KINETICS AND THERMOCHEMISTRY OF SI(CH3)(3)- DIRECT DETERMINATION OF ASI-N BOND-ENERGY(NO REACTION )

The reaction of trimethylsilyl radicals with nitric oxide, Si(CH3)(3) + NO reversible arrow Si(CH3)(3)NO, has been studied using pulsed exci mer-laser photolysis coupled with time-resolved photoionization mass s pectrometry over the temperature range 300-812 K. The reaction rate co nstant was measured as a function of temperature and density (He) in t he range 0.82 x 10(16) [He] 15.7 x 10(16) molecules cm(-3). Equilibriu m constants for the reaction were measured between 685 and 787 K. The standard enthalpy of the reaction Si(CH3)(3) + NO reversible arrow Si( CH3)(3)NO was obtained from the measured equilibrium constants using b oth second- and third-law methods. The two procedures yielded results in good agreement: Delta H-298(o) = -183 + 11 kJ mol(-1) (second law) and Delta H-298(o) = -190.2 +/- 3.6 kJ mol(-1) (third law), the latter being more accurate. The high-pressure-limit rate constant of reactio n Si(CH3)(3) +/- NO --> Si(CH3)(3)NO, obtained by a short extrapolatio n of the experimental data using the Troe factorization technique, has a small negative temperature dependence: k(infinity.rec) = (3.8 +/- 0 .4) x 10-(11) (T/298)(-(0.6) (+/-) (0.2)). Ab initio calculations with empirical bond additivity corrections have been performed to determin e the structure, vibrational frequencies, and energies of the low-lyin g electronic singlet and triplet states of the Si(CH3)(3)NO molecule. The calculated thermodynamic functions of the molecule were used to ob tain the standard entropy of the reaction (Delta S-298(o) = -147.9 J m ol(-1) K-1) that was used in the third-law thermochemical calculations . The theoretical standard enthalpy of the reaction, Delta H-298(o)(ca lc) = -190.9 kT mol(-1), is in excellent agreement with that determine d experimentally using the third-law procedure. The intrinsic Si(CH3)( 3)-NO bond strength of 187.4 +/- 4.0 kJ mol(-1) (-Delta H-0(o) for the reaction) was determined using the measured enthalpy of the reaction at 298 K (by the third-law method) and the calculated relative enthalp y functions. This study provides the first direct experimental determi nation of a Si-N bond strength.