Mode- and bond-selective reaction of Cl(P3∕22) with CH3D: C–H stretch overtone excitation near 6000cm−1

Abstract

Experiments explore the influence of different C-H stretching eigenstates of CH3D on the reaction of CH3D with Cl(2P3/2). We prepare the mid |110>|0>(A1,E), mid |200>|>0(E), and mid |100>|0> +nu3 +nu5 eigenstates by direct midinfrared absorption near 6000 cm(-1). The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the |200>|0>(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to|100>|0> +nu3 +nu5 excitation, indicating mode-selective chemistry in which the C-H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, |110>|0>(A1) and |110>|0>(E), the ratio of reaction cross sections is 1.00 +/- 0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excited local modes corresponding to one quantum of C-H stretch in each of two distinct oscillators react exclusively to form C-H stretch excited CH2D products. Conversely, eigenstates containing stretch excitation in a single C-H oscillator form predominantly ground vibrational state CH2D products. Analyzing the product state yields for reaction of the |110>|0>(A1) state of CH3D yields an enhancement of 20 +/- 4 over the thermal reaction. A local mode description of the vibrational motion along with a spectator model for the reactivity accounts for all of the observed dynamics.