The Totally Symmetric Vibrations of NH3···HF and NH3···HCN: a CCSD(T) Study Including Anharmonicity Effects

Abstract

Abstract The coupled cluster variant CCSD(T) has been employed to calculate potential energy and dipole moment surfaces for the hydrogen-bonded complexes NH3···HF and NH3···HCN, restricting the treatment to the totally symmetric coordinates. Both complexes have C 3ν symmetry, with equilibrium dissociation energies (D e) predicted to be 52.1 and 27.1 kJ mol−1, respectively. Vibrational term energies and wavefunctions as well as absolute IR intensities are calculated under inclusion of anharmonicity effects. In contrast to previous theoretical work for NH3···HF, the umbrella bending vibration of this complex as well as for NH3···HCN is found to be rather anharmonic. The familiar double harmonic approximation fails to yield reliable values for the IR intensities of the NH3 (ND3) symmetric stretching vibrations of either NH3···HF or ND3···DF. The wavenumbers of the intermolecular stretching vibrations are predicted as ν4 = 277 cm−1 (NH3···HF) and ν5 = 154 cm−1 (NH3···HCN). Owing to a large quadratic coupling term in the electric dipole moment function, the band ν4(NH3···HF) has virtually zero intensity. The effects of deuterium substitution on wavenumbers and IR intensities have been studied for both complexes.