摘要:
Spectroscopy of reactive potential energy surfaces Daniel M. Neumark Department of Chemistry, University of California, Berkeley, CA 94720, USA, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. elsewhere. While most of these studies are pump-probe experiments with two laser pulses, success has recently been achieved using ultrafast electron diffraction6 as a probe, in which one can directly probe the geometries of the transient species produced by the pump pulse. The most successful frequency-domain TSS experiments have been the negative ion photodetachment experiments performed in our laboratory and by Lineberger and coworkers.7–9 In these experiments, one photodetaches a stable negative ion similar in structure to a neutral transition state, and the resulting photoelectron spectrum can yield resolved vibrational structure characteristic of the transition state.In many cases, these experiments have revealed vibrational frequencies characteristic of nuclear motion perpendicular to the reaction coordinate. These are clearly of interest, since, for example, these are the frequencies one would use in the calculation of the transition state theory rate constant of a chemical reaction. The intensity distribution in the photoelectron spectra is also of interest, as it reflects the change in geometry between the anion and neutral. A more elusive goal in TSS experiments has been the detection of dynamical resonances, loosely defined as levels of the transition state complex that are bound or quasi-bound along the reaction coordinate. Resonances have been predicted in reactive scattering calculations on model potential energy surfaces for many years,10 and are a very appealing target for TSS because they can be quite narrow and precisely located, thereby providing a particularly sensitive probe of the transition state.Thus far, however, the only TSS experiment that has definitively revealed the existence of resonances was the zero-electron kinetic energy (ZEKE) spectrum of IHI2,11 which showed clear evidence for quasi-bound IHI levels and not just the broader vibrational structure characteristic of motion perpendicular to the reaction coordinate. The only other definitive observation of reactive resonances to date comes from scattering experiments on the F 1 H2 and F 1 HD reactions by Liu and co-workers,12–14 in which structure in the total reaction cross section and rapid changes in the differential cross section as functions of collision energy provided an unambiguous signature of FH2 resonance states.There are several promising new directions in the area of TSS. In the time-domain, the development of increasingly shorter laser pulses15 and the recent implementation of timedependent photoelectron spectroscopy16 in several laboratories E-mail: neumark@cchem.berkeley.edu Received 4th March 2002, Accepted 25th March 2002 Published on the Web 2nd April 2002 This perspective reviews experiments in which spectroscopy rather than scattering is used to probe reactive potential energy surfaces.The application of negative ion photodetachment to the transition state spectroscopy of benchmark reactions is described, followed by a brief description of recent transition state spectroscopy experiment starting from clustered precursor anions that probe the effects of solvation on transition state spectroscopy and dynamics. Experiments on the spectroscopy of open-shell ‘‘pre-reactive’’ complexes are also discussed. Introduction The concept of potential energy surfaces is central to the field of reaction dynamics. Potential energy surfaces describe the interactions between reacting species and, in the absence of non-adiabatic effects, the development of an accurate potential energy surface is equivalent to mapping out the microscopic, interatomic forces that govern a particular chemical reaction.The process by which a potential energy surface is developed generally involves a close interaction between scattering experiments and theory, in which experimental quantities such as rate constants, angular distributions, translational and internal energy distributions, and angular momentum polarization are compared to scattering calculations on model potential energy surfaces that are determined through some combination of ab initio electronic structure calculations and empirical corrections. In this case, one is dealing with asymptotic properties of the reaction, through measurement and calculation of the reaction attributes long after the products have separated and ceased to interact with one another.An alternative and conceptually appealing approach is to develop spectroscopic probes of the transition state region, the region of the potential energy surface in which chemical bonds are broken and formed.1–3 Kinetics and scattering experiments are of course sensitive to the attributes of the transition state region, but direct, spectroscopic probes of the transition state can, at the very least, complement scattering experiments, and often yield much richer detail through the determination of the geometry, vibrational frequencies and lifetime of the transition state complex. This perspective summarizes past efforts to characterize the spectroscopy of the transition state and includes some of the more recent and likely future directions in this area. Esssentially all successful transition state spectroscopy (TSS) experiments performed to date are ‘‘half-collision’’ experiments in which the transition state is accessed through photoexcitation of a stable transition state precursor, such as a closed shell molecule, a van der Waals complex, or a negative ion.These experiments can be divided into time-domain and frequencydomain studies. Since the transition state by its nature is short-lived, with passage through a transition state typically occurring on a time scale of 10–100 fs, experiments with ultrafast lasers would seem to be a natural way to probe reacting or dissociating systems, and this has indeed been the focus of ‘‘femtochemistry’’ experiments in the Zewail laboratory4,5 and 76 PhysChemComm, 2002, 5(11), 76–81 This journal is # The Royal Society of Chemistry 2002 Perspective DOI: 10.1039/b202218d2 2 2 Fig.1 Photodetachment of FH probes the F 1 H2 transition state region. The ground state vibrational wavefunction for FH2 (shaded region) is superimposed on a model collinear potential energy surface for F 1 H2 reaction. The saddle point on the reactive surface is marked with an X. (Adapted from ref. 34). holds considerable promise for probing more rapid transition state dynamics in greater detail, as does further development of ultrafast electron and X-ray light sources.17,18 Negative ion photodetachment on mass-selected cluster anions has been used recently to probe the effects of solvation on transition state spectroscopy and dynamics,19,20 and as such provides a key link between gas phase and condensed phase reaction dynamics.Finally, the recent observation of pre-reactive van der Waals states of the OH ? H2 and OH ? CO complexes21,22 points out the importance of studying a relatively neglected region of reactive potential energy surfaces, the shallow van der Waals well between reactants. The interplay between this well and the transition state may have a significant effect on transition state geometry as well as the overall reaction dynamics, so characterization of this well may be of comparable importance to the transition state itself.The main body of this perspective focuses on TSS using negative ion photodetachment. Results on several benchmark bimolecular reactions are summarized, followed by a description of recent work on TSS in cluster anions and possible future experiments on reactant van der Waals wells. 2 TSS of benchmark chemical reactions The promise of TSS using negative ion photodetachment is exemplified by our studies of transition states for the F 1 H and OH 1 H2 reactions by photoelectron spectroscopy of Fig. 2 Solid lines are experimental photoelectron spectra of F2 ? para-H2 (left) and F2 ? n-H2 (right). Dashed lines are exact simulations using anion surface from ref. 32 and F 1 H2 surface from ref. 33. The peaks around 1 eV are a progression in FH2 internal rotor states, while the smaller peaks around 0.5 eV involve H2 stretch excitation.(Adapted from ref. 35). 2 2 2. FH 3O2, respectively. These are benchmark reactions and H that have been studied extensively by experiment and theory. The F 1 H2 reaction, in particular, has defined the state-of-theart in experimental reactive scattering studies, with increasingly sophisticated crossed molecular beams experiments yielding progressively more detail on the dependence of the integral and differential cross sections on collision energy, the partitioning of product translational, vibrational, and rotational energy, and the possible reactivity of spin-orbit excited F atoms.12,13,23–29 Although some features of the differential cross section, namely state-selected forward scattering of the HF(v ~ 3) product, were originally attributed to dynamical resonances,23 these features could be largely reproduced by classical trajectory calculations30,31 on potential energy surfaces with bent as opposed to linear transition states.We therefore hoped to address the issue of the FH2 bend potential by photoelectron spectroscopy of FH2 2 2 2 2 As shown in Fig. 1, this reaction is ideal from the TSS perspective, because electronic structure calculations indicate that the geometry of FH 2 is similar to that of the FH2 transition state;32,33 in both, the distance from the F atom the H2 center-of-mass is relatively long, and the H–H bond distance is similar to that in diatomic H2.These geometries reflect the fact that FH is essentially F2 weakly bound to H2 (D0$0.20 eV), while the F 1 H2 reaction has an early barrier. The calculations indicate FH2 is linear, so the existence or extent of bend progressions in the anion photoelectron spectrum would provide a direct probe of the bend potential at the neutral transition state. 2 The FH2 photoelectron spectrum34,35 shown in Fig. 2 indeed shows progressions in the two vibrational modes perpendicular to the reaction coordinate: the H–H stretch and the F–H–H bend, which is more like an H2 internal rotor. The bend progression is quite extended, indicating that the FH2 transition state is bent rather than linear. This conclusion is supported by quantum mechanical simulations of the spectrum performed on a high level potential energy surface for the F 1 H2 reaction.35 This system thus represents an example in which TSS resolved a key issue in a benchmark chemical reaction.2 The FH 2 photoelectron spectrum shows no clear evidence for resonances, i.e. states of the FH2 complex bound along as opposed to perpendicular to the reaction coordinate. The 77 PhysChemComm, 2002, 5(11), 76–81signature for resonances would be additional, low frequency vibrational progressions corresponding to either F ?? H2 or HF ?? H vibrational motion, depending on whether the resonance wavefunction is localized on reactant or product region of the potential energy surface. Calculations by Manolopoulos36 indicate that these progressions should be observable at higher resolution, and experiments of this type are planned in our laboratory in the near future.2 While the F 1 H2 reaction is one of several three-atom benchmark reactions, along with H 1 H2, O 1 H2, and Cl 1 H2, the OH 1 H2 A H2O 1 H reaction is unique as a prototypical four atom reaction, since the presence of three hydrogen atoms makes both electronic structure calculations of potential energy surfaces and the execution of scattering calculations on these surfaces tractable. As a consequence, high quality surfaces have been constructed and scattering calculations on these surfaces have been performed by several groups.37–41 Scattering experiments by Casavecchia37 and Davis42 have yielded product angular distributions and detailed vibrational energy distributions for the H2O product.These experiments are complemented by studies of the OH ? H van der Waals complex by Lester and co-workers.21,43 TSS of the OH 1 H2 reaction via photoelectron spectroscopy of H3O2 is complicated by the existence of two anion structures:44–47 H2 ? H2O, which has good Franck–Condon overlap with the neutral H2O 1 H product valley, and OH2 ? H2, which overlaps the neutral transition state. As shown in Fig. 3, the H2 ? H2O structure is calculated to lie y0.08 eV lower in energy, which might appear problematic from the perspective of TSS. Nonetheless, the anion photoelectron spectrum48 was quite revealing. The top spectrum in Fig. 4 is dominated by photodetachment from H2 ? H2O, showing a resolved vibrational progression in which the peak spacing was slightly lower than the H2O antisymmetric stretch frequency.This progression was assigned to transitions to the H 1 H2O exit valley, with the lowered frequency a signature of non-negligible interaction between the separating products in the Franck–Condon region. An even lower frequency progression was seen in simulations48,49 on the best surface available at the time of the experiment.50,51 This discrepancy indicates that the barrier on the model surface was too ‘‘late’’ along the reaction coordinate (i.e. the OH ? H2 distance is too small at the saddle point), resulting in an overly large effect on the peak spacings in the Franck–Condon region.In fact, the saddle points on more recent OH 1 H2 surfaces occur at larger OH ?? H2 distances (by 0.14 A°, on average39), and a simulation of the H2 ? H2O photoelectron spectrum on one of these new surfaces47 is in better agreement with experiment. Hence, even though the Fig. 3 Reaction coordinates for OH2 1 H2 and OH 1 H2 reactions, showing energetics of the H2(H2O) and OH2(H2) minima. (Adapted from ref. 48). 78 PhysChemComm, 2002, 5(11), 76–81 Fig. 4 Photoelectron spectra of H3O2 at laser polarization angles h ~ 0u (top) and 90u (bottom). Top spectrum is dominated by photodetachment of H2(H2O) structure. Vibrational level spacings of free H2O are shown in parentheses (n1n2n3). Broad peak in lower spectrum around 2.1 eV is from OH2(H2) structure.(Adapted from ref. 48). H2 ? H2O vibrational wavefunction has little or no overlap with the OH 1 H2 saddle point, the nature of the vibrational structure in the photoelectron spectrum is quite sensitive to the location of the saddle point and therefore provides important information regarding the transition state of the reaction. 2 structure? The H3O2 photoelec-2 What about the OH2 ? H tron spectra show a strong angular dependence, with a broad feature appearing at high electron binding energy (bottom spectrum, Fig. 4) when the laser polarization angle is perpendicular to the direction of photoelectron detection. This feature becomes more intense when the ion source temperature is raised, suggesting it might be from the higher energy OH2 ? H2 structure and therefore corresponding to photodetachment to the OH 1 H2 transition state.We constructed a two-dimensional potential energy surface for the H3O2 anion and determined the 2-D vibrational wavefunctions and energy levels supported by this surface. While the v ~ 0 wavefunction was localized in the H2 ? H2O well, as expected, the v ~ 2 wavefunction showed significant amplitude in the OH2 ? H well, and simulations of the photoelectron spectrum from the v ~ 2 wavefunction approximately reproduced the broad experimental feature. Hence, vibrational excitation of the anion results in considerably better overlap with the OH 1 H2 entrance valley and transition state, a result supported in more sophisticated simulations of the photoelectron spectrum.47,49,52 2 It also appears that vibrational excitation of H3O2 results in a marked change in the photoelectron angular distribution, a ather unusual observation in negative ion photoelectron spectroscopy.This effect can be understood in terms of the nature of the orbital from which detachment occurs; near threshold, H2 undergoes p-wave detachment, because an s-electron is being detached,53,54 whereas OH2 undergoes s-wave detachment. The photoelectron angular distributions therefore reflect the dominance of the H2 ? H2O structure in the v ~ 0 wavefunction and the dominance of the OH2 ? H structure in the v ~ 2 wavefunction.Transition state spectroscopy of clustered TS precursors One of the more intriguing new directions for TSS via anion photoelectron spectroscopy is the investigation of clustered transition state precursors.It is straightforward to generate mass-selected cluster anions of the type ABC2(Sn), where ABC2 is a transition state precursor anion and S is a solvent atom or molecule. The photoelectron spectroscopy of clusters of this type can probe the effects of solvation on the neutral transition state spectroscopy and dynamics, thereby providing an important link between gas phase and condensed phase reaction dynamics. We have recently initiated experiments of this type using the bihalide transition state precursors BrHI2 and IHI2 clustered to one or more Ar atoms.19,20 Photodetachment of the bare ions accesses the transition state region of the Br 1 HI and I 1 HI heavy 1 light-heavy reactions, and the photoelectron spectra of these anions55,56 are dominated by progressions in the antisymmetric H-atom stretch. In addition, the higher resolution ZEKE spectrum of IHI2 showed lower frequency progressions in IHI hindered rotor states and symmetric stretch states;11 the latter represent a definitive observation of reaction resonances as defined above.In any case, the rich vibrational structure seen for both bihalide species provides a suitable template for determining the effects of solvation. Our PE spectra of BrHI2(Ar) and IHI2(Ar), displayed in Fig. 5, showed that even a single weakly bound solvent species such as Ar induced significant changes in the photoelectron spectrum of the bare anion.20 These changes are primarily due to cooling effects, since a cluster in which an Ar atom is clustered to a vibrationally excited bihalide anion will undergo predissociation before it passes through our mass spectrometer, leaving only those clusters in which the anion chromophore is vibrationally cold; similar effects have been seen in the PE spectrum of I22(Ar) and in the infrared spectrum of clustered halide anions.57,58 As a consequence, the vibrational features in the BrHI2(Ar) PE spectrum are considerably narrower than in the BrHI2 spectrum,56 enabling a more detailed comparison with theory.While the bare IHI2 spectrum shows a wellresolved progression in the relatively high frequency IHI antisymmetric stretch,55 the IHI2(Ar) PE spectrum shows additional structure from progressions in low frequency hindered rotor states of the IHI complex.Fig. 5 Effect of complexation of single Ar atom on photoelectron spectra of IHI2 (a) and BrHI2 (b). Addition of an Ar atom results in narrower peaks and, for IHI2, the observation of additional fine structure from progressions in IHI hindered rotor states. (Adapted from ref. 20). 2 The PE spectra of IHI2(Ar)n (n ~ 1–15) clusters, shown in Fig. 6, exhibit several trends that reflect the effects of solvation on the anion and the neutral, and sorting these out is perhaps the most challenging aspect in the interpretation of the spectra.19 We observe shifts with EA as a function of cluster size that are very similar to those seen for I 2(Ar)n clusters,57 implying that the first six Ar atoms cluster around the waist of the IHI2, where they can interact with the partial negative charges on the two I atoms.Subsequent Ar atoms bind primarily to a single I atom. We also observe a change in the spacing of the IHI antisymmetric stretch frequency as a function of cluster size, an effect attributed to solvent-induced distortion of the IHI2 geometry. However, the most intriguing effect is that the low-frequency hindered rotor structure seen in the binary IHI2(Ar) cluster becomes more pronounced for the largest clusters we studied, an unusual result since clustering generally obscures vibrational structure in photoelectron spectra.This effect is attributed to caging of the neutral IHI complex by the surrounding Ar atoms, resulting in a longer lifetime and hence sharper structure than seen in the smaller clusters. It represents a remarkable demonstration of the effect of clustering on transition state dynamics, with clear analogies to the caging of photoexcited chromophore molecules such as I2 and I22 in cryogenic matrices and clusters seen in several laboratories.59,60 The Ar cluster experiments also provided evidence for an additional structural isomer of the form I2(Ar)nHI,19 suggesting that even a weakly interacting solvent species such as Ar can result in structural perturbations of the anion chromophore. These results point the way toward future studies aimed at the effects of more strongly solvating species such as water on the photoelectron spectrum of symmetric (XHX2) and asymmetric (XHY2) bihalides.Photoelectron spectra of sizeselected anions XHY(X)2 ? (H2O)n will provide a novel probe of the effects of hydration in the negative ion. As water molecules are added, we expect vibrational structure to reflect structural changes induced by hydration. We might, for example, expect the first few water molecules to asymmetrically hydrate one of the halogen atoms, resulting in partial localization of the charge on the more hydrated atom in analogy to predicted and observed effects in I22(CO2)n and I22(Ar)n clusters,61–64 with the addition of further waters possibly resulting in dissociation of the bihalide within the cluster to a hydrated halide anion clustered to HX.79 PhysChemComm, 2002, 5(11), 76–81Fig. 6 Photoelectron spectra of IHI2(Ar)n, n ¡ 15. (Adapted from ref. 19). Probing the van der Waals reactant well 2 Transition state spectroscopy is most generally defined as an experiment that yields spectroscopic information on the reaction complex, as opposed to separated products or reactants. The conventional wisdom in reaction dynamics is that a successful transition state spectroscopy experiment is one that probes the classical transition state, e.g. the barrier region for a direct reaction. However, for any direct reaction between neutral species, one expects shallow wells along the reaction coordinate on either side of the barrier due to the attractive van der Waals interaction between reactants or products.These wells have often been used to launch bimolecular reactions through either electronic excitation65 or photodissociation66,67 of one of the reactants constituting the complex. For example, photoexcitation of the metal atom in HgH2 or NaFH complexes initiates an excited state chemical reaction,68,69 and by probing the product yield as a function of excitation energy one can essentially perform transition state spectroscopy of the excited state potential energy surface. The possible role of vdW wells on ground state dynamics is an issue of considerable interest. Even though these wells occur at considerably larger internuclear distances than are characteristic of transition states, they can play an important role in the ground state reaction dynamics. For example, recent experiments and calculations on Cl 1 HD by Liu and coworkers70 showed that the presence of a reactant vdW well was necessary to reproduce the experimental HCl : DCl product branching ratio.This effect arises because the vdW minimum is T-shaped, whereas the transition state geometry is linear, so the shallow vdW well results in a torque on the reactants en route to the transition state. In the F 1 H2 reaction, the vdW well is also T-shaped,33 and occurs at only a slightly larger F ?? H distance than the transition state geometry. The barrier for F 1 H2 is much lower than that for Cl 1 H2, 1.9 kcal mol21 vs.7.6 kcal mol21,71,72 so one can speculate that the bent nature of the transition state in F 1 H2 arises in part from its 80 PhysChemComm, 2002, 5(11), 76–81 proximity to the T-shaped well. Hence, experiments that probe the spectroscopy of ground state complexes between reactants may be quite important for gaining a global picture of the corresponding reaction. The reactant vdW well has been probed in an elegant series of experiments by Lester and co-workers.21,22 In these studies, open-shell, ‘‘pre-reactive’’ vdW complexes such as OH ? H2 and OH ? CO are formed in a free jet expansion, and their infrared and Raman predissociation spectra are measured by action spectroscopy through detection of the OH radical. These complexes lie to the reactant side of small barriers for the OH 1 H2 and OH 1 CO reactions.One question underlying these studies is whether vibrational excitation of the complexes can actually drive the chemical reaction; there is strong indirect evidence for this, although reaction products have yet to be observed experimentally. Regardless, these rotationally-resolved spectra provide a detailed probe of the van der Waals well and therefore provide new experimental insight into what now appears to be a very important region of the reactive potential energy surface. Experiments of this type can also be carried out with nega- 2 2 2 tive ion photoelectron spectroscopy, so long as there is good overlap between the anion and neutral vdW well.This condition favors reactions with relatively ‘‘late’’ barriers; otherwise the reactants will be too far apart in the vdW well to be accessed by photodetachment. 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ISSN:1460-2733
DOI:10.1039/b202218d
出版商:RSC
年代:2002
数据来源: RSC