An investigation of the gas phase thermolysis of hexaborane (10) by quantitative mass spectrometry
A method has been developed, after extensive evaluation of several different types of experimental system, which allows a gaseous reaction mixture to be sampled by a mass spectrometer. Samples from the reaction mixture flow into the spectrometer under a regime of viscous flow in an excess background of helium. This technique preserves the composition of the sample as it passes from the reaction vessel into the source of the mass spectrometer. Analysis of the resultant mass spectrum gives the composition of the mixture. The gas phase thermolysis of hexaborane(10) has been studied by this method for pressures in the range 1-10 mmHg, and at temperatures between 75 and 153°C. Hexaborane(10) is found to decompose by a second order process having an activation energy of 79 ± 5 kJ mol -1 and a pre- exponential factor of ~10 7 m3 mol -1-1s The main products are hydrogen and a non-volatile solid, though small amounts of pentaborane(9) and decaborane(14) are also produced. In addition there is evidence for a B12 intermediate, and octaborane(12) is observed in trace amounts during the early stages of the thermolysis in the lower temperature runs. Some preliminary work on the effects of deuterium on the reaction is also described. Additionally, studies of a red glassy solid obtained from the room temperature, liquid phase decomposition of hexaborane(10) suggest that this reaction may be related to that in the gas phase. The overall results are consistent with a mechanism involving polymerization via a reactive B12 intermediate formed in a bimolecular reaction between two hexaborane(10) molecules. The pentaborane(9) and decaborane(14) are considered to be products of a relatively minor side reaction. Possible mechanisms are proposed and discussed.