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The kinetics of the reactions CI(zPJ) + SiH4 (1) and Br(ZPJ) + SiH4 (2) have been investigated by time-resolved atomic resonance fluorescence spectroscopy. Halogen atoms were generated by flash photolysis of CCl, and CH2Br2. Experiments over 295-472 K yielded kl = (1.56 f 0.1 1) X exp[(2.0 f 0.2) kJ mol-'/Rq cm3 s-I and studies over 295-575 K yielded k2 = (9.0 f 1.5) X lo-" exp[(-17.0 f 0.6) kJ mol-'/Rq cm3 s-I. 95% confidence intervals are about f10% and f20%, respectively. These expressions are in good agreement with most earlier measurements. An expression for k-' is also derived. Alternative interpretations of the negative activation energy for k , , based on transition-state theory or formation of an intermediate adduct, are discussed.

The flash-photolysis resonance-fluorescence technique has been employed to measure the rate constant for O+SiH4+products from 295-565 K, and yielded kr = 1.23 X lo-" exp( -14.6 kJ mol-'/RT) cm3 s-r with an accuracy of about f 15%. The transition state for direct H-atom abstraction has been characterized at up to the Gaussian-2 ab initio level of theory. With small adjustments it is possible to model kinetic data for O+SiH, in terms of an abstraction channel leading to OH + SiH, . This agreement does not rule out minor participation by addition or insertion channels, but there is no theoretical evidence for bound triplet intermediates in the potential energy surface. A transition state theory analysis suggests that kr at 1000 K is 16 times larger than previously thought.

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Abstract:The kinetics of the elementary reactions of C1 and Br with HSi(CH3), (1 and 2) have been measured by flashphotolysis/time-resolved atomic resonance spectroscopy over the approximate temperature range 300-460 K. The results are3 kJ mol-I/RT) cm3 s-l, with confidence limits of about *20%. The activation energy of 2, combined with an estimated activation energy for the reverse reaction, yields a bond dissociation enthalpy of D298(H-Si(CH3)3) = 398 * 6 kJ mol-', which is about 14 kJ mol-l larger than D298(H-SiH3). This difference is supported by ab initio calculations. Implications for AH&Si(CH3),) and the Si-Si bond strength in disilanes are discussed in the text. exp(l.3 & 0.8 kJ mol-I/RT) cm3 s-l and k2 = (7.6 3.3) X

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The reaction O(,PJ) + (CH&SiH -products (1) has been investigated by the discharge/fast-flow method with mass spectrometric detection (DF-MS) over 298-773 K and by the flash photolysis/resonance fluorescence (FP-RF) technique over 293-549 K. The rate constants from both methods are in close accord and may be summarized by kl(T) = 5.6 X lo-" exp(-1005 K/7') cm3 molecule-' s-l for 290-770 K. There is reasonable agreement with previous room-temperature studies. kl is larger than k2 for 0 + SiH4 (2), similar to results for triethylsilane and triisopropylsilane [Buchta, Chr.; Stucken, D.-V.; Vollmer, J.-T.; Wagner, H. Gg. Z. Phys.Chem. Neue Folge, to be published], and tri-alkylation is seen to increase the reactivity of the Si-H bonds by similar amounts largely independent of the nature of the alkyl group. This silane activation occurs through lowering the energy barrier to abstraction to form O H + (CH3)3Si*. Ab initio analysis of this pathway at the MP2/6-31G*//HF/3-21G(*) level of theory gives relative energies for the transition states of reactions 1 and 2 that agree well with experiment.