KINETIC AND THERMOCHEMICAL STUDY OF THE SIH3 + HBR REVERSIBLE SIH4 + BR AND SIH3 + HI REVERSIBLE SIH4 + I EQUILIBRIA

The reactions between SiH3 and HI (2) and SiH3 and HBr (5) have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry over the temperature range 295-550 K, giving k2 = 7.3 (+/- 2.8) X 10(-12) exp{2.0 (+/- 1.4) kJ mol-1/RT} and k5 = 1.2 (+/- 0.4) X 10(-12) exp{0.7 (+/- 1.2) kJ mol-1/RT} cm3 molecule-1 s-1. The reverse of reaction 5 was also studied by using excimer laser flash photolysis/resonance fluorescence over the temperature range 298-483 K, giving k-5 = 1.6 (+/- 0.6) X 10(-10) exp{-18.0 (+/- 1.3) kJ mol-1/RT} cm3 molecule-1 s-1. The enthalpy change for reaction 5, DELTA-H-degrees-298, was calculated from k5(T)/k-5(T) using second law {-18.4 (+/- 1.8) kJ mol-1} and third law {-17.4 (+/- 1.6) kJ mol-1} techniques, and these values were then employed to provide a recommended value for DELTA-H-degrees-f,298 for SiH3 of 200.5 (+/- 2.5) kJ mol-1. The SiH3-H bond enthalpy at 298 K was also determined, 384.1 (+/- 2.0) kJ mol-1. The determinations of k2(T) were combined with literature values of k-2(T) in an additional third law determination of the SiH3 heat of formation. The result, DELTA-H-degrees-f,298 = 200.8 (+/- 3.4) kJ mol-1, is of reduced accuracy but in complete accord with the recommended value based on the study of reaction 5. The latter determination also provides a reconciliation of formerly disparate literature values for this heat of formation. The reactivity of SiH3 is compared to that of the alkyl radicals with both HI and HBr.