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.