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

The thermal unimolecular decomposition of hexamethyldisilana (HMDS), M e(3)SiSiMe(3), has been investigated over the temperature range 893-12 48 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 pat hway is Si-C bond fission but this accounts for <5% of the HMDS decomp osition. RRKM calculations yield the extrapolated high-pressure rate p arameters 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 f ission. The A factor for reaction 1 was assigned from the reaction the rmochemistry combined with recent measurements of the Me(3)Si recombin ation 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 disso ciation 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 m ol(-1). Observed secondary molecular products of HMDS decomposition un der VLPP conditions are CH4, C2H2, and C2H4. Their formations are cons istent with known or plausible reaction's initiated by partial unimole cular decomposition of Me(3)Si radicals under reaction conditions.