Mechanistic Study of Pyrolysis of Small Boron Hydrides

Abstract

Theoretical background and computational methods are introduced in Chapter 1. In Chapter 2 the rate constant for the association of two boranes to form diborane is investigated using several methods. The most sophisticated method is variable reaction coordinate - variation transition state theory (VRC-VTST) which has been developed to handle reactions with no enthalpic barriers. The rate constant was computed using conventional VTST with the IRC from the G4 and W1DB methods. Two variations of the multi-step mechanisms for diborane pyrolysis are presented. The initial steps in the B4H10 pyrolysis mechanism have been elucidated in Chapter 3. The mechanism can be divided into three stages: initial formation of B4H8, production of volatile boranes with B3H7 acting as a catalyst, and formation of nonvolatile products. The first step is B4H10 decomposition to either B4H8/H2 or B3H7/BH3 where the free energy barrier for the first pathway is 5.6 kcal/mol higher (G4, 333 K) than the second pathway when transition state theory (TST) is used. We suggest that the rate-determining step is B4H10 + B3H7  B4H8 + H2 + B3H7 where B3H7 acts as a catalyst. The role of reactive boron hydrides such as B3H7 and B4H8 as catalysts in the build-up of larger boron hydrides may be more common than that previously considered.