Mode- and bond-selected reactions of vibrationally excited methane and monodeuterated methane with chlorine atoms.

摘要

Direct infrared absorption prepares CH₄ in two nearly isoenergetic vibrationally excited states, the symmetric stretch-bend combination (ν ₁ + ν₄) and the antisymmetric stretch-bend combination (ν₃ + ν₄), for a study of the effect of stretching vibrations of CH₄ on the reaction, CH₄ + Cl( 2P_3/2) → CH₃ + HCl. Comparison of intensities in the action spectra with those in the simulated absorption spectra shows that vibrational excitation of methane to the ν₁ + ν₄ state promotes the reaction more efficiently than excitation to the ν₃ + ν₄ state by a factor of 1.9 ± 0.5.; The reduced symmetry of CH₃D allows us to explore the relative reactivity of the fundamental symmetric and the antisymmetric C-H stretches. We excite three vibrational eigenstates of CH₃D near 3000 cm −1 that contain different amounts of symmetric C-H stretch (ν ₁), antisymmetric C-H stretch (ν₄), and two quanta of bend (2ν₅). Analyzing the action spectra with the simulation and the composition of the eigenstates reveals that the ν₁ vibration is 6 ± 1 times more reactive than the ν₄ vibration. Ab initio calculations of the vibrational eigenfunctions along the reaction coordinate show that as the Cl atom approaches, the ν₁ vibration of CH₃D is transformed into localized vibrational excitation in the C-H bond pointing toward the Cl atom, promoting the reaction, and the ν ₄ vibrational energy flows into the distal C-H bonds that remain unaffected during the reaction, consistent with our experimental results.; Selective vibrational excitation permits control of the outcome of a reaction with two competing channels. Vibrational excitation of the first overtone of C-D stretch (2ν₂) of CH₃D at ∼4300 cm−1 exclusively increases the probability of breaking the C-D bond, yielding CH₃ but no CH₂D. By contrast, excitation of the ν₁ vibration, the ν₄ vibration, or the combination vibration of C-H stretch and CH₃ umbrella bend to produce CH₂D over CH₃. The vibrational action spectra for the two products permit the separation of the two sets of interleaved transitions to give band origins and rotational constants of the 2ν ₂ state and the ν₄ + ν₃ state of CH ₃D.