Overtone-induced dissociation and isomerization dynamics of the c radical (CH₂OH and CD₂OH). Part I: A theoretical study.

E. Kamarchik, C.P. Rodrigo, J.M. Bowman, H. Reisler, and A.I. Krylov
J. Chem. Phys. 136, 084304 (2012)

The dissociation of the hydroxymethyl radical, CH₂OH, and its isotopolog, CD₂OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH₂OH and CH₃O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH₂OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wave functions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about 3000 cm-1 above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the metoxy radical. The computed dissociation energies are: D0(CH2OH -> CH2O + H)=10,188 cm-1, D0(CD2OH -> CD2O + H)=10,167 cm-1, D0(CD2OH -> CHDO + D)=10,787 cm-1. All are in excellent agreement with the experimental results. For CH2OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm^-1, respectively.

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