Mechanism and Kinetics of Hexamethyldisilazane Reaction with a Fumed Silica Surface

Quantum chemical calculations of the reaction of hexamethyldisilazane with silica indicates a two-step mechanism. The first, rate-determining step results in trimethylsilylation of a surface silanol and formation of trimethylaminosilane. The second step involves reaction of a trimethylaminosilane reactive intermediate to form a trimethylsilyl surface species and ammonia. This two-step mechanism was applied to analyze data of the kinetics of hexamethyldisilazane reaction with fumed silica using custom interactive software. Both the extent of surface reaction and the loss of hexamethyldisilazane from toluene solution were monitored by FT-IR spectroscopy as a function of time. Data analysis indicates that under the conditions studied the reaction is adsorption rate limited. The entire reaction can be explained in terms of reaction with non-hydrogen-bonded surface silanols, though hydrogen-bonded silanols act as adsorption sites. Copyright 2000 Academic Press.