The dissociation of the hydroxymethyl radical, CH 2 OH, and its isotopolog, CD 2 OH, 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 CH 2 OH and CH 3 O 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

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... ted vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH 2 OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions 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 fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: All are in excellent agreement with the experimental results. For CH 2 OH, the barriers for the direct OH bond fission and isomerization are: 14 205 and 13 839 cm −1 , respectively.