VERY-LOW-PRESSURE PYROLYSIS OF HEXAMETHYLDISILANE - KINETIC DETERMINATION OF THE BOND-DISSOCIATION ENTHALPY DH-DEGREES(ME(3)SI-SIME(3)) AND IMPLICATIONS FOR THE ENTHALPY OF FORMATION OF THE TRIMETHYLSILYL RADICAL

The thermal unimolecular decomposition of hexamethyldisilana (HMDS), Me(3)SiSiMe(3), 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 Si-Si bond fission to form the trimethylsilyl radical, Me(3)Si. A minor primary reaction pathway is Si-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 k(1,infinity) = 10(16.5+/-0.3) exp(-314.3 +/- 8.0 kJ mol(-1)/RT) s(-1) for Si-Si fission and k(6,infinity) approximate to 10(17.3) exp(-352 kJ mol(-1)/RT) for Si-C fission. The A factor for reaction 1 was assigned from the reaction thermochemistry combined with recent measurements of the Me(3)Si recombination rate and the intrinsic A factor for reaction 6 was chosen to be the same. The rate parameters for Si-Si fission lead to the bond dissociation enthalpy DH degrees(300)(Me(3)SiSiMe(3)) = 332 +/- 12 kJ mol(-1). This value, combined with a recent reaction-solution calorimetric measurement of -303.7 +/- 5.5 kJ mol(-1) for Delta H-f degrees(300)(Me(3)SiSiMe(3)) leads to Delta H-f degrees(300)(Me(3)Si) = 14 +/- 7 kJ mol(-1). Observed secondary molecular products of HMDS decomposition under VLPP conditions are CH4, C2H2, and C2H4. Their formations are consistent with known or plausible reaction's initiated by partial unimolecular decomposition of Me(3)Si radicals under reaction conditions.