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Photoelectron Spectroscopy in a New Light Zero Kinetic Energy (ZEKE) Photoelectron Spectroscopy with Coherent Vacuum Ultraviolet Light John W. Hepburn Centre for Molecular Beams and Laser Chemistry Department of Chemistry University of Waterloo Waterloo Ontario N2L 3G1 Canada 1 Introduction A great deal of what we know about the electronic structure of mole- energy analysecules and solids has resulted from the study of photoemission. photoelectronsAlthough studies of the photoelectric effect began in the 19th century the modern study of photoelectron spectroscopy began with the development of suitable short wavelength light sources in the 1950s and 60s. For gas-phase high-resolution photoelectron spectroscopy -TPm:(PES) the differentially pumped He discharge lamp was the necessary prerequisite.' This light source along with other rare-gas discharge AB' detecte-sources provides high intensity reasonably monochromatic light at withE-0 fixed wavelength in the vacuum ultraviolet (VUV). Not very long after the development of high resolution PES a different form of photo- electron spectroscopy based on monochromatized continuum or pseudo-continuum radiation evolved out of the field of photoioniza-tion mass spectrometry.' This method called threshold photoelectron spectroscopy (TPES) is based on scanning the photon energy of the ionizing light while detecting only those photoelectrons that have essentially zero kinetic energy. In this way the signal is detected only when the photon energy matches an ionization threshold. The result- ing spectrum provides the same information as a conventional photo- electron spectrum although there are important differences between these two types of spectroscopy. In Fig. 1 both types of photoelectron spectroscopy are illustrated schematically. TPES is ideally suited to synchrotron light sources which are excellent sources of continuously tunable short-wavelength light. Most of the work in TPES has been done with synchrotrons. The advent of third generation synchrotron sources such as the Advanced Light Source in Berkeley California improves both the resolution and light flux dramatically compared with previous synchrotrons. Currently the state of the art for energy resolution in gas-phase photoelectron spectroscopy with rare gas lamps is about 24 meV while TPES can do slightly better. Since the resolution for both techniques is limitedlargely by thelight source,one should beable togainanother factoroffive bydoingTPES witha high- resolution monochromator with a third generation synchrotron. John Hepburn is Professor of chemistry and physics at the University of Waterloo. He did his undergraduate studies in chemistry at Waterloo completing his BSc degree in 1976. HeJinished his PhD under John Polanyi's supervision at the University of Toronto in 1980,and then spent two years in Yuan Lee's laboratory in Berkeley as a NATO postdoctoral fellow. He returned to Waterloo in I982 as an NSERC University Research Fellow and became a Professor in 1990. Hepburn S research has focused on photofragment spec- troscopy of small molecules photoionization and photoelec- tron spectroscopy and the dynamics of ion-molecule reac- tions. He has been an A.P. Sloan Foundation Fellow and is cur- rently a Canada Council Killam Fellow. In 1992 he received the Noranda Award from the Canadian Society for Chemistry and the Rutherford Memorial Medal in Physicsfiom the Royal Society of Canada. Figure 1 Schematic diagram of conventional PES and threshold PES. Note that since hu-El = hu,,both measure ionization energies. In parallel with the development of TPES using synchrotron radiation the application of coherent light sources to the study of valence-shell photoionization has created a revolution in photoelec- tron spectroscopy over the past decade and a half. The beginnings of this leap forward came with the development of resonance- enhanced multiphoton ionization (REMPI) by Johnson in the 1970s.?This technique based on sequential absorption of visible or ultraviolet photons allows one to excite to the ionization threshold in molecules with coherent light. Although originally developed as a sensitive method for excitation spectroscopy REMPI has also been used in elegant and detailed studies of photoionization dynam- ics,4 There are several advantages for photoionization with coherent light. The first and most important is energy resolution as even conventional pulsed laser systems have a resolving power of 200 000 and that can be improved to 4 X lo6 with single mode pulsed lasers. The current state of the art in scanning VUV mono-chromators is less than 150 000 resolving power. Because of the intensity of lasers very dilute targets such as skimmed supersonic beams of free radicals and clusters can be easily studied. The use of multiple resonance techniques provides a means to photoionize a state-selected aligned target. Because the excitation is pulsed sen- sitive and of high resolution time of flight techniques can be used for photoelectron spectroscopy following REMPI .5 A very important new technique for extremely high-resolution photoelectron spectroscopy which combines REMPI and thresh- old photoelectron spectroscopy was developed about a decade ago by Schlag and Miiller-Dethlefs.6 The technique has come to be known by the acronym PFI-ZEKE (for pulsed-field ionization zero kinetic energy) photoelectron spectroscopy or the simpler and more euphonius ZEKE (/zi:ki:/) spectroscopy.' The ZEKE method takes advantage of both the high photon energy resolution and the pulsed excitation provided by tunable pulsed-dye lasers. As with conventional TPES the photon energy is scanned through the 28 1 various ionization thresholds and the signal is detected only from threshold ionization The key to the high photoelectron energy resolution is the technique used for threshold detection which is delayed pulsed field ionization Although it was not realized until a few years after the initial development of ZEKE the detection of ionization thresholds is based on the excitation and subsequent field ionization of high principal quantum number Rydberg states lying just below the ionization thresholds Since there are Rydberg series converging to every quantum state of every ion the ZEKE method is quite universal The use of optimized ionizing pulse sequences has led to ‘photoelectron’ energy resolutions of 0 I cm I or ca 0 01 meV This spectacular energy resolution is suffi cient to fully resolve rotational structure in small and medium sized molecules and can easily resolve very low frequency vibrational structure in larger systems Furthermore because the threshold detection is based on pulsed field ionization one can monitor either the field ionized electrons as in ZEKE spectroscopy or the corre- sponding positive ions This latter type of threshold spectroscopy first developed by Phil Johnson has come to be known as MATI (/ma1 / for mass-analysed threshold ionization) spectroscopy The spectroscopic information is the same for ZEKE and MATI although MATI is typically of somewhat lower resolution MATI has the enormous advantage that because one can mass-analyse the product ions the carrier of the photoelectron spectrum is unambiguously identified For studies of unstable species such as molecular clusters this is a major advance since it is impossible to make pure clusters of one type for spectroscopic study and it can be quite difficult to tell them apart spectroscopically Most of the work in ZEKE and MATI spectroscopy up until now has been done using multiple resonance with pulsed ultraviolet lasers While there are many significant advantages to this approach not the least of which is large signals one must have an intermediate resonant state to step from to get to the ionization thresholds In systems where the neutral spectroscopy is unknown or where potential intermediate states are dissociative there is some advantage in exciting to the ionization threshold in one step using short wavelength coherent light The combination of coherent VUV with the ZEKE or MATI detection creates a universally applicable method for doing ul trahigh-resolution threshold photoelectron spectroscopy at ionization energies up to 19 eV This review will discuss the use of coherent VUV in ZEKE photoelectron spec- troscopy focusing on a few unique features of this type of photo- electron spectroscopy The next four sections will describe the generation of coherent VUV the principles of ZEKE spectroscopy and two examples of unusual features of ZEKE spectroscopy with coherent VUV 2 Generation of Coherent VUV Since there are currently no suitable broadband gain media for amplifying light at wavelengths shorter than 190nm coherent VUV must be generated by frequency-mixing longer-wavelength coher- ent light This is routinely done to generate coherent light in the ultraviolet region of the spectrum using birefringent crystals to mix light from visible dye lasers The current tuning limit for commer cially available crystals is the 189 nm limit of P-barium borate (BBO) meaning that photon energies up to 6 5 eV can be reached with current tunable pulsed-laser systems Beyond this photon energy gaseous media must be used for frequency mixing Since symmetry restrictions forbid second-order non-linear processes in materials which have a centre of inversion gases cannot be used for second-order frequency mixing (e g second harmonic generation) under normal circumstances The first non-linear term for gases is the third-order term which leads to frequency mixing between three input frequencies resulting in sum and difference mixing of the applied frequencies Because this third-order non-linear process involves four different waves three input (fundamental) and one generated it is called four-wave frequency mixing The theory of four-wave mixing is described in several places?-” so it will not be considered here What will be discussed is some of the practical details about generation of tunable coherent VUV and the current capabilities of coherent sources CHEMICAL SOCIETY REVIEWS 1996 There are two general types of four-wave mixing in gases used for coherent VUV generation and two wavelength ranges which need to be considered The simplest kind of four-wave mixing and probably the most commonly used is non-resonant third-harmonic generation where a single colour is focused into a gaseous medium and coherent light at three times the fundamental frequency is generated The second type of four-wave mixing is two-photon res onant sum and difference frequency mixing in which two different colours are used to generate coherent light at the sum and difference frequencies vOIJT= 2u * u2 The two wavelength ranges are based on purely practical considerations wavelengths above and below the LiF transmission cut-off at about 105 nm For coherent VUV at wavelengths transmitted by LiF the non-linear medium can be con- tained in a gas cell with an LiF output window while below the cut- off windowless designs typically pulsed gas jets must be used While any gaseous medium can be used for frequency mixing atomic gases have the best characteristics and tend to be the most efficient non-linear media For simplicity we shall restrict our dis- cussions to rare gases although in some cases metal vapours such as Hg or Mg may be better media In any case the general princi- ples of frequency mixing are the same no matter what gas is employed The easiest way to generate coherent VUV is by third-harmonic generation in gas cells All that IS required is to focus the funda- mental light fairly tightly (with a 15-30 cm focal length lens) into a cell containing rare gas and VUV is generated at the focus Although the process is quite inefficient under ideal conditions one can convert one part in los of the input light into VUV This means that by using 10 mJ 5 ns pulses of 365 nm light one can generate up to 10” photons/pulses of Lyman aradiation with typical yields being one or two orders of magnitude lower The major drawback to third-harmonic generation is the need to phase-match? which requires that the index of refraction be lower in the VUV than at the fundamental frequency Such behaviour called anomalous dispersion occurs when the third harmonic frequency is just to the blue of a strong absorption line Phase matching requires a certain relationship between the refractive indices depending on focusing conditions which means that only a certain range of nonlinear medium pressures will be phase matched Since the refractive index is a strong function of fre- quency near a resonance phase matching is also very wavelength sensitive All this combines to make third harmonic generation useful over small tuning ranges within windows of frequency close to absorption lines Furthermore to increase conversion effi- ciency up to the one part in los level one must increase the density of nonlinear gas and maintain phase matching with a bal ancing pressure of normally dispersive rare gas (1 e a mixture of Kr and Ar for 121 6 nm generation) This makes phase matching very frequency dependent limiting the tuning range to tens of wavenumbers for a given gas mixture However for essentially fixed-frequency applications such as detection of atomic hydro- gen this is not a serious limitation and third-harmonic generation can be very useful For wavelengths less than 105 nm a pulsed jet is used in place of the gas cell Although the phase matching conditions are somewhat different in pulsed jets because of the short length of the nonlinear medium relative to the confocal parameter roughly the same wave- length limitations exist for jets and gas cells Furthermore one cannot obtain the same densities in ajet that are possible in a cell which makes the conversion efficiency substantially lower Nevertheless one can still generate useful amounts of VUV of the order of lo9 photons/pulse by this method A good description of an apparatus employing third-harmonic generation in gas jets is given by Tonkyn and White l2 To obtain higher conversion efficiency and a much broader tuning range resonant-enhanced four-wave frequency mixing must be used The expression for the third-order nonlinear susceptibility (x‘?)),which determines the strength of the generated nonlinear response has resonance terms at one two and three photon ener gies Since one-photon and three photon resonances will lead to one-photon absorption of the fundamental or generated radiation two photon resonance is used to enhance the value of x(~)by orders PHOTOELECTRON SPECTROSCOPY IN A NEW LIGHT-JOHN W HEPBURN of magnitude What this means in practical terms is that one uses two independently tunable lasers at frequencies v,and v2,to gener- ate coherent VUV The first frequency is fixed on an allowed two- photon resonance for the atom being used as the nonlinear medium and the second laser is tunable While phase matching still applies for the sum mixing (vour = 2u,+ v2j one can achieve negative dispersion over a very broad spectral range by choosing a medium such that vol,r is above the lowest ionization limit Below the ion- ization limit or in regions of strong autoionizing resonances the conversion efficiency will vary quite markedly with frequency but one can obtain quite spectacular conversion efficiencies near Rydberg resonances In our own laboratory we have used Mg Hg Kr and Xe for this type of sum mixing and they all work very well with routine conversion efficiencies of one part in lo4being obtain- able The combination of Hg Kr and Xe provides continuous cover- age of the 10 to 19 eV spectral range for sum mixing For photon energies below 1 I eV the best way to generate coher- ent light is by difference-mixing using Kr or Xe in a gas cell The phase matching conditions for difference-mixing are very much less restrictive than in sum-mixing which means that just about any output wavelength can be generated efficiently within the tuning range of the v2 laser For example an efficient way to generate Lyman 01 is to use difference-mixing in Kr with the uI laser set at 212 5 nm and the v2 laser at 842 nm A summary of the tuning ranges available for different choices of media and ui is shown in Fig 2 For more information readers are referred to refs 10 and 1 1 Generated Photon Energy / eV Figure 2 Continuous tuning ranges for two photon resonant frequency mixing in Hg Kr and Xe The wavelengths corresponding to v1are given For sum-mixing. only the tuning range above the ionization limit (2P in Kr. Xe) is shown For this review it is sufficient to understand the practical end result Using standard nanosecond dye lasers and frequency-mixing crys- tals one can generate between 1O9 and 1Oi I photons/pulse of VUV continuously tunable from 6 5 eV where frequency-mixing crystals currently cut off to 19 eV This VUV has the coherence properties of the input fundamental light meaning that photon energy resolu- tions of a fraction of a wavenumber at 100 000 cm I are easily achievable The data discussed in the following sections were recorded using light generated by sum-mixing in pulsed jets of Kr or Xe 3 The ZEKE Process Because ZEKE spectroscopy has created such excitement there have been numerous recent reviews which have treated many differ- ent aspects in great detail Probably the best single source of general information about several aspects of ZEKE is the book edited by Powis Baer and Ng I3 The intention of this review is to draw atten- tion to some unusual features of ZEKE spectroscopy with coherent VUV light sources so we shall begin with a recap of what is cur- rently understood about this type of spectroscopy ZEKE spectroscopy was originally developed as a very high resolution version of TPES exploiting the pulsed nature of REMPI to introduce a long (of the order of a microsecond) delay between excitation to the ionization threshold under field-free conditions and extraction of the resulting threshold electrons with a small pulsed electric field The basic idea was that during this long delay any electrons formed with small amounts of kinetic energy (even just a few wavenumbers) would drift away from the excitation volume leaving only those with zero kinetic energy to await extraction and detection This explains the ZEKE acronym which remains despite the true nature of this spectroscopy The first hint that something was wrong in this interpretation was that the directly measured ion- ization energy of NO was found to be 747 17 2 cm !,6 later revised to 74719 0 2 0 5 cm I ,I4 substantially below the then accepted value of 74721 5 t0 5 cm I determined by extrapolation of very well understood Rydberg series Is The explanation of this srgnifi- cant discrepancy was that the signal detected in ZEKE spectroscopy came not from electrons formed by ionization at photon energies just above threshold but instead was the result of field ionization of very high lying Rydberg states just below the ionization threshold Ih This difference in signal source is actually good news from an experimental standpoint as the care and feeding of extremely low energy electrons is a substantial chore while field ionization detec- tion of Rydberg states is a relatively straightforward task Furthermore because of continuity of oscillator strength through the ionization limit of any Rydberg series the information available from field ionization is in principle the same a? from threshold ion- ization The ZEKE process is shown schematically in Fig 3 Fig 3 shows the limits to electron energy resolution or more correctly threshold resolution in ZEKE spectroscopy One can obviously do no better than the bandwidth of the exciting light but -50 500 1000 1500 2000 rl nrn Figure 3 Schematic of the ZEKE process Rydberg states around n = 100 are excited at t = 0 in the absence of applied fields Prompt electrons are formed by excitation of degenerate ionization continua After a delay the remaining n = 100 Rydberg states are Stark ionized and the resulting PFI-ZEKE signal 15 detected that is generally not what limits ZEKE resolution Resolution is normally determined by the pulsed ionizing field In the presence of a pulsed electric field the ionization limit is lowered by between 4dEand 6dE cm I (where Eis in cm I) depending on whether the Stark ionization is diabatic or adiabatic I8 This means that for even fairly modest pulsed fields a wide range of Rydberg states will be detected at each threshold Because of stray DC fields present in any apparatus and the fields caused by ions formed simultaneously with the Rydberg molecules one cannot decrease the pulsed ionizing field arbitrarily since there is a pre-existing reduction of the ionization threshold To get higher resolution combinations of pulsed fields must be used to create an energy window of Rydberg states for detection As an example if one uses a 0 5 V cm-’ pulse followed by a 0 6 V cm-I pulse and detects the electrons formed by the second pulse the band of Ryberg states between 2 8 and 3 1 cm-I below threshold will be detected It turns out that the time evolution of the Stark states formed in the pres-ence of the pulsed ionizing field means that the best strategy is to apply two pulses of roughly equal magnitude but opposite polar-ity + While the mechanism of Stark ionization leading to a ZEKE spectrum explains the observed shifts in thresholds it introduces a new conceptual problem how do the originally excited Rydberg states survive for several microseconds3 In the case of NO one would expect the Rydberg states to decay by predissociation as was pointed out by Muller-Dethefs Schlag and co-workers l6 In many cases especially for ionization limits above the lowest one would expect even high-n Rydberg states to decay through autoionization or predissociation on a timescale much shorter than the delay time The solution first pointed out by Chupka a few years ago,18was that the initially excited low-f high-n Rydberg states could be rapidly mixed with energy-degenerate high-l high-rn Rydberg states because of the perturbing effects of weak stray electrical fields and the inhomogeneous fields caused by nearby ions created with the Rydberg states These high-l,m states interact very weakly with the ion core and thus decay very slowly The effects of electric fields on autoionization rates had been previously noted in a study of nd Rydberg states of Na2,19and the lengthening of the nd state lifetimes was interpreted in terms of Stark-mixing with high-f states The idea of optically exciting a particular ‘state,’ whose lifetime properties are modified by coupling to optically inaccessible states is very familiar in photophysics and the analogy between radiationless transitions and suppression of Rydberg state decay is made in some recent articles by Bixon and Jortner20 These articles also have a comprehensive bibliography and readers are referred to them for a more detailed discussion of the stabilization of the Rydberg states involved in ZEKE spectroscopy The extensive literature discussion from over the past three years can be summarized in the following way The optical excita-tion is to a band of high-n Rydberg states which lie within the bandwidth of the exciting laser These states have low-f character typically with 1s 3 and in many cases are expected to have rela-tively short lifetimes (sometimes sub-nanosecond) because of autoionization or predissociation The unperturbed decay life-times for the typical states involved in ZEKE spectroscopy (150 <n <300)can be estimated by scaling the measured lifetimes for lower nl states using the n3 scaling law for lifetimes Dunng the initial penod after excitation there is a competition between decay Time 0 Q high-Z,m Rydberg I Field Ionization w Figure 4 Mechanism for ZEKE spectroscopy Initially excited low-f state can decay by autoionization or predissociation + H J Dietnch and K Muller Dethefs Phys Rev Lett 19% 70 3530 CHEMICAL SOCIETY REVIEWS 1996 of the initially excited low-f Rydberg state and stabilization by f,m mixing This stabilization occurs by two different mechanisms mixing which is caused by Stark mixing due to weak DC fields present in the excitation volume and m,mixing which requires the inhomogeneous electnc fields created by nearby ions 2’ Both 1 and rn mixing lead to an increase in lifetime by a factor of roughly n about two orders of magnitude increase for each type of mixing This means that if the unperturbed lifetime of the ini-tially excited low-l states is significantly less than one micro-second both f and rn,mixing will have to occur to observe a ZEKE spectrum The significance of this is that in some cases there may be a requirement of an ion density of the order of lo4-lo5cm in the excitation volume for ZEKE spectroscopy or observed linestrengths may be ion density-dependent Recent experiments have probed ion density effects on ZEKE spectra2?24 and we have observed these effects in ZEKE spectroscopy on H 22 While these ion densities are easily achieved under REMPI conditions they may not be present when VUV excitation is employed especially if the VUV is from a synchrotron light source For coherent VUV if one assumes a light flux of 10’ photons/pulse in a spot of 1 mm2,with a target density of loi2cm 3 about lo6 ions cm are formed for a typical 10 Mb ionization cross section With more dilute targets such as a radical beam it may be necessary to seed an easily ionized molecule into the beam to achieve the ion den-sities necessary for stabilization Finally for synchrotron sources which are discussed in more detail in the final section the greatly reduced instantaneous light flux will reduce the ion density by four or five orders of magnitude which may have strong effects on Rydberg state stabilization While understanding the detailed Rydberg state dynamics behind ZEKE spectroscopy can provide important new information about interpretationof spectral intensities or lead to technical advances such as resolution improvement one should not lose sight of the experimental fact that ZEKE has proved to be a very generally applicable technique In our own experience we have never failed to find the ZEKE signal at any threshold where we have looked for it In fact one of the surprises of ZEKE spec-troscopy is that the signal can be found even where you may not expect to find it as shall be described in the next section 4 ZEKE and the Franck-Condon Principle In conventional photoelectron spectroscopy with 2 1 22 eV photons from a He I resonance lamp the band intensities observed in the spectrum are generally well described by Franck-Condon factors between the starting neutral ground state and the final vibrational levels of the ion 25 However when lower-energy resonance lines are used for PES such as the 16 85 eV Ne I resonance line one can observe dramatic changes in the band intensities in PES An example is provided by oxygen shown schematically in Fig 5 The 0,photoelectron spectra 0 2 4 6 8 10 12 14 16 18 v’ 1 11 I I I I,,,cI *‘‘I #“‘I 12 13 14 15 16 Ionization Energy/ eV Figure 5 Schematic of oxygen PES with He I and Ne I resonance lines Adapted from spectra shown in ref 9 PHOTOELECTRON SPECTROSCOPY IN A NEW LIGHT-JOHN W HEPBURN band intensities seen in the He I spectrum (21 22 eV) are the same as the Franck-Condon factors while the relative intensities for Ne I photonization (16 84 eV) are bimodal with substantial intensity for high vibrational levels of 0; The reason for the change in band intensities is autoionization there is a broad autoionizing resonance at 16 84 eV and the intensities of the high vibrational levels reflect the Franck-Condon factors between the autoionizing Rydberg state and the vibrational levels of the ion 26 In TPES the observation of Franck-Condon forbidden peaks (meaning the Franck-Condon factor to the starting neutral state is zero) is very common and can produce ions in highly vibrationally excited states In oxygen the TPES spectrum shows significant intensity for vibrational levels up to v+ =24 of the PII state of 0; 27 In the case of TPES the reason for this breakdown of the Franck-Condon approximation is not the same as in the NeI PES of 0 For TPES the ionizing light is tuned continuously and bands are observed in TPES whether or not there is a coinciding autoion- izing Rydberg state While there is some ambiguity about this last statement if one examines (relatively) low-resolution TPE and PIE spectra there is none if one looks at the high-resolution ZEKE and PIE spectra of jet-cooled oxygen recorded with coherent VUV In the spectra shown in Fig 6 part of a much larger data set that has ~'=14 i 1 120700 I\ v'=15 ~ c/\I \'\-123500 VUV Energy /cm-' Figure 6 PIE (dashed) and ZEKE (solid) spectra of jet-cooled 0 The PIE specta have been shifted upwards for clarity All three ZEKE spectra and all three PIE spectra have the same relative scales The VUV energy axis is 100 cm-I per division been published,2* 29 one can see that the peaks in the ZEKE spectra which can all be assigned to 0; quantum states are not always coincident with resonances in the PIE spectrum Furthermore not only are all the resonances observed in the ZEKE spectra assignable to ionization thresholds in oxygen but for all the 16 vibrational bands ~nvestigated~~ the observed rotational linestrengths can be reproduced using a standard model for photoelectron spectra called the BOS model 30 These observations can be explained in terms of a modified resonant autoionization mechanism 3i In this mech- anism the initial excitation is to a Franck-Condon-allowed Rydberg state which is coupled through a dissociative valence con- tinuum to Rydberg states converging to a highly vibrationally excited ion core Vibrational and rotational autoionization from this final Rydberg state produces low-energy electrons which are detected in TPES The modification suggested by our ZEKE results is that it is not necessary to be in resonance with a Franck-Condon- allowed Rydberg state for the excitation to occur Rather one excites the valence continuum directly and it is coupled to the vibrationally excited Rydberg state The interesting end result is that one obtains at least a partial yield of stabilized Rydberg states which are subsequently field-ionized rather than the neutral frag- ments one might expect This is similar to the commonly observed ZEKE dynamics where Rydberg state stabilization competes successfully with predissociation and autoionization The inter- esting difference in this case is that the oscillator strength for the initial excitation is not carried by the high-n Rydberg states but by the valence continuum (or continua) The result which will be fairly common based on TPES results is that it will be possible to inves tigate the spectroscopy of Franck-Condon-forbidden levels by ZEKE as well as using PFI to state select highly vibrationally excited ions for reaction dynamics studies 5 ZEKE at Thresholds Corresponding to Unstable States of the Ion Since as we have seen ZEKE spectroscopy seems to work in cases where it is somewhat surprising that it does we investigated whether there were instances where ZEKE did not work because the initially excited Rydberg states simply could not survive the subsequent dynamics One simple example where it is not really a surprise that ZEKE works is the ZEKE spectrum at the N20+A2Z+ionization limit 32 Since this excited state of N20+ has a fluorescence lifetime of about 200 ns the initially excited Rydberg states with the N20+A2Z+ ion core would undergo a radiative decay during the microsecond delay before field ionization resulting in a Rydberg state with an N20+ X2H ion core This radiative relaxation of the ion core does not have any effect on the Rydberg electron as the inten- sity of the ZEKE spectrum is not a function of time on the hundreds of nanoseconds timescale In this particular case it is not too sur- prising that the high-l,m Rydberg state formed after excitation should be essentially unaffected by the relatively minor change of the ion core relaxing from the A2Z to the x2FI state A more dramatic case of core relaxation is provided by the A2Z state of HBr+ which is strongly predissociated for all vibrational levels above v+ = 1 In this case the ZEKE spectrum is quite interesting since for the v+ =0 and 1 levels the ion is stable fluorescing to the ground state with several microseconds lifetime while for v+ = 2 the lifetime is ca 1O-Io s and for v+ =3 the lifetime is 10 l3 s In our experi- ments we measured ZEKE spectra for all of the vibrational levels from v+ = 0 to 3 for the HBr+ A2Z state 33 The spectra for two of these levels is shown in Fig 7 The spectrum for the v+ = 1 level is exactly as one would expect given the high-resolution spectro- scopic results for this level The simulation shown below the spec- trum was calculated using the BOS model for rotational linestrengths 3o The spectrum for v+ =2 shown in the lower panel is precisely what one would expect based on the v+ = 0 and I ZEKE spectra This means that the spectral constants obtained from the ZEKE spectrum are what one would expect (there are no high resolution data for this level) and the parameters used for the BOS simulation are the same as those for v+ = 1 In fact even the rela- tive intensities of the v+ =0,l and 2 ZEKE bands are in reasonable agreement with the Franck-Condon factors All this in spite of the fact that the v+ =2 level predissociates in ca 10 lo s How can we explain the observation of a ZEKE spectrum at a threshold where the ion core is unstable? The answer is the same as the N,O case mentioned above As long as the decay of the ion core does not affect the Rydberg electron significantly it will still be available for pulsed-field ionization microseconds later In this extreme case the dissociation of the HBr+ core into H + Br+ has little effect on the Rydberg electron which continues orbit ing the Br+ ion core after the H atom has departed Since the signal obtained after field ionization is larger than at the V+ = I threshold where the ion core is stable we can conclude that in this particular case the dissociation of the ion core has essentially no effect on the Rydberg electron For the v+ = 3 level the results are even more striking as shown in Fig 8 In this case the lifetime of the ion core is extremely short orders of magnitude shorter than the period of the CHEMICAL SOCIETY REVIEWS 1996 0.5 nv1 5C =1 . 0.0 2 W d 124.6 124.7 vlt +2 i - +I - 0- -1 - -2 - 1O3 VUV Energy/ cm-' Figure 7 ZEKE spectra at the HBr+A2C+v+ = 1 and 2 thresholds. Assignments are given on the spectra as are the results of a simulation using the BOS model. Adapted from ref. 33. Rydberg orbit for the n = 200 state. However even for this level we observe a ZEKE spectrum with an integrated intensity that is comparable with that for the v+ = 1 and 2 levels in agreement with Franck-Condon factors. The solid line drawn through the data in Fig. 8 is the result of taking the v+ = 2 ZEKE spectrum shown in t 126.8 126.9 127.0 127.1 127.2 127.3 1O3 Ionization Energy/ cm-' Figure 8 PFI-ZEKE spectrum at the HBr+A*C+v+ = 3 threshold. The line drawn through the data is the result of convoluting the shifted V+ = 2 ZEKE spectrum with a Lorentzian line corresponding to a 0.05 ps life-time. Fig. 7 and convoluting it with a Lorentzian lineshape correspond- ing to a lifetime of 5 X 10-l4s. This lifetime is consistent with pre-vious theoretical work and our measured ZEKE spectrum is the same as high-resolution photoelectron spectra for this band. Because of lifetime broadening the ionization energy for this level is 'uncer- tain' by ?50 cm-I yet we are observing the ZEKE spectrum using Rydberg states that are bound by less than 5 cm-I to the ion core. 6 Future Directions ZEKE photoelectron spectroscopy is now a well-established field with many different systems studied. Although this article has focused on ZEKE spectroscopy of small molecules there has been quite extensive work done on large molecules and molecular clus- ters as well as metal clusters and metal compounds. It would appear that ZEKE is an essentially universally applicable technique which can even be used to probe quantum states of ions that cannot be reached through Franck-Condon allowed transitions. Improve- ments in resolution can still be made and it is conceivable that the transform limit for pulsed-dye lasers may be achieved in the near future. There is beginning to be some interest in using PFI to create state-selected ions for reaction dynamics studies although to date only preliminary work has been done.34 Given the high state-selec- tivity provided by PFI it is easy to predict that this will become a method of choice for creation of state-selected molecular ions. From the point of view of this review we can look forward to developments in two areas of technology. The current maximum photon energy of coherent VUV sources is 19 eV a limit established by frequency-mixing crystals. As this technology improves making it possible to generate millijoule pulses of tunable light at wave- lengths shorter than 189 nm the maximum photon energy will increase accordingly. However to achieve a substantial increase in tuning range the best currently available solution is provided by third generation synchrotron light sources. As mentioned in the introduction a long period undulator with a high-resolution mono- chromator can produce 1O1O to 10" photons/ s-' at a resolution of 2 cm-1 (0.25 meV) at 20 eV.35 While the resolution is somewhat inferior to that of a coherent source the photon flux is comparable and the upper limit of photon energy can be above 30 eV. Furthermore the synchrotron source can be easily tuned over very broad photon energy ranges and the flux can be increased at the expense of energy resolution. Efforts have just begun at the Advanced Light Source in Berkeley California to utilize third generation synchrotron radiation for threshold ionization experi- ments. If these efforts are successful the resulting threshold ioniza- tion mass spectrometer will be a powerful addition to this field enabling us to carry out experiments at much higher photon ener- gies and allowing for very broad survey spectra which will be extremely useful for systems where little is known such as free rad- icals and clusters. Acknowledgements The data from our lab which are discussed in this review were produced by the hard work of my collaborators Profs W. Kong and D. Rogers Drs A. Mank and C. Alcaraz and T. Nguyen and J. D. D. Martin. Funding for the research was provided by the NSERC (Canada) and the Canadian Federal Networks of Centres of Excellence program administered by the NSERC. I thank the Canada Council for a Killam Research Fellowship. 7 References 1 F. I. Vilesov B. L. Kurbatov and A. N. Terenin Sovi. Phys. (Dokl.) 1961,6,490; M. I. Al-Joboury and D. W. Turner,./. Chem. Phys. 1962 37,3007. 2 D. Villarejo R. R. Herm and M. G. Inghram J. Chem. Phys. 1967,46 4995; W. B. Peatman T. B. Borne and E. W. Schlag Chem. Phys. Lett. 1969,3,492. 3 P. M. Johnson and C. E. Otis,Acc. Chem. Res. 1980,32,20; Annu. Rev. Phys. Chem. 1981,32,139. 4 D. J. Leahy K. L. Reid H. Park and R. N. &re J. Chem. Phys. 1992 97,4948; H. Park D. J. Leahy and R. N. &re Phys. Rev. Lett. 1996 76 1591; J. B. Milan W. J. Buma C. A. de Lange K. Wang and V. McKoy J. Chem. Phys. 1995,103,3262. 5 P. Kruit and F. H. Read J. Phys. E Sci. Instrum. 1983,16,313. 6 K. Muller-Dethlefs and E. W. Schlag Chem. Phys. Lett. 1984 112 291. 7 K. Muller-Dethlefs and E. W. Schlag Annu. Rev. Phys. Chem. 1991,42 109. 8 L. Zhu and P. M. Johnson J. Chem. Phys. 1991,94,5769. 9 D. C. Hanna M. A. Yuratich and D. Cotter Nonlinear Optics of Free Atoms and Molecules Springer-Verlag Berlin 1979. 10 R. Hilbig G. Hilber A. Lago B. Wolff and R. Wallenstein Comments At. Mol. Phys. 1986 18 157; A. Lago in Half Collision Resonance Phenomena in Molecules; Proceedings of the Escuela Latinoamericana de Fisica ed. M. Gacia-Sucre G. Raseev and S. C. Ross AIP Proc. No. 225 AIP New York 1990. PHOTOELECTRON SPECTROSCOPY IN A NEW LIGHT-JOHN W HEPBURN 287 11 J W Hepburn in Laser Techniques in Chemistry ed A B Meyers and 24 M J J Vrakking I Fischer D M Villeneuve and A Stolow J Chem T R Rizzo Wiley New York 1995,pp 149- 183 Phys ,1995,103,4538 12 R G Tonkyn and M G White Rev Sci Instrum ,1989,60,1245 25 J H D Eland Photoelectron Spectroscopy Butterworth London 1984 13 High Resolution Laser Photoionization and Photoelectron Studies ed 26 J N Bardsley Chem Phys Lett ,1%8,2,329,A L Smith Phil Trans I Powis T Baer and C Y Ng Wiley New York 1995 Roy Soc Lond A 1970,268,169 14 M Sander L A Chewter K Muller-Dethlefs and E W Schlag Phys 27 F Merkt P M Guyon and J W Hepburn Chem Phys ,1993,173,479 Rev A 1987,36,4543 28 W Kong D Rodgers and J W Hepburn Chem Phys Lett ,1993,203 15 E Meischer Can J Phys 1976,54,2074 497 16 G Reiser W Habenicht K Muller-Dethlefs and E W Schlag Chem 29 W Kong and J W Hepburn Can J Chem ,1994,72,1284 Phys Lett 1988,152,119 30 A D Buckingham B J Orr and J M Sichel Philos Trans Roy Soc 17 T F Gallagher Rydberg Atoms Cambridge University Press London A 1970,268,147 Cambridge 1994 3 1 P M Guyon T Baer and I Nenner J Chem Phys ,1983,78,3665 18 W A Chupka J Chem Phys ,1993,98,4520 32 W Kong D Rodgers and J W Hepburn Chem Phys Lett ,1994,221 19 C Bordas P F Brevet M Broyer J Chevaleyre P Labastie and J P 301 Perrot Phys Rev Lett 1988,60,917 33 A Mank T Nguyen J D D Martin and J W Hepburn Phys Rev A 20 J Jortner and M Bixon J Chem Phys 1995 102 5636 1995 103 1995,51,RI 443 1 34 S R Mackenzie and T P Softley J Chem Phys 1994 101 10 609 21 F Merkt andR N Zare,J Chem Phys ,1994,101,3495 S R Mackenzie E J Halse F Merkt and T P Softley in Luser tech- 22 J D D Martin C Alcaraz W Koz and J W Hepburn in ‘Laser niques for State-Selected and State-to-State Chemistry Ill ed J W Techniques for State-Selected and State-to-State Chemistry 111’ ,ed J Hepbum SPIE Proc 2548 SPIE 1995,p 293 W Hepburn SPIE Proc 1995,74,2548,J D D Martin C Alcaraz and 35 M Koike P A Heimann A H Kung T Namioka R DiGennaro J W Hepburn to be published B Gee and N Yu,Nucl Instrum Meth Phys Res A 1994,347,282,P 23 M J J Vrakking and Y T Lee,/ Chem Phys 1995,102,8833 A Heimann personal communication of unpublished data

ISSN:0306-0012    

DOI:10.1039/CS9962500281

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

The Changing Face of Arene Oxide-Oxepine Chemistry Derek R. Boyd* and Narain D. Sharma School of Chemistry, The Queen‘s University of Belfast, Belfast, UK BT95AG 1 Introduction The term ‘arene oxide’ is widely used to describe oxirane (epoxide) derivatives from chemical, i.e. non-enzyme-catalysed, and enzyme- catalysed epoxidation of mono-and poly-cycl ic arenes. The increasing range of arene oxides reported in the literature, since the last major reviews of this topic appeared,’.* requires that the subject area be revised and updated. Particular emphasis will be placed upon advances in arene oxide chemistry which have appeared since 1983 and citations of earlier work will be covered by reference to the most recent review.* Scheme 1 Possible arene oxide regioisomers from epoxidation of toluene 1 Epoxides of toluene 1 and naphthalene 5 exemplify the range of possible monoarene oxide regioisomers in monocyclic and bicyclic arenes respectively.A simple epoxidation of such arenes would in principle yield three (2-4; Scheme 1) or four (6-9; Scheme 2) different oxirane regioisomers. In practice, however, due to aromatization and further epoxidation, only one of these seven arene oxides, i.e. oxide 6, has been isolated from the direct chem- ical or enzyme-catalysed oxidation? An additional factor is the preference for epoxidation of polycyclic arenes to occur at bonds of ~~~ Derek Boyd has been involved in teaching and research at The Queen j. University of Belfastfor almost three decades. Afrer gradu- ating with a BSc ( 1963) and a PhD ( 1966) at Queen k he then joined the academic staflas a lecturer (I 967).Research interests in the chemical and enzyme-catalysed syn- thesiJ of chiral compounds followed from postgraduate and post- doctoral Jtudies under the supervision of Professors H. B. Henbest and M. F. Grundon. This area of research was further stimulated by sabbatical leave spent at the National Institutes of Health, Bethesdu, MD, in the laboratories with Drs. B. Witkop, J. Dalv and D. M.Jerina (I 968-69) and at the Massachusetts Institute of Technology, Cambridge. MA, in collaboration with Professor G. A. Berchtold (1977-78). The award of a NufJield Foundation Science Research Fellowship and a NATO travel grant (with NIH and MIT) provided a further opportunity for full-time research and travel in the USA (I 980-81).He was promoted to Reader (1978) and Professor of Organic Chemistry (I989) at Queen j. where current research interests include chiral oxidations by chemical and enzyme-catalysed routes, applications of chiral bio-products in synthesis, and the chemical and chemoenymatic synthesis of enantiopure alka- loids. .--54 Scheme 2 Possible arene oxide regioisomers from epoxidation of naphtha-lene 5 highest electron density and thus arene oxides 7-9 were not pro- duced by this method. One type of arene oxide nomenclature, associated only with poly-cyclic members, is exemplified by reference to arene oxides of the pentacyclic arene benzo[g]chrysene 10 (Scheme 3).This is the sim- plest pol ycyclic aromatic hydrocarbon (PAH) containing non-K, K-, bay- and fjord-regions in the same molecule. In the literature? the terms non-K-region arene oxide (e.g. 6,16),K-region arene oxide (e.g. 15), bay-region arene oxide (e.g. 12-14) and fjord-region arene oxide (e.g. 11,17),are used to identify epoxides prox- imate to these positions (bay and fjord region) or formed at the region of highest electron density (K-region) as shown in Scheme 3. Benzene oxides, e.g. of type 2-4, are distinguished by reference to the approximate bond of the corresponding monosubstituted arenes, e.g. the arene 1,2-oxide 2 derived from toluene. Multiple epoxide derivatives of a PAH have been classified as ‘polyarene oxides’ where the epoxides are in different arene rings, e.g.the 1,2,9,10-diarene oxide of benzolglchrysene 18,jor as ‘arene poly- oxides’ where the epoxides are in the same arene ring, e.g. the 8,9,10,1 l-arene dioxide of benzlajanthracene 19.4 Fixed molecular geometry and configurational stability are normal characteristics of epoxide derivatives of cyclic alkenes. These static features are however less common in arene oxides where dynamic stereochemistry may result in spontaneous epoxide ring inversion between the faces of the arene via an oxepine (e.g. arene oxides 2-4 to the corresponding oxepines 20-22) with con-comitant racemisation at two chiral centres. Arene oxide derivatives of monocyclic arenes, e.g.benzene (23, R = H), toluene (2-4), Narain Sharma was born in Simla, India in 1944. He obtained his MSc in 1967 from Kuruks hetra University (Haryana, India) and a PhD from Delhi Universitv in 1972. He served as a Lecturer and later as a Senior Lecturer in a Postgraduate College of Delhi University from 1972 to 1989. He is working as u Research Fellow in The Queen5 University of Belfast. 289 CHEMICAL SOCIETY REVIEWS, 1996 NoII-K-Scheme 3 Possible arene oxides from epoxidation of benzo[g Jchrysene 10 benzyl benzoate (24), and bromobenzene (25) are generally assumed to exist in a state of rapid equilibration at ambient temper- ature with the corresponding valence tautomeric oxepines (26,20-22,27and 28)via an electrocyclic rearrangement (Scheme 4).Scheme 4 Arene oxide-oxepine equilibration In contrast, the arene 1,2-0xide 6of naphthalene showed no evi- dence of spontaneous isomerization to benzoxepine 29,while oxe- pines 30-32 appeared to have no significant contribution from the tautomeric arene oxides 7-9. In the context of the changing shape and colour of benzene oxide (23,colourless), as it equilibrates with the seven-membered oxepine (26,bright yellow), this tautomeric system may be described as a 'molecular chameleon.' 2 Structure and Stereochemistry The static stereochemistry of arene oxides, in both the cystalline state and in solution, has been demonstrated for the relatively stable K-region arene oxide series, by X-ray cystallography, and by the configurational stability of enantiomers, e.g.benzo[a]pyrene 45-oxide 33.2K-region arene oxides are considered closer in structure, stereochemistry and reactivity to alkene epoxides. Acridine 1,2-0xide 34,a non-K-region arene oxide of sufficient '0 (33) B2-0 / (35) stability at room temperature to permit analysis by X-ray crystal- lography,5,6was also found to have fixed geometry with the epoxide ring at a similar external angle (ca. SO0) to the planar diene ring system and epoxide bonds of lengths similar to those of K-region arene oxides. The observed geometry of polycyclic arene oxides in the crystalline state was also predicted to be close to that for non- crystalline monocyclic arene oxides, e.g.benzene oxide 23,based on molecular orbital calculation^.^ Evidence for the preferred oxepine tautomeric form (e.g.20) of arene 12-oxides of mono- substituted benzene oxides (e.g. 2)is largely based on spectral data.2 X-Ray crystallographic structure analysis of the relatively stable compound 1-tert-butoxycarbonyloxepine 35 confirmed unequivo- cally the predicted structure and the non-planar boat c~nformation.~ Arene oxide 34was synthesised in enantiopure form and existed exclusively as a single valence tautomer in solution as well as in the crystalline state. Similarly oxepine 35 was the strongly preferred tautomer in solution and could be crystallized out exclusively. However, the parent arene oxide-oxepine system (benzene oxide-oxepine, 23+26)was found to be in a finely balanced equi- librium in hydrocarbon solvents with the epoxide tautomer slightly favoured at lower temperatures and in hydroxylic solvents.* The enigmatic preference for an arene oxide or oxepine valence tau- tomer in the monosubstituted benzene series can now be predicted, from MIND0/3 calculations, and rationalized simply in terms of the maximum number of dipolar resonance structures which can be drawn.2 Epoxides at the 2,3-bond of monosubstituted arenes bearing either a n-electron donating or withdrawing substituent will equilibrate rapidly showing a preference for arene 2,3-oxides (e.g.THE CHANGING FACE OF ARENE OXIDE OXEPINE CHEMISTRY -DEREK R BOYD AND NARAIN D SHARMA 29 1 3) over oxepines (e g 21) at ambient temperature Conversely arene 1,2 oxides (e g 2) and arene 3,4-oxides (e g 4), derived from the same arenes, will be very minor contributors to the equilibrum com pared with the corresponding oxepines (20and 22) Kinetic studies of arene oxide-oxepine valence isomerisation have been largely precluded by this marked substituent effect which causes the position of equilibrium to be heavily biased towards one particular valence tautomer Preference for the arene 2,3-oxide tau- tomer can, however, be helpful during dynamic NMR studies of enantiomerisation when a prochiral substituent is present For example, at -100°C,in dimethyl ether solvent, the methylene protons Ha and H, of arene oxide 24 are effectively enantiotopic and give a singlet signal due to rapid equilibration (via oxepine 27) while at a lower temperatures (<-135“C) the latter protons become diastereotopic and give an AB quartet (Scheme 4) This type of study has enabled the barrier to isomerization, of substituted benzene oxides to oxepines, to be measured (AG* ca 7 6 kcal mol I, 1 cal = 4 184 J) * A more direct demonstration of the spontaneous enantiomerisation of monocyclic arene oxides, at ambient temperature, was provided when the arene 2,3-oxide of bromobenzene25, synthesised from the corresponding enantiopure 2,3-crs-dihydrodioI derivative of bromobenzene, was found to have totally racemised via the oxepine 2S9(Scheme 4) Monocyclic oxepine tautomers, generally considered as achiral in solution due to rapid ring inversion, can account for the con- comitant racemisation of two chiral centres in the corresponding arene oxides Molecular orbital calculations2 have led to the conclu- sion that the barrier to inversion, of the boat conformation in the oxepine ring in solution, is very small (AG ca 0 7-1 2 kcal mol I) Using dynamic ‘H NMR spectroscopy, the barrier to ring inversion of the disubstituted oxepine 36 bearing a prochiral group (CHaH,Me) has been found7 to be higher (A@ ca 6 5 kcal mol-I) (Scheme 5) Scheme 5 Oxepine ring inversion Me (37) A similar attempt to measure the barrier to valence isomerisation and degenerate racemisation of the substituted naphthalene oxide 37, by dynamic NMR methods, was unsuccessful due to (I) the much higher value (AG >> 23 kcal mol I) and (11) compound instability above ca 100 “C 8 The high barrier to racemisation of naphthalene 1 J-oxide 6 is consistent with the relatively large loss in resonance energy involved in isomerisation to the corresponding oxepine 29 2 A similar explanation in terms of resonance energies can be used to account for the exclusive preference for the oxepine tautomers 30-32 over the corresponding arene oxides 7-9 Perturbational molecular orbital (PMO) calculations have allowed the relative loss in resonance energy, associated with the tautomer rsation of polycyclic arene oxides to the corresponding oxepines (and thus the configurational stability), to be predicted * This approach led to the prediction and experimental verification that spontaneous racemisation, at ambient temperature, would only occur in particular arene oxides from members of the polycyclic aromatic hydrocarbon (PAH) series including phenanthrene ,chry sene, triphenylene, benz[a]anthracene, benzo[ clphenanthrene, benzo[e]pyrene, dibenz[a,h]anthracene, dibenzl ajlanthracene, dibenz[a,c]anthracene and benzo[g]chrysene Configurational stability was predicted for other arene oxides of the latter PAHs includingall K-region arene oxides and also for the non K-and bay region arene oxides of naphthalene, anthracene and benzo[ a1-pyrene A significant proportion of these predictions has been con- firmed experimentally (ca 10 examples of spontaneously racemising arene oxides and ca 12 examples of configurationally stable arene oxides) and, to date, no exception to the rule has been found2I0 Furthermore, it now appears that the results of PMO calculations can be extended to the prediction of ease of racemisa- tion of azaarene oxides? and should also be applicable to diarene oxides I Where an arene oxide derivative from the PAH series, e g tri phenylene 12-oxide 38, is predicted2 and foundlz to racemise spontaneously,i e AG < 23 kcal mol I, this may be accounted for by valence tautomerisation viu a very minor proportion of the cor- responding transient oxepine (e g 39, Scheme 6) Direct evidence for the presence of this elusive valence tautomeric oxepine has not yet been obtained in any of the PAH systems studied It is important to note that the undetected and less stable oxepine 39, responsible for spontaneous epoxide ring inversion or racemisa- tion of the arene oxide enantiomers of triphenylene 1 2-oxide 38’ (viaa disrotatory electrocyclic rearrangement mechanism), is struc- turally distinct from the more stable isomeric oxepine 40 The latter oxepine was of sufficient stability as to be isolated after photoisom- erisation of arene oxide 38 (via a sigmatropic rearrangement mech- anism) (Scheme 6) l2 Eight examples of relatively stable oxepines, structurally similar to compound 40, have been found either as a result of a photochemically induced ‘circumambulatory’ (‘oxygen walk’) rearrangement of the arene oxide, or as a by-product formed during an attempted synthesis of the corresponding arene oxide from a dibromoester precursor (Scheme 7) Formation of the more stable oxepines can also be predicted on Seheme 6 PhotoisomerisatIan of arene oxide 38 to a stable oxepine 40 and racemisation via an unstable oxepine 39 CHEMICAL SOCIETY REVIEWS, 1996 IBr B Ii 1) Scheme 7 Reagents: i, NaOMe; ii, MeC(OMe),; iii, Me,SiCI-Et,N; iv, N-bromosuccinimide (NBS) the basis of the PMO calculations of resonance energy loss previ-ously applied to rationalise the ease of racemisation of arene oxide enantiomers.2J3 3 Synthetic Methods The recent availability of arene oxides by chemical (non-enzyme- catalysed) syntheses, and the resulting stability studies, have facil- itated the first direct detection of monocyclic and polycyclic arene oxides in biological systems.The non-enzymatic synthesis, spectral characterisation and stability studies of methyl benzoate 1,2-0xide 41: which prefers to exist as the oxepine tautomer 42, were instrumental in its sub- sequent identification as a secondary metabolite from the wood- rotting fungus Phellinus tremulae.I4 The synthesised quinoline 5,6-oxide 43, and the isomeric arene 7,8-oxide 44, were also remarkably stable. The stability of compound 43 allowed its isola- tion as a xenobiotic metabolite from monooxygenase-catalysed epoxidation of quinoline in the presence of an epoxide hydrolase inhi bitor. I Significant advances in the direct epoxidation of pol ycyclic arenes by chemical methods have been made by using dimethyl- dioxirane (DMD, 45), a powerful neutral oxidant requiring a minimal workup pr~cedure.~J~- l9 This method has been applied to the synthesis of relatively stable K-region monoarene oxides4J6 and to the more labile arene oxide derivatives of five-membered aro- matic heterocycle^.^^-l9 Thus substituted benzofuran and indole derivatives have been epoxidised at the 2,3-bond to yield the corre- sponding heterocyclic arene oxides, e.g.46I7J8and 47.'9.20N-Acetylindole 2,3-oxide 47 is a crystalline solid while benzofuran 2,3-oxide 46 was only detected by IH NMR spectroscopy; it decom- posed during attempted isolation. 43 (X=N,Y=CFQ (45) (46) X=O 44 (X=CE,Y=N) (47) X=NCOMe A problem associated with the direct epoxidation of PAHs, at non-K region positions, is the relatively slow rate of formation of monoarene oxides, e.g.6, compared with the much faster epoxidation of the alkene group of the arene oxide to yield diarene oxides, e.g. 48.4J6As a result, direct non-enzymatic epoxidation IBr t does not generally provide a satisfactory route to non-K or bay- region monoarene oxides. The results of DMD oxidation of poly- cyclic arenes and heteroarenes have shown that, as expected, epoxidation occurs preferentially at the bond and ring having less aromatic and more alkene character. To date arene oxide deriva- tives, of aromatic heterocycles e.g. 46 and 47, have only been obtained in racemic form and, therefore, their propensity to racemise or to retain configurational stability has not yet been examined.0m-_. The spherically symmetrical molecule C,, ([6O]fullerene) con- tains an array of fused unsaturated five- and six-membered rings. The superficial similarity of C,, to strained and activated members of the PAH series has led to extensive studies of its direct epoxidation by a range of oxidants including DMD21,22 and several model systems for monooxygenase enzymes (cytochrome P450).23 Thus, epoxidation of one of the thirty equivalent double bonds of [60]fullerene yielded 1,2-epoxy [60]fullerene 49 without evidence of the corresponding oxepine tautomer which would have a structure similar to that of annulene 322' Sequential epoxidation of [60]fullerene, to yield the monoepoxide 49, cis-diepoxide 50 and cis, cis-triepoxide 51, has been reported using the tetraphenylporphyrinatoiron(Ii1) chloride-iodosylbenzene P450 chemical model system.23 Although compounds 49-51 have some features similar to those of arene oxides in the PAH series, e.g.9, 18 and 19, the reactivity of C,, is closer to that of an arene K-region or an alkene. Multiple site epoxidation of C,, can only occur on the outer surface to yield cis-products, e.g. di-and tri-epoxy[60]fullerenes 50 and 51 respectively. This contrasts with the epoxidation of planar PAHs where epoxides are formed on either face to yield trans-arene dioxides, e.g. 19. The most widely used synthesis of PAH oxides involves the treat- ment of dibromoesters with base? Arene oxides 6: 34: 4315 and 4415were obtained in good yields and in enantiopure form using this method (Scheme 7, route A).Arene oxides 13,3and 143and THE CHANGING FACE OF ARENE OXIDE-OXEPINE CHEMISTRY-DEREK R BOYD AND NARAIN D SHARMA 38'2 were accompanied by the corresponding stable oxepine tautomers, e g 40, formed by competitive S,2' displacement (Scheme 7, route B) An alternative synthetic approach is based on the conversion of cis-diol precursors via dioxolane and chloroacetate intermediates, and offers significant advantages including allowing the arene oxide tautomer,e g triphenylene 1,2-oxide 38, to be obtained exclusively (Scheme 7, route C) 24 The availability of enantiopure cis-dihydro- diol metabolites of monocyclic and bicyclic arenes (e g bromoben-zene, naphthalene and quinoline), from dioxygenase-catalysed oxidation using mutant strains of the soil bacterium Pseudomonas putida, and their regioselective catalytic hydrogenation to the cor- responding cis-tetrahydrodiol enantiomers, has also facilitated the synthesis of arene oxide enantiomers Thus, naphthalene, 1,2-oxide 6, quinoline 5,6-oxide 43, and quinoline 7,8-oxide 44 were all obtained in enantiopure form from the corresponding cis-dihydro- diol precursors 24 The synthetic method (Scheme 7, route C) pro-vides a valuable link between the two major metabolic pathways for aromatic rings in nature Thus, the readily available initial metabo- lites of arenes (cis-dihydrodiols) in procaryotic systems (bacteria) can, in turn, become chiral precursors for the non-enzymatic syn- thesis of the elusive initial arene metabolites (arene oxides) in eucaryotic systems (plants, animals, and fungi) Racemic samples of arene oxides 38,52and 53 derived from tri- phenylene, benzol elpyrene and dibenzl a,c]anthracene respectively (previously unavailable without contamination by the correspond- ing oxepine isomers of structure similar to that of compound 40) were obtained in pure form from the appropriate cis-tetrahydrodiol precursors 24 The synthesis of racemic arene 2,3-oxides of mono- substituted benzenes, e g bromobenzene 2,3-oxide 25 from cis-2,3- dihydroxy-2,3-dihydrobromobenzene,has also been achieved by this approach9 (Scheme 8) 8' 8' jiii Br BrI IOo--oov 1v Scheme 8 Reagents I, H, Rh, A120,, 11, AcOCMe2COBr, 111, NaOMe, IV, NBS, v, DBU Preliminary have shown that the cis-dihydrodiol methods used in Schemes 7 and 8 are also applicable to the syn- thesis of 3,4-arene oxides of monosubstituted benzenes and to arene oxides of aromatic heterocycles For example, the cis-2,3-diol metabolite of benzothiophene can also be converted via the diox- olanexhloroacetate route to the corresponding benzothiophene 2,3-oxide 54 (Scheme 9) 2s The labile compound 54 was only detected in solution by IH NMR spectroscopy Since arene 2,3-oxides of benzothiophenes have not yet been synthesised via the direct (DMD) oxidation r~ute'~-~Oused for arene 2,3-oxides of the corresponding benzo- furan 46 and indole 47, the cis-diol route (Scheme 9) may therefore be complementary (54) Scheme 9 Reagents I, MeC(OMe),, 11, Me,SiCI-Et,N, 111, NaOMe The arene 2,3-oxides 46 and 47 have also been synthesised by photosensitized oxygenation of the corresponding benzofuran and indole precursors using singlet oxygen to yield the 2,3-dioxetanes 55 and 56 followed by partial deoxygenation using dimethyl sulfidei7l9 (Scheme 10) e e Me x=o X=NCOMe Scheme 10 Reagents 1, O,, 11, Me,S 4 Non-enzymatic and Enzymatic Reactions of Arene Oxides Amongst the more widely studied reactions of arene oxides are (1) isomerisation to yield oxepines, ketones and phenolic products, (rr) ring opening with nucleophiles and (iii) oxidation-reduction pro- cesses * Arene oxides are commonly found as mammalian metabo- lites of carbocyclic and heterocyclic arenes and, in this context, transformations (1)-(iii) are of considerable interest since their products may be linked to particular biological activity, eg cytotoxicity, mutagenicity and carcinogenicity of arenes The development of new synthetic routes to arene oxides from hetero- cyclic arenes,17-2024 and larger PAHs,23 has also extended the range of possibilities for enzyme-catalysed studies Spontaneous interconversion of arene oxide-oxepine tautomers has been observed in monocyclic and PAH systems (Section 2) Thus the oxepine tautomers, 20 and 21, of substituted 1,2- and 3,4- benzene oxides are generally preferred, whereas substituted benzene 2,3-oxides, e g 3, and most PAH arene oxides, e g 6,ll-17 and 38, exist almost exclusively in the arene oxide form (Schemes 1-3 and 6) Synthesis and spontaneous racemisation of the arene oxide 38 of triphenylene is consistent with equilibration via the undetected oxepine 39 The stable oxepine 40 was, however, formed upon exposure of arene oxide 38 to sunlight This facile photoisomerisation process has only been observed among arene oxides in the tetracyclic and larger members of the PAH series I R OH OH x=oJk=Ph(57) X=NCOMe,R=Me (58) Isomerisation of arene 2,3-oxides in the benzofuran and indole series has been reported to yield a series of products containing a carbonyl group Products isolated include lactone 5718and lactam 58*0 which involve migration of a methyl group from the C-2 to the C-3 position The migration of an atom (e g D or C1) or a group (eg Me or C0,Me) from one carbon atom and retention on an adjacent carbon during the enzyme-catalysed hydroxylation of an arene has been described as the 'NIH shift' (after its discovery at the National Institutes of Health, Bethesda, USA) (Scheme 11) To date more than 100 examples of the 'NIH shift' have been reported.2 This phenomenon has become widely associated with the monooxygenase-catalysed formation of arene oxides and their in situ isomerisation to ketodienes.Compounds 57 and 58 parallel the ketodiene products associated with the ‘NIH shift.’ The thermal or acid-catalysed isomerisation of specifically labelled samples of 1-and 2-deuterio-naphthalene oxides (6) and 5-,6-, 7-and 8-deu- terio-quinoline oxides (43, 44) to the corresponding phenols showed typical ‘NIH shift’ behaviour.26 Recent evidence26 sug- gests that the assumption of a link between arene oxide metabolites and the occurrence of the ‘NIH shift’ during enzyme-catalysed aro- matic hydroxylation may not be correct in all cases, particularly in the context of bacterial hydroxylations. An alternative explanation for the ‘NIH shift’ has been provided by results obtained from bac- terial hydroxylation of arenes and heteroarenes, where the pheno- lic metabolites have been obtained from the dioxygenase-catalysed oxidation of arenes to yield unstable cis-dihydrodiol metabolites which can readily dehydrate.Phenols resulting from dehydration of cis- and trans-dihydrodiols or from the isomerisation of arene oxides all showed similar evidence of the migration and retention of label, i.e. the ‘NIH shift’ (Scheme 11). D 0 OH O2 Monooqgenase1.dr“.Dioxygenase 12.70; Scheme 11 Aromatic hydroxylation and the ‘NIH shift’ Hydrolysis of mono-and poly-cyclic arene oxides to form trans- dihydrodiols is catalysed by epoxide hydrolase enzyme systems but, with the exception of K-region arene oxides, until relatively recently this could not be achieved by non-enzymatic methods.2 The 0bservation~3~ that trans-dihydrodiols 59 and 60 can be obtained by non-enzymatic hydrolysis of the corresponding stable non-K arene oxides of the aromatic heterocycles quinoline 43 and acridine 34 under a range of pH conditions is hence unusual.Arene oxide derivatives of the heterocyclic ring of benzofuran, e.g. 46, proved to be even more reactive and readily hydrolysed to an equi- librating mixture of cis-and trans-diols 61 and the isomeric acyclic ketones 6217(Scheme 12). tl ~ OH Scheme 12 Hydrolysis of benzofuran 2,3-oxide 46 Confirmation that 1,2-epoxy[60]fullerene 49 should be treated as an ‘abnormal’ arene oxide is provided by the unprecedented observation that it undergoes a thermal cycloaddition with [60]fullerene to give the novel structure 63(Scheme 13).28 Epoxidation of arene oxides with the neutral oxidizing agent DMD 45 occurs either in the same ring, e.g. to form arene dioxides 19 and 48, or in a more remote ring containing a K-region, e.g.to CHEMICAL SOCIETY REVIEWS, 1996 Scheme 13 Conditions: i, Heat (solid mixture), 6 h yield diarene oxides 64 and 65.4J6Evidence of enzyme-catal ysed arene oxide formation as the initial step in the ‘bay-region diol- epoxide pathway’ for the mammalian metabolism of PAHs is now widely accepted? Reports4J suggest that the initial arene oxide metabolites may also undergo monooxygenase-catalysed epoxida- tion to yield both arene dioxide, e.g. and diarene oxide, e.g. 65,29metabolites. Scheme 14 Reagents: i ,MCPBA; ii ,NaOCl The ‘bay-region diol-epoxide pathway’ for the mammalian metabolism of PAHs involves the metabolic sequence: arene -arene oxide -trans-dihydrodiol-vicinal diol epoxide, occurring in one arene ring proximate to a bay-region.* This bioactivation route has been confirmed for many members of the PAH series including the procarcinogens benzo[a]pyrene, benz[a]anthracene and benzo[c]phenanthrene.Recent reports,’ ,30however, have sug- gested that alternative metabolic pathways may involve the initial formation of an arene oxide followed by further epoxidation in a different ring of the PAH. Mammalian metabolism of the PAHs chrysene, benzo[a]pyrene, dibenz[aJz]anthracene and cyclo-penta[cd]pyrene thus appear to involve two independent epoxida- tion steps during the formation of the corresponding bis-arene oxide, trans-dihydrodiol monoarene oxide, phenol monoarene oxide, phenol trans-dihydrodiol , and bis-trans-dihydrodiol prod- ucts.I *30 Non-enzymatic oxidation of polycyclic azaarene oxides may result in competition between epoxidation or heteroatom oxidation, to yield either an arene dioxide or an N-oxide arene oxide, accord- ing to the choice of oxidant.Use of a peroxyacid resulted in epoxidation of the arene oxides 43 within the same ring to yield the trans-diarene oxide 66,while oxidation using NaOCl gave the N-oxide 673’ (Scheme 14). Although the epoxidation of arene oxide derivatives of PAHs is a relatively fast reaction, the epoxidation of monocyclic arene oxides-oxepines is much slower and attempts to isolate arene dioxides, arene trioxides, or oxepine epoxides have THE CHANGING FACE OF ARENE OXIDE-OXEPINE CHEMISTRY -DEREK R BOYD AND NARAIN D SHARMA generally been unsuccessful Peroxyacid epoxidation of benzene oxide-oxepine has been assumed to proceed by oxidation of the vinyl ether bond to yield an unstable oxepine epoxide 68 which readily opened up to yield the isomeric Z,Z-mucondialdehyde 69 * 0 .LCHO (70,R =ribose) Scheme 15 Reagent I, PhCO,H The reaction of mucondialdehyde isomers with cellular amines to yield cyclic adducts, e g compound 70 from reaction with guano- sine, is of interest in the context of the carcinogenicity of benzene3*33 (Scheme 15) It has been postulated that nucleophilic attack resulting in opening of the epoxide ring in the labile oxides 68 and 46, occur-ring in wvu,is an important step in the formation of DNA adducts, it may also contribute to the mutagenicitykarcinogenicity of benzene and furan derivatives 32-34 OH OH OH The reduction of benzene oxide using LiAIH, provides a conve- nient route to the racemic benzene hydrate 71 This method has been extended to the synthesis of enantiopure arene hydrates of PAHs, several (e g 72,73) of which have been reported as metabo- lites of dihydroarenes 3s To date, however, no evidence for the enzyme-catalysed reduction of arene oxide metabolites, to yield arene hydrates, has been reported Studies on the relative rates of aromatisation of arene oxides (phenol formation) and arene hydrates (dehydration) have shown that arene oxides appear to have an unexpected additional stability due to homoaromaticity 3h This enhanced stability of arene oxides is relevant to their formation and reactivity in biological systems 5 Summary Alternative and improved methods of synthesis have recently opened up new possibilities In arene oxide chemistry The syn- thesis of oxide derivatives of heterocyclic arenes both in the car- bocyclic (quinoline, isoqumoline, acridine) and heterocyclic rings (rndole, thiophene, furan), which had previously only been postu- lated as transient intermediates in metabolism, has finally been realised in the laboratory Some of these advances have only been possible with the development of powerful new oxidants such as dimethyldioxirane (which can effect direct oxidation of arenes under mild neutral conditions) Epoxide derivatives of [60]fulle- rene can also be produced by direct oxidation and these, in turn, provide entry into a new range of structures including linked C,, units The availability of cwdihydrodiol derivatives as chiral syn- thons from bacterial metabolism of mono- and poly-cyclic arenes has also provided a simple new approach to the synthesis of arene oxides The role of arene oxides in biological systems continues to attract attention The earlier focus upon two separate epoxidation steps, occurring within one PAH bay-region ring to yield diol epoxides, has been successfully linked to the mutagenic-carcinogenic prop- erties of PAHs Attention has also recently moved toward consideration of the biological consequences of monoepoxidation in two different arene rings and their derivatives While some progress has already been made in the synthesis of diarene oxides, further studies are required to develop regioselective routes to the putative phenol-arene oxide, and trans-dihydrodiol-arene oxide types of metabolites and to examine their biological properties Based upon current understanding of the molecular basis for haemotoxicity in benzene, it is anticipated that the enzyme-catal- ysed epoxidation of monocyclic arenes, to yield oxepines and their transient epoxide derivatives, will receive further attention The development of synthetic routes to the elusive oxepine epoxides would greatly facilitate such studies Enzyme-catalysed synthesis of arene oxides has arguably been in progress since the origin of life itself Despite their origins in remote antiquity, studies of the structure, chemistry, and biochemistry of these generally unstable compounds have only been carried out over the past thirty years The face of these chameleon-like mole- cules will undoubtedly continue to change and attract attention from both chemists and biologists well into the new millenium Acknowledgements We gratefully acknowledge financial support from the BBSRC (to N D S ) 6 References 1 G S Shirwaiker and M V Bhatt, Adv Heterocvcl Chem , 1984,37, 67 2 D R Boyd and D M Jerina.in Small Ring Heterocycles. 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ISSN:0306-0012    

DOI:10.1039/CS9962500289

出版商:RSC    

年代:1996

数据来源: RSC