Dynamics of the reactions of O( ¹D) with CD ₃OH and CH ₃OD studied with time-resolved Fourier-transform IR spectroscopy

We investigated the reactivity of O( ¹D) towards two types of hydrogen atoms in CH ₃OH. The reaction was initiated on irradiation of a flowing mixture of O ₃ and CD ₃OH or CH ₃OD at 248 nm. Relative vibration-rotational populations of OH and OD (1 ≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O( ¹D) CD ₃OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is bimodal; the product ratio OHOD is 1.56 ± 0.36. In O( ¹D) CH ₃OD, the rotational distribution of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio OHOD is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O( ¹D) inserts into the C-H and O-H bonds of CH ₃OH to form HOCH ₂OH and CH ₃OOH, respectively. The observed three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH ₂OH or decomposition of CH ₃OOH. The former decomposition channel of HOCH ₂OH produces vibrationally more excited OH because of incomplete intramolecular vibrational relaxation, and decomposition of CH ₃COOH produces OH with greater rotational excitation, likely due to a large torque angle during dissociation. The predicted OHOD ratios are 1.31 and 0.61 for O( ¹D) CD ₃OH and CH ₃OD, respectively, at collision energy of 26 kJ mol ^-1, in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy.