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 Delta(f)H(298)(o)(SiCl,g) = 36.8 kcal mol(-1) and Delta(f)H(298)(o)(SiBr,g) = 48.7 kcal mol(-1), along with the dissociation energies D-0(o)(SiCl) = 98.9 kcal mol(-1) and D-0(o)(SiBr) = 84.7 kcal mol(-1), all +/-2 kcal mol(-1). Thermochemical data were also obtained for the species SiCl2 and SiBr2, confirming earlier data in the literature. Nonobservance of the species SiCl3 and SiBr3 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 SiCl3 and SiBr3. 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.