PNNL

Abstract

The thermochemistry of the formation of Lewis adducts of BH3 in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane (AB) by triethylamine are reported. The dative bond energy of Lewis adducts were predicted using density functional theory at the B3LYP/DZVP2 and B3LYP/6-311+G** levels and correlated ab initio molecular orbital theories, including MP2, G3(MP2), and G3(MP2)B3LYP, and compared with available experimental data and CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or B3LYP geometries reproduce the benchmark results usually to within ~1 kcal/mol. Energies calculated at the MP2/aug-cc-pVTZ level for geometries optimized at the B3LYP/DZVP2 or B3LYP/6-311+G** levels give dative bond energies 2-4 kcal/mol larger than benchmark values. The enthalpies for forming adducts in THF were determined by calorimetry and compared with the calculated energies for the gas phase reaction: THF:BH3 + L ? L:BH3 + THF. The formation of AB in THF was observed to yield significantly more heat than the reaction in gas phase, consistent with strong solvation of AB. Substitution of NEt3 on AB is an equilibrium process in THF solution (K ? 0.2 at 25 °C). The reaction obeys a reversible bimolecular kinetic rate law with the Arrhenius parameters: log A = 14.7 ± 1.1 and Ea = 28.1 ± 1.5 kcal/mol. Simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical SN2 mechanism. Pacific Northwest National Laboratory is operated by Battelle for the USDOE.