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.