Intramolecular energy transfer and mode-specific effects in unimolecular reactions of disilane

Intramolecular energy transfer rates and pathways in disilane Si2H6 have been investigated in detail by analysis of the envelope functions of the time variation of the uncoupled normal-mode kinetic energies [J. Chem. Phys. 89, 5680 (1988)] and by a new method that involves the Fourier transform of the local-mode ‘‘bond energies.’’ The results show that the total intramolecular vibrational relaxation (IVR) rate out of a given mode is generally much faster than the total dissociation rate. However, many of the individual mode-to-mode rate coefficients are significantly smaller than this rate. Consequently, IVR is not globally rapid on the time scale of the reactions. The Si–Si and local modes relax over a much longer time scale than the Si–H modes. This observed decoupling of sets of internal modes is interpreted to mean that phase space is not explored ergodically on the time scale of the reactions, even at internal energies significantly greater than the dissociation thresholds. The present results are consistent with and complementary to our earlier observation of trajectory rate coefficients that are considerably larger than corresponding statistical phase space predictions computed on the same potential-energy surface [J. Chem. Phys. 94, 0000 (1991)]. As a consequence, we find numerous mode-specific effects present in the system. Trajectory rates are found to be very sensitive to the nature of the initial energy partitioning. The computed kinetic isotope effects also show evidence of mode-specific chemistry. These data are consistent with the principle that a total intramolecular energy transfer rate from a given vibrational mode that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure statistical behavior and an absence of mode-specific chemistry.