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Heats of Formation and Bond Energies of the H(3−n)BXn Compounds for (X = F, Cl, Br, I, NH2, OH, and SH)

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journal contribution
posted on 29.01.2009, 00:00 by Daniel J. Grant, David A. Dixon
Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for the borane compounds H(3−n)BXn for (X = F, Cl, Br, I, NH2, OH, and SH) and various radicals from coupled cluster theory (CCSD(T)) calculations with an effective core potential correlation-consistent basis set for I. In order to achieve near chemical accuracy (±1.5 kcal/mol), three corrections were added to the complete basis set binding energies calculated from frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin−orbit effects. Vibrational zero point energies were computed at the MP2 level. The calculated heats of formation are in excellent agreement with the available experimental data for the closed shell molecules, but show larger differences with the reported “experimental” values for the BX2 radicals. The heats of formation of the BX2 radicals were also calculated at the G3(MP2) level of theory, and the values were in excellent agreement with the more accurate CCSD(T) values. On the basis of extensive comparisons with experiment for a wide range of compounds, our calculated values for these radicals should be good to ± 1.5 kcal/mol and thus are to be preferred over the experimental values. The accurately calculated heats of formation allow us to predict the B−X and B−H adiabatic bond dissociation energies (BDEs) to within ± 1.5 kcal/mol. The B−F BDEs in the H(3−n)BFn compounds and in BF (1Σ+) are the largest BDEs in comparison to the other substituents that were investigated. The second and third largest B−X BDEs in the H(3−n)BXn and BX compounds are predicted for X = OH and NH2, respectively. The substituents have a minimal effect on the B−H BDEs in HBX2 and H2BX compared to the first B−H BDE of borane. The differences in adiabatic and diabatic BDEs, which are related to the reorganization energy in the product, can be estimated from singlet−triplet splittings in these molecules, and can account for the large fluctuations in adiabatic BDEs observed, specifically for the BX2 and HBX radicals, during the stepwise loss of the respective substituents.