Quantum chemical characterization of the structural and energetic properties of HCN-BF₃

The structure, dipole moment, binding energy, and vibrational frequencies of HCN-BF₃ are investigated via 12 DFT methods as well as MP2, MC-QCISD, and MCG3 calculations. By comparing the DFT results to both experimental data and results from 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 provide 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 D. 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 Schrödinger equation for motion along this bond-stretching coordinate is solved to obtain vibrational energy levels, wave functions, and expectation values of the B-N distance and dipole moment. In every instance, average bond lengths differ by only a few thousandths of an angstrom from the corresponding equilibrium values, and dipole moments are 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, dipole moment, and vibrational frequencies, all DFT methods fail to predict a binding energy that compares favorably to the MCG3//MC-QCISD result of-5.7 kcal/mol.