Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

The scattering dynamics leading to the formation of Cl (P-2(3/2)) and Cl-* (P-2(1/2)) products of the CH3+HCl reaction (at a mean collision energy < E-coll >=22.3 kcal mol(-1)) and the Cl (P-2(3/2)) products of the CD3+HCl reaction (at < E-coll >=19.4 kcal mol(-1)) have been investigated by using photodissociation of CH3I and CD3I as sources of translationally hot methyl radicals and velocity map imaging of the Cl atom products. Image analysis with a Legendre moment fitting procedure demonstrates that, in all three reactions, the Cl/Cl-* products are mostly forward scattered with respect to the HCl in the center-of-mass (c.m.) frame but with a backward scattered component. The distributions of the fraction of the available energy released as translation peak at f(t)=0.31-0.33 for all the reactions, with average values that lie in the range < f(t)>=0.42-0.47. The detailed analysis indicates the importance of collision energy in facilitating the nonadiabatic transitions that lead to Cl-* production. The similarities between the c.m.-frame scattering and kinetic energy release distributions for Cl and Cl-* channels suggest that the nonadiabatic transitions to a low-lying excited potential energy surface (PES) correlating to Cl-* products occur after passage through the transition state region on the ground-state PES. Branching fractions for Cl-* are determined to be 0.14 +/- 0.02 for the CH3+HCl reaction and 0.20 +/- 0.03 for the CD3+HCl reaction. The difference cannot be accounted for by changes in collision energy, mass effects, or vibrational excitation of the photolytically generated methyl radical reagents and instead suggests that the low-frequency bending modes of the CD3H or CH4 coproduct are important mediators of the nonadiabatic couplings occurring in this reaction system. (C) 2008 American Institute of Physics.