Transition-state spectroscopy

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

The above measurements are asymptotic , m that they involve looking at the products of reaction long after the collision has taken place. These very valuable experiments are now complemented by transition-state spectroscopy ... 

Figure A3.7.3. Principle of transition-state spectroscopy via negative-ion photodetaclunent.

The above discussion represents a necessarily brief simnnary of the aspects of chemical reaction dynamics. The theoretical focus of tliis field is concerned with the development of accurate potential energy surfaces and the calculation of scattering dynamics on these surfaces. Experimentally, much effort has been devoted to developing complementary asymptotic techniques for product characterization and frequency- and time-resolved teclmiques to study transition-state spectroscopy and dynamics. It is instructive to see what can be accomplished with all of these capabilities. Of all the benclunark reactions mentioned in section A3.7.2. the reaction F + H2 —> HE + H represents the best example of how theory and experiment can converge to yield a fairly complete picture of the dynamics of a chemical reaction. Thus, the remainder of this chapter focuses on this reaction as a case study in reaction dynamics. 

The transition-state spectroscopy experiment based on negative-ion photodetachment described above is well suited to the study of the F + FI2 reaction. The experiment is carried out tln-ough measurement of the photoelectron spectrum of the anion FH,. This species is calculated to be stable with a binding energy of... 

Neumark, D. M. (1992), Transition State Spectroscopy of Bimolecular Chemical Reactions, Ann. Rev. Phys. Chem. 43 153. 

General Discussion on Transition-State Spectroscopy and Photodissociation 809... 

A. H. Zewail If we solve for the molecular Hamiltonian, we will be theorists I do, of course, understand the point by Prof. Quack and the answer comes from the nature of the system and the experimental approach. For example, in elementary systems studied by femtosecond transition-state spectroscopy one can actually clock the motion and deduce the potentials. In complex systems we utilize a variety of template-state detection to examine the dynamics, and, like every other approach, you/we use a variety of input to reach the final answer. Solving the structure of a protein by X-ray diffraction may appear impossible, but by using a number of variant diffractions, such as the heavy atom, one obtains the final answer. 

GENERAL DISCUSSION ON TRANSITION STATE SPECTROSCOPY AND PHOTODISSOCIATION... 

Importantly, motion along the reaction coordinate at the saddle point has no restoring force, so the transition structure has only 3 N — 7 real vibrations. Indeed, passage through the saddle corresponds to conversion of the vibration along the reaction coordinate into a translation, so that ksT = hv, where v is frequency of this hypothetical vibration. At 25°C, v = 6 x 1012 s 1 equivalently, the lifetime of a transition structure is 1.7 x 10-13 s and methods capable of observing labile species on this time scale ( transition state spectroscopy ) have been developed, permitting many of the assumptions about their behaviour to be tested directly [1],... 

Although theoretical techniques for the characterization of resonance states advanced, the experimental search for reactive resonances has proven to be a much more difficult task [32-34], The extremely short lifetime of reactive resonances makes the direct observation of these species very challenging. In some reactions, transition state spectroscopy can be employed to study resonances through "half-collision experiments," where even very short-lived resonances may be detected as peaks in a Franck-Condon spectrum [35-38]. Neumark and coworkers [39] were able to assign peaks in the [IHI] photodetachment spectrum to resonance states for the neutral I+HI reaction. Unfortunately, transition state spectroscopy is not always feasible due to the absence of an appropriate Franck-Condon transition or due to practical limitations in the required level of energetic resolution. The direct study of reactive resonances in a full collision experiment, such as with a molecular beam apparatus, is the traditional and more usual environment to work. Unfortunately, observing resonance behavior in such experiments has proven to be exceedingly difficult. The heart of the problem is not a... 

Indirect photodissociation involves two more or less separate steps the absorption of the photon and the fragmentation of the excited complex. Resonances, which mirror the quasi-bound states of the intermediate complex in the upper electronic state, are the main features. They have an inherently quantum mechanical origin. If we consider — in very general terms — the inner region, before the fragments have obtained their identities, as the transition state, then the resolution of resonance structures in the absorption spectrum manifests transition state spectroscopy in the original sense of the word (Foth, Polanyi, and Telle 1982 Brooks 1988). 

Bowman, R.M., Dantus, M., and Zewail, A.H. (1990). Femtosecond transition-state spectroscopy of iodine From strongly bound to repulsive surface dynamics, Chem. Phys. Lett. 161, 297-302. 

Neumark, D.M. (1990). Transition state spectroscopy of hydrogen transfer reactions, in Electronic and Atomic Collisions Invited Papers of the XVIICPEAC, ed. A. Dalgamo, R.S. Freund, P. Koch, M.S. Lubell, and T.B. Lucatorto (American Institute of Physics, New York). 

Yamashita, K. and Morokuma, K. (1990). Ab initio study of transition state spectroscopy C1HC1- photodetachment spectrum, J. Chem. Phys. 93, 3716-3717. 

At the end of your Nobel lecture, as if projecting into the future, you mentioned two directions. One was transition-state spectroscopy and the other was surface-aligned photochemistry. That was nine years a o. Was this a prediction ... 

I was involved with both early on. In the case of transition-state spectroscopy, we had the idea that we could see the very short-lived intermediate between reagents and products by means of something like line broadening. The spectral line broadening would be due to the strong repulsion for a millionth of a millionth of a second seconds) between the pair of... 

The way new fields are born is as a consequence, first of all, of a surmise that there is something new that can now be done. In the case of transition-state spectroscopy, the surmise was that one could study the interaction of light with the very short-lived collocation of atoms that we are talking about here. These are subpicosecond collocations which constitute the successive intermediate configurations between reagents and products. As it turned out, it could be done in much better ways than were initially dreamed of. 

So much for transition-state spectroscopy, a field in which I am still enthusiastically involved. 

When you speuk about transition-state spectroscopy, it seems to me to have a close relationship to Michael Polanyi. 

That lasers have played a key role as promoters and as probes of chemical reactions is well known and extensively documented.1,4,7,62-72 In many of these applications the laser is employed as an intense, nearly monochromatic, light source whose characteristics ensure species selectivity, a well-characterized spectroscopy, and adequate intensity for multiphoton processes. Some possible applications, notably laser-assisted collisions 73,74 and transition-state spectroscopy,75,76 are yet in their infancy, but the extant studies already suggest considerable promise for influencing and probing chemical reactions. 

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