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

Abstract

The kinetics of thermal decomposition of tetraborane(10) have been investigated by a mass spectrometric technique in the pressure range 0.86–38.69 mmHg and temperature range 40.3–77.9 °C (313.4–351.0 K). The initial rate of consumption of B₄H₁₀ follows the first-order rate equation –d[B₄H₁₀]/dt= 6.4 × 10¹¹ exp(–99 400/RT)[B₄H₁₀], with no evidence for the -order dependence claimed in the most recent literature. In the initial homogeneous gas-phase reaction, B₅H₁₁is produced with an order slightly greater than unity, and at a rate of 0.4–0.5 mole per mole of B₄H₁₀ consumed; hydrogen is also produced, with first-order kinetics, together with non-volatile solid hydride. Significant induction periods are observed in the build-up of species other than B₅H₁₁(i.e. B₂H₆, B₆H₁₂, and B₁₀H₁₄), indicating that they are the result of reactions subsequent to those involved in the initial stages. The initial reaction is interpreted in terms of the three-step mechanism, (1a), (2), and (3), in which (1a) is rate-limiting. A possible fourth step (5) may be an additional minor reaction favoured at low pressures. B₄H₁₀⇌{B₄H₈}+ H₂(1a), {B₄H₈}+ B₄H₁₀→ B₅H₁₁+{B₃H₇}(2), {B₃H₇}+{B₃H₇}→{‘B₆H_{14 – 2m}’}(polymerizes)+mH₂(3), {B₄H₈}+{B₄H₈}→{‘B₈H_{16 – 2n}’}(polymerizes)+nH₂(5)