Computational Study of the Initial Stage of Diborane Pyrolysis

Abstract

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.

Production of B Atoms and BH Radicals from B2H6/He/H2 Mixtures Activated on Heated W Wires

“…The contributions of the bimolecular reactions, 2B 2 H 6 → B 3 H 9 + BH 3 and B 2 H 6 + H → B 2 H 5 + H 2 , must be minor because of their large activation energies. The activation energy for the former reaction has been reported to be 120 kJ mol –1 , while the barrier height for the latter reaction is 61 kJ mol –1 according to our calculation. The unimportance of the latter reaction is also supported by the mass spectrometric observation that the decomposition efficiency of B 2 H 6 is decreased by the addition of H 2 .…”

“…The production mechanism of BH 3 from B 2 H 6 , reaction , is not clear. Both thermal decomposition in the gas phase ,− and catalytic decomposition on the wire surfaces are possible. Mankelevich and co-workers suggested the importance of gas-phase decomposition rather than the catalytic decomposition, , although the contribution of catalytic decomposition was also mentioned .…”

Decomposition processes of diborane and borazane (ammonia-borane complex) on hot wire surfaces

“…The question is how BH 3 is produced. It has been shown that B 2 H 6 can be decomposed thermally in the gas phase [5][6][7][8][9][10][11][12]. Of course, catalytic decomposition on hot wire surfaces can also be expected [12].…”

Evaluating the Role of Triborane(7) As Catalyst in the Pyrolysis of Tetraborane(10)

“…At the G4 level, the free energy maximum on the dissociation path of diborane was 6.7 kcal/mol at 420 K larger than two separated BH 3 units. 11 When 6.6 kcal/mol is added to the free energy of B 3 H 7 (c) + BH 3 , the activation free energy B 4 H 10 (a) → B 4 H 8 (d) + H 2 is more favorable by 1.0 kcal/mol than B 4 H 10 (a) → B 3 H 7 (c) + BH 3 at 333 K. The preference does not change at different temperatures as seen in Table 3.…”

“…The reaction mechanism can be summarized in Figure 9. Briefly, B 4 H 10 eliminates hydrogen, then B 4 H 8 reacts with 11 , and H 2 ) in the gas mixture were collected after a particular time interval of at least 60 s after the reaction started. Even at the very first experimental point, both H 2 and B 5 H 11 were already produced.…”