A Theoretical Study of the Unimolecular Dissociation of Diborane

Abstract

Presently, we are involved in a theoretical study of the thermal pyrolysis of diborane (B₂H₆), with an emphasis on the early stages of the reaction. It is generally agreed that the first three elementary reactions which occur in this process are

$$ {{\rm{B}}_2}{{\rm{H}}_6} \to 2{\rm{B}}{{\rm{H}}_3} $$

((1))

$$ {\rm{B}}{{\rm{H}}_3} + {{\rm{B}}_2}{{\rm{H}}_6} \to {{\rm{B}}_3}{{\rm{H}}_9} $$

((2))

$$ {{\rm{B}}_3}{{\rm{H}}_9} \to {{\rm{B}}_3}{{\rm{H}}_7} + {{\rm{H}}_2} $$

((3))

Kinetic studies have shown that the initial reaction rate is 3/2 order in diborane [1], implicating (2) or (3) as the rate-determining step. Results of early investigations by Clark and Pease [2] and by Bragg et al. [3] favored assignment of step (2) as rate limiting. Enrione and Schaeffer [4], however, studied the isotopic dependence of the rate, and found that it was substantially decreased when perdeuterated diborane was used (k _H/k _D = 5). On the basis of this result, and calculations which indicated that B₂D₆ was dissociated to a far greater extent than B₂H₆ under the reaction conditions, it was predicted that (3) represented the rate-limiting process. Preliminary calculations [5], however, have predicted that the activation energy for reaction (2) is larger than that for (3). Efforts to calculate these barriers at very high levels of theory are currently underway. If the results of the more primitive calculations are substantiated in these studies, it is conceivable that (2) represents the true slow step in the reaction sequence.