Thermochemistry of Lewis Adducts of BH₃ and Nucleophilic Substitution of Triethylamine on NH₃BH₃ in Tetrahydrofuran

Journal Article · Mon Nov 15 04:00:00 EST 2010 · Inorganic Chemistry

DOI:https://doi.org/10.1021/ic101481c· OSTI ID:1001122

Potter, Robert G.; Camaioni, Donald M.; Vasiliu, Monica; Dixon, David A.

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 E_a = 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 S_N2 mechanism.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1001122
• Report Number(s): PNNL-SA-73673; 39979; EB4202000
• Journal Information: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 22 Vol. 49; ISSN 0020-1669
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ADDUCTS
AMMONIA
BENCHMARKS
BORANES
CALORIMETRY
Environmental Molecular Sciences Laboratory
FUNCTIONALS
KINETICS
SIMULATION
SOLVATION
TETRAHYDROFURAN