A MASS SPECTROMETRIC INVESTIGATION OF LOW-PRESSURE PYROLYSIS OF BORANE CARBONYL . BOND DISSOCIATION ENERGY OF DIBORANE

The thermal decomposition of borane carbonyl, BH3CO, has been examined in a low-pressure glass flow reactor over the temperature range of 400-560°K. The loss of BH3CO and the formation of CO, B2H6, BH3, and H2 were measured. The rate of the reaction, BH3CO → BH3 + CO, was observed near the low-pressure limit. The probability of reaction per collision has been examined over a range of average number of gas and wall collisions. The wall collisions are about 10 times more effective than gas-phase collisions. The homogeneous reaction exhibits a statistical-weight inverse secondary kinetic isotope effect of 0.38 ± 0.05 at 450°K while the wall reaction exhibits a normal isotope effect of 1.2 ± 0.3. The rate constant and the kinetic isotope effect at the low-pressure limit have been calculated as a function of the critical energy using the vibrational frequencies of BH3CO and BD3CO. The critical energy that fits the data is used to fix D(H3B-BH3) at 36 ± 3 kcal. Evidence for an interesting role of the surface in the heterogeneous reaction is presented.