Tracking the energy flow along the reaction path

Abstract

We report a comprehensive study of the quantum-state correlation property of product pairs from reactions of chlorine atoms with both the ground-state and the CH stretch-excited CHD 3. In light of available ab initio theoretical results, this set of experimental data provides a conceptual framework to visualize the energy-flow pattern along the reaction path, to classify the activity of different vibrational modes in a reactive encounter, to gain deeper insight into the concept of vibrational adiabaticity, and to elucidate the intermode coupling in the transition-state region. This exploratory approach not only opens up an avenue to understand polyatomic reaction dynamics, even for motions at the molecular level in the fleeting transition-state region, but it also leads to a generalization of Polanyi's rules to reactions involving a polyatomic molecule. Over the past decades, there has been tremendous progress in experimental characterization of the structure of the transition state, notably by using the spectroscopic probes (2-4). Transition-state spectroscopy experiments performed to date are essentially the half-collision type in which the transition state is directly accessed either through photodetachment of negative ion precursor in a frequency-resolved experiment (3) or by the femtosecond pump-probe, time-resolved approach (4). As elegant and informative as those experiments are, half-collision results, in general, do not depict a full picture of how the reactants transform into the products. One way to think of this is as follows. The basic idea of a typical half-collision experiment is to initiate the reaction at transition state by a photoexcitation process. By virtue of photoabsorption, the total angular momentum, that is, the partial wave or the impact parameter, of the reactive system is then well specified and often limited to the lowest few quantum numbers in a restricted geometry of the Franck-Condon region. Consequently, the half-collision results are greatly simplified and more amenable to theoretical tests. In contrast, a chemical reaction inevitably constitutes the contribution from collisions with a full range of impact parameters and orientations. The resultant wave-interference patterns, arising from the coherent sum of scattering amplitudes of many partial waves, are manifested in the full-collision attribute such as product angular distribution (5, 6), which cannot be readily accounted for by the few-partial-wave, half-collision approach. On the horns of a dilemma, a full-collision experiment usually deals with asymptotic properties of the reaction, thereby rendering direct probes of the fleeting transition state difficult.Here, we propose an approach to delineate the dynamical aspects of the transition state in a full-collision experiment by tracking the energy flow along the reaction path. We previously introduced an experimental method to unfold the state-specific correlation of coincident product pairs in polyatomic reactions (7-9). More recently, we exploited the product pair-correl...

Relative efficacy of vibrational vs. translational excitation in promoting atom-diatom reactivity: Rigorous examination of Polanyi's rules and proposition of sudden vector projection (SVP) model

“…70 At this point, it might be interesting to comment on the vibrationally adiabatic model based on the reaction path Hamiltonian 71 that has been widely used for understanding mode selectivity, particularly in polyatomic systems. [72][73][74][75][76][77] The basic idea is to assume that the reaction follows an adiabatic reaction path. The enhancement of reactivity by exciting a particular vibrational mode in the reactant is interpreted as the lowering of the vibrationally adiabatic barrier relative to that for the ground vibrational state, stemming from the socalled mode softening near the transition state as the reactants approach each other.…”

Rotational mode specificity in the Cl + CHD3 → HCl + CD3 reaction

“…[14][15][16][17][18][19][20][21][22][23] These studies have stimulated several new models to understand the energy flow and mode specificity in polyatomic reactions. 21,24,25 Despite the tremendous progress, there have been very few experimental studies on the effect of reactant rotation on reactivity at a quantum state resolved level. [26][27][28] On the a) Authors to whom correspondence should be addressed.…”

Communication: Imaging the effects of the antisymmetric-stretching excitation in the O(3P) + CH4(v3 = 1) reaction

“…In this regard, the propensity toward translational energy at low collision energies can also be found in many other activated reactions. [29][30][31][35][36][37][38][39][40][41][42][43][44] On intuitive grounds, a finite momentum of the two reactants seems necessary in order to drive reactions over barriers. If correct, a translational propensity in the post-threshold region of activated reactions may well be a general trait; regardless it is an early-, central-, or late-barrier reaction.…”