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The thermal unimolecular decomposition of hexamethyldisilane (HMDS), MesSiSiMes, has been investigated over the temperature range 893-1248 K by using the technique of very-low-pressure pyrolysis (VLPP). The major primary reaction pathway is the expected S i S i bond fission to form the trimethylsilyl radical, Me3Si. A minor primary reaction pathway is S1-C bond fission but this accounts for <5% of the HMDS decomposition. RRKM calculations yield the extrapolated high-pressure rate parameters at 1000 K given by the expressions kl,, = 1016-5M.3 exp(-314.3 f 8.0 kJ mok1/R7') s-l for S i S i fission and ka,, EC: 1017.3 exp(-352 kJ mol-l/RT) for S i 4 fission. The A factor for reaction 1 was assigned from the reaction thermochemistry combined with recent measurements of the Me3Si recombination rate and the intrinsic A factor for reaction 6 was chosen to be the same. The rate parameters for S i S i fission lead to the bond dissociation enthalpy DH03~(Me3Si-SiMe3) = 332 f 12 kJ mol-'. This value, combined with a recent reaction-solution calorimetric measurement of -303.7 f 5.5 kJ mol-' for Mf03~(Me3SiSiMe3) leads to AHfo3m(Me3Si) = 14 f 7 kJ mol-l. Observed secondary molecular products of HMDS decomposition under VLPP conditions are CH4, C2H2, and CzH4.Their formations are consistent with known or plausible reactions initiated by partial unimolecular decomposition of Me3Si radicals under reaction conditions.

A simple model for the anharmonic coupling constants has been used to calculate vibrational state mixing in S1 anthracene. The aim of the calculations is to provide insight into the vibrational state mixing responsible for intramolecular vibrational energy redistribution (IVR). The calculations include all vibrations of the appropriate symmetry within a 100 cm−1 interval centered about the state of interest. The calculations are compared with experimental measurements of quantum beats in S1 anthracene [P. M. Felker and A. H. Zewail, J. Chem. Phys. 82, 2975 (1985)]. These experiments involved an investigation of rotational effects that established the coupling to be anharmonic in origin. We show that in order for the experimental data to be explained by anharmonic coupling alone, the high-order anharmonic terms must be reasonably large. This implies that the anharmonic expansion converges quite slowly for EVIB≲2000 cm−1 in anthracene, in contrast with spectroscopic data for small molecules. Anthracene does not appear to be unique with regard to its IVR behavior, and consequently we suggest that slow convergence of the anharmonic expansion will prove to be the norm for large molecules. As a consequence of the slow convergence, direct coupling through high-order anharmonic terms is an important coupling mechanism. The model used to determine the anharmonic coupling constants is not specific to anthracene, and it is anticipated that it will be possible to predict vibrational state mixing in other molecules using the parameters deduced for anthracene.

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