Computational Study of the Initial Stage of Diborane Pyrolysis

The rate constants for the association of two boranes to form diborane are investigated using several methods. The most sophisticated method is the variable reaction coordinate-variational transition state theory (VRC-VTST) which has been developed to handle reactions with no enthalpic barriers. The calculated rate constant of 8.2 x 10(-11) cm(3).molecule(-1).s(-1) at 545 K is in good agreement with experiment. The rate constant was also computed using conventional VTST with the G4 composite method. Two variations of the multistep mechanisms for diborane pyrolysis are presented. One is initiated by the step B2H6 reversible arrow 2 BH3 while the other begins with 2 B2H6 reversible arrow B3H9 + BH3 as the initial elementary step. Both variations are 3/2 order in cliborane and have the same activation energy (G4, 28.65 kcal/mol at 420 K). In contrast, the traditional mechanism involving a B3H9 intermediate with C-3v symmetry has a higher activation energy (33.37 kcal/mol). The two variations involve a C-2-symmetry penta-coordinate B3H9 structure that, while an electronic minimum, is not a stationary point on the free energy path between B2H6 + BH3 -> B3H7 + H-2. While the calculated activation barrier is higher than the recently determined experimental barrier, the variation in reported values is large (22.0-29.0 kcal/mol). We discuss possible sources of disagreement between experiment and theory.