Intramolecular energy transfer and modehyphen;specific effects in unimolecular reactions of 1,2hyphen;difluoroethane

摘要

The unimolecular decomposition reactions of 1,2hyphen;difluoroethane upon modehyphen;specific excitation to a total internal energy of 7.5 eV are investigated using classical trajectory methods and a previously formulated empirical potentialhyphen;energy surface. The decomposition channels for 1,2hyphen;difluoroethane are, in order of importance, fourhyphen;center HF elimination, Cndash;C bond rupture, and hydrogenndash;atom dissociation. This order is found to be independent of the particular vibrational mode excited. Neither fluorinendash;atom nor F2elimination reactions are ever observed even though these dissociation channels are energetically open. For fourhyphen;center HF elimination, the average fraction of the total energy partitioned into internal HF motion varies between 0.115ndash;0.181 depending upon the particular vibrational mode initially excited. The internal energy of the fluoroethylene product lies in the range 0.716ndash;0.776. Comparison of the present results with those previously obtained for a random distribution of the initial 1,2hyphen;difluoroethane internal energy lsqb;J. Phys. Chem.92, 5111 (1988)rsqb;, shows that numerous modehyphen;specific effects are present in these reactions in spite of the fact that intramolecular energy transfer rates for this system are 5.88ndash;25.5 times faster than any of the unimolecular reaction rates. Modehyphen;specific excitation always leads to a total decomposition rate significantly larger than that obtained for a random distribution of the internal energy. Excitation of different 1,2hyphen;difluoroethane vibrational modes is found to produce as much as a 51percnt; change in the total decomposition rate. Modehyphen;specific effects are also seen in the product energy partitioning. The rate coefficients for decomposition into the various channels are very sensitive to the particular mode excited. A comparison of the calculated modehyphen;specific effects with the previously determined modehyphen;tohyphen;mode energy transfer rate coefficients lsqb;J. Chem. Phys.89, 5680 (1988)rsqb; shows that, to some extent, the presence of modehyphen;specific chemistry is correlated with the magnitude of the energy transfer rate. However, the particular pathways for energy flow seem to be more important than the magnitude of the rate coefficients. It is suggested that the propensity for the energy to remainisolated in small subset of modes, such as the CH2F deformation modes or the rocking modes, is primarily responsible for the observation of modehyphen;specific chemistry. The results clearly demonstrate that an intramolecular energy transfer rate that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure the absence of modehyphen;specific chemicaleffects.