Thermochemistry of Lewis Adducts of BH3 and Nucleophilic Substitution of Triethylamine on NH3BH3 in Tetrahydrofuran
The thermochemistry of the formation of Lewis base adducts of BH3 in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane 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 accurate CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or the 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: THFBH3 + L → LBH3 + THF. The formation of NH3BH3 in THF was observed to yield significantly more heat than gas phase dative bond energies predict, consistent with strong solvation of NH3BH3. Substitution of NEt3 on NH3BH3 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. Finally, simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical SN2 mechanism.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1001122
- Report Number(s):
- PNNL-SA-73673; 39979; EB4202000; TRN: US201101%%853
- Journal Information:
- Inorganic Chemistry, Vol. 49, Issue 22; ISSN 0020-1669
- Publisher:
- American Chemical Society (ACS)
- Country of Publication:
- United States
- Language:
- English
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