Mode-specific tunneling in the unimolecular dissociation of cis-HOCO to H + CO2

We report mode-specific tunneling in the unimolecular dissociation of cis-HOCO to H + CO2 using a recent projection theory that makes use of a tunneling path along the imaginary-frequency normal mode, Qim, of a relevant saddle point. The tunneling probabilities and lifetimes are calculated for the ground vibrational state of cis-HOCO and highly excited overtones and combination bands of the modes that have large projections onto the Qim path. To go beyond the harmonic approximation, which is important for the OH stretch, energies and classical turning points are calculated using the anharmonic one-dimensional (1D) potential in that normal mode and used in the projection theory. The tunneling lifetimes calculated for a number of combination states of the OCO bend and CO stretch are in good accord with those estimated in a previous five degrees-of-freedom quantum wavepacket simulation of the dissociative photodetachment of HOCO(-). The present results are also consistent with the interpretation of the tunneling of cis-HOCO to H + CO2 seen in recent experiments.