Ei k-I-----/M+ -Fig. 2. A diagram showing the ionization of M to M+ and the formation of a typical photoelectron band.Table 1. Nl spectroscopic constants obtained from photoelectron spectrum parameter NT (X22i) NT (A211,) Nl (B 'X:) ionic equilibrium bond length/A re (p.e.s.) 1.115 1.177 1.078 re (spectroscopic)" 1.116 1.175 1.074 vibrational constant/cm-' 6,(p.e.s.) 6,(spectroscopic)" 2220 2207 1900 1904 2415 2420 a Ref. (3) length in the ion can be obtained if the equilibrium bond length in the molecule is known. The method which involves the calculation of Franck-Condon factors for a range of trial ionic equilibrium bond lengths can be summarized in the following way? For a diatomic molecule the intensity of individual components, Iut,ufl,in a photo- electron band is given by: where Photoelectron Spectroscopy of Unstable Molecules and In these equations, qUlruIfis the so-called Franck-Condon factor, defined as the squared overlap of the vibrational wavefunctions belonging to the ion (t,bUf) and the neutral molecule ( t,hutr), integrated over the internuclear distance, r.R+.(r) is the electronic transition moment and is the expectation value of the electronic dipole moment operator, Me, with respect to the electronic wavefunctions of the ionic (+;) and neutral (+;) states, integrated over the electron coordinates qe. If &(r) is assumed to be a slowly varying function of r and of the kinetic energy of the ejected electron, eqn (1) can be approximated by: in which R, is evaluated at the r centroid, Fut,ugo,a weighted mean of the internuclear separation.In the determination of ionic equilibrium bond lengths from photoelectron band envelopes it is usual to assume that the first term in eqn (4) is a constant (ie. that the electronic transition moment does not vary over a photoelectron band) and hence the relative intensities of vibrational components in a photoelectron band are determined by the Franck-Condon factors, qur,u8t. In order to calculate values of qur,uetat trial values of the ionic equilibrium bond length the following procedure has been adopted by the Southampton p.e.s. group. (1) A Morse potential is assumed for the molecular and ionic state. This is determined by the parameters G,,Gexeand re in each state. For the neutral molecule, these values are usually already available from other spectroscopic studies, whereas Ge and 6,xe in the ion are obtained from measurement of the vibrational component separations in the photoelectron band.The equilibrium bond length in the ion is set at its trial value. (2) Vibrational energies and wavefunctions are then calculated as numerical solutions of the radial Schrodinger equation. The required values qof,vffcan then be obtained from the computed wavefunctions in the initial and final states. (3) The procedure is repeated for different values of re (ion) until good agreement is obtained with the experimental envelope. In fact, a least-squares procedure is used to determine the value of re (ion) which gives the closest agreement with the experimental vibrational component intensities.The ionic equilibrium bond lengths obtained from the three bands observed in the He I photoelectron spectrum of N2are shown in table 1. As can be seen from this table, the agreement between the values of 6,and re derived from the photoelectron spectrum with the corresponding values obtained from optical emission spectroscopy is good. Similar comparisons can also be made for ionic states observed in the He I photoelectron spectra of NO, CO, O2 and CS, where the ionic states have also been investigated by independent spectroscopic experiments. For the ionic states where clearly resolved, unoverlapped vibrational structure is observed in the photoelectron spectrum, the general result is that when the photoelectron data are used to derive ionic spectroscopic parameters, re values are obtained to within k0.005A of the spectroscopic value and 6,is obtained to within *30 cm-'.This result indicates that the observed vibrational component intensities are determined almost completely by the quf,urrvalues and that the variation of the electronic transition moment over the band is small. This assumption can be tested by plotting the observed vibrational component intensities in a band, corrected for the analyser transmission function, divided by the qur,u.tvalue [computed with the spectroscopically determined re (ion) value] against the vibrational numbering in the upper state. This has been done for the second band of N2 (see fig. 3) and, as expected, this plot deviates only slightly from a horizontal straight line.J. M. Dyke 0 1 2 3 v Fig. 3. An investigation of the variation of the electronic transition moment over a photoelectron band for the Nl(A 211,)6N2(X*Zg)ionization recorded with He I, radiation. This general result has been obtained for all diatomics that have been investigated by p.e.s. where ionic spectroscopic constants are available from other studies for comparison, with the notable exception of H2, where the variation of the electronic transition moment with internuclear distance has to be taken into account to reproduce the experimental photoelectron relative inten~ities.~’~ As well as the assumption of a constant electronic transition moment over a photoelectron band, two other assumptions are made.First, it is assumed that the electronic states involved can be satisfactorily described by Morse potentials and secondly it is assumed that the photoelectron band is not distorted by autoionization. This latter assumption can be investigated experi- mentally by recording a photoelectron band with several different photon sources. The application of this method to a spectrum of a short-lived molecule is illustrated by considering the photoelectron spectrum of S2 obtained by heating solid sulphur to 600 K.* The He I photoelectron spectrum of S2 shows nine bands, five of which clearly exhibit resolved vibrational structure (fig. 4). As an example, an expanded scan of the first band of S2 is shown in fig.5. This band, which corresponds to the ionization S, +X 211s +S2X ’Xi, shows vibrational structure as well as evidence of spin-orbit splitting in the X 211gstate.The ionic parameters obtained from analysis of the S2 photoelectron spectrum are shown in table 2. These values are, of course, subject to an error of k0.005 A in re and *30 cm-’ in Ge. The relative positions of the ionic states of S2 observed in the photoelectron spectrum and the ionic spectroscopic constants derived from the photoelectron spectrum allow ionic emission envelopes to be computed. For example, S,f(A21X,)-+ (x21X,) is an optically allowed transition. From the photoelectron data, the energy of transitions from the lowest nine vibrational levels in the A 21Xu state to the lowest nine vibrational Photoelectron Spectroscopy of Unstable Molecules hh II ~ 18.0 17.0 16.0 15.0 14.0 ionization potential/eV Fig.4. (a) The He I photoelectron spectrum of S2(X ’Xi). (b) -3 V acceleration. 3000 4 ‘m mU a s 9.70 9.60 9.50 9.40 9.30 ionization potential I eV Fig. 5. The band assigned to the Sl(X211,) tS2(X3H;) ionization ndenotes spin-orbit splitting. levels in the lower X 211g state can be calculated and the Franck-Condon factors for these transitions can be computed. The advantage of this procedure is that although the position of a (d-u”) transition may be in error by up to 60 cm-’, the relative intensities of the components for a transition from a given vibrational level (u’) in the upper state is reasonably accurately predicted. This allows the vibrational numbering (d,u”) of the bands in the emission spectrum to be established and as a result, improved spectroscopic constants of the Sl states can be obtained (see fig.6). The ST spectroscopic constants 28 26 24 22 20 18 16 lo3 energylcm-' Fig. 6. Comparison of the emission spectrum computed from p.e.s. data' and observed experi- mentally' for the SZ(A'nu)-(X211g) transition. (-) Optical data, (---) p.e.s. data. Table 2. Spectroscopic constants obtained for Sl from p.e.s.8 S,' state re/ A a (3Jcm-l a 1.824 (1.823) 790 (807 f3)2% 4nu 2.058 620 2Kd 2.047 (2.044) 547 (551 *3) "z, 1.936 58 1"z, 1.983 546 a Values in brackets are those derived subsequently from optical emission studie~.~~~~ obtained from the emission approximately five years after the initial photo- electron study have been included in table 2.This demonstrates the point that photoelec- tron measurements are often important precursors to higher-resolution spectroscopic studies. The application of these methods to a molecule produced by a rapid atom-molecule reaction can be illustrated by considering the NF radical produced from the F+N3 reaction.' '-12 This reaction is sufficiently exothermic to produce both the ground elec- tronic state of NF (the X 3C-state) and the first excited state (the NF a 'A state). Removal of one electron from both of these states produces the NF+ (X 'n) state and the measurement of the difference in the experimental adiabatic ionization energies (see fig. 7) allowed the separation of the zeroth vibrational levels of the X '2-and a 'A NF) states to be obtained as 1.42*0.01 eV, in agreement with a value of 1.418 eV obtained by observing the a 'A-X 'C-emi~sion.'~Also, measurement of the vibrational separ- ations in each band allowed 6,to be obtained in the NF+ (X 'n) state as 1520* 40 cm-' and use of the vibrational component intensities in each band allowed the equilibrium bond length in this state to be derived as 1.180f 0.006 At present this appears to Photoelectron Spectroscopy of Unstable Molecules Ar (He[@1 n NF?X Zfl)--NF(X3E) m NF+(X 'n)-NF(o'A) m 14 13 12 1.1 10 9 Ei/eV Fig.7. The first bands observed in the He I photoelectron spectra of NF(X 3X)and NF(a 'A).be the only experimental determination of these quantities for the NF+ (X 'II) state, although values of 0, = 1499 cm-' and re = 1.182 A were subsequently computed from ab initio calculations which include the effects of extensive configuration intera~tion.'~ Fig. 8 illustrates the least-squares fitting method used to obtain the equilibrium bond length of a diatomic ion from vibrational component intensities in a photoelectron band. As discussed earlier, Franck-Condon factors are calculated at various trial re values for the NF+ (X 'II) state and the least-squares error, E uI [Iur(calc) -I,,!(expt)]', is calculated for each value of the ionic re. The least-squares error can then be plotted against re for both the NF' (X 'n) +NF (X 'E-) and NF+ (X 21-1) +NF (a 'A) band envelopes to yield the plots shown in fig.8. Ideally the minima of both these curves should occur at the same value of the equilibrium bond length in the ion. As can be seen from fig. 8, however, a small difference was observed and the equilibrium length for NF+ (X 'II) was derived as 1.178k0.004 A and 1.182f0.004 A from the bands associated with the NF (X 'Z-) and NF (a 'A) states, respectively. Combining these results gave a value for re of 1.180k 0.006A for NF+ (X 'II).'' A typical spectrum of a molecule produced by high-temperature vaporization is shown in fig. 9. This shows the spectrum, recorded in the 7.0-8.5 eV ionization energy region, of the VO molecule obtained by heating stoichiometric VO(s) to 1980 *30 K.The first band shown in fig.9 corresponds essentially to a metal (4s)-' ionization, whereas the second band is much stronger and corresponds essentially to a metal (3d)-' ionization. These relative intensities are consistent with those seen in the photoelectron spectrum of the metal15316 and the experimental band envelopes are consistent with computed band envelopes obtained from a6 initio molecular-orbital calculations and Hartree-Fock-Slater ~alculations.'~ Only the first band in fig. 9 shows vibrational structure and measurement of the vibrational component separations allows 0,to be obtained in the ground state of the ion (the X 3Zc-state) as 1060*40 cm-' and the experimental component intensities can be used to obtain re in the VO+ (X 'Z-) state as lS4* 0.01 A.The error in re quoted here is larger than that normally quoted because of the possibility of weak 'hot band' contributions to the band shown in fig. 9. This is thought likely as it has been found in previous studies that for the high-temperature J. M. Dyke 0.10 0 0.08 N n nc,2 0.06 a v %a I h ri-cf 0.04 v %a U w -9 0.02 10 Y/ ' 1,170 1.180 1.1 90 Fig. 8. Results of the least-squares fit performed at various trial re values for the NF+ (X 'n) state. The experimental vibrational component intensities are from fig. 7 for (a) the NF+(X 'II) tNF(X 3E-) ionization and (b) the NF+(X 'n) tNF(a 'A) ionization. 8.5 8.0 7.5 7.0 Ei /eV Fig.9. The first two bands seen in the photoelectron spectrum of VO. (a)VO+(A 'A) +VO(X 'E-), (b)VO+(X 'E-) tVO(X 'X-). Photoelectron Spectroscopy of Unstable Molecules -lo01 (a I Ili v)I c 0 11.0 10.0 9.0 EJeV Fig. 10. The first band seen in the He I photoelectron spectrum of PF2, produced as a secondary product of the F+ PH3 reaction. spectrometer used, evaporation of a metal oxide at a furnace temperature of ca. 2000 K leads to molecules in the photoionization region with a vibrational temperature of ca. 800 K. This difference arises because of collisional deactivation between the furnace and the photoionization region.17 In contrast, similar p.e.s. studies on metals'6-18 show that if a metal is evaporated at ca.2000 K, the electronic excitation temperature of the metal in the photoionization region may be ca. 1500 K, indicating that, as expected, much more inefficient deactivation of electronic excitation on collision occurs than vibrational deactivation. In the study of triatomic molecules by p.e.s., it is not generally possible to determine the equilibrium geometry of the ion from the experimental spectrum as usually at best only one vibrational series is resolved. However, measurement of the adiabatic ionization energy allows the heat of formation of the ion to be determined, provided the heat of formation of the neutral molecule is known. An example of a triatomic band where vibrational structure is resolved and the adiabatic component can be readily identified occurs in the first band of the PF2 As shown in fig.10,the first band of PF2 shows regular vibrational structure and the adiabatic and vertical ionization potentials were measured as 8.84*0.01 and 9.09*0.01 eV, respectively. On the basis of the bonding character of the molecular orbital from which ionization occurs and the known frequencies of the normal modes in the neutral molecule, PF2 (X 2B1),20,21the observed structure in fig. 10 is assigned to excitation of vl ,the P-F stretching mode, in the ionic state, PF,f (X 'Al). Measurements of the vibrational separations gave (3, = 980* 30 cm-' for vl in the PF,f (X 'Al) state. As observed in the first band of the photoelectron spectrum of NF2,22this value is higher than vl in the ground state of the neutral molecule because ionization occurs from the outermost half-filled level which is antibonding in the P-F direction and bonding the F-F direction. In the case of a triatomic molecule where a large equilibrium geometry change occurs on ionization, identification of the adiabatic ionization energy may be very difficult.Such a case occurs in the first band of the formyl radical where the ionization process corresponds to a transition from a neutral molecule with a bent C, equilibrium structure to an ion with a C,, equilibium geometry. In the first photoelectron study of the HCO radical with a single detector ~pectrometer~~ a long vibrational series was seen in the HCO deformation mode, as anticipated. Unfortunately, however, it was not possible to observe the adiabatic ionization energy.A study of this band with a multidetector spectrometer allows two extra vibrational components to be observed in the low ioniz- ation energy region of the HCO photoelectron band (fig. 11) with the lowest component J. M. Dyke 9.2 8.8 8.4 8.0 EJeV 6000 2000 w ~.~ 9.2 8.8 8.4 8.0 Ei/eV Fig. 11. The low ionization energy region of the first band of HCO recorded with (a)He1 and (b) Ne I radiation. being seen at 8.35 * 0.01 eV. It is clear, however, that the adiabatic component is not directly observed in fig. 11, as the HCO adiabatic ionization energy has recently been determined from a series of photoionization mass-spectrometric measurements on a number of organic compounds as 8.10k0.05 eV.24 In order to establish the ionic vibrational numbering in the series seen in fig.11 it is necessary to utilize two pieces of information derived from independent studies; the adiabatic ionization energy of 8.10f 0.05 eV determined by photoionization mass spe~trometry~~ and the IJ = 0-1 ( v2) band origin for HCO+ measured by infrared diode laser spectroscopy as 829.72 ~m-'.~' This added information shows that two vibrational components are unobserved in fig. 11. Having established the ionic state vibrational numbering, the separation of the observed vibrational components in the photoelectron spectrum can be plotted against (u'+ l), where v' is the upper state numbering, to yield (3, = 850f25 cm-' and O,x, = 9 f10 cm-'.The HCO adiabatic ionization energy is then derived as 8.14f0.04 eV. The adiabatic ionization energy of DCO can then be calculated from the HCO value, by making appropriate zero-point energy corrections, and used to determine the ionic state vibra- tional numbering in the DCO photoelectron spectrum. Plotting the separation of the observed vibrational components in the DCO case against (v'+l) gives (3,= 655 f25 cm-', O,x, = 4* 10 cm-' and a IJ = 0-1 (v,) band centre of 647 f25 cm-'. This value is important as u2in DCO+ has not been measured by high-resolution spectroscopy and it is hoped that the photoelectron work will assist the search for this fundamental band by higher-resolution methods.26 Photoelectron Spectroscopy of Unstable Molecules A I*O* I 15 14 13 12 11 10 9 8 Ei/eV co+ O+ \ Ar+ 70 60 50 40 30 20 10 amu Fig. 12.(a) The He I photoelectron spectrum of A120and (b)The electron impact mass spectrum recorded at the same time as (a)using 60 eV electron energy. Some triatomic molecules show no resolved vibrational structure in their photoelec- tron spectra. Even in these cases, however, it usually proves possible to derive some structural information. An example of this type occurs in the photoelectron spectrum of A1202' obtained by heating an Al-A1203 mixture to 1600K. The photoelectron spectrum obtained [fig. 12(a)]was confirmed as being attributable to A120by recording the mass spectrum of the high-temperature vapour [fig. 12(6)] using electron im act at 60 eV electron kinetic energy simultaneously with the photoelectron spectrum.27 'P A120 is an interesting molecule as, although it seems likely from previous spectro- scopic studies that the valence isoelectronic molecules Ga20, In20 and T120 have C2" equilibrium geometries, the equilibrium geometry of A120is not well established.If A120has a Dmhequilibrium geometry, it has the following valence electronic configur- ation: 4ai 2~: 6a2, 5a; and in practice the photoelectron spectrum can be assigned on this basis because four bands were seen. Experimentally, the first two bands were sharp with the same experi- mental half-width and are assigned to the (5uU)-'and (6a,)-' ionizations. Hartree-Fock-Slater calculations show that these two ionizations correspond essentially to removal of electron density from the Al3s atomic orbitals with a small reduction in J.M. Dyke 81 Al3p electron density. In contrast, the second two bands correspond to ionization from the 27ru and 4uu molecular orbitals, which are composed mainly of Al3p and 0 2pcontribution^.^^ If the equilibrium symmetry of A120 is lowered from Dmhto C2, then the valence electronic configuration of A120 changes in the following way: 4a2, 6a2, 502,I I I bl a1 bl. This indicates that five bands would be expected in the photoelectron spectrum. Also, it might be expected that the 2Al determinants obtained by the two (al)-' ionizations would be close in energy and would interact causing the lower 2A1state to acquire some character of the upper 2Al state and vice versa.This effect would be seen in the experimental spectrum by a broadening of the second band compared to the first. However, as already stated, only four bands were seen experimentally and the observed half-widths of the first and second bands were the same. Hence the experimental photoelectron spectrum is consistent with a Dcohequilibrium geometry for the ground state of A120.This result has been confirmed by a6 initio, minimum-energy geometry calculations for this molecule using a double-zeta quality gaussian basis set with two added d-polarization functions on each centre and incorporating the effects of electron correlation via fourth-order many-body perturbation theory.27 After the photoelectron investigation of A12027was complete, a Dmhequilibrium geometry was independently derived from an investigation of the Raman spectrum of A120trapped in an argon matrix28 and further a6 initio molecular-orbital calculation^.^^ The type of information to be obtained from the spectra of tetratomic and polyatomic molecules can be illustrated by considering the photoelectron spectra of the hydroxy- methyl (CH20H) and methoxy (CH30) radicals.30931 These can both be conveniently produced by the reaction of fluorine atoms with methanol, i.e.F+CH,OH --* CH20H+HF AH?& = -163 kJ mol-' (5) F+CH,OH --* CH,O+HF = -141 kJ mo1-l. (6) Hydroxymethyl is more stable than the methoxy radical and, at a reagent mixing distance of 1.0cm above the photon beam, the first band of CH20H can be clearly identified (see fig.13). As can be seen from fig. 13, the band assigned to CH20H shows one vibrational series with evidence of a second series. The main series could be analysed to give 6,= 1650+ 30 cm-' for the vibration excited, whereas tFe second series had an average separation of 1370*30 cm-' (see table 3). In the ground electronic state of CH20H, the half-filled molecular orbital can be described in terms of a C 2p,-O 2p, antibonding atomic orbital combination with C-H bonding and H-H bonding contributions in the CH2 group. On this basis, the 1650 cm-' structure is assigned to excitation of a C-0 vibrational mode increased from the neutral molecule C-0 value of 1183 cm-', whereas the 1370 cm-' structure is assigned to a CH2 deformation mode decreased from the value of 1459 cm-' in CH20H (X 2A).In CD,OH, two series were again observed (see fig. 14). The main series now increases slightly and analysis of the component separations yields 6,= 1810 f 30 cm-', whereas the smaller series decreases to yield an average separation of 1100f30 cm-'. The decrease in the average vibrational separation of the second series from 1370 to 1100 cm-' on going from CH,OH+ to CD20H+ is consistent with assignment of this structure to excitation of a CH2 deformation mode. However, the slight increase in the C-0 mode on deuteration is more difficult to explain. It is thought, however, that this is due to a 'C-D' stretchkg mode lying below the 'C-0' mode in (CD,OH)+.These modes interact strongly causing the 'C-0' mode to be increased slightly. Some support 82 Photoelectron Spectroscopy of Unstable Molecules 1000 I I 9.5 9.0 8.5 8.0 7.5 Ei/eV Fig. 13. The first band of the CH,OH radical produced from the F+CH,OH reaction. Table 3. Summary of the data obtained far the first p.e. bands of the CH20H and CH30 radical^^'*^' ionization energy/ eV radical vertical adiabatic (3,/cm-' c/cm-' CH20H 8.14*0.01 7.56*0.01 1650*30 1370*30 CH20D 8.14*0.01 7.55*0.01 1610*30 1390* 30 CDZOH 8.13*0.01 7.55*0.01 1810*30 ll00*30 CD2OD 8.14*0.01 7.56*0.01 1770*30 1130*30 CH30 8.13*0.02 7.37*0.03" 1530*40 -(I Obtained from analysis of main vibrational series; see for example fig.13 and 14. Average separation of secondary series; see fig. 13 and 14. " Band onset. Average separation of vibrational structure; see fig. 15. for this suggestion has been provided by force-field calculations which have been performed on CH20H+ (X 'A)and CD20H+ (X 'A).Also, the suggested assignments are consistent with the vibrational structure observed in the photoelectron spectra of other deuterated CHzOH species (see table 3). The main results to be derived from this study are an improved value for the first adiabatic ionization potential of CH20H and measurement of two of the vibrational frequencies in CH20H+(X 'A). Also, since it appears from the literature that the heat of formation AH&,8 of the ion, as calculated from the proton affinity of f~rmaldehyde,~~ is better determined than the heat of formation of the neutral molecule, use of the p.e.s.value for the adiabatic ionization energy of CH20H of 7.56k0.01 eV with AHgg8(CH20H+)of 7.30 f0.12 eV32 leads to a redetermination of AH&8(CH20H) of -0.25 f0.13 eV. It should also be noted that some weak structure was seen in the F+ CH30Hspectra in the 8.5 eV ionization energy region (see fig. 13 and 14). This structure was not seen J. M. Dyke 3500 e 'v) v)U 18 0 9.0 8.5 8.0 7.5 Ei/eV Fig. 14. The first band of the CD20H radical produced from the F+CD,OH reaction. CH,O (He I) 1 I L 1 9.0 8.0 7.0 EJeV Fig. 15. The first band of the methoxy radical obtained from pyrolysis of dimethyl peroxide.in spectra obtained from the Cl+CH,OH reaction, which is less exothermic than the F+CH,OH reaction and produces CH20H as the only detectable radical product. It was thought that this structure at ca. 8.5 eV ionization energy may be due to the methoxy radical and, as a result, attempts were made to make CH30 in the absence of CH20H. A suitable way of achieving this is via pyrolysis of dimethyl peroxide and a spectrum obtained from this route is shown in fig. 15. As can be seen a broad band was observed centred at 8.13 f0.02 eV ionization energy and this has been assigned to the CH30+ (X 3A2)+CH,O(X 2E)ionization. The vibrational series seen in fig. 15 had an average separation of 1530 *40 cm-'. Assignment of this structure can be achieved in an analogous way to assignment of the Photoelectron Spectroscopy of Unstable Molecules Fig.16. A schematic representation of chemi-ionization in the 0+CH reaction. main series in the CH20H first band, as the molecular orbital from which ionization occurs is antibonding in the C-0 direction. This structure is therefore assigned to excitation of the C-0 stretching mode in the ion, with a vibrational frequency increased over the corresponding value (1053 cm-l) for the neutral molecule. Having described some existing examples of photoelectron spectroscopy of unstable molecules, it is useful to describe one relatively new area of electron spectroscopy where information on small molecular ions can also be obtained. This involves the use of electron spectroscopy to study chemi-ionization. A chemi-ionization reaction can be defined as a reaction in which new chemical bonds are formed and the number of charge carriers increases. Reactions of this type in which electrons are produced have been termed chemi-electron reactions.33 The only chemi-ionization reaction to be investigated previously by electron spectroscopy is the reaction of oxygen atoms with the CH radical, i.e. O+CH + HCO++e-. (7) Jonathan et al.33 measured the electron energy distribution obtained from the reaction and obtained a sharp, unstructured band centred at 0.23h0.01eV. As the heats of formation of 0, CH and HCO+ are well estab!ished, the exothermicity of reaction (7) can be calculated readily as 0.23eV, in good agreement with the maximum in the experimental electron distribution for the 0+CH reaction.With the aid of potential- energy surfaces computed for HCO and HCO+ by MacGregor and Berry,34 the 0+CH reaction can be envisaged as occurring as shown in fig. 16. As illustrated in this diagram, the 0 and CH reactants can combine to give either HCO in its ground state or HCO in an excited state. For certain reaction coordinates, the HCO excited state will lie above HCO+ in its ground state and, as a result, electron emission will occur. The maximum in the electron kinetic energy of the ejected electrons will correspond to the line marked (A) on fig. 16. Unfortunately, the observed electron energy di~tribution~~ J. M. Dyke Ce+O, photon source (He I) off on I-’ O:(b4E,) ‘m2000 I I I I I I I I ,-I‘v) v) Ic, 1 I 0 I I I I I I I 0 0 1 2 kinetic energy/eV Fig. 17.The electron energy spectrum obtained from the Ce+02 reaction. showed no vibrational structure and hence it was not possible to obtain any additional information on HCO+ over that which can be obtained from direct ionization of HCO. At the time that the first chemi-electron study was made, extension of this method to the study of other reactions proved difficult. This arose because most known chemi- ionization reactions which yield electrons involve reactions of metals, produced in the vapour phase by heating the solid metal, with a suitable oxidant.Unfortunately, the high-temperature capability in electron spectroscopy had not at that time been developed. However, with the development of high-temperature pyrolysis methods in p.e.s., these reactions can now be studied and should provide an alternative way of probing the ground state of metal oxide ions other than direct ionization from the neutral molecule. An example of a spectrum obtained from a reaction of this type is shown in fig. 17, which was recorded for the Ce+0, reaction. The chemi-electron band observed has a peak maximum of 0.90 f0.04 eV and showed clear vibrational structure with an average separation of 790* 30 cm-’. The way in which this spectrum is interpreted is shown in fig. 18. As shown in this diagram, transitions take place from a CeO, excited state to CeOl, with the maximum observed electron energy corresponding to a transition to the lowest vibrational level of CeOl.Measurement of this quantity should allow the heat of formation of CeOl to be obtained and hence the adiabatic ionization energy of CeO, can be determined if the heat of formation ofthe neutral molecule is known. Also, if, as seems llkely,35336 the electronically excited state of CeO, shown in fig. 18 and the ground state of Ce0; have C2vequilibrium Photoelectron Spectroscopy of Unstable Molecules Fig. 18. A schematic representation of chemi-ionization in the Ce +O2 reaction. structures, then the observed vibrational structure can be assigned to excitation of the v1 mode in the ion.Other metal plus oxidant reactions are currently being studied by chemi-electron spectroscopy and it is anticipated that the information obtained on metal-oxide ions will complement that obtained by direct ionization. It is a pleasure to acknowledge that the results used in this report have been obtained by members of the Southampton p.e.s. group, both past and present. The current research group consists of Dr Alan Morris, Vivienne Butcher, Andrew M. Ellis, Miklos Feher, Steven Harris, Julian Stevens and Hadi Zamanpour. Dr Alan Morris is acknowledged for skillfully designing and building the apparatus used in these studies and Prof. Neville Jonathan is thanked for numerous stimulating discussions. This research has been financed by grants from the S.E.R.C.and C.E.G.B. and was also supported in part by the Air Force Office of Scientific Research (grant no. AFOSR-83-0283) through the European Office of Aerospace Research (EOARD), United States Air Force. References 1 D. K. Bulgin, J. M. Dyke, N. Jonathan, E. Lee and A. Morris, J. Electron. Spectrosc. Relat. Phenom., 1977, 12, 67. 2 A. Moms, J. M. Dyke, M. P. Hastings, G. D. Josland and P. D. Francis, High Temperature Science, 1986, accepted for publication. 3 (a)A. E. Douglas, Can. J. Phys., 1952,30,302; (b)E. A. Colbourn and A. E. Douglas, J. MoZ. Spectrosc., 1977, 65, 332; (c) K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure IV (Van Nostrand, New York, 1979). 4 J. M. Dyke, N.Jonathan and A. Moms, Int. Rev. Phys. Chem., 1982, 2, 3. 5 J. M. Dyke, N. Jonathan and A. Moms, Electron Spectroscopy (Academic Press, London, 1979), vol. 3, p. 189. 6 J. Berkowitz and R. Spohr, J. Electon Spectrosc. Relat. Phenom., 1973, 2, 143. J. M. Dyke 7 Y. Itikawa, J. Electron Spectrosc. Relat. Phenom., 1973, 2, 125. 8 J. M. Dyke, L. Golob, N. Jonathan and A. Moms, J. Chem. SOC.,Faraday Trans. 2, 1975,71, 1026. 9 M. Tsuji, I. Murakami and Y. Nishimura, Chem. Phys. Lett., 1980, 75, 536. 10 A. J. Capel, J. H. D. Eland and R. F. Barrow, Chem. Phys. Lett., 1981,82,496. 11 J. M. Dyke, A. E. Lewis, N. Jonathan and A. Morris, Mol. Phys., 1982,47, 1231. 12 J. M. Dyke, A. E. Lewis, N. Jonathan and A. Moms, J. Chem, SOC.,Faraday Trans. 2, 1982,78, 1445.13 W. E. Jones, Can. J. Phys., 1967, 45, 21. 14 M. BettendorfT and S. D. Peyerimhoff, Chem. Phys., 1985,99, 55. 15 J. M. Dyke, B. Gravenor, M. P. Hastings and A. Morris, J. Phys. Chem., 1985,89, 4613. 16 J. M. Dyke, 8. Gravenor, M. P. Hastings, G. D. Josland and A. Moms, J. Electron Spectrosc. Relat. Phenom., 1985,35,65. 17 J. M. Dyke, B. Gravenor, R. A. Lewis, G. D. Josland and A. Moms, Mol. Phys., 1984,53,465. 18 J. M. Dyke, B. Gravenor, R. A. Lewis and A. Moms, J. Phys. B, 1982, 15,4523. 19 J. M. Dyke, A. Moms and A. M. A. Ridha, unpublished results. 20 J. K. Burdett, L. Hodges, V. Dunning and J. H. Current, J. Phys. Chem., 1970,74, 4053. 21 D. Solan, Ph. D. Thesis (Brooklyn College, City University of New York, 1965). 22 (a) A.B. Cornford, D. C. Frost, F. G. Herring and C. A, McDowell, J. Chem. Phys., 1971, 54, 1872; (b) A. B. Cornford, D. C. Frost, F. G. Hemng and C. A. McDowell, Faraday Discuss. Chem. SOC., 1972, 54, 56. 23 J. M. Dyke, A. Moms, N. Jonathan and M. J. Winter, Mol. Phys., 1980, 39, 629. 24 J. C. Traeger, Znt. J. Mass Spectrom. Ion Processes, 1985, 66, 271. 25 P. B. Davies and W. J. Rothwell, J. Chem. Phys., 1984, 81, 5239. 26 K. Kawaguchi, A. R. W. McKeIlar and E. Hirota, J. Chem. Phys., 1986, 84, 1146. 27 J. M. Dyke, M. Feher, M. P. Hastings, A. Moms and A. J. Paul, Mol. Phys., 1986, 58, 161. 28 I. V. Ovchinnikov, L. V. Serebrennikov and A. A. Maltsev, Russ. J. Phys. Chem., 1985, 59,923. 29 V. G. Solomonik and I. G. Sazonova, Rum. J. Znorg. Chem., 1985, 30, 1100.30 J. M. Dyke, A. R. Ellis, N. Jonathan, N. Keddar and A. Morris, Chem. Phys. Lett., 1984, 111, 207. 31 J. M. Dyke, A. R. Ellis, N. Jonathan and A. Moms, unpublished results. 32 K. Tanaka, G. I. MacKay and D. K. Bohme, Can. J. Chem., 1978, 56, 193. 33 N. Jonathan, A. Moms, M. Okuda and D. J. Smith, J. Chem. Phys., 1971,55,3046. 34 M. MacGregor and R. S. Berry, J. Phys. B, 1973, 6, 181. 35 S. D. Gabelnick, G. T. Reedy and M. G. Chasanov, J. Chem. Phys., 1974,60, 1167. 36 R. L. DeKock and W. Weltner, J. Phys. Chem., 1971, 75, 514. Paper 6/1202; Received 13th June, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300069

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987,83, 89-109 Thermodynamics of Some Proton-transfer Reactions Dynamic Ion Structures and the Measurement of Entropies of ‘Internal Translation’ Rod S. Mason” Chemistry Department, University College of Swansea, Singleton Park, Swansea SA28PP M. Tereza Fernandez and Keith R. Jennings Chemistry Department, University of Warwick, Coventry CV4 7AL Entropies well in excess of expected values have been measured for some protonated aromatic compounds (benzene, halogenobenzenes, halogeno- toluenes and xylenes). It is proposed that this occurs only when very facile proton migration is possible and that the large excess arises out of rapid ‘internal translation’ of the proton across a broad potential well within the molecule, An attempt has been made to model these systems, using a ‘particle-in-a-box’ approach to estimate the molecular partition functions.It was possible to generate sections of the potential-energy sur- faces involved by using ab initio calculations at the 4-31G basis set level. The ideas presented are consistent with independent experimental evidence not based on thermodynamic measurements. In particular, the measure- ments and calculations involving protonated benzene in the gas phase can be compared with a low-temperature study in the liquid phase, using n.m.r. spectroscopy. Making some assumptions with regard to the total excess entropy available in the dynamic protonated species, it is possible to estimate barrier heights for proton migration. The general implications are that such barriers are probably somewhat lower than those currently predicted by the best ab initio methods and that the relative proton affinities measured are not ground-state values and may therefore appear to be significantly lower than expected.Although the idea of ‘internal translation’ has been used in the past in the context of the transition-state theory, the concept of a stable, but dynamic ion structure, as suggested here, is new. Proton transfer is probably the most widely studied class of reaction in gas-phase ion-molecule chemistry.’ The driving force for much of the early work in this field came from a desire to study acid-base reactivity in terms of molecular structure and bonding, in the absence of solvent effects.As a consequence, extensive compilations of relative acidity and basicity data have become available covering a wide range of compounds? Based on these, reactivity-structure relationships can be rationalised both qualitatively and quantitatively (by comparison with the data generated from molecular- orbital theory calculations) and it is probably true to say that the gross features of proton transfer are well understood and well reproduced by the~ry.~ The task of filling in the details, however, still remains. It is often in the detail that chemistry reveals surprising and interesting phenomena. This paper describes a case in point -the ring protonation of aromatic compounds. The aim of this paper is to bring together the results from an extensive investigation of proton transfer in systems containing benzene, halogenobenzenes, halogenotoluenes, toluene and the xylenes in order to rationalise very surprising, but consistent measure- ments of anomalously large entropy changes which accompany the transfer of a proton to most of these species. Since the data have either already been published, are in press, or in some instances, are still in preparation, no description of the experimental details is given, except to 89 Entropies of 'Internal Translation' say that the measurements were made by the now well known technique of pulsed high-pressure-source mass ~pectrometry.~'~ Temperature Dependence of Proton-transfer Equilibria Relative basicity measurements are usually made by observing a proton-transfer equili- brium of the type represented in the equation: AH++B eA+BH+.(1) The free energy change, AG*, is derived, as usual, from the equilibrium constant, Ke, and is a direct measure of the relative basicities of the two neutral compounds, A and B. Again, in the usual manner, measurements of K0 as a function of temperature lead, via the van't Hoff equation (In K*= -AHe/RT+ASe/R) to the more fundamental thermodynamic quantity, the enthalpy change AH*, which is here equal to the difference in proton affinities (PA)between the two neutrals [AH0 = PA(A)-PA(B)]. The proton affinity is a measure of the binding energy of the proton to the neutral molecule and for most compounds lies in the range 400-1000 kJ mo1-1.2 At any particular temperature, T, AGO and AH* are significantly different only if the entropy change, ASe, is large.In fact for most proton-transfer reactions AS is insignificant.'16 This is readily seen by consideration of AS in terms of its individual components as in AS = Astrans,.+Aselect. +ASSvib.+Asrot. (i) The translational component depends on a change in mass, which is negligible. ASelect. is also zero because, in this study, we are concerned only with the ground electronic states of the protonated species in thermal equilibrium with the bath gas. Furthermore, since the proton binding energy is very strong, vibrational frequencies associated with the proton attachment will usually be too high to make significant contributions.Even so, vibrational cont?-ibutions that do arise would be expected to cancel out for isodesmic processes. The most significant term is usually ASrot.when there are changes in rota- tional symmetry involved. It is easily calculated by statistical mechanics to be R In (cTAH+cTB/cTBH+cTA) where 0 represents the rotational symmetry number and R the gas constant. It is still small because for most compounds and their protonated analogues CT = 1 or 2. It is for such reasons that a study of temperature effects on proton-transfer equilibria is usually considered to be chemically uninteresting, although of course there are exception^.^ In the past there were also practical reasons for avoiding temperature- dependence studies.' Proton Transfer between Benzene and the Halogenobenzenes One apparently straightforward example of the above is provided by the equilibrium (2), first studied by Kebarle8 who measured a AS of -15 J K-' mol-'. This value has since been confirmed in other laborat~ries~.~ and by different methods and it has been rationalised by assuming that the 'proton' in both protonated species is associated with a specific carbon atom which adopts an sp3 valence R.S. Mason, M. T. Fernandez and K. R. Jennings O800 t 1 Fig. 1. The anticipated influence of ortholpara directing effects on sites of protonation in halogeno toluenes. structure, as suggested by molecular-orbital theory. This structure has actually been observed in the liquid phase using n.m.r.spectroscopy,'op" but only at temperatures <200 K. In protonated fluorobenzene it is presumed that the fluorine substituent 'directs' the proton exclusively to the carbon atom at the para position. Based on these structures the rotational symmetry correction term is calculated to be -14.9 J K-' mol-', in excellent agreement with experiment. The similar reaction with chlorobenzene, however, has a lower experimental value for AS (-11 J K-' mol-'), which could be rationalised if protonation took place at both the ortho and para position^.^ Sites of Protonation on Halogenotoluenes Relative basicity measurements on various halogenotoluene systems were initiated12 in order to study the combined effects of two substituents on the benzene ring towards protonation.Both methyl and halogen substituents are ortholpara directing towards electrophilic attack. Using a purely qualitative rationalisation it can be argued that the 'directional' influences of the two substituents interact either coherently or incoherently, depending on the positions of the substituents relative to each other. This is most easily understood by reference to fig. 1, reproduced from ref. (12). Thus, coherent interaction occurs in the meta isomer because unsubstituted carbon atoms which are ortho to one of the substituents are always ortho or para to the other. On the other hand, incoherent interaction occurs in the para isomers where the prime sites are blocked by the sub- stituents themselves and because the unsubstituted carbon atoms are always meta with respect to one or other of the substituents.The ortho isomer lies between the two, so relative basicities of the isomers are expected to be in the order meta > ortho >para, as is indeed found by experiment.'* Apart from this it appears that on any single molecule there may be more than one preferred site of protonation. The possibility arises, therefore, that a number of distinguishable proton isomers (protomers) could be formed from some or all of these compounds. This would give rise to an entropy of mixing contribution to the free energy change. Take the para isomer as an example; if a proton can attach at any of the unsubstituted carbon atoms and at the carbon ips0 to the methyl group (a possibility that has often been suggested for similar compound^'^) there would be three distinguishable protomers yielding R In 3 J K-' mol-' for the entropy of mixing.Originally it was hoped that this sort of information would be revealed by experimental measurements of the entropy changes for proton transfer between these compounds. Entropies of ‘Internal Translation’ meta ortho 28b 23‘ ortbo Dam para &a Fig. 2. Overlapping proton affinity ladder for halogenotoluene systems. Experimental error *20% (2 x standard deviation). For an explanation of see text. Entropy Changes and Relative Proton Affinities for Halogenotoluene Systems Fig. 2 and 3 reproduce data from ref. (14) and summarise the results of a study of seven systems represented generally by the equation; CH3 CH3 CH3 CH3 Q+@:Q+--I -,--* +HQ+H ‘-.* \ ‘--* (3) X Y X Y where X and Y represent atoms of either fluorine or chlorine attached at various positions around the ring.The relative proton affinity and entropy changes were extracted, in the usual way, from the slopes and intercepts, respectively, of van’t Hoff plots. The overlap- ping relative proton affinity values (fig. 2) allow a thermodynamic self-consistency check on the data. Thus the sum of values marked with an ‘a’ superscript is 38 kJ mol-’, in good agreement with the 37 kJ mol-’ obtainable by summing the ‘6’ superscript values, as it should be if the measurements are of true thermodynamic equilibria. This is emphasised because the data in fig.3 are very surprising. Here the entropy changes are presented in a similar manner to fig. 2, and again the self-consistency check is good, R. S. Mason, M. T. Fernandez and K. R. Jennings Para-23' 56' ortho-ortho-meta meta Fig. 3. Overlapping entropy changes during proton transfer in halogenotoluene systems. Experi- mental error *20% (2 x standard deviation). For an explanation of u9b see text. but the entropy changes for four out of the seven systems are extraordinarily large. It is the protonated p-halogenotoluenes which have the largest entropies followed by the ortho, then the meta compounds. Moreover, there is an almost linear (but inverse) correlation here between the relative proton affinity of the molecule and the degree of extra entropy conferred by the attachment of a proton.Entropy Changes involving Protonated Xylenes The above observations caused us to look for similar effects in the protonated isomers of ~y1ene.l~In this case the equilibria in eqn (4) were studied over the range 500-800 K. The higher temperature range was chosen because the mass spectrum of the ion-molecule reaction products showed a significant peak for the (CH30CH&H+ dimer ion at lower temperatures and it was felt that the formation of this ion could cause interference to CH3 CH3 Entropies of ‘Internal Translation’ 4 ? I 1 I0‘ 1 1 I 1.8 2.2 2.6 3.0 lo3K/ T Fig. 4. In K* vs. 1/T for the system: C&+p-FC6H4CH3 *C6H6+p-FC6H4CH3H+.t marks the point corresponding to the temperature above which isomerisation is thought to become a significant interference.the equilibrium of interest (although, in fact, equilibrium data collected at lower tem- peratures were consistent with the high-temperature measurements). Although the entropy changes were found to be smaller than for the halogenotoluene systems, some were still significantly high (e.g. 28 J K-’ mol-’ for the p-xylene system) with entropies again increasing in the order meta < ortho <para. A further point of experimental significance is that the proton affinity of dimethyl ether lies closest to that of para- and ortho-xylene. This gives rise to a small AH (hence a shallow slope on the van’t Hoff plot), but a large AS (i.e.high intercept) for these systems. The rn-xylene gives a steep slope but small intercept. This is opposite, experimentally, to the halogenotoluene systems, where large AH values were correlated with large AS values. It thus removed any doubts as to whether the behaviour observed in the halogenotoluenes could somehow be an artefact arising from systematic errors (albeit unrealistically large) in the slopes of the van’t Hoff plots and exaggerated by the procedure of extrapolation to the intercept to determine AS. (This sort of problem is discussed at length in papers investigating ‘isokinetic’ relationships in so1ution.16) The Benzen+p-Fluorotoluene System The new relative proton affinities from the halogenotoluene work and comparison with proton affinity compilations led us to predict that the relative proton affinities of benzene and p-fluorotoluene ought to be similar (at least, at the temperatures of the experiments).A study of the reaction F F in the temperature range 300-450 K not only confirmed this prediction but also indicated that benzene itself has a high excess entropy when protonated. Above 450 K, however, a new aspect to the problem emerges because the van’t Hoff plot suddenly diverges R. S. Mason, M. T. Fernandez and K, R. Jennings at high temp. -=== fast ~FTH*/BH* -.-.fast \ Fig. 5. Energy diagram (schematic) illustrating proton transfer and possible isomerisation Drocesses in the benzene-o-fluorotoluene svstem. BH++PFT e B+PFTH+ isomerisation at T > 500 KIMFTH+(?> proton sink Scheme 1 upwards (see fig.4) in favour of the protonated fluorotohene, PFTH+. This divergence is not accompanied by any new ionic products in the reaction mixture. It therefore appears as if PFTH+ is isomerising to a new species at the higher temperatures from which the proton cannot easily transfer back to either of the neutrals, benzene and p-fluorotoluene (scheme 1 and fig. 5). The most obvious candidate for the new species is protonated rn-fluorotoluene (MFTH+), since it requires only that either the fluorine atom or the methyl group shift across to one of its neighbouring carbon atoms and similar processes are seen in acid solution.'' [Preliminary examination of the potential- energy surfaces (using MIND0/3 calculations) supports the contention that the fluorine atom is induced to move when the proton approaches the fluorine-bonded carbon atom.] In addition, the basicity of MFT is -14 and -21 kJ mol-' relative to p-fluorotoluene and benzene, respectively, compared with the -1.4 kJ mol-' of PITrelative to benzene (B).Therefore back-transfer of the proton from MFTH' to either of the neutrals present in the reaction mixture would have a much lower probability and would cause an anomalously large value for the apparent equilibrium constant. Whereas the bimolecular proton transfer being studied is relatively temperature insensitive, the unimolecular Entropies of ‘Internal Translation’ isomerisation can occur, to a significant extent, only at high temperatures because of the inevitably high energy barrier for such a process.This effect does not show up in the systems containing only halogenotoluene mixtures. This is explained if both fluoro- and chloro-toluenes indulge in similar behaviour so that in the PF-T/PClT system both sides of the equation are affected equally. The other systems, however, contain either an ortho or rneta compound, which lie closer in proton affinity to that of the supposed isomerised species, and there is therefore more chance that the proton can be fed back into the proton-transfer equilibrium of interest. This is represented in scheme 2. PFTH+ +OC~TZ= PFT+ OC~TH+ MFTH+ MCITH+ Scheme 2 That there is no noticeable effect on the measured equilibrium constant of interest in any of these systems is supported by kinetic modelling.? In any case, no curvature of the van’t Hoff plots could be detected outside the experimental scatter, in addition to which significant effects would have destroyed the thermodynamic self-consistency of the data.The identification of such isomerisation processes by this method is of interest because it is possible, again using kinetic modelling and fitting to the data, to estimate quite precisely the barrier height. This is not pertinent to the main theme of this paper, except to emphasise that in these systems we are aware of other reactions besides proton transfer and their possible effects on the thermodynamic data. In fig. 4 only data at T 6450 K have been used to calculate AH and AS.Comparison with Independent Data The temperature dependence of the rn-fluorotoluene-toluene system has also been measured.” The systems discussed in this paper lead to proton affinity values which can, therefore, be related back to that of toluene. Using this compound as the anchor point, the data are compared in fig. 6 with the values recommended by Lias et aL2based on a compilation of many different measurements. The agreement over a wide range of proton affinities is very good except where we have measured large entropy effects. This is to be expected since most of the previous data were obtained from single- temperature measurements. There are no previously reported entropy measurements for the systems under discussion, except ihe benzene- fluoroben~ene~,~ and benzene- chlorobenzene’ systems, for which the agreement is good.Excess Entropies due to Protonation If the measured entropy change in the proton-transfer reaction is expressed in terms of the entropies of the individual species involved: (ii) 7 The computer model, included in which were the proton-transfer reactions, loss of ions by diffusion and the isomerisation process, used the transition-state theory expression for the rate of isomerisation, i. e. kisom.= (kT/k)exp (-AG”/RT), where AG# is the relative free energy of the barrier to isomerisation and the only variable in the model, other quantities being deduced from the experiinental data. The excellent fit to the data was very sensitive to AG#, for which a value of 102*4kJmol-’ was obtained.The type of isomerisation proposed here is known to occur easily in solution, under acid conditions” [see also ref. (1711. R. S. Mason, M. T. Fernandez and K. R. Jennings (a) independent (b) this work duta MOtheory m-x ylene ----__ ~ -o-xylene -dimethyl ether I -_.e -----800-p-xylene 3 to1 ue ne 1 ..... . . ........ :U ---__ __---.-m-Ft oluene 0 --UE 780--o-Ftoluene -----__ a / I / / I-/ / 1 760--P-Ftohene,' -_ benzene -Fig. 6. Comparison of experimental proton affinity measurements with (a)published data' and (b) molecular-orbital theory calculati~ns.~~~~~,~~ Sa represents the excess entropy acquired by a molecule A when it has become protonated.The entropy data for all the compounds discussed so far can be reduced to S' values relative to rn-fluorotoluene through the ladder of measured entropy changes. These are represented in fig. 7. From the earlier discussion of these entropies (based on conventional ideas) the excess can be due to changes in only three possible ways: (a)the number and frequencies of vibrations, (b) rotational symmetry and (c) entropy of mixing due to isomer formation. It is already established that contribution (c) could amount to no more than 9.1 J K-' mol-' for the p-halogenotoluene systems. It is conceivable that the ortho compounds could be protonated on 5 distinguishable sites (giving 13.4 J K-' mol-'), but it is most unlikely. Among the xylenes there are again a maximum of only 3 distinguishable protomers.External rotational symmetry changes could account for no more than R In 2 (5.8 J K-' mol-') in any system except those involving benzene. The contribution from (a)is also not normally large. Exceptions occur where there are significant structural changes which cause either the arrest or the release of internal rotation,2b or where the proton becomes attached to the end of a long molecule with basic groups attached to both ends. In this latter case, ring closure via a 'hydrogen bond' occurs and causes a substantial loss of entropy.' Neither alternative is feasible in the systems under discussion. There would be a large release of entropy if the benzene ring were to be broken in the protonated species, but this is unrealistic because of the very large energy that would be required when the maximum available exothermicity in any of the equilibrated systems is <30kJmol-'.Clearly, if the very large and Entropies of 'Internal Translation' 56PXT----o CITT-----Fig. 7. Entropy S' of protonated species in excess of the neutral molecule (S' = SAH+S,) relative to rn-fluorotoluene. anomalous entropy values are to be believed a less conventional explanation is required. In this regard there is independent experimental evidence which points the way. Mobile Protons The first experimental evidence for the existence of the benzenium cation was provided in an n.m.r. study by Olah et al., looking at benzene dissolved in a low-temperature superacid liquid medium." This also showed that as the temperature is increased there is a rapid 1,2-shift of protons around the ring, the protons becoming indistinguishable above 193 K.The energy of activation for this intramolecular process was estimated to be 38-44 kJ mol-', with a high pre-exponential factor of 1015.9*1.5. There have since been a number of gas-phase studies. An ion-cyclotron resonance (ICR) investigation at 300 K of isotope exchange between D20 and various protonated aromatic compounds showed18 sequential ring deuteration to varying degrees. In particular, reaction (6) leads rapidly to the preferred C6D; ion: The degree of exchange varied significantly for different disubstituted structural isomers.Taking the xylenes as an example, ortho and para compounds rapidly exchange all 4 ring hydrogens, whereas the meta isomer exchanges only a single hydrogen, and this is slow. Compounds such as cyanobenzene, in which the proton is known to attach to the substituent group and not the ring, did not undergo exchange. The authors concluded, without further elaboration, that whilst ring protonation is necessary for the observation of this exchange it is not a sufficient condition. It is clear, however, that complete exchange requires the ring positions to be roughly equivalent in their proton affinities and, therefore, that protonated m-xylene has only one favoured ring site. The speed of complete exchange would be enhanced where the protons are internally labile.This is possible only where there are a number of adjacent equivalent sites. R. S. Mason, M. T. Fernandez and K. R Jennings In an independent study" Bruins and Nibbering were able to generate species (1) in a gas-phase ICR experiment, again at 300 K. They observed its subsequent proton or deuteron transfer with an added based B. D5D+B---+or BHD + D BD' The statistical ratio of BH+/BDf obtained showed that there was complete internal scrambling in (1) within the s timescale of the experiment. More recent experiments by Kuck2' generated various protonated aromatic compounds and their deuterated isotopomers in a chemical ionisation source (at 200°C). Scrambling was detected by MIKES analysis of the unimolecular decomposition products.They observed complete internal scrambling of up to 21 protons within the 10 ps timescale of their experiment. The accumulated evidence points to the presence of highly mobile protons in these gas-phase ions. Although it is difficult to estimate the true temperature of the ions in these non-equilibrated systems, it can be shown, using the simple transition-state theory for unimolecular processes, that a barrier height of 40 kJ mol-1 is too high to account for the gas-phase observations unless there is a substantial contribution from the 'entropy of activation' term. Ab Initio and Semi-empirical MO Calculations Protonated Benzene Protonated benzene has been the subject of a number of theoretical studies21 which show that the benzenium structure (2) (in which the proton is attached to a carbon atom in its tetrahedral sp3 configuration) is indeed the most stable form of the ion.Although the best proton affinity calculation2'" so far (at the 4-31G basis set level) puts the absolute proton affinity of benzene at ca. 27 kJ mol-' above the experimental value, relative values of ground-state structures are generally much more reliable. The bridged structure (3) is thought to be the saddle point for proton migration and its energy, relative to (2), is therefore the barrier height, AE. At the 4-31G level AE is calculated2*" to be ca. 86 kJ mol-', but becomes significantly reduced, to 33-40 kJ mol-', when recalculated22 at the higher MP2/6-31G** level. This calculation is much closer to reality because it includes polarisation effects, necessary for non-classical cations such as this.It is also much closer to Olah's value, determined by n.m.r. spectroscopy" to be 38-46 kJ mol-' in the liquid phase. Even so, the very significant difference between the theoretical values (which are the more relevant to the gas phase) indicates that convergence has not yet necessarily been reached and that they may have to be revised downwards to account for both electron correlation at higher levels and the zero-point energy diff eren~es.~~ H Entropies of 'Internal Translation' 150 I -0E 50 01 L 1-2-3-4-5-6-1 F CH3 distance/ 8, m A A A A v V v VD QF CH3 F Fig. 8. A section (around the ring perimeter) of the potential energy surface for protonated fluorotoluenes: (a) rneta, (b) ortho, (c)para.Semi-empirical calculations (MIND0/3) predict 21e924 much the same sort of barrier height as the high-level ab initio method. In addition it is predicted that the face- protonated (rr-complex) structure (4) has a similar energy to (3) (dismissed in the lower level ab initio studies as having too high an energy to be considered and not yet tackled by the high-level method). This structure is of interest because if its energy is low enough it would allow the proton to roam over the whole planar surface of the ring at the temperatures of the experiments discussed in this paper. Protonated Halogenotoluenes The proton affinities at all 6 ring carbon positions have been calculated25 for the fluorotoluene isomers, at the 4-3 1G level using STO-3G-optimised molecular parameters.Limitations on the total number of basis functions, imposed by the computer program, as well as cost, precluded similar calculations on protonated chlorotoluenes. The relative energies for the protomers of each fluorotoluene isomer are plotted in fig. 8. Qualitatively R. S. Mason, M. T. Fernandez and K. R. Jennings similar patterns were generated24 for the chlorotoluenes and the xylenes at the STO-3G level of calculation. MIND0/3 gave disappointing results because of its tendency to exaggerate hydrogen bonding grossly whenever protons came too near to the halogen atom (a generally re~ognised~~ deficiency of the method).Absolute (4-31G) values are overestimated again by ca. 30 kJ mol-’. However, relative values are generally in much better agreement with experiment. Thus the values for the highest proton affinity sites in m-and o-fluorotoluene are apparently good to within 4 kJ mol-’ relative to either toluene or benzene. On the other hand, the disparity for p-fluorotoluene is much larger (> 18 kJ mol-’), implying that the observed ion does not have the ground-state structure. Once more the para compound stands out in its eccentric behaviour and it is reasonable to suppose that the unusual entropy measure- ments and deviation from the ground-state structure are connected. A clearer and more dramatic correlation is seen between the excess entropy measure- ment and the appearance of the potential-energy surface as shown in fig.8. These diagrams represent sections around the aromatic ring of the potential-energy surface for proton attachment. Since energies for the proton at sites bridging the ring carbon atoms cannot be estimated at the 4-31G level (see above), these parts, indicated by dotted lines and representing the barriers to proton migration, are assumed to be low, consistent with the previous discussion. It is readily apparent that all the ring sites in p-fluorotoluene, except the fluorine-bonded carbon atom, are equivalent in proton affinity (to within 3 kJ mol-*), whilst the ortho compound has only three equivalent and adjacent sites. In contrast, the meta compound has two sites, but they are not adjacent and they sit in comparatively deep proton-affinity wells. There is, therefore, a direct correlation between the number of equivalent adjacent sites for protonation and the excess entropy measured.It is very reasonable to suppose that this is associated with the freedom of the proton to migrate from one site to another because if the temperature is high enough for the proton to become free of the small energy wells it has the freedom to move over a very wide flat potential-energy surface. The same motion is not possible in the meta compounds, whilst the ortho isomers represent an intermediate situation. Clearly the distribution of ions between the two states of the molecule and, therefore, the extent to which the full excess entropy arising out of this motion can be realised experimentally depends on the barriers to proton migration (height AE) and the temperature.If AE was large, migration over the barrier would be slow and correctly described as an isomerisation process giving rise only to an entropy of mixing term. As discussed earlier, protons distributed between all five sites in the para compound would, therefore, yield a total of only 3 externally distinguishable isomers and hence a maximum entropy contribution of only R In 3 (9.1 J K-’ mol-I). If, on the other hand, AE is low enough, the proton will spend most of its time above the barrier, undergoing rapid motion across the much wider potential-energy well. This motion is best described as ‘internal transla- tion’ (by direct analogy with the concept of ‘internal rotation’), where the excess entropy is the same as that envisaged in the transition-state theory for unimolecular isomerisation processes and referred to as the ‘entropy of activation’. Entropy Calculations The excess entropy S’ can, in principle, be calculated from the molecular partition function q.To estimate this quantity accurately requires a detailed knowledge of the potential-energy surface, which is not currently available. However, it is possible to estimate the magnitude of the contributions roughly involved by adopting a very simplified model. For the purposes of this model it is assumed that there are two ionic states involved: the fixed ground state, whose structure conforms to the conventional vim of the proton attached to one carbon atom, and a dynamic upper state, which lies at an energy AE above the ground state and in which the mobile proton is free to move Entropies of 'Internal Translation' Fig.9. (a)Schematic diagram of the section of the potential energy surface of protonated benzene, around the ring perimeter. (b) Translational motion of the proton as a particle-in-a-ring. in a very broad potential well. The well encompasses all the adjacent carbon atoms with the highest equivalent proton affinities (and therefore the lowest energies). This model is illustrated in fig. 9(a) for protonated benzene. Excess entropies of the ground-state ions, Si,,,,, are determined only by the changes in rotational symmetry, whereas the excess in the upper state, Skpper,is determined by the sum of three contributions: Sb, S:ran,l,(ld) or S&.,,,.(2d), and S:ib..Sb is due to the change in external rotational symmetry between the neutral and the dynamic species and is calculated in the usual way. S:ransl.is the contribution from the translational motion of the proton in the broad energy well; this is estimated by making use of either the one- or the two-dimensional particle-in-a-box, or the particle-on-a-ring solution of the Schrodinger equation. Skib. arises from changes to low-frequency out-of-plane motions of the atoms other than, but due to, the motion of the mobile proton. It is difficult to calculate, explicitly, what this effect might be and here it is necessary to make comparison with known neutral systems.The entropy excess SLxpt.actually measured depends on the relative proportion of ions in the upper and lower states. This in turn will depend on AE and the temperature. Protonated Benzene Protonated benzene is the system most readily amenable to the above treatment. Since protonation is equally likely at all 6 carbon atoms, the section of its potential-energy surface taken around the perimeter of the ring can be drawn schematically as in fig. 9( a). From the previous discussion of barrier heights, AE 9 40 kJ mol-'. When motion is restricted to the perimeter of the molecule [fig. 9(b)],protons which acquire sufficient energy to overcome this barrier enter into a potential well whose geometry approximates to that of a ring of radius r.When rapid motion across the plane of the ring is also possible the surface will have a bowl shape which, for the purposes of the calculation, is approximated to a flat-bottomed cylinder of surface area A. Textbook solutions provide convenient relationships of use in calculating the molecular entropy.26 These are listed in table 1. Strictly, for small-dimensioned systems, q should be summed explicitly. However, use of the integrated summations of q, shown in table 1, does not introduce significant errors. Since there is an equal possibility of motion in the planes above and below the ring it was assumed that g = 2. Using ab initio derived geometries, one obtains a value of t=2 A.At 400 K (the mid-point of the experimental range), this gives S:ransl. = R. S. Mason, M. T. Fernandez and K. R. Jennings Table 1. Formulae used in calculating internal translational entropy values26 E, = (box)= n2R2/8mL2; &,(ring)= n2h2/8r2mt2; q =C g, exp (-E$) n q(Id) = 0.115M1/2LT1'2; q(ring)=3.6M'/2rT1/2 q(2d) =0.1152MAT S = [ U -U(O)]/T +R In q; U -U(0)= -(N/q) aq/ap S:rans~.(ld)=R/2+RlnCq(1 -d)I; SL-,,,.(2d)=R+Rln[q(Z-d)] Siing=R/2+R In [q(ring)] units and symbols m =mass of proton E, =energy of nth quantum level M =mass of proton (a.m.u.) L =length of one-dimensional box (d) h = Planck's constant A = area of two-dimensional box (A2) R =gas constant r = radius of ring (A)N = Avogadro's number g, = degeneracy of nth quantum level" T =temperature (K) U, U(0)=internal energy at T/K and O/K, respectively p = l/kT There is an equal possibility of motion in the plane both above and below the ring :.g = 2.26.3 J K-' mol-' as the particle-on-a-ring entropy contribution. Although the ground- state protonated structure, (2) has u=2,the dynamic structure proposed here is more like (3) in its symmetry (within the timescale of a molecular rotation) and is assumed to have (T = 1 giving Sb =R In (12/1) =20.7 J K-' mol-'. The calculated sum, S:ransl.+ Sb=47 J K-' mol-' is in very good agreement with the experimental value S&. = 53 f6 J K-' mol-' and it is tempting to believe that the significant contributions have already been fully accounted for. However, this is unlikely because it would require 390% of the ions to be in the dynamic state (measured values of the equilibrium constant are in fact reproducible to only *lo%), which in turn would require the well depth for the ground-state structure to be <10 kJ mol-'.Moreover, the contribution, SLib., could be quite large because there will be a significant change to out-of-plane bending modes of the hydrogen atoms and it is probable that a low frequency 'puckering' mode could be induced into the motion of the carbon atoms in phase with the passing proton. Puckering of the ring is reckoned27 to contribute ca. 21 J K-' mol-' to cyclo- hexane relative to benzene. Benson has expressed the view that loss of most of this excess entropy in benzene occurs in the final step of the ring-tightening process during the transformation from cyclohexane to benzene.27 To obtain 20 J K-'mol-' excess entropy would require a frequency assignment of ca.50 cm-'. This is the same sort of frequency (ca. 10l2s-') that can be calculated for classical motion of the proton round a benzene-sized ring at ca. 400 K (energy =$kT). Stib.would be increased further if the mobile proton was free to move not only parallel to the plane of the molecule, both above and below the ring, but also perpendicularly across from one plane to the other, between carbon atoms. This is least likely, however, because the theoretical calculations indicate24 that the barrier to this sort of motion is significantly higher than the parallel motion. Even so, it is conceivable that Stib.will add to the entropy of the dynamic species quite considerably. There is evidence for this in the liquid-phase measurements on proton migration. Olah measured" the pre-exponential factor, A, for proton migration in the liquid s-'phase to be 1015.9*1.6at T= 173 K. According to transition-state theory A= (kT/h) exp (AS*/R), where AS*here is the entropy difference between the ground-state ion (2) Entropies of 'Internal Translation' Table 2. Estimates of contributions to the entropy excess, Skpper,of protonated aromatic compounds in their dynamic upper state halogenotoluenes compound benezene" para ortho meta (TIuH+ 12 2 1 1 box dimensions (A) r=2 A=12.6 L=9.4 A=8.9 L=5.2 L=O Siransl,(1 or 2 dimensional) 26.3 44 30 41.3 25.2 0 20.7 5.8 0 0 ? ? ? 0 47 65 36 47 25 0 T = 400 K.T =450 K. (+ and uH+ are rotational symmetry numbers of the neutral and protonated species, respectively. Table 3. Comparison of calculated and experimental excess entropy values for protonated benzene and halogenotoluenes compound one-dimensional two-dimensional Skxpt. Slib.(min)" Siowerb ~___ 53*6 benzene 47 65 15 79 f30" 14 p-fluorotoluene 58*3 47 13 5.8 p-chlorotoluene 36 63 *3I o-fluorotoluene 8* 1.5 10 0 o-chlorotoluene 25 35*3I m-fluorotoluene 0 -0 rn-chlorotoluene 7zt1 " Minimum value required to fit theory to experiment, obtained by subtracting the maximum value of (SipperStib.)from Sixpt,.Obtained by calculating RIn ((+/uH+)for ground-state -structures. Liquid-phase (superacid) entropy of activation +RIn 6 (see text). and the activated form of (3), for which a value of 64 f30 J K-' mol-' is derived. Because the gas-phase measurement and the calculation of entropy excess start from neutral benzene, for the purpose of comparison, Rln6JK-'mol-' must be added, giving 79 f30 J K-' mol-' [this extra term accounts for the symmetry change in going from benzene to (2)]. This very high value for the liquid phase (albeit with a wide margin of error) is too large to be accommodated by one-dimensional motion. A considerable boost to the entropy excess is given when the proton is allowed to migrate over the whole plane of the ring (giving S' =65 J K-' mol-', see table 2), with a further increase when Stib.is added (indeed SLib.== 14 J K-' mol-' would bring S' close to the average value measured in the liquid phase). These numbers are summarised in tables 2 and 3.R. S. Mason, M. T. Fernandez and K. R. Jennings Fig. 10. Region of the halogenotoluene molecule over which proton translational motion is assumed: (a) para, (b) ortho. Protonated Halogenotoluenes Fig. 8 shows that in the protonated p-halogenotoluenes the proton is free to roam (again depending on AE!) over most, but not all, of the ring perimeter, because the proton affinity of the halogen-bound carbon atom is too low. Both one- and two-dimensional motions have been considered in the calculations summarised in table 2, based on either the ‘particle-in-a-one-dimensional box’ or the ‘particle-on-a-surface’ model.In the protonated ortho isomers, according to fig. 8, motion is limited to only one half of the ring perimeter (see also fig. 10) for which one-dimensional motion only has therefore been considered. Again, one-dimensional motion is not enough to account for the experimental data in the p-halogenotoluene systems and again it is necessary in all cases to postulate additional low-frequency motions with a minimum contribution of 10 J K-’ mol-’. The extent to which any of these entropies contribute to the experimental measure- ment depends upon the AE values and T. Any curvature in the van’t Hoff plots which might occur, in fact, would not show up because of the scatter of the experimental data over the experimental temperature range (300-600 K).Estimation of Barrier Heights At 400K protonated benzene is considered here as existing in its two forms. The ground-state ion is represented as BIH+ [=(2)] and has an excess entropy of R In (12/2) = 15J K-’ mol-’ (due only to changes in c),whilst the dynamic form B2H+ has an excess entropy Sg and a proton affinity which is lower than B,H+ by AE, but both as yet unspecified. In its reaction” with p-fluorotoluene (PFT) it is assumed for the sake of simplicity that only one species of PFTH’ is formed. [In fact, PFTH’ will also have two states, but it is probable that only one is populated significantly (see below).] Moreover, provided equilibrium is maintained, they can be treated as one without affecting the scheme, using the composite proton affinity and entropy values actually found experimentally.) Then, at equilibrium, three systems are involved having equili- brium constants K1, K2 and K3, respectively.B represents neutral benzene. B,H++PFT e B+PFTH+ (1) By making the reasonable assumption that the rate constants are equal for each proton transfer in the exothermic direction, it can be that the observed equlibrium Entropies of 'Internal Translation' constant, Kobs,measured experimentally as [PITH'][ B]/[ BIH' +B2Hf][PFT], is given by In Kobs=h K,+h K2-11l (K,+K,). (iii) If the enthalpy change in (I) is AH, then its value in (11) is AH +AE. Putting AS, and AS, for the respective entropy changes, substitutions for K, and K2 can be made giving (AE -AH) +(AS1+AS,)In (Kobs) = RT R -In [exp (AS,/R)+exp (AS2/R) exp (-AEIRT)].(iv) If BIH+ and B,H+ were degenerate isomers (AE =0, AS, = AS,) then eqn (iv) would predict the conventional entropy of mixing term (-Rln2), as expected. This is not the case. AS1 is easily estimated to be S'(p-fluorotoluene) -R In 6 =43 J K-' mol-', whilst AS2=S'(p-fluorotoluene) -Sb = (58 -Sg) J K-' mol-'. According to Olah's data Sf3 could fall anywhere between 49 and 109 J K-' mol-'. The mid-point of this range corresponds to the reasonable maximum that can be envisaged theoretically at which point AS2 = -21 J K-' mol-'. This fits to the gas-phase data when AH =3 kJ mol-' and AE =23 W mol-'.(The lower and upper limits of Olah's data lead to AE values of <13 and 38 kJ mol-', respectively, whilst AH varies in the region only from 3 to 6 kJ mol-'.) Even though the above treatment leads to a rather imprecise value for AE in the gas phase, the most reasonable considerations indicate that its value is s23 W mol-'. This is somewhat lower than the liquid-phase value and less than the best (so far) ab initio derived value. A further consequence is that the ground-state proton affinity of benzene is actually 6 to 9 kJ mol-' higher than the composite value (i.e.uncorrected for dynamic-state contributions) for p-fluorotoluene, and hence 3 to 6 kJ mol-' higher than the value quoted by Lias et aZ? (which is also uncorrected).Similar observations can be made with regard to the p-halogenotoluenes. Here experimental values of S' are much higher than the estimates of [S:ransl.(Id)+Sb]. Indeed the calculated entropies can be made to balance with the experimental data, only if two-dimensional motion is postulated, and ca. 13 J K-' mol-' are assigned to S:ib.. In any case, the experimental value is the minimum for the dynamic protonated state and may be larger depending on AE. As described above, the proton affinity measurement at 759 kJ mol-' [taking toluene as the anchor point (see fig. 6)] is in fact a composite of the ground-state value minus a proportion (depending on relative populations of upper and lower states) of the barrier height for the proton migration. The highest (ab initio) calculated proton affinity (again taking toluene as the anchor point) is 777 kJ mol-'.If this is closer to the ground-state proton affinity it puts AE in p-fluorotoluene at 318 kJ mol-'. Taking AE = 18 kJ mol-' and the excess entropy of the dynamic upper state (over and above the ground-state ion) as ca. 55 J K-' mol-' (see table 3) then at 450 K ca. 90% of the protonated p-fluorotoluene ions would be in the dynamic upper state at equilibrium. The general implication is that the barriers may be even lower than in benzene. The protonated fluoro- and chloro-toluenes show similar entropic behaviour, except for the ortho isomers. If this reflects differences in barrier heights, then the low excess entropy in the protonated o-fluorotoluene implies that the barrier is significantly larger than in protonated o-chlorotoluene.A relatively higher barrier than in the p-fluoro R. S. Mason, M. T.Fernandez and K. R. Jennings compound is also consistent with the rather better agreement between measured and calculated proton affinities. Similar arguments would apply to the protonated xylenes, for which a similar pattern of behaviour is observed. The main difference is that the measured excess entropies are somewhat lower than those for the halogenotoluenes, which implies that the barriers to proton migration are greater than those in benzene. Amongst the halogenotoluene isomers the highest entropies are associated with the lowest experimentally measured proton affinities. It is of interest to note that a plot of the AH values us.AS leads to a strong 'isokinetic relati~nship'.'~ In solution chemistry such plots are well known but are usually less clearly defined and ascribed to solvent effects.l6 Conclusions The unusually large entropy excess in the ring-protonated forms of compounds such as benzene and a variety of substituted and disubstituted benzenes very probably arises when the proton is free to migrate rapidly between adjacent ring carbon atoms. The fact that this can be experimentally measured in an equilibrium experiment must indicate that a dynamic state of the ion exists which is not transitory, as envisaged in transition- state theory applied to an isomerisation process, but is a stable state of the ion. The degree to which this state is populated, and hence the degree to which the full 'dynamic' entropy can be achieved experimentally depends upon the energy difference between the ground state and the dynamic structure, and of course on the temperature.This energy difference is the same, in effect, as the barrier to isomerisation (in this case proton migration), except that with a low barrier at high enough temperatures, the ions actually spend more of their time in the 'transition state' than in the ground state. It is suggested here that the entropy actually arises out of the internal translational motion of protons in the very broad energy well which, above the barriers encompasses two or more adjacent ring carbon atoms and sometimes the whole or part of the planar surface of the ring.The entropy is thought to be increased still further by low-frequency out-of-plane motions of the atoms other than (but due to) the mobile proton. The entropy of the dynamic state of protonated benzene, in excess of the neutral molecule, could be as high as 80JK-'mol-' at 400K. About half can be accounted for by a contribution from the entropy of internal translation, provided it is assumed that motion across the planar surface of the ring, and not just the perimeter, is possible. In addition to changes in external symmetry it is thought that the remainder arises from the out-of-plane motions of the other atoms. Amongst the halogenotoluenes the number of likely adjacent protonation sites increases in progressing through the meta, ortho and para isomers, as shown by ab initio calculations of the proton affinities at each ring carbon atom.This correlates well with the measured entropy excess. Again, calculations based on internal translational motion (using the particle-in-box model) can account for a large part of the measured entropy. The excess entropies actually measured are composite values, which reflect the distribution of the ions between the ground and dynamic states, which at equilibrium is determined in the usual way by the value of AE, the barrier height, and the change in entropy between the two states. The benzene data are then consistent with 4EG 23 kJ mol-' (assuming S'==80 J K-' mol-'). This is less than that measured for the liquid phase, and less than the best calculated result.Adopting this value, ca. 66% of protonated benzene molecules are in their dynamic state at 400K. The barrier height in the p-halogenotoluenes appears to be lower, at 318 kJ mol-', and has a significantly larger portion of ions in their excited state, evidenced by their larger measured entropy. By these criteria the barrier in o-fluorotoluene is significantly larger than o-chlorotoluene, whereas the para compounds are essentially similar, as are the meta. 108 Entropies of ‘Internal Translation’ A further implication is that the relative proton affinities measured for those com- pounds with high excess entropy are not ground-state values but again composite, being depressed by a fraction of the barrier height, again depending on relative populations of the ground and excited states. It is thought that this is the reason that the measured ‘apparent’ proton affinity of p-fluorotoluene deviates significantly from the a6 initio derived value, whereas theory and experiment are in relatively good agreement for the rneta and ortho compounds.The xylenes behave in a similar manner to halogenotoluenes except that, judged by the entropy excess, the barriers to proton migration are actually higher than in protonated benzene. If one accepts the above ideas, the rationalisation (based only on symmetry differences between ground-state ion structures), originally given8 for the entropy change when a proton is transferred between benzene and fluorobenzene cannot be correct. However symmetry differences between their dynamic states would lead to a similar result, provided that both species exhibit essentially the same degree of internal translation.Toluene, like the rn-halogenotoluenes, has only a very small excess entropy, which would be expected if a deep proton affinity well is induced at the fourth ring carbon atom by the single methyl substituent. The concept of ‘internal translation’ is not new, since it has been invoked in transition-state theory to describe the movement of atoms over the barrier during isomerisation.28 What is new is the idea that such a motion can be accommodated within a stable, albeit dynamic, molecular structure, and that an entropy of ‘internal translation’ can be identified as a contribution to systems in thermodynamic equilibrium.As such this concept is directly analogous to but different from ‘internal rotation’. We thank the S.E.R.C., the Gulbenkian Foundation and INIC (Portugal) for financial support. IReferences 1 (a) D. H. Aue and M. T. Bowers, in Gas Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1979), vol. 2, pp. 2-51; (b)R. S. Mason, in Thermochemistry and its Applications to Chemical and Biochemical Systems, ed. M. A. V. Ribeiro de Silva, NATO Series C, vol. 119, (D. Reidel, Dordrecht, 1984), pp. 627-652. 2 (a) S. G. Lias, J. F. Liebman and R. D. Levin, J. Phys. Chem. Re$ Data, 1984, 13, 695; (b) J. E. Bartmess and R.T. McIver, in Gas Phase Ion Chemistry, ed. M.T. Bowers (Academic Press, New York, 1979), vol. 2, pp. 87-121. 3 W. J. Hehre, L. Radom, P. v. R. Schleyer and J. A. Pople, Ab Initio Molecular Orbital Theory (Wiley, New York, 1986), chap. 6, p. 133. 4 P. Kebarle, in Interaction between Ions and Molecules, ed. P. Ausloos (Plenum Press, New York, 1975), p. 459. 5 D. K. Bohme, J. A. Stone, R. S. Mason, R. S. Stradling and K. R. Jennings, Int. J. Mass Spectrom. Ion Phys., 1981, 37,283. 6 S. G. Lias, in Kinetics oflon-Molecule Reactions, ed. P. Ausloos, NATO Series B (Plenum Press, New York, 1979), vol. 40, p. 223. 7 R. Yamagni and P. Kebarle, J. Am. Chem. SOC.,1973,95, 3504. 8 Y. K. Lau and P. Kebarle, J. Am. Chem. SOC.,1976,98, 7452. 9 K. G. Hartmann and S.G. Lias, Int. J. Mass Spectrom. Ion Phys., 1978,28, 213. 10 G. A. Olah, R. H. Schlosberg, R. D. Porter, Y. K. Mo, D. P. Kelly and G. D. Mateescu, J. Am. Chem. SOC.,1972, 94, 2034. 11 G. A. Olah, J. S. Staral, G. Asencio, G. Liang, D. A. Forsyth and G. D. Mateescu, J. Am. Chem. SOC., 1978, 100,6299. 12 R. S. Mason, D. K. Bohme and K. R. Jennings, J. Chem. SOC.,Faraday Trans. 1, 1982,78, 933. 13 J. F. Wolf, J. L. Devlin, D. J. DeFrees, R. W. Taft and W. J. Hehre, J. Am. Chem. SOC.,1976,98, 5907. 14 M. T. Fernandez, K. R. Jennings and R. S. Mason, J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 159. 15 (a) M. T. Fernandez, K. R. Jennings and R. S. Mason, Adu. Mass Spectrum., 1985, 10, 1167; (b) unpublished results. R. S. Mason, M.T. Fernandez and K. R. Jennings 16 0. Exner, Prog. Phys. Org. Chem., 1973, 411. 17 J. H. Lawry and K. S. Richardson, Mechanism and Theory in Organic Chemistry (Harper and Rowe, London, 2nd edn, 1981), and references therein. 18 B. S. Freiser, R. L. Woodin and J. L. Beauchamp, J. Am. Chem. SOC., 1975, 97, 6893. 19 A. P. Bruins and N. M. M. Nibbering, Org. Mass Spectrom., 1976, 11, 950. 20 D. Kuck, Int. J. Mass Spectrom. Ion Phys., 1983, 47, 499. 21 (a)W. J. Hehre and J. A. Pople, J. Am. Chem. SOC., 1972,94,6901; (b)M.J. S. Dewar and D. Landman, J. Am. Chem. SOC., 1977,99,7439; (c)W. C. Ermler and R. S. Mulliken, J. Am. Chem. Soc., 1978, 100, 1647; (d) J. Catalan and M. Yanez, Chem. Phys. Lett., 1974, 60,499; (e) T. Sordo, J. Bertran, E. Canadell, J. Chem. SOC., Perkin Trans. 2, 1979, 1486, and references therein. 22 P. v. R. Schleyer, personal communication. 23 T. Clark, A Handbook of Computational Chemistry (Wiley, Chichester, 1985), chap. 4. 24 All calculations were performed using QCPE packages, either Gaussian 76 (ab initio) or MIND0/3 (semi-empirical). 25 R. S. Mason, M. T. Fernandez and K. R. Jennings, Adv. Mass Spectrom., 1985, 10, 1167. 26 For example, P. W. Atkins, Physical Chemistry (Oxford University Press, Oxford, 2nd edn, 1982), chap. 20 and 21. 27 S. W. Benson, Thermochemical Kinetics (Wiley, Chichester, 2nd edn, 1976), chap. 2. 28 See for example, K. J. Laidler, Chemical Kinetics (McGraw-Hill, New York, 2nd edn, 1965), p. 77. Paper 6/1299; Received 27th June, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300089

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 111-126 Infrared Absorption Spectroscopy of Molecular Ions using Tunable Lasers Trevor J. Sears Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, U.S.A. For many years molecular ions have represented a challenge to the high- resolution spectroscopist. Their high reactivity makes it difficult to generate sufficient concentrations in the laboratory for traditional gas-phase spectros- copy in all but a few cases. During the past five years, however, advances in infrared laser technology and the development of sensitive experimental methods to identify absorptions by charged species have led to the observa- tion of the infrared spectra of approximately forty molecular ions. Many of them had not been detected previously except in mass spectrometric studies. In this paper the experimental methods and results are reviewed and the type of information obtainable is illustrated by reference to recent results for the H30+ (D30+)and CO; molecular ions.High-resolution spectroscopy of molecular ions has undergone a revolution during the past ten years as a variety of new laser-based experiments has been developed which possess increased resolution and sensitivity compared to classical techniques. Prior to this recent work, most information came from electronic emission spectra of ions formed in electrical discharges.'72 Infrared studies were fewer in number and almost entirely confined to condensed phases, typically rare-gas matrices at low temperature^.^'^ While valuable information can be obtained from such studies, much more is available from rotationally resolved studies in the gas phase, and little progress was possible along these lines because of the relative insensitivity of classical gas-phase infrared spectros- copy.In the present paper we are entirely concerned with developments in infrared spectroscopy which have occurred during the past six years owing to the ready availability of several different infrared laser technologies covering the entire infrared region of the spectrum. When I first agreed to present this paper, a little over a year ago, it would have been possible to review the entire field in a single talk; however, the recent pace of discoveries in the field has been very fast and some selectivity is necessary.In the present paper then, we begin by reviewing the various devices currently available for producing tunable infrared radiation and describing the typical experimental set-up. Along with the developments in laser technology, experimental techniques which allow the separation of the relatively weak ionic absorptions from much stronger absorption lines due to stable or semi-stable neutral species have been most important. All reported experimental results have been collected together in the form of a table, and the kind of information obtainable is illustrated by more detailed discussion of recent results on H30+and CO;. Experimental Details and Overview of Results The various tunable infrared sources currently available are summarized in table 1.Each has its advantages and disadvantages, some of which are noted in the table. The colour centre (CC) and infrared difference frequency (DF) systems have the significant advan- tage over the diode (DL) and C02 sideband (SB) systems in that they are broadly and 111 I. R. Spectroscopy of Molecular Ions using Lasers Table 1. Summary of currently available tunable infrared sources wavelength source coverage /Wn power /mw linewidth / MHz comments colour-centre laser 1.4- 1.6 1-10 1-2 Need visible ion or dye laser for pump- 2.3-3.6 ing. Major noise due to amplitude difference 2.2-4.2 0.01-0.03 1-2 fluctuation in pump laser. Single mode Ar+ and dye laser mixed frequency in LiNbO, crystal.Noise due to ampli- tude fluctuations in ion and dye laser. semiconductor 2.8-28.6 0.1- 1.0 10-20 Noise due to cold head vibration, low diodes frequency. Limited single mode tuning (typically <2 cm-'). Discontinuities in frequency coverage. infrared sidebands 8.5-1 1.2 0.02-0.5 aO.l C02 laser line and microwave source mixed in CdTe or GaAs crystal. Good noise characteristics, very narrow linewidth, limited and discontinuous tuning. continuously tunable. The CC laser requires several different crystals to cover its entire range, while the DF system tuning range is limited by the dye laser and the LiNb03 crystal oven temperature. The broad tuning range of these systems means that search problems are much less severe in the wavelength regions that they cover.On the negative side, these sources (in particular the DF system) are significantly more complicated (and expensive) than the others since they require pump lasers in addition to the infrared hardware. In both the CC and DF systems infrared amplitude instability caused by the pump lasers is generally the major noise source and maximum sensitivity requires a two-beam experiment with some form of ratioing for noise suppression. Diode lasers can now be obtained covering almost the entire infrared from ca. 350 cm-' to above 3500 cm-l. They are small mixed Pb; Sn; Se or Pb; Sn; Te (typically) crystals, and the laser wavelength depends on the detailed crystal composition.Lasing usually occurs in two to five simultaneous modes separated by several wavenumbers. Changes in the operating temperature of the crystal results in coarse tuning of the laser output by modification of the diode bandgap, while finer tuning is accomplished by varying the laser injection current. Single-mode continuous tuning occurs until the band- gap shift pulls beyond the single-mode range. Typical single-mode tuning ranges are <2cm-' and there are usually many discontinuities in the laser spectrum making extended searching difficult and time consuming. In addition, the laser beam profile is often rather large, especially at longer wavelengths. Nevertheless, these devices offer the only tunable source in many regions of the infrared spectrum and they have been used extensively to measure ion spectra.The final source noted in table 1, infrared sideband frequency generation, suffers from many of the same disadvantages as the DL and it has not yet been applied to the study of transient species. However, of all the sources considered, the technology here is the least mature and the SB source has very high spectral purity and could be important in the future as the techniques are refined, possibly in sub-Doppler spectroscopy. Analogous techniques in the far-infrared region of the spectrum are more developed, and rotational spectra of several ionic species have been detected using radiation generated from the mixing of a far-infrared laser and a microwave s~urce;~ however, these studies are outside the scope of the present article.T. J. Sears 113 reference ,gas inlet Pump absorption cell 7 monochromator LIPI etalon detector 1 I Ioc k -in 2 I 1,. II 1 1osc. 2 1 I current control data I acquisition lock-in 1 osc. 1 Fig. 1. Block diagram of diode-laser absorption experiment. The experiment shown uses the velocity modulation technique developed by Saykally and coworkers [see ref. (6) and text] to discriminate between absorptions by ions and those by other short-lived species formed in the plasma. Fig. 1 shows diagrammatically the typical experimental set-up for infrared absorption spectroscopy of molecular ions. In this particular case it refers to the diode laser system specifically, but the principles for all types of source are the same.Conceptually the experiment is simple: there is a tunable source of infrared radiation, an absorption cell where the ions are generated and a detector which monitors the infrared power as a function of wavelength. Additional complications include a small monochromator to isolate the particular laser mode of interest (this feature is not needed for the other infrared sources considered) and some method for measuring absolute and relative frequency. Frequencies are normally measured relative to some known reference gas line and interpolation performed using etalon fringes. In order to increase the absorption sensitivity and eliminate background power variations, the laser output is normally frequency modulated and the absorption lines detected by phase-synchronous amplification at the laser modulation frequency.The Hl ion was the first detected by this kind of technique. In 1980 Oka22 reported the observation of a number of lines in the v2band of this ion by absorption of infrared radiation generated by a laser difference frequency system in a large liquid-nitrogen- cooled d.c. discharge in H2. The DF radiation traversed ca. 30 m through the plasma in a multipass arrangement, and an example of a single vibration-rotation line is shown in fig. 2. Quite soon after this, several other groups detected vibrational spectra of small protonated species, e.g. HCO+ and HN;. An example is shown in fig. 3, which shows some rotational lines in the v3 (CO stretch) band of HCO+.35 The absorptions were detected by diode-laser absorption in a hollow cathode d.c.discharge in H2 with a trace of CO by frequency modulation of the laser and detection at twice the modulation frequency. Fig. 2 shows immediately the serious problem in this type of spectroscopy, the ion absorption lines are weak compared to the absorptions of the stable molecules present in the plasma. While this effect is less serious in the simple cases considered up to now, it becomes much more of a problem in chemically more interesting mixtures. Fortunately, an experimental procedure has been developed by Saykally and CO-workers6 which allows the unique detection of the absorption lines of a charged species I.R. Spectroscopy of Molecular Ions using Lasers II IIII IIII IIII Fig. 2. Vibration rotation line in the v2 fundamental band of the HT molecular ion [ref. (22)] (2725.885 cm-'). This was the first molecular ion detected using the experimental techniques discussed in the present paper. The spectrum was detected by the absorption of laser difference frequency radiation in a flow discharge using frequency modulation of the infrared radiation and lock-in detection at the modulation frequency. I I I 2218.05 2218.15 wavenumber/ cm-' Fig. 3. Lines in the v3 (CO stretch) fundamental band of HCO+ from ref. (35). This spectrum was detected by diode laser absorption in a large hollow-cathode discharge arrangement. Again the spectrum was detected by frequency modulation of the laser, with 2F detection to eliminate baseline drifts.The much stronger absorptions by stable precursor or product molecules in the plasma are clearly evident. T. J. Sears 115 input waveform 3 kV a.c. 9.6 kHz step -up transformer ho I1ow cat hode volt age resistor ( 500n) waveform to vacuum pump(Liquid nitrogen cooled) Fig. 4. Schematic of modulated hollow-cathode ion source, showing typical voltage waveform. in a chemically complex plasma. The method makes use of the fact that charged species in the mixture experience a coulombic force in the potential gradient of the discharge. The result is that ions have a net drift velocity in one direction which is superimposed on their random thermal motions.When the plasma is probed by a laser at frequency v then the ions see a Doppler-shifted frequency ( v fAv) with Av given by where V,is the drift velocity and c the velocity of light. The drift velocity depends on the plasma characteristics and can be estimated quite simply and reliably; details are given in the review article by Gudeman and Saykally.6 If the polarity of the discharge is reversed, the ion drift velocity changes sign and the Doppler shift will be in the opposite direction. The breakthrough made by Gudeman and Saykally was to realize that if the discharge polarity was switched rapidly, absorptions by charged species would be modulated in phase with polarity while those due to neutrals would, to a first approximation, be unaffected. The experimental set-up required is shown in detail in fig.1; instead of a d.c. glow discharge, the ions are now formed in an ax. discharge which is usually operated at high audio frequencies and powered by a commercial power amplifier. The ion absorptions are then detected by lock-in detection at the modulation frequency. An alternative method for uniquely identifying absorptions due to molecular ions which makes use of their rather short lifetimes compared to neutral species uses a large hollow cathode electrode arrangement and takes advantage of the fact that ion concentra- tions are generally enhanced in a hollow cathode plasma. The experimental set-up is shown in fig. 4. Here, instead of the glow discharge tube used in the velocity modulation technique, the plasma is contained in the large copper cathode and the discharge current I.R. Spectroscopy of Molecular Ions using Lasers Fig. 5. (a) Pure rotational transition in the OH-ion showing opposite phases of signals for positively (unassigned H30+transition) and negatively charged species in the velocity modulation experiment [ref. (13)]. (b) 13CS2reference spectrum. II I1 I I It I 2: 5.0 2273.1 22' 3.2 wavenumber/ cm- ' Fig. 6. Absorption line in the v3 fundamental band of HN; (HN-N stretch). This band is calculated to be 30 times weaker than the analogous one in HCO+ (fig. 3) [ref. (38)]. is rapidly switched on and off by making use of the fact that the electrode arrangement acts like a diode and only allows current to pass in one direction.Current flows, and ions are present, only when the copper tube has negative polarity relative to the stainless-steel anode. Examples of the results obtainable using these two techniques are shown in fig. 5 and 6. Fig. 5 shows an example of a spectrum obtained by Oka and coworker^'^ and illustrates another important advantage of the velocity modulation technique, namely that positively and negetively charged species appear with opposite phases in the synchronously amplified signal. The OH-line is a pure rotational one, while the H30+ one is unassigned but believed to be due to a transition in the inversion spectrum of T. J. Sears 117 the H30+molecule. Fig. 6 shows a weak line in the v3 band of HN; detected using the modulated hollow-cathode technique.At present both techniques provide good discrimi- nation against absorptions by neutral species, although the velocity modulation has the bonus that it distinguishes the relative sign of the charge on the ion; however, the hollow cathode is generally a cooler source. In many cases the two methods are complementary. Table 2 gives a complete list, with references, of molecular ions detected to date using tunable infrared laser absorption spectroscopy. The overwhelming majority have been detected using either the velocity modulation or the modulated hollow-cathode techniques; and in most cases detection would have been difficult or impossible without such a method of signal enhancement.About forty species have now been detected and much current work is focussed on the spectroscopy of negative ions, species for which rather little spectroscopic information has been available up to now. There are obviously far too many experimental studies listed in table 2 for me to cover in any detail. Instead, I have decided to concentrate on two examples in order to illustrate the kind of data and information now being obtained. The oxonium ion has been the subject of a large number of theoretical and experimental studies, reflecting its importance in many areas of chemistry. Until quite recently, however, spectroscopic studies of the ion had been restricted to measurements in condensed phases. In 1977 Sch~arz~~ reported the measurement of the infrared spectra of H30f and some higher solvated clusters in the 0-H stretching region by traditional infrared absorption spectroscopy following pulse radiolysis of a mixture of water vapour and argon.The resolution of this study was modest, and only little structural information could be inferred. However, this pioneering study supplied a reliable starting estimate for the laser-based studies initiated by Saykally and coworkers,60 who in 1983 reported high-resolution measurements of the v3 symmetric stretching mode by colour-centre laser absorption using the velocity-modulation method. A stick diagram showing the assigned spectrum is reproduced in fig. 7. These assigned lines represent only a fraction of the total number of ion lines in this region, albeit the stronger ones.Soon after the report of the detection of the v3band, several groups began to attack the spectroscopically interesting v2 inversion spectrum of the ion by diode-laser absorption. H30+is isoelec-tronic with ammonia, and various quantum-chemical calculations had predicted that the equilibrium structure of the ion was pyramidal with a rather low barrier to inversion; most of the later calculations had converged on a barrier height of 1000 cm-' or less.78 The consequence of this low barrier to planarity is a large splitting in the zero point and higher excited inversion mode levels of the ion caused by quantum-mechanical tunnelling through the barrier. For H30+the inversion splitting in the lowest level was estimated to be of the order of tens of wave number^.^^ The approximate inversion potential function and the positions of some of the lower vibrational levels for H30+ are shown in fig.8. The strongest electric dipole allowed transitions obey the selection rules Av2= 0, f1 and + e* -. For H30+,rotational lines in all of the allowed transitions involving v2=0 and 1 have now been detected. The first observations were made by the Chicago group on the 1- +--O+ band around 950 cm-' ; subsequently, measurements on the 1+-O-band and the same two bands in D,O+ were made. An example of the spectra is shown in fig.9. Most recently, the experi- mentally difficult 1-+-1+ spectrum was detected by Oka et aZ.,56and the combination of all of the observations enables a very precise determination of the effective inversion potential function for this important species.Direct observation of the complex ground- state inversion doubling spectrum in H,O+ (or D30') awaits technical advances in tunable far-infrared spectroscopy; however, the predictions based on the infrared work should be extremely good. I. R. Spectroscopy of Molecular Ions using Lasers Table 2. Molecular ions studiesd by tunable-laser infrared spectroscopy diatomics molecule ref. comments' HeH+ (1-0) band (DF) NeH+ (1-0) band in 20NeH+ and 22NeH+ (DF) ArH+ mobility measurements in t, = 1, 0 (DL) NH-(1-0) band (CC); auto-detachment in ion beam OH+ (1-0) band (DF), pure rotation (DL) OH+ (1-0) band (DF), pure rotation (DL) OH-(1-0) band (CC), pure rotation (DL) OD-(1-0) band (DF) HCl+ (1-0), (2-1) bands; 35Cl and 37Cl (DL) co+ (1-0) band (DL) C F+ fundamental and hot bands to v"=6 (DL) c, (A-X) electronic band (DL); (0,O) and (1,l) bands CCl+ fundamental and hot bands (DL) NO+ ? DL (1-0 and 2-1) absorption coefficients vibrational mode molecule (ref.) comments H;H2D+ HD; D;HCO+ DCO+ HN; DN; H2F+ HCS+ co; HBF+ FHF-H,Cl+ H20+ NH, H30+ DL(53-59,20), CC(60,61), v2 is inversion band D30+ DL(59), DF(20) CH; DF H3S+ DF HCO; DF HN20+ DF HCNH+ DF; also 13C, HCND+ DCNH+ DF and hot bands more than four atoms NH,+ ~~(73-79,v4(20) DF(73), CC(74,75) NH3D+ v4(76) DF.One component of v3 (degenerate mode of NH3;C2H; (63) no details reported CH,CNH+ (64) ' DF = diff erence-frequency laser system; DL = diode laser; CC = colour centre laser.T. J. Sears 119 U1u12 -11 33 8 31 10 wavenumber/cm-' Fig. 7. Stick diagram of lines assigned in the v3 fundamental (asymmetric stretching mode) of H30+. These were detected by colour-centre laser absorption in a velocity modulation experiment [ref. (60) and (61)]. A representative sample of all of the high-resolution data available was fitted by Sears and coworkers59 to an analytic rovibronic potential function for the species. Experimental information is now available on v2 and v3, while transitions in the v4 fundamental have been detected but as yet not assigned by Saykally and coworkers. As is the case in ammonia, the v4= 1 level is probably perturbed via Coriolis-type interac- tions with the 2+ level, and this could explain the assignment problems encountered by Saykally et aL2' Approaching the problem from the opposite direction, predictions based on numerical calculations using the potential function shown in fig. 8 were made of the positions of lines in the v2=2+ +1-band.This was subsequently observed;58 its origin is around 521 cm-'. It is not possible to fit the spectrum to within the measurement precision, even though assignments are unambiguous, lending further evidence to the perturbation hypothesis. An example of a line in the 2+ +1-spectrum is shown in fig. 10, together with a stronger line due to a transition in the 1+ +0-band.Further work is required before details of the possible interaction can be evaluated. Among future experimental challenges in the infrared is the detection of spectra of higher hydrates. Tantalizing indications of the presence of higher hydrates have be\ n found in both the 3 pm region and at long wavelengths (20-25 pm), where complex ion spectra have been found which peak at higher oxygen concentrations than H30+ signals. The early work of Schwar~'~ shows that these species are undoubtably present; however, high-resolution spectra will be difficult to assign and provide a significant challenge to both experimental and theoretical spectroscopists. co; Historically, the CO,' ion was one of the first studied by high-resolution spectroscopic techniques.The complex emission spectrum can be observed quite easily in the visible and near-u.v. regions of the spectrum in a discharge through carbon dioxide, and there I. R. Spectroscopy of Molecular Ions using Lasers I 2000 1500 3 I E 2 1000F2 500 0 72.42' PI" Fig. 8. The inversion potential function for H30+ (with r = re fixed), showing the positions of the lowest few inversion states of the ion. The electric-dipole-allowed transitions are indicated. (&/2 sinp = sin (a/2);re = 0.975 819 A; rp = 0.960 897 A. , I I I l l I I I I 642.95 643.00 643.05 wavenumber/ cm-' Fig. 9. Vibration-rotation lines in the 1-+0' component of the v2 (inversion) spectrum of D30+. The assignments give the (4K) quantum numbers of the lines (AK = 0).The spectrum was obtained by diode laser absorption spectroscopy in a modulated hollow-cathode discharge with an amplifier time constant of 400ms. The lower trace shows an etalon fringe pattern with a spacing of 899 f1 MHz [ref. (59)]. T. .ISears 121 R(5,2) (a++-1-1 638.9839 639.2 142 wavenumber/ cm-' Fig. 10. Vibration-rotation lines in the v2 mode of H30+. These were obtained using diode laser absorption and the velocity modulation technique with detection at twice the discharge frequency (hence the true absorption lineshape). The weaker line has been assigned to a transition out of the vibrationally excited 1-level [ref. (58)]. The insert represents 0.3% absorption.2F-100 kHz, 1 s t.c.has recently been considerable interest in it because of its observation in cometary spectra and planetary atmospheres. From a spectroscopic point of view, CO,' is of considerable interest because it provides a prime example of a Renner-Teller type vibronic interaction due to the coupling between the electronic orbital and nuclear vibrational angular momentum. While examples of the Renner-Teller effect in linear molecules are much more common than the analogous Jahn-Teller effect in non-linear molecules, most of the experimental information is derived from visible and microwave studies, while the most direct manifestation of the effect is a splitting of the excited bending vibrational levels of the molecule. Fig. 11 shows the lowest few bending levels in the electronic ground (X 211i)state of CO,'.Their positions were determined approxi- mately from a detailed study of the A-X and B-X electronic band systems,79 but the g f* u selection rule prevents the direct measurement of the u2 fundamental frequency in the electronic spectrum and its position has to be estimated from detailed calculations involving the positions of various overtones and hot bands. Infrared spectroscopy of CO,' in the gas phase was non-existent until 1985, when Kawaguchi and Hir~ta~~ reported the observation of the v3band by diode laser absorption in a hollow cathode discharge arrangement. Despite some effort, these workers were unable to detect hot bands involving excited bending levels of the ion, or to detect the highest-frequency vibronic component of the u2 fundamental (the 'Z-+ 211!/2 transition) predicted to occur higher than 700 cm-l.Recently, however, stronger vibronic com- ponents of the u2fundamental have been detected at longer wavelengths; many rotational lines have been observed in the vibronic transitions indicated in fig. 11. Fig. 12 shows an example of some of the observed spectra, the low-J Q-branch region of the 2A3,2-2111/2 transition. The resolution of the technique is nicely illustrated by the observation of the staggering of the rotational lines due to the A-doubling in the lower level. Since the lower level involved in these transitions is ca. 160 cm-' above the lowest spin-orbit component and the experiment is conducted at close to 200 K, the lines in this branch are expected (and found) to be only 1/3-1/2 as intense as those in the 2A5/2-2113/2 sub-band Q-branch head, which is observed some 3 cm-' higher.I. R. Spectroscopy of Molecular Ions using Lasers I200 -(010)22-I100 /--(010) 2A3/2 I/I/// I000 900 ,-I E 800 2 c 700 600 500 400 Fig. 11. Positions of the lowest few vibronic levels associated with the Renner-Teller active v2 vibration of CO; (X *n).The transitions indicated have been observed by diode laser absorption. The analysis of the observed spectra can be divided into two stages. First, the rotational structure within each vibronic band is fitted to a standard effective Hamiltonian in order to extract effective rotational and fine-structure parameters for the various vibronic states.The results of this process are summarized in tables3 and 4; these parameters fit the observed frequencies to within the experimental measurement pre- cision. The relative energies of the different vibronic levels are treated as free parameters at this stage of the fitting, to be interpreted in terms of vibronic coupling model for the Renner-Teller and spin-orbit coupling in the molecule. The simplest model available treates the vibrational motion as a harmonic oscillator and considers the vibronic coupling to be caused by the multipole moments of the nuclear framework as it distorts T. J. Sears 123 J I 1 I 1 1 1 1 507.30 507.34 507.38 507 wavenumber/cm-' Fig.12. Example of the observed CO; spectra. The absorption lines are the low-J Q-branch spectrum of the 2A3/2-2111/2 vibronic transition in the ion. The 2111/2level is the higher of the two spin-orbit components of the ground state and has only ca. 1/3-1/2 of the population of the lower 2113/2level at the temperature (ca. 200-220 K) of the experiment. The staggering in the spectrum due to the A-doubling in the ground state is clearly evident in the spectrum. Table 3. Fitted parameters for (0OO)X211iCOl/cm-' Aeff= -159.7202(92)" 102(p+2q) =0.3857(33) 104Ab,,,= -0.371(49) 104q= -0.515(51) B=0.380511(12) 106D= 0.1531(93) " Numbers in parentheses are one standard deviation of the least-squares fit. For more details including notation see ref.(47). Effective parameter since y=O was fixed in the fit. Table 4. Fitted parameters for (010) levels of COT/cm-' E" 510.02065( 32)b 3 87.398 1 (46) 639.4383(47) Aef f -156.5639(92) --104ADerr -0.657(50) --B 0.381685(12) 0.381703 (12) 0.381414( 13) lo6D 0.1 559(97) 0.1680(98) 0.150( 10) Y -0.166714(48) 0.16782( 10) lo6YD --0.564( 14) -0.525(46) a Energy in cm-' above the vibrationless level. Numbers in parenthesis are one standard deviation of the least-squares fit. Y(~A)=0 was fixed. from the linear configuration. Details of the theory are beyond the scope of this paper and they have been considered extensively in the literature.80'81 The results of fitting the empirical vibronic intervals to the harmonic model are summarized in table 5.The parameters o and E are the unperturbed oscillator frequency in wavenumbers and the Renner parameter, respectively. The latter gives direct informa- tion on the degree of vibronic coupling and the vibronic potential function in the 124 I. R. Spectroscopy of MoZecular Ions using Lasers Table 5. Derived vibronic parameters for X 'II Cola quantity harmonic approximation ref. (77) anharmonic calculationd w 516.332 513(1) 510.26(7) 2.914 3.22( 10) 3.18(6)-0.189 -0.188(4) -0.21 16(2) A(2w -159.436 -158.80 -159.75" ASO ---162.34(9) g22= g:2 --1.68f a In cm-' except for E, which is dimensionless. Calculated using eqn (9)-(13) of ref. (47) and the energy positions of the (010) levels given in table 4. E is conventionally defined to be negative since E(010)2X-> E(010)2X+.From a fit to the (010)2X+, (010)2Z-, (010)2A5/2,3/2 level positions and A,,,(000). " Calculated from eqn (15) of ref. (47). Fixed. The data were not sufficient to determine these parameters and they were held at values predicted by the model. See ref. (47) and (83) for details. harmonic approximation. The smaller parameter g, takes account of K -dependent contributions to the vibronic energies (K=R+Z, where A is the electronic orbital momentum quantum number and I the vibrational angular momentum quantum number) and gives information on the contamination of the electronic ground state by excited I: and A electronic states. All of the parameters entering in the harmonic approximation model are uniquely fixed by the experimental data; the test of the model is how well it predicts the positions of vibronic states associated with higher bending states of the molecule.The positions of these are not known nearly as well, and the situation is complicated by the fact that the (020) level positions are modified by Fermi-resonance effects with the first excited stretching state (100). Larcher et aLS2 have determined the approximate positions of the levels by applying a deperturbation procedure to data obtained from the electronic emission spectrum; perhaps not surprisingly, the harmonic model does not do a very good job predicting these level positions. A more realistic model for the combined effects of Renner-Teller and spin-orbit coupling in molecules like CO;has been developed by Jungen and coworkers recently.83 This model includes anharmonic effects and is more difficult to implement.However, preliminary calculations using the derived vibronic energy splittings obtained here give the results summarized in the final column of table 5. In the new model the spin-orbit constant A is an explicit function of the vibronic state and the additional anharmonic parameters g,, and ga2enter. The resulting fit is fairly satisfactory, although it does not approach the experimental accuracy. It does, however, do a much better job of reproduc- ing the known vibronic structure in higher bending levels. The latest infrared data show evidence of absorptions from the lowest state (2Z+) in the (010) manifold and when this is analysed in detail, much more accurate information will be available for at least some of the (020) vibronic levels and a better estimate of the accuracy of the anharmonic calculation will be possible.The Future The future of infrared spectroscopy using tunable laser sources looks very good indeed. First, the laser devices themselves are constantly being improved, giving better power, tuning range, and/or mode stability. This is especially true of diode lasers, which have improved dramatically over the past three years. The limited single-mode tuning range of these devices is the major drawback; however, in the mid-infrared (750-2500 cm-') single-model tuning of 2-3 cm-' is now not uncommon, and with luck and two or three lasers, continuous frequency coverage is now possible over extended regions between these limits.Above 2500 cm-' diff erence-frequency and colour-centre lasers provide T. J. Sears 125 good coverage; however, below ca. 700 cm-' the available diodes become poorer, and spectroscopy of transient species below 600 cm-' becomes progressively more challeng- ing, although certainly not impossible, as many studies have shown. The current practical limit at long wavelengths currently appears to be ca. 350 cm-', almost to the far-infrared. There are certainly no signs that the pace of new molecular ion discovery is slowing down, and much current interest is devoted to the detection of spectra of negative ions, species for which there has been relatively little information available to now.The recent, unexpected discovery that negative ions are quite abundant in the types of laboratory plasmas traditionally used to produce positively charged species will no doubt lead to the characterization of many more exotic anions. Prime candidates would appear to be light molecules containing highly electronegative atoms such as fluorine, and such species may be of importance in the chemistry of plasmas used in the semiconductor industry. Other areas of continuing interest are solvated species, such as H501, and various hydrocarbon ions. There is still much to be learned on the ion chemistry of even the relatively simple hydrogen-oxygen plasma, where it is several workers' experience that if one looks hard enough, weak ion absorption lines can be found all through the infrared.I am grateful to Drs T. Amano, P. B. Davies, E. Hirota, A. R. W. McKellar, T. Oka and R. J. Saykally for illuminating discussions and for providing details of their experimental results prior to publication. Research carried out at Brookhaven National Laboratory was performed under contract DE-AC02-76CH00016 with the U.S. Depart-ment of Energy and supported by its Division of Chemical Sciences. References 1 G. Herzberg, Q. Rev. Chem. SOC., 1971,25, 201. 2 G. Herzberg, in Molecular Zons: Spectroscopy, Structure and Reactivity, ed. T. A. Miller and V. E. Bondybey (North Holland, New York, 1983). 3 L. Andrews, in Molecular Ions Geometricand Electronic Structures, ed.J. Berkowitz and K-0.Groeneveld, NATO AS1 Series B, Physics Vol. 90 (Plenum Press, New York, 1983). 4 L. Andrews, in Molecular Ions: Spectroscopy, Structure and Reactivity, ed. T. A. Miller and V. E. Bondybey (North Holland, New York, 1983). 5 See, for example, F. C. Van den Heuvel and A. Dymanus, Chem. Phys. Lett., 1982, 92, 219. 6 C. S. Gudeman and R. J. Saykally, Annu. Rev. Phys. Chem., 1984, 35, 387. 7 P. Bernath and T. Amano, Phys. Rev. Lett., 1982, 48, 20. 8 M. Wong, P. Bernath and T. Amano, J. Chem. Phys., 1982, 77, 693. 9 N. N. Haese, F. S. Pan and T. Oka, Phys. Rev. Lett., 1983, 50, 1575. 10 D. M. Neumark, K. R. Lykke, T. Anderson and W. C. Lineberger, J. Chem. Phys., 1985, 83,4364. 11 M. W. Croften, R. S. Altman, M-F.Jagod and T. Oka, J. Phys. Chem., 1985, 89, 3614. 12 J. C. Owrutsky, N. H. Rosenbaum, L. M. Tack and R. J. Saykally, J. Chem. Phys., 1985, 83, 2426. 13 D-J. Liu and T. Oka, J. Chem. Phys., 1986,84, 2426. 14 T. Oka, personal communication. Rotational transitions in NeH+, ArH+ and OH+ have been recorded by DL absorption, D. J. Liu, W. C. Ho and T. Oka, to be published. 15 P. B. Davies, P. A. Hamilton, W. Lewis-Bevan and M. Okamura, J. Phys. E, 1983, 16, 289. 16 P. B. Davies, P. A. Hamilton and S. A. Johnson, Mol. Phys., 1986, 57, 217. 17 P. B. Davies and W. J. Rothwell, J. Chem. Phys., 1985, 83, 5450. 18 K. Kawaguchi and E. Hirota, J. Chem. Phys., 1985, 83, 1437. 19 M. Gruebele, M. Polak and R. J. Saykally, Chem. Phys. Lett., 1986, 125, 165. 20 R.J. Saykally, personal communication. 21 F. Bien, J. Chem. Phys., 1978, 69, 2631. 22 T. Oka, Phys. Rev. Lett., 1980, 45, 531. 23 T. Oka, Philos. Trans. R. SOC. London, Ser. A, 1981, 303, 543. 24 J. K. G. Watson, S.C. Foster, A. R. W. McKellar, P. Bernath, T. Amano, F. S. Pan, M. W. Croften, R. S. Altman and T. Oka, Can. J. Phys., 1984, 62, 1875. 25 C. S. GCldeman and R. J. Saykally, unpublished work (reported in Annu. Rev. Phys. Chem., 1984,35, 387). 26 T. Amano and J. K. G. Watson, J. Chem. Phys., 1984, 81, 2869. 27 T. Amano, J. Opt. Soc. Am., 1985, B2, 790. I. R. Spectroscopy of Molecular Ions using Lasers 28 S. C. Foster, A. R. W. McKellar, I. R. Peterkin, J. K. G. Watson, F. S. Pan, M. W. Croften, R. S. Altman and T.Oka, J. Chem. Phys., 1986,'84, 91. 29 K. G. Lubic and T. Amano, Can. J. Phys., 1984, 62, 1886. 30 S. C. Foster, A. R. W. McKellar and J. K. G. Watson, J. Chem. Phys., submitted. 31 S. C. Foster, A. R. W. McKellar and J. K. G. Watson, to be published. 32 C. S. Gudeman, M. H. Begemann, J. Pfaff and R. J. Saykally, Phys. Rev. Lett., 1983,50, 727. 33 T. Amano, J. Chem. Phys., 1983, 79, 3595. 34 K. Kawaguchi, C. Yamada, S. Saito and E. Hirota, J. Chem. Phys., 1985,82, 1750. 35 S. C. Foster, A. R. W. McKellar and T. J. Sears, J. Chem. Phys., 1984, 81, 518. 36 P. B. Davies, P. A. Hamilton and W. J. Rothwell, J. Chem. Phys., 1984,81, 1598. 37 K. Kawaguchi, A. R. W. McKellar and E. Hirota, J. Chem. Phys., 1986,84, 1146. 38 S. C. Foster and A. R.W. McKellar, J. Chem. Phys., 1984, 81, 3424. 39 C. S. Gudeman, M. H. Begemann, J. Pfaff and R. J. Saykally, J. Chem. Phys., 1983, 78, 5837. 40 J. C. Owrutsky, C. S. Gudeman, C. C. Martner, L. M. Tack, N. H. Rosenbaum and R. J. Saykally, J. Chem. Phys., 1985, 84, 605. 41 T. J. Sears, J. Opt. SOC.Am., 1985, B2,786. 42 D. J. Nesbitt, H. Petek, C. S. Gudeman, C. B. Moore and R. J. Saykally, J. Chem. Phys., 1984,81,5281. 43 T. J. Sears, J. Chem. Phys., 1985,82, 5757. 44 E. Schafer and R. J. Saykally, J. Chem. Phys., 1984, 81, 4189. 45 N. H. Rosenbaum, J. C. Owrutsky, L. M. Tack and R. J. Saykally, J. Chem. Phys., 1985,83,4845. 46 P. B. Davies and W. J. Rothwell, J. Chem. Phys., 1985, 83, 1496. 47 T. J. Sears, Mol. Phys., to be published. 48 K. Kawaguchi, C.Yamada and E. Hirota, J. Chem. Phys., 1985, 82, 1174. 49 K. Kawaguchi and E. Hirota, Chem. Phys. Lett., 1986, 123, 1. 50 K. Kawaguchi and E. Hirota, J. Chem. Phys., 1986, 84, 2953. 51 K. Kawaguchi and E. Hirota, J. Chem. Phys., 1986, to be published. 52 L. M. Tack, N. H. Rosenbaum, J. C. Owratsky and R. J. Saykally, J. Chem. Phys., in press. 53 N. N. Haese and T. Oka, J. Chem. Phys., 1984,80, 572. 54 D-J. Liu, N. N. Haese and T. Oka, J. Chem. Phys., 1985, 82, 5368. 55 P. B. Davies, P. A. Hamilton and S. A. Johnson, J. Opt. SOC. Am.,1985, B2, 794. 56 D-J. Liu and T. Oka, Phys. Rev. Lett., 1985, 54, 1787. 57 D-J. Liu, T. Oka and T. J. Sears, J. Chem. Phys., 1986, 84, 1312. 58 P. B. Davies, S. A. Johnson and T. J. Sears, to be published; H,O+ vibrationally hot band v,=(2+-1-). 59 T.J. Sears, P. R. Bunker, P. B. Davies, S. A. Johnson and V. Spirko, J. Chem. Phys., 1985, 83, 2676. 60 M. H. Begemann, C. S. Gudeman, J. Pfaff and R. J. Saykally, Phys. Rev. Lett., 1983, 51, 554. 61 M. H. Begemann and R. J. Saykally, J. Chem. Phys., 1985, 82, 3570. 62 P. R. Bunker, T. Amano and V. Spirko, J. Mol. Spectrosc., 1984, 107, 208. 63 M. W. Croften, W. A. Kreiner, M-F. Jagod, B. D. Rehfuss and T. Oka, J. Chem. Phys., 1985,83,3702. 64 T. Amano, personal communication. 65 T. Amano and K. Tanaka, J. Chem. Phys., 1985, 82, 1045. 66 T. Amano and K. Tanaka, J. Chem. Phys., 1985, 83, 3721. 67 T. Amano Chem. Phys. Lett., to be published. 68 R. S. Altman, M. W. Croften and T. Oka, J. Chem. Phys., 1984, 80, 3911.69 R.'S. Altman, M. W. Croften and T. Oka, J. Chem. Phys., 1984,81, 4225. 70 T. Amano and K. Tanaka, J. Mol. Spectrosc., 1986, 116, 112. 71 K. Tanaka, K. Kawaguchi and E. Hirota, J. Mol. Spectrosc., in press. 72 T. Amano, J. Chem. Phys., 1984, 81, 3350. 73 M. W. Croften and T. Oka, J. Chem. Phys., 1983, 79, 3157. 74 E. Schafer, M. H.Begemann, C. S. Gudeman and R. J. Saykally, J. Chem. Phys., 1983,79, 3159. 75 E. Schafer, R. J. Saykally and A. G. Robiette, J. Chem. Phys., 1984, 80, 3969. 76 T. Nakamaga and T. Amano, Can. J. Phys., to be published. 77 H. A. Schwarz, J. Chem. Phys., 1977,67, 5525. 78 (a) P. Botschwina, P. Rosmus and E-A Reinsch, Chem. Phys. Lett., 1983, 102, 299; (b) P. R. Bunker, W. P. Kraemer and V. Spirko, J. Mol. Spectrosc., 1983, 101, 180. 79 D. Gauyacq, C. Larcher and J. Rostas, Can. J. Phys., 1979,57, 1634. 80 C. Jungen and A. J. Merer, in Molecular Spectroscopy: Modem Research, ed. K. N. Rao (Academic Press, New York, 1976). 81 J. M. Brown, J. Mol. Spectrosc., 1977, 68, 412. 82 C. Larcher, D. Gauyacq and J. Rostas, J. Chem. Phys., 1980,77, 655. 83 D. Gauyacq and Ch. Jungen, Mol. Phys., 1980, 41, 383 (and references therein). Paper 611021; Received 23rd May, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300111

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC.,Furuday Trans. 2, 1987, 83, 127-137 Dynamics of Chemical Reactions of Ions from Beam Scattering and State-selected Studies Zdenek Herman" J. Heyrovsky' Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia Inosuke Koyano Institute for Molecular Science, Okazaki 444, Japan Information on the dynamics of ion-molecule reactions has been obtained mostly from beam scattering, product internal state analysis by spectroscopic methods and from state-selection of reactants. A combination of beam- scattering studies and state selection of reactants experiments aided better description of several exoergic and endoergic reactions. Results obtained by both methods on the reactions CHT(CH,, CH,)CHl, Hl(He, H)HeH+ and Hl(Ne, H)NeH+ are discussed.The subject of ion-molecule reaction dynamics revolves very much about two main problems: the question of the molecular collisional mechanism of the elementary reaction act and the question of energy transformation in it. Answers to the first question are contained in the angular scattering of reaction products and, therefore, information on it is usually obtained from beam-scattering studies."* Answers to the energy transforma- tion question require information on the terms of the basic equation of reactant-product energy balance in an elementary collision event ( Etr,E, and E, are the relative translational, vibrational and rotational energy of reactants, respectively, and primed quantities refer to products; -AH: = C (AH;)'-E (AH:) is the reaction enthalpy; we deliberately omit the electronic energy, which is included in the reaction enthalpy term).A variety of methods has been used to explore the particular terms of eqn (1). Beam and beam-scattering methods1.* have been used to investigate the translational energy terms: the selective influence of the relative translational energy of reactants (over the entire 'chemical' range of collision energies, from quasithermal to tens of eV) and the partitioning of energy between relative translational and internal energy of products. The selective influence of reactant internal energy (vibrational, electronic and in the future hopefully rotational) has been explored by preparing state-selected reactants by means of coincidence Product vibrational and rotational energy has been analysed by spectroscopic methods: product chemiluminiscence,6 product infrared chemil~minescence~~'and product laser-induced fluorescence.' It is evident that combination of several approaches helps us to arrive at a more detailed description of the dynamics of the system in question.Joint efforts of this type are well known, e.g. in beam scattering and spectroscopic product analysis?-'' The same is true of joint efforts in experimental and theoretical studies [Ie.g.ref. (12) and (13)]. The aim of this communication is to show several examples of how beam scattering and state-selection of reactants join in a fuller description of a system. We will discuss 127 Beam Scattering and State-selected Studies results for three reactions: CHl+CH, + CHl+CH, -AH",00.17eV (2) Hl+He ---* HeH++H -AH; = -0.80 eV (3) Hi+Ne + NeH++H -AH: = -0.57 eV.(4) Reactions (2)-(4) differ in exoergicity: the first is slightly exoergic, while reactions (3) and (4) are endoergic with ground-state reactants. First, however, we will briefly describe the experimental methods used in these investigations. Experimental Beam Scattering The crossed beam scattering apparatus EVAII, used in Prague, has been described previously [e.g. ref. (lo)]. Briefly, it consists of a scattering chamber and a detector- magnetic mass spectrometer. Reactant ions are formed by electron impact, formed into a beam, mass analysed at ca.100-300eV, decelerated to a desired energy (from ca. 0.5 eV upwards) by a multi-element lens system. The reactant ion beam is crossed with a collimated neutral reactant beam of thermal energy emerging from a multichannel jet. The two beams cross at right angles and can be pivoted about the scattering centre (size ca. 1 mm3). Product ions formed in collisions in the scattering zone and reactant ions are detected by a detection slit (resolution ca. lo), energy analysed and registered after they pass through the detection mass spectrometer. Primary data are angular distribu- tions and LAB-energy profiles at a series of scattering angles. From these data contour scattering diagrams are constructed which show the probability density distribution of the product ion in the velocity space.By integration of the scattering diagram other dynamical characteristics, namely differential cross-sections (c.m. angular distribu- tions) of the product ion and product relative translational energy distributions can be derived. Reactant Ion State-selection by the TESICO Technique The TESICO (threshold electron-secondary ion coincidence) technique and the TEPSICO apparatus, used in Okazaki, have been described previously.334 The technique makes it possible to select internal states (vibrational and electronic) of reactant ions. The apparatus consists of a helium Hopfield continuum light source, a 1 m Seya-Namioka monochromator, an ionization chamber, a steradiancy threshold electron analyser, a deceleration multi-element lens, a reaction chamber and a detection quad- rupole mass spectrometer. Reactant ions are produced from parent molecules by photoionization at the threshold wavelength, corresponding to a specific internal excita- tion (or a specific internal state).The reactant ions are extracted, decelerated to a desired energy and injected into the reaction chamber; product and reactant ions are efficiently extracted by a small field applied across the chamber, mass analysed and counted in coincidence with the threshold electron signals, as registered by the steradiancy analyser. Primary data are coincidence time-of-flight spectra of the reactant and product ions. Integrated intensities of the time-of-flight peaks are used to obtain relative total cross-sections for reactant ions of a specific internal energy at a particular relative reactant translational energy.Z. Herman and I. Koyano Fig. 1. Scattering diagram of CH; [reaction (2)] at E,, = 0.67 eV. Insert: c.m. angular distributions; lower curve, all scattered product; upper curve, central portion corresponding to complex formation. Results and Discussion CHf +CH4+CH; +CH3 Reaction (2) has been one of the most widely studied ion-molecule reactions since the time it was first described in 1952.14 Numerous papers deal with measurements of the total cross-section and its dependence on collision energy, using isotope labelling and beam studies. Only recently, however, a more detailed beam-scattering investigation has been carried out.” It confirmed the occurrence of three parallel mechanisms of CHf formation in the eV collision energy region.Fig. 1 shows a scattering diagram of CHf at a collision energy of 0.67 eV. Three distinct features can be observed: a peak in the direction of the reactant ion flight with respect to the centre of mass (right-hand side of the figure), resulting from H-atom transfer to the reactant ion CH,’; the peak occurs at the point where the product is expected, if formed by the impulsive spectator-stripping mechanism. Similarly, there is an even bigger peak on the left-hand side of the figure (in the direction of flight of the neutral reactant CH4)and its position corresponds very well to the impulsive spectator- stripping mechanisms of proton transfer to the methane molecule.In addition, there is a small but distinct peak at the centre-of-mass which corresponds to the product formed by the decomposition of an intermediate complex C2Hl. The insert in fig. 1 shows the c.m. angular distributions derived from the scattering diagram. The latter exhibits fully developed forward-backward symmetry and confirms the complex formation. The mean lifetime of C2Hi at E,, 0.6-2.5 eV could be estimated as longer than s and the dissociation energy as greater than 0.5 eV. Recent theoretical calculation^'^^'^ confirm the existence of C2Hi as a species stable towards dissociation by ca. 0.7 eV. Analysis of the potential-energy surfaces of the system” indicates that the relatively small probability of complex formation results from the location of the minimum on the potential-energy surface in the hardly accessible region deep in the corner of a narrow Beam Scattering and State-selected Studies 30C 20c UR ioa Etr=2.69eV 4 Etr = 1.44eV's-11 2 3 Et0,IeV Fig.2. Dependence of the relative total cross-section of reaction (2) on total initial energy Etot.l8 valley. It also suggests that the occurrence of the two direct mechanisms (proton and H-atom transfer) results from a different initial configuration of the reactants and that the H-atom transfer involves a non-adiabatic transition. From the scattering diagrams information on the branching ratios of the three mechanisms were obtained: proton transfer prevails in the eV collision energy region and the ratio complex formation: H-atom transfer varies between 0.08 and 0.15. In the abovementioned scattering experiments the internal energy of reactant CHZ (formed by electron impact) was not controlled.Its distribution was estimated to be quite large; ranging from ca. 0.2 to 1.3 eV, with a mean value 1.0 eV. The influence of internal energy, Ei, of the reactant ion on the course of reaction (2) was investigated by coincidence experiments." By photoionization at a series of wavelengths, ions CH;(v) with a defined internal energy were formed [0.13, 0.43, 0.60, 0.70, 1.07 eV, assuming IP(CH,) = 12.62 eV]. These ions were then used to measure relative total cross-sections of reaction (2) at several collision energies.Because of the complicated, overlapping structure of the threshold electron spectrum of CHZ," no attempt was made to assign a specific vibrational state to the state-selected ions; the ions CHZ are regarded as having the abovementioned amount of internal (vibrational) energy, without further specification. The results of the measurements are summarized in fig. 2, where the relative total cross-section uRis plotted against the total energy of reactants, E,,, = E,,+ Ei.It can be seen that uRof reaction (2) decreases with E,,, and that this decrease is caused by the decrease both with the relative collision energy E,, and with the internal energy Ei.A decrease of the total cross-section with the internal energy of the reactant ion has been observed for several other ion-molecule It may be a result of interplay of many state-to-state microscopic cross-sections of a different behaviour, depending on the details of the potential-energy surfaces involved.Changes in the branching ratio of the mechanisms of the product ion formation were investigated using isotope-labelling experiments with reactant ions of a defined Ei and by measuring relative total cross-sections of the processes: CHZ(v) +CD, + CH,D'+ CD3 (54 + CH,D,++CHD, (5b) Z. Herman and I. Koyano 131 I %/ b" ostb Etr= 1.43 eV Etr= 0.77LV I Etr=2.71 eV I Em,/ eV Fig. 3. Branching ratios of mechanisms of reaction (2) in the system CHi( v) + CD,: (a) H+-transfer, (b) D-atom transfer, (c) complex formation." --* CHD:+CH3 -+ CD,++CH, CD,+(V)+ CH~4 CHD: + CH~ --* CH3Dl+CHD, --* CH,D++CD, -+ CH,++CD,.The check, if intensities of ion species in reactions (5) and (6) may be used to interpret the branching ratios of the mechanisms was done by comparing the results of beam studies with the results derived from ion intensities in these reacti0ns.l' Branching ratios of the mechanisms for the two systems are shown in fig. 3 and 4 plotted against E,,,. It can be seen that the dominant mechanism is proton transfer, whose weight slightly decreases with decreasing E,,,. The fraction of H-atom transfer does not change much over the range of E,,, and the fraction of complex formation slightly increases with decreasing E,,,.No appreciable difference exists in the depen- dence of the branching ratios on the relative translational energy and on the internal energy. There is a large isotope effect in the fractions of the direct mechanisms (1.3-2) depending on whether the transferred particle is an H or D atom or Htor Df (cf. fig. 3 and 4). From the investigation of reactions (5) and (6) information on the relative role of charge transfer vs. chemical reaction in the system was obtained. It turned out that the relative cross-section for the charge-transfer process [reactions (5d) and (6d)l showed little dependence on E,,, and at the highest values of E,,, studied (ca.3.5 eV) it represented a.25% of the reaction channels in the case of reactions (5) and 50% in the case of 132 Beam Scattering and State-selected Studies I I Et,=2.70 eV Etr=O.77 eV c I T Etcl,IeV Fig.4. Branching ratios of mechanisms of reaction (2) in the system CD:(v)+CH,: (a) D+-transfer, (b) H-atom transfer, (c) complex formation.’* reactions (6). The relative cross-section for reaction (6d), i.e. formation of CHZ, was constantly ca. twice as large as that for reaction (5d) (re. formation of CD;). H;+He ---* HeH++H Reaction (3) is a prototype of a simple elementary chemical process which is endoergic with ground-state reactants and becomes exoergic only for reactant ion vibrational excitation Hl( v 23). Therefore, the influence of vibrational energy in its cross-section and the efficiency of vibrational vs.relative translational energy in overcoming the endoergicity barrier can be investigated. Early mass-spectrometric work culminated in Chupka’s pioneering photoionization-mass spectrometry study” for specific vibrational states of Hl, which showed that vibrational energy was much more effective in overcom- ing the endoergicit barrier than relative translational energy. Also, several beam studies were carried The simplicity of the three-electron system attracted theoretical investigation: the potential-energy surface is well known22.23 for this adiabatic chemical reaction and quasiclassical trajectory studies estimated the total cross-sections depen- dence on vibrational and translational er~ergy.~~?~~ A joint beam-scattering and quasiclassical trajectory in~estigation’~ of reaction (3) confirmed that the prevailing mechanism of the process with a vibrationally non-selected beam was spectator stripping.Fig. 5 shows the scattering diagram of HeH+ from this study at E,,= 3.36 eV for the ‘Franck-Condon’ mixture of Hl vibrational states as obtained by electron impact (vibrationally non-selected beam). Besides the prominent spectator-stripping peak, there is a portion of almost isotropic scattering at large angles; a minimum about the centre of mass is due to the dissociation of the product HeH+ excited above the dissociation limit. Other parts of fig. 5 compare experimental and calculated quantities at several collision energies: c.m. angular distributions (b) and product relative translational energy distributions (c).The theoretical and experimental results agree quite well. The trajectory calculations also predicted experimentally 2. Herman and I. Koyano A SS 180' CB 1.5t n 0.5 0.o v,g0.5 0.OPT? 0.5rin-:;i 0 .o 012345 e,rnlo E:,IeV Fig. 5. A, Scattering diagram of HeH+ from reaction (3) (non-selected beam) at E,, = 3.36 eV. B, C.m.angular distributions of HeH+. C, Product relative translational energy distributions [experi- ment; histogram, trajectory calculations]. B, C = E,,/eV: (a) 0.84, (b) 1.37, (c) 2.70, (d) 3.36. inaccessible product ion vibrational and rotational excitation and the overall partitioning of energy in the reaction products: with increasing collision energy (0.5-4 eV) the fraction of energy in product relative translation increases from 20 to 45%, the fraction in vibration is about constant (20%) and the fraction in rotation correspondingly decreases.13 The TESICO technique made it possible to investigate directly the effect of reactant vibrational energy on the relative total cross-section of reaction (3) at several collision energies in the eV region.26 H~(v) ions were prepared in specific vibrational states Beam Scattering and State-selected Studies 0 1 2 3 4 vibrational state of Hl Fig.6. Dependence of the relative total cross-section of reaction (3) on vibrational excitation of H:( v);data are arbitrarily normalized to v =4.26E,,/eV: 0,0.40* 0.20; A, 0.93f0.30; 0,2.0 *0.3. 0 1 2 3 4 EmtIeV Fig.7. Dependence of the relative total cross-section of reaction (3) on E,,, (excitation functions).26 (v=0-4) by photoionization and were selected by the coincidence technique. The results are summarized in fig. 6 and show a dramatic effect of reactant vibrational energy on the cross-section (results are arbitrarily normalized to the value at v =4); increasing translational energy can make the reaction go below the threshold (v=3), but its effect is by no means as large as the effect of vibrational energy. The data are consistent with previous mass spectrometric resultslg and also with recent coincidence data for thermal energy ions.27 The behaviour is typical of reactions occurring on a potential-energy surface with the endoergicity barrier located in the product valley.28 2.Herman and I.Koyano -I--,;,-----0' 180' .,.-,kQW? 876543 2 10.5 ss I 2x103cm 5-1 -0 30 60 90 120 150 100 a/ Fig. 8. (a)Scattering diagram and (b)c.m. angular distribution of HeH+ from reaction (3) with Hl( z, = 0, 1) (selected beam).30 The influence of vibrational us. translational energy can be seen even better in fig. 7, where the relative total cross-sections of reaction (3) are plotted in the form of 'excitation curves', i.e. the dependence on the total initial energy Etot= E,,+ E,(H;). The experiments were carried out at a series of collision energies between 0.4 and 3 eV. Solid curves connect results at a particular collision energy with increasing vibrational Beam Scattering and State-selected Studies 0.36'eV IECm= 1 0 1 2 3 4 5 EtoJeV Fig.9. Dependence of the relative total cross-section of reaction (4) on E,,, (excitation functions).26 energy, whereas dashed lines are relative total cross-sections for a particular vibrational state. The cross-sections for endoergic channels (v =0, 1,2) rise with increasing E,,, from the threshold, but adding translational energy never increases them very much. However, for exoergic channels (u23) the cross-sections are large, and decrease with increasing E,,, (an effect of increasing collision energy). Theoretical quasiclassical trajectory calculation^^^^^^ are in good agreement with the experimental results. Also, recent quantum-mechanical calculations29 using the RIOSA method reproduce the experimental data well.The question now arises if the features of dynamics change for the endoergic channels of the reaction. To investigate this aspect, a special beam-scattering experiment was carried out3' with a beam of hydrogen ions preferentially in v =0.1 at E,, =3.6 eV. The resulting scattering diagram is given in fig. 8. It shows that, in comparison with fig. 5 (comparable collision energy, non-selected beam), there is a considerable (ca. twofold) increase in large-angle scattering and a decrease of the stripping peak. The corresponding c.m. angular distribution of HeH' shows the difference even better (solid line for the selected beam). Trajectory calculations support the selected-beam iesults (histogram in fig.8). The increase of the large-angle scattering implies that small impact parameter, hard collisions are necessary in order to use efficiently the initial translational energy in overcoming the barrier for endoergic channels (n = 0,l) of reaction (3). Hl+Ne -NeH++H Reaction (4) is of a similar type to reaction (3). A DIM potential-energy surface for this process is available:31 it exhibits features analogous to those of reaction (3). The reaction is an adiabatic process and (because of a slightly different reaction enthalpy) channels with (u32) are exoergic. So far there are no beam-scattering experiments for this kinematically rather inconvenient system. Coincidence TESICO experiments, analogous to those described in the preceding paragraph, have been performed.2h The results are summarized in fig.9 in the form of the excitation functions, i.e. the dependence of the relative total cross-sections for selected vibrational states (v =0-4) on E,,, = E,,+ &(Hi). The cross-sections increase almost linearly with vibrational energy of the reactant ion at a given collision energy, but appear to decrease less sharply with increasing E,,, than in reaction (3). Preliminary quasi- classical trajectory calculation^^^ show a reasonable agreement with the experimental results at low collision energies. 2. Herman and I. Koyano The authors gratefully acknowledge contributions of their colleagues to various parts of the research described in this article; these are V.PacSlk, B. Friedrich, K. Birkinshaw and M. J. Henchman in Prague, K. Tanaka, T. Kato and S. Suzuk-i in Okazaki. Also, they wish to express their appreciation of discussions and collaboration with quantum chemists, in particular with M. Baer, H. Nakamura, K. Morokuma, R. Zahradnik, Ch. Zuhrt; L. Zulicke, F. Schneider, U. Havemann. Z.H. thanks the Yamada Science Foundation for a stipend which made possible his three month visit to Okazaki. References 1 W. R. Gentry, in Gas Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1979),vol. 2. 2 Z. Herman, Int. J. Mass Spectrom. Zon Processes, 1982, 45, 293. 3 K Tanaka and Z. Koyano, J. Chem. Phys., 1978,69, 3422; I. Koyano and K. Tanaka, J. Chem. Phys., 1980, 72,4858. 4 K. Tanaka, T.Kato and I. Koyano, J. Chem. Phys., 1981, 75, 4941. 5 L. Squires and T. Baer, J. Chem. Phys., 1976, 66, 4001. 6 Ch. Ottinger, in Gus Phase Ion Chemistry, ed. M. T. Bowers (Academic Press, New York, 1984),vol. 3. 7 J. C. Weisshaar, T. S. Zwier and S. R. Leone, J. Chem. Phys., 1981,75, 4873. 8 V. M. Bierbaum, G. B. Ellison and S. R. Leone, in Gas Phase Ion Chemistry,ed. M. T. Bowers (Academic Press, New York, 1984), vol. 3. 9 N. A. Sondergaard, I. Sauers, A. C. Jones, J. J. Kaufman and W. Koski, J. Chem. Phys., 1979,71,2229. 10 B. Friedrich and 2. Herman, Chem. Phys., 1982, 69, 433. 11 Ch. Ottinger and H. Reichmuth, J. Chem. Phys., 1981, 74, 928. 12 J. Krenos, R. K. Preston, R. Wolfgand and J. C. Tully, J. Chem. Phys., 1974, 60,1634. 13 F.Schneider, U. Havemann, L. Ziilicke, V. Pacak, K. Birkinshaw and Z. Herman, Gem. Phys. Lett., 1975, 37, 323. 14 V. L. Talroze and A. K. Lyubimova, Dokl. Akad. Nalik SSSR,1952,86, 909. 15 2. Herman, M. J. Henchman and B. Friedrich, to be published. 16 2. Havlas, E. Bauwe and R. Zahradnik, Chem. Phys. Lett., 1985, 121, 330. 17 K. Kamiya and K. Morokuma, Chem. Phys. Lett., 1986, 123, 331. 18 Z. Herman, K. Tanaka, T. Kato and I. Koyano, J. Chem. Phys., in press. 19 W. A. Chupka, in Zon-Molecule Reactions, ed. J. L. Franklin (Plenum Press, New York, 1972), vol. 1, p. 72 and references therein. 20 J. J. Leventhal, J. Chem. Phys., 1971, 54, 3279. 21 R. H. Neynaber and G. D. Magnuson, J. Chem. Phys., 1973,59, 825. 22 P. J. Brown and E. F. Hayes, J.Chem. Phys., 1971, 55,922. 23 P. J. Kuntz, Chem. Phys. Lett., 1972, 16, 581. 24 W. N. Whitton and P. J. Kuntz, J. Chem. Phys,, 1976, 64, 3924. 25 F. Schneider, U. Havemann, L. Zulicke and Z. Herman, Chem. Phys. Lett., 1977,48, 439. 26 K. Tanaka, Z. Herman, S. Suzuki and I. Koyano, to be published. 27 D. Van Pijkeren, E. Botjes, J. Van Eck and A. Niehaus, Chem. Phys., 1984, 91, 293. 28 P. J. Kuntz, in Dynamics of Molecular Collisions, ed. W. H. Muller (Plenum Press, New York, 1976, part B and references therein. 29 M. Baer, S. Suzuki, K. Tanaka, I. Koyano, H. Nakamura, Z. Herman and D. J. Kouri, to be published. 30 V. Pacak, U. Havemann, Z. Herman, F. Schneider and L. Ziilicke, Chem. Phys. Lett., 1977,49, 273. 31 P. J. Kuntz and A. C. Roach, J. Chem. SOC., Faraday Trans. 2, 1972, 68, 259. 32 Ch. Zuhrt, unpublished results. Paper 6/1019; Received 23rd May, 1986

ISSN:0300-9238    

DOI:10.1039/F29878300127

出版商:RSC    

年代:1987

数据来源: RSC