A kinetic study of the gas-phase thermolysis of hexaborane(10)

Abstract

The gas-phase thermolysis of B₆H₁₀, has been studied kinetically by mass spectrometry for pressures in the range 1–7 mmHg, and at temperatures between 75 and 165 °C (348–438 K). Hexaborane(10) was found to decompose by a second-order process with an activation energy of 79.7 ± 3.7 kJ mol^–1. The rate of decomposition is much slower than implied by qualitative statements in the literature. The main products are hydrogen and a non-volatile solid. The initial rates of hydrogen production and B₆H₁₀, consumption are in a ratio of 1 : 1, but the final amount of hydrogen produced is nearer twice that of the B₆H₁₀ consumed; added hydrogen has little effect on the initial rates. The simplest mechanism consistent with these observations involves an initial bimolecular rate-controlling step, with rapid elimination of two molecules of hydrogen from the (B₁₂H₂₀)‡; activated complex to give {B₁₂H₁₆}, a previously unconsidered reactive intermediate, which is immediately removed as a solid by condensation polymerization. Other modes of decomposition of the activated complex, although less likely as routes to the solid, do provide a plausible explanation for the minor amounts of B₅H₉ and B₁₀H₁₄ that are produced. A B₁₂ species is observed in the mass spectrum, but this is shown to be a secondary-process ion resulting from an ion–molecule reaction in the source of the mass spectrometer.