Direct Observation of the Transition State John C
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Acc. Chem. Res. 1995, 28, 119—132 119 Direct Observation of the Transition State John C. Polanyi* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A1 Ahmed H. Zewail* Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology,f Pasadena, California 91125 Received December 14, 1994 I. Introducing the Transition State product. Consequently the intervals at which one needs to record its configuration and energy are ~1% The direct observation of the “transition state” for as great as that indicated above: in the range 10~14— chemical reaction merits a in this series clearly place HT13 s (10-100 fs). Observation of a labile intermedi- devoted to the Grails” of The transi- “Holy chemistry. ate in this time interval is, therefore, the task of the tion state is neither one chemical thing, namely, transition state spectroscopist. nor the reaction Instead it reagents, other, products. In the time domain, if the resolution is sufficiently illustrates the event of trans-substantiation. mystical short and the zero of time sufficiently well established, The time for the electronic of bond required process it is possible to clock the transition state in real time and bond formation would be rupture unmeasurably and to follow the nuclear motion. The key here is the short were it not for the fact that the electronic ability to localize the reactive system to a spatial transformations are substantial triggered by changes resolution of about 0.1 A or less. Such localization in internuclear This became evident in separation. refers quantally to a wave packet, a concept introduced the of mechanics when it was early years quantum in the of mechanics by Schrod- for the first to calculate the early days quantum possible, time, binding inger and Heisenberg. between an atomic as a function of energy pair, BC, The definition of the transition state used internuclear (TS) here, separation.1 namely the full family of configurations through which This seminal calculation was quantum rapidly the reacting particles evolve en route from reagents extended to the case of three interacting particles2 to products, may seem broad to those accustomed to such as one encounters for the simple exchange seeing the TS symbol, 4, displayed in solitary splendor reaction, at the crest of the energy barrier to reaction. It should A + BC —* AB + C (1) be recognized, however, that even if one restricts one’s interest to the overall rates of chemical reactions, one However, the step from the physics of AB and BC to a of the of intermediates the of ABC necessitated so requires knowledge family chemistry approximations collisions of different collision severe that the of and sampled by reagent empiricism Henry Eyring and The variational Michael was needed in order to render the energy, angle, impact parameter. Polanyi of reaction rates further extends the of first surface” (PES) for a chemical theory range “potential energy TS considerations extend reaction credible.3 of interest, quantum the and the concern with rates to The first PES for a reactive as range yet further, yield process, anticipated in states and since the time of showed the products specified quantum angles Arrhenius, reagents extends the most of all. A definition of approaching, crossing an energy barrier, and then requirements the TS that embraces the entire process of bond descending the far side of that barrier as the products as is therefore The whole from to breaking and bond making, used here, separated. trip reagents products to the most involved changes in internuclear separation totaling likely prove enduring. ~10 A. If the atoms moved at 104-105 cm/s, then the II. Characterizing the Transition State entire 10 A would take 10-12—10-11 s. If the trip Not transition state is defined to all surprisingly, spectroscopy (TSS) “transition state”, [ABC]4, encompass in the inverse domain of before ABC the began frequency, configurations of significantly perturbed from on to the time domain. It was also to be the A + BC or the moving potential energy of reagents products the nature of AB + then this of 1-10 is the time expected, given gradually evolving C, period ps that TSS would have its roots in a well- available for its observation. science, established field. At a time when the feasibility of TSS In fact, of course, the interest for the reaction was the subject of vigorous debate, it was remarked lies in the manner in which [ABC]4 evolves dynamicist that “the best sign that something is in reach in from a resembling reagent to one resembling species science is that, from a selected vantage point it can as been achieved”.4 Ref- John Polanyi received his training at Manchester University. Subsequently he already be regarded having was a postdoctoral fellow at the National Research Council in Ottawa (1952-1954) erence was being made to the fact that early in the and at Princeton University (1954-1956). He has been at the University of Toronto Lorentz had established that the broadening since that date, with scientific interests in reaction dynamics, transition state century spectroscopy, and surface-aligned photochemistry. In 1986 he shared the Nobel of the line spectrum of an atom A was due to the prize in chemistry with D. R. Herschbach and Y. T, Lee. + Ahmed Zewail received his B.S. from the University of Alexandria and his Ph.D. Contribution No. 9023 from Caltech. from the University of Pennsylvania. After two years of postdoctoral (IBM) fellowship (1) Heitler, H.; London, F. Z. Phys. 1927, 44, 455. at the University of California at Berkeley, he joined the faculty at Caltech, where he (2) London, F. Probl. Mod. Phys. (Sommerfeld Festschrift) 1928,104; holds the Linus Pauling Chair in Chemistry and Physics. His current scientific interests Z. Elektrochem. 1929, 35, 552. are in the development of ultrafast laser techniques and their applications in chemistry (3) Eyring, H.; Polanyi, M. Z. Phys. Chem. 1931, B12, 279. and biology. (4) Polanyi, J. C. Faraday Discuss. Chem. Soc. 1979, 67, 129. 0001-4842/95/0128-0119$09.00/0 © 1995 American Chemical Society Polanyi and Zewail the clocking of the evolution of the TS directly in the time domain. Not surprisingly, the young field of TSS involves a number of studies of alkali metal atoms. Previously alkali metals had provided a proving ground for collisional line broadening6 as well as for the study of simple exchange reactions.7 Accordingly, the first TSS experiments involved alkali metal atomic exchange reactions, studied at pressures that were orders of magnitude lower than those at which collisional pressure broadening would normally be observed. Nonetheless there was undoubted evidence of far-wing emission8 and absorption9 extending ~100 nm away from the alkali metal atom line center. This was indicative of the occurrence of a strong collision.6’10 The “collision” in question was that between the products of reactions 2 and 3 (see refs 8 and 9), F + Na2 — NaF + Na* (2) and K + NaCl + hv — KC1 + Na* (3) as illustrated schematically for reaction X + Na2 in Figure 1. Panels a and b show the origins of collisional line the lower panels of Figure 1. for colli- broadening in emission and absorption (respectively) In the case of reaction 2,8 wings to the red and the sions between sodium atoms and sodium halides; panels c and blue of the D-line evidenced themselves in emission d show the comparable phenomenon of “transition state spec- (see The of the troscopy” (TSS) in emission and absorption for the case in which Figure lc). integrated intensity wing sodium atoms and sodium halides are formed in the chemical emission was ~10~4 that of the sodium D-line emission exchange reaction X + Na2 -* NaX + Na* (reaction on the upper (lifetime ~ 10”8 s), giving a total lifetime for the potential, u) or alternatively X + Na2 — NaX + Na (reaction on [FNa2]* of ~10-12 s. For reaction 3,9 laser absorption the lower potential, 1). The symbols R, TS, and P indicate the by [KCINa]*, tens of nanometers to the red of the transition and the reaction reagent, state, product regions along sodium D-line and also a little to the blue, was detected coordinate. This figure is based on Figure 12 of ref 4. by recording the resulting D-line wavelength. The fleeting interaction of A with a collision partner B, laser absorption is shown in the more detailed render- yielding a sufficient concentration of what today we ing of Figure 2, based on the ab initio calculations of might call transition states [AB]* in order to interact Yamashita and Morokuma for this system.11 with light, i.e., give rise to TSS. A limitation in both of these early experiments, as Each frequency in the Lorentzian “wing” of the in line broadening in general, was the sparsity of atomic line corresponds to an internuclear separation structure in the wings. Early calculations10 had tab- Frequencies further removed from the line center shown that structure should be present, corresponding correlate in general with smaller tab- The longer (AB)* to the marked variation in density of TS species along remains at a given tab, the more intense the emission the reaction coordinate. In recent work12 on the or absorption at the corresponding frequency. Here, exchange reaction K + NaBr, Brooks’s laboratory has in 1915, was TSS in the frequency domain.5 The broad found clear evidence of structure centered 20 nm to wings, corresponding to a wide range of collisional the red of the sodium D-line, when one component of internuclear separations tab, observed with modern the line, D2 (2P3/2 — 2Si/2), is monitored but not when instrumentation give clear evidence of the possibilities Di is the product.