Experimental Thermochemistry of the SiCl and SiBr Radicals; Enthalpies of Formation of Species in the Si−Cl and Si−Br Systems

Gaseous reaction equilibria involving the radical species SiCl and SiBr were studied by mass spectrometric monitoring of the molecular beam issuing from a heated effusion cell source, and the results were used to derive the enthalpies of formation = 36.8 kcal mol -1 and = 48.7 kcal mol -1, along with the dissociation energies = 98.9 kcal mol -1 and = 84.7 kcal mol -1, all ±2 kcal mol -1 . Thermochemical data were also obtained for the species SiCl 2 and SiBr 2, confirming earlier data in the literature. Nonobservance of the species SiCl 3 and SiBr 3 was used to set lower limits of −84 and −40 kcal mol -1 to the enthalpies of formation of the gaseous trichloride and tribromide, respectively. The results are compared with experimental and theoretical values in the literature, clarifying previous discrepancies regarding the species SiCl 3 and SiBr 3 . From a full evaluation of the literature and this work, a recommended set of enthalpies of formation of the gaseous silicon mono-, di-, tri-, and tetrahalide species is given.