Thermochemistry of Gaseous OSiI, OSiI2, SiI, and SiI2

Gaseous reaction equilibria in the Si−O−I system were investigated by mass spectrometry, leading to identification and thermochemical characterization of the species OSiI, OSiI 2, SiI, and SiI 2 . Species were generated by the addition of gaseous I 2 to an effusion cell reactor containing a Si/SiO 2 mixture of composition “SiO”. Equilibria involving the Si−O−I species were measured in the range 1350 to 1540 K. From third law analysis, the enthalpies of formation of OSiI, OSiI 2, SiI, and SiI 2 were derived as ≥−21.7 ± 5, ≥−74.5 ± 5, 61.0 ± 2, and 21.6 ± 3.3 kcal mol -1, respectively. Although the values for SiI and SiI 2 are in good agreement with selected values in a recent compilation, the results for OSiI and OSiI 2 are 25−30 kcal mol -1 less stable than semiempirical estimates developed for modeling of metal halide lamp chemistry. In some instances, other estimated thermochemical database values differ substantially from both theoretical and experimental results; reasons for these discrepancies are discussed.