Quantum Monte Carlo calculations of the dimerization energy of borane

Accurate Computational Thermodynamics Using Anharmonic Density Functional Theory Calculations: The Case Study of B-H Species

The thermal decomposition of boron-hydrogen compounds is complex and multistep and involves the formation of various intermediates. An accurate description of the thermodynamics of the reactants, products, and intermediates is required for an in-depth understanding of their reactivity. In this respect, we have proceeded to the accurate determination of the key thermodynamic functions (ΔH(T), S(T), and C _P (T)) of 44 isolated B-H molecular species involved in the decomposition of B-H solids, with the inclusion of anharmonic effects. An excellent agreement is observed with available experimental data. We report the analytic expressions of these functions obtained by fitting them with NASA functions in the 200-900 K temperature range. Because the vibrational spectra of these species are their fingerprints, we also report the predicted IR and Raman spectra. The calculated anharmonic spectra show an excellent agreement with experiments and allow for a clear-cut identification of fundamentals, combinations, and overtones.

Computational study of the initial stage of diborane pyrolysis

The rate constants for the association of two boranes to form diborane are investigated using several methods. The most sophisticated method is the variable reaction coordinate-variational transition state theory (VRC-VTST) which has been developed to handle reactions with no enthalpic barriers. The calculated rate constant of 8.2 × 10(-11) cm(3)·molecule(-1)·s(-1) at 545 K is in good agreement with experiment. The rate constant was also computed using conventional VTST with the G4 composite method. Two variations of the multistep mechanisms for diborane pyrolysis are presented. One is initiated by the step B2H6 ⇄ 2 BH3 while the other begins with 2 B2H6 ⇄ B3H9 + BH3 as the initial elementary step. Both variations are 3/2 order in diborane and have the same activation energy (G4, 28.65 kcal/mol at 420 K). In contrast, the traditional mechanism involving a B3H9 intermediate with C3v symmetry has a higher activation energy (33.37 kcal/mol). The two variations involve a C2-symmetry penta-coordinate B3H9 structure that, while an electronic minimum, is not a stationary point on the free energy path between B2H6 + BH3 → B3H7 + H2. While the calculated activation barrier is higher than the recently determined experimental barrier, the variation in reported values is large (22.0-29.0 kcal/mol). We discuss possible sources of disagreement between experiment and theory.