Mode- and bond-selective reaction of Cl(P-2(3/2)) with CH3D: C-H stretch overtone excitation near 6000 cm(-1)

摘要

Experiments explore the influence of different C-H stretching eigenstates of CH3D on the reaction of CH3D with Cl(P-2(3/2)). We prepare the vertical bar 110 >vertical bar 0 >(A(1),E), vertical bar 200 >vertical bar 0 >(E), and vertical bar 100 >broken vertical bar 0 >+nu(3)+nu(5) 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 broken vertical bar 200 >broken vertical bar 0 >(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to vertical bar 100 >vertical bar 0 >+nu(3)+nu(5) 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, vertical bar 110 >vertical bar 0 >(A(1)) and vertical bar 110 >vertical bar 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 vertical bar 110 >vertical bar 0 >(A(1)) 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. (c) 2006 American Institute of Physics.