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Quantum Chemical Characterization of the Structural and Energetic Properties of HCN-BF3

Phillips, J. A.; Cramer, C. J.
J. Chem. Theory Comput. 2005, 1, 827.

The structure, dipole moment, binding energy, and vibrational frequencies of HCN-BF3 are investigated via 12 DFT methods as well as MP2, MCQCISD, and MCG3 calculations. By comparing the DFT results both to experimental data and to the results from the post-Hartree-Fock molecular orbital methods, we gauge the effectiveness of various density functionals in modeling this fairly weak donor-acceptor system. For structural data, B3PW91, B98, and mPWPW91 give results that compare favorably with experiment. All DFT methods that yield a reasonable structure predict dipole moments that are only slightly larger than the experimental value by 0.1 to 0.2 debye. Moreover, to ensure that a comparison of calculated (equilibrium) and experimental (vibrationally-averaged) data is indeed valid for this system, the B-N distance potential is calculated using B3PW91, MP2, and MCG3, and the one-dimensional Schroedinger equation for motion along this bond-stretching coordinate is solved to obtain vibrational energy levels, wave functions, and expectation values. In every instance, average bond lengths differ by only a few thousandths of an angstrom from the corresponding equilibrium values and dipole moments are also unchanged to within hundredths of a debye. For vibrational frequencies, B3PW91 agrees most closely with gas-phase experimental data for BF3 and also with MP2 calculations of the BF3-localized modes in the complex; mPW1PW91 and B3LYP agree nearly as well. However, despite the effectiveness of DFT for structure and vibrational frequencies, all DFT methods fail to predict a binding energy that compares favorably to the MCG3//MCQCISD result of -5.7 kcal/mol.

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