Kinetics and mechanism of the gas-phase pyrolysis of hexafluorodisilane in the presence of iodine and some reactions of silicon difluoride

Abstract

The gas-phase thermal decomposition of Si₂F₆ in the presence of I₂ has been found to follow first-order kinetics with the formation of SiF₄, SiF₂I₂ and SiF₃I. The reactions appear to be homogeneous and consistent with the mechanism: Si₂F₆→ SiF₂+ SiF₄(slow)(1), SiF₂+ I₂→ SiF₂I₂(fast).(2) SiF₃I is a minor product and its mechanism of formation is uncertain. Rate measurements over the temperature range 603–652 K fit the Arrhenius equation: log (k₁/s^–1)=(12.41 ± 0.24)–(193.5 ± 2.9 kJ mol^–1)/RT ln 10. In competitive scavenging reactions at 630 K, SiF₂ is found to react with I₂ 3.6 times faster than with O₂ and 17 times faster than HI. The reaction of SiF₂ with I₂ is also at least a factor of 10² faster than with HBr or benzene and greater than 10⁴ times faster than with SiF₄. The absence of an I˙ atom displacement process is consistent with D(F₃Si—SiF₃) 337 kJ mol^–1, which leads to the value ΔH^⊖_f(Si₂F₆)⩽–2337 kJ mol^–1.