Differential steric effects in Cl reactions with aligned CHD3(v1 = 1) by the R(0) and Q(1) transitions. I. Attacking the excited C-H bond.

When a CHD3 molecule is pumped to the C-H stretching-excited state by absorbing a linearly polarized infrared (IR) photon via the R(0) branch of the v1 = 1←0 transition, the rotational angular momentum j of the prepared state jK=10 preferentially lies in a plane perpendicular to the IR polarization axis εIR. By way of contrast, when the Q(1) branch is used, the state of jK=1±1 is prepared with j aligned along the direction of εIR. Reported here is a detailed study of the title reaction by actively controlling the collision geometries under these two IR-excitation schemes at collision energy Ec = 8.6 kcal mol-1, using a crossed molecular beam, product imaging approach. We found that under the R(0) excitation, the polarization-dependent differential cross sections for the HCl(v = 0) + CD3(00) channel can largely be understood by invoking dual reaction mechanisms. The forward-scattered products are most likely mediated by a time-delayed resonance mechanism-as the formation of the HCl(v = 1) + CD3(00) channel, whereas the backward/sideways scattered products are governed by a direct abstraction mechanism. Compared to the previous results at lower Ec of 3.8 kcal mol-1, the sighting of opening-up the attack angle at the transition state of the direct pathway is proposed. Results under the Q(1) excitation are, however, perplexing and bear no obvious correlation to the corresponding ones for the R(0) excitation, defying simple intuitive interpretation. Possible reasons are put forward, which call for theoretical investigations for deeper insights. The results on the alternative isotope channel, DCl + CHD2, will be reported in the following paper.