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Transition State Spectroscopy Ated with Vibrational Levels of the Transition State for Unimolecular Dissociation (4)

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Transition State Spectroscopy Ated with Vibrational Levels of the Transition State for Unimolecular Dissociation (4) -PERSPECTIVES energy showed clear step-structure associ- Transition State Spectroscopy ated with vibrational levels of the transition state for unimolecular dissociation (4). Another frequency-resolved method for Daniel M. Neumark probing the transition state is to use a stable negative ion as the transition state precursor and to photodetach order to access the tran- sition state on the ground-state potential One of the most useful concepts in under- tured in the figure. In these experiments, the energy surface for a neutral reaction (5). The standing chemical reactivity is the idea of transition state is accessed by photoexcitation overall process is AB- h> ABt + e-, where the transition state. When two reactants un- of a suitably chosen stable precursor, which the energy hv is typically from a visible or dergo a chemical reaction to form products, can be the ground state of a molecule, a van ultraviolet photon. If the negative ion and new chemical bonds are formed while others der Waals complex, or a negative ion. Once neutral transition state have similar geom- are broken; one can therefore imagine a mo- on the uppersurface, the reactants pass through etries, then the photoelectron spectrum of lecular configuration intermediate between the transition state en route to products, and the anion can reveal vibrational structure reactants and products, and this is what chem- a suitable probe ofthe dynamics on the upper associated with the neutral transition state. ists refer to as the transition state. The tran- state can yield both spectroscopic and dy- The photodetachment method has been sition state often occurs at the top ofa poten- namical information on the transition state. used with considerable success to study tran- tial energy barrier and acts as a bottleneck Two general types of experiments of this sition states for bimolecular reactions. For in a chemical reaction. The properties of example, the geometries of the FH2- anion the transition state therefore determine the and FH2t transition state for the F + H2 reac- reaction rate, as well as many of the more tion are quite close. The photoelectron spec- Downloaded from detailed attributes that one measures in scat- trum of FH2§ shows an extended progression tering experiments. Thus, the experimental in a vibrational mode ofFH2 transition state, characterization ofthe transition state through and from the peak spacings, it is clear that "transition state spectroscopy" is one of the this is a progression in the FH2* bending grand challenges in the field of physical mode (6). This observation provides direct chemistry (1). There has been considerable evidence that the FH2t transition state is http://science.sciencemag.org/ progress in this area during the last 10 years, bent, thereby settling a long-running contro- but the paper by Wenthold et al. in this issue hv versy concerning the F + H2 reaction. More- (2) on the transition state for inversion in over, simulations of the FH2- photoelectron cyclooctatetraene (C8H8) represents a quali- spectrum using the most recently developed tative jump in the complexity of reactions potential energy surface for the F + H2 reac- accessible to transition state spectroscopy. tion are in excellent agreement with the ex- In terms of the potential energy surface periment, showing that this new surface is for a chemical reaction, the transition state is accurate in the vicinity ofthe transition state. typically located at a saddle point (see figure) 'J In the work by Wenthold et al., photo- between reactants and products. At the tran- -.. MW detachment of the cyclooctatetraene anion, sition state for a reaction involving N atoms, C8H8-, probes the transition state for the on May 17, 2018 the reaction complex is unstable along one In transition. Absorption of light hv excites a bound species to the transition state of a poten- ring inversion in neutral C8H8. The anion is coordinate (the reaction coordinate) and tial energy surface for a chemical reaction. The planar, whereas the neutral molecule has bound along the other 3N-5 coordinates. El- transition state lies at the saddle point between four equivalent nonplanar minimum-energy ementary transition state theory tells us that reactants and products. structures separated by relatively low (-1 the rate constant for a chemical reaction de- eV) energy pathways [see figure 1 of (2)]. pends on the transition state geometry, the type have been carried out. In time-resolved The minimum-energy path to get from one barrier height at the transition state, and the experiments (3), the transition state precur- structure to another involves passage through frequencies of the vibrational modes along sor is photoexcited with an ultrafast (-100 a planar transition state; this transition state its bound coordinates. These are the charac- fs) laser pulse, and a second ultrafast pulse has good geometric overlap with the nega- teristics of the transition state that one probes the dynamics on the upper potential tive ion and is therefore accessed by photo- would like to extract from a transition state energy surface. Typically, one monitors the detachment of the anion. spectroscopy experiment. appearance of products on the upper surface In spite of the large number of atoms, the The evanescent nature of the transition as a function of delay time between the laser photoelectron spectrum shows resolved vi- state makes it particularly challenging to study pulses; this is sensitive to the slope and cur- brational structure. This structure is assigned experimentally; during the course of a bi- vature of the reaction coordinate in the vi- to the "bond-alternation" vibration in which or unimolecular reaction, passage through cinity of the transition state. In frequency- the four single and double C-C bonds in the the transition state occurs on a time scale of resolved experiments, the transition state planar geometry ofC8H8 synchronously switch 10-13 to 10-14 s. Whereas the first attempts at precursor is excited with a relatively long position with one another; the net result is a transition state spectroscopy focused on prob- pulse (several nanoseconds), and one "pseudorotation" in which the molecule ap- ing the transition state formed during a full bi- searches for spectral features associated with pears to have rotated by 450 [see figure 1 of molecular collision, considerably more success the transition state, particularly vibrational (2)]. This vibrational mode is clearly associ- has been achieved by use of the basic idea pic- motion along the bound coordinates perpen- ated with the planar transition state; its char- dicular to the reaction coordinate. For ex- acter is completely different in the nonplanar The author is in the Department of Chemistry, University ample, in a study of ketene (H2C20) pho- equilibrium geometry of C8H8. Hence, as in of California, Berkeley, CA 94720, USA. E-mail: tolysis to make triplet CH2 + CO, the disso- the bimolecular photodetachment experi- [email protected] ciation rate k(E) as a function of excitation ments, one is observing vibrational motion 1446 SCIENCE * VOL. 272 * 7 JUNE 1996 along a bound coordinate at or near the tran- References for liquid water on ancient Mars. sition state geometry. It is remarkable that 1. J. C. Polanyi and A. H. Zewail, Acc. Chem. Res. The earliest history of Mars, before 4.0 only a single such progression is seen in 28,119(1995). billion years ago, is important in itselfand as Wenthold's spectrum; this is the result ofthe 2. P. G. Wenthold et al., Science 272, 1456 (1996). a proxy for Earth's earliest history; little is high symmetry of the anion and neutral spe- 3. A. H. Zewail, J. Chem. Phys. 87, 2395 (1987); S. known of this on Earth because no Pedersen. J. L. Herek, A. H. Zewail, Science 266, epoch cies. Overall, the paper represents an impor- 1359 (1994). rocks from that time are preserved. So, it was tant expansion of transition state spectros- 4. E. R. Lovejoy, S. K. Kim, C. B. Moore, Science exciting when Rb-Sr and Sm-Nd radio-iso- copy from the type of model chemical reac- 256,1541 (1992). tope studies showed that the ALH84001 tions usually studied by chemical physicists 5. R. B. Metz, T. Kitsopoulos, A. Weaver, D. M. Neumark, J. Chem. Phys. 88,1463 (1988). martian meteorite formed 4.5 billion years to much more complex species of interest to 6. D. E. Manolopoulos et al., Science 262, 1852 ago, and therefore must be from Mars's most the wider chemistry community. (1993). ancient terrane, the southern highlands. The K-Ar radio-isotope system yields a younger age of 4.0 billion years, which probably rep- resents impact metamorphism (5). These To See a World ages are comparable to the oldest crystalliza- tion ages and the most common impact ages in 80 Kilograms of Rock found in lunar highlands rocks, further sug- gesting a common history of planet forma- tion and impact cratering. It seems likely Allan Treiman that early Earth shared these experiences, but it is not clear that ALH84001, a pyroxen- ite, sheds any light on the nature of Earth's Downloaded from early crust. Of the more than 7500 known meteorites, lion years ago, so their water-deposited Four ofthe martian meteorites, the Chas- only 12 are certain to have come from Mars. grains must be younger still. The most abun- signy and the nakhlites, provide a strong As the only available samples of Mars, they dant of the alterations are veinlets of clays, constraint on Mars's very early history. have become the basis for much of what is hydrous iron oxides, and salt minerals in These meteorites contain excesses ofthe iso- known (or hypothesized) about Mars (1). the Lafayette meteorite. These veinlets tope 142Nd, which arose from a decay of http://science.sciencemag.org/ In one sense, the martian meteorites are formed from reaction between the rock and '46sm (half-life, 103 million years).
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