Theoretical Study of the Validity of the Polanyi Rules for the Late-Barrier Cl + CHD₃ Reaction

Abstract

The Polanyi rules, which state that vibrational energy is more efficient in promoting a late-barrier reaction than translational energy, were questioned recently by an experimental unexpected finding that the CH stretch excitation is no more effective in promoting the late-barrier Cl + CHD 3 reaction than the translational energy. However, the present quantum dynamics study on the best-available potential energy surface for the title reaction reveals that the CH stretch excitation does promote the reaction significantly, except at low collision energies. Further studies should be carried out to solve the disagreements between theory and experiment on the reaction. SECTION: Kinetics and DynamicsI n a typical chemical reaction with an energetic barrier, there is a saddle point that the reactants must surmount to reach the product side. Which form of energy initially deposited in reactants, translational or vibrational, is more efficacious in surmounting the barrier is one of the central topics in the field of reaction dynamics. In 1972, on the basis of theoretical studies of some model atom−diatom reactions, Polanyi proposed the well-known Polanyi rules, which state that vibrational energy is more efficient in promoting a late-barrier reaction (that is, a transition state resembling the products) than translational energy, whereas the reverse is true for an early barrier reaction.1 Over the past decades, the rules have enjoyed great successes not only on the atom−diatom reactions 2,3 but also on the H and Cl + H 2 O reactions and their isotope analogues. 4−9However, the generality of these rules for the Cl + CH 4 reaction was questioned recently by an unexpected experimental finding on the title reaction.10 Following the early work, carried out in the groups of Crim, 11 Zare, 12 and Orr-Ewing, 13Liu and co-workers performed crossed molecular beam experiments on the title reaction with the CH vibration in the ground and first stretch vibrational excited states and measured integral and differential cross sections for product CD 3 in the ground vibrational state (CD 3 (ν = 0)). They found that at the same total energy, the integral cross section for CD 3 (ν = 0) for the initial CH stretch excited state, σ s (ν = 0), is smaller than that at low collision energies and becomes comparable at higher collision energies to the corresponding one for the ground initial state, σ g (ν = 0). On the basis of their estimation that the CD 3 (ν = 0) product accounts for 2/3 of the total product distribution for the initial CH excited state, they concluded that the CH stretch excitation is no more effective 10 or slightly more effective 14 in promoting the late-barrier Cl + CHD 3 reaction than an equivalent amount of translational energy, in contradiction with the Polanyi rules. Equally interesting, the same group also found that the CH stretch excitation inhibits CH bond cleavage in the early-barrier F + CHD 3 reaction. 15To study the reaction theoretically, Czakóand Bowman recently developed a high-quality, full-dimensi...

Quantum dynamics of polyatomic dissociative chemisorption on transition metal surfaces: mode specificity and bond selectivity

“…This is a reasonable approximation, which has been extensively used in gas phase X + CH 4 reactions. [131][132][133][134][135] For the DC of methane, the reduceddimensional Hamiltonian is given as follows: 78…”

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

“…Recently, Polanyi's rules have been extensively tested in reactions involving polyatomic molecules both experimentally [6][7][8][9][10][11][12][13] and theoretically. [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.…”

Communication: Mode specific quantum dynamics of the F + CHD3 → HF + CD3 reaction

“…Indeed, in the recent studies of the Cl + CHD 3 reaction, the reactivity for the vibrationally excited reactant was found to increase when more rotational states of the products were included in the experiment, 57,58 bringing the vibrational enhancement factor closer to theoretical prediction. 59 Theoretically, the 8D QD model still contains large uncertainties concerning approximations introduced by the reduced dimensionality. In addition, the rotational state of the CHD 3 reactant in our calculations (| J, K⟩ = 0, 0) is different from the experiment (| J, K⟩ = 2, 0 and ±1).…”