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

Experiments explore the influence of different C–H stretching eigenstates of $CH3D$ on the reaction of $CH3D$ with $Cl(P3∕22)$⁠. We prepare the $∣110⟩∣0⟩(A1,E)$⁠, $∣200⟩∣0⟩(E)$⁠, and $∣100⟩∣0⟩+ν3+ν5$ eigenstates by direct midinfrared absorption near $6000cm−1$⁠. 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 $∣100⟩∣0⟩+ν3+ν5$ 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.