The totally symmetric vibrations of NH3 center dot center dot center dot HF and NH3 center dot center dot center dot HCN: a CCSD(T) study including anharmonicity effects

Abstract

The coupled cluster variant CCSD(T) has been employed to calculate potential energy and dipole moment surfaces for the hydrogen-bonded complexes (NH3HF)-H-... and (NH3HCN)-H-..., restricting the treatment to the totally symmetric coordinates. Both complexes have C-3v symmetry, with equilibrium dissociation energies (D-c) 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 (NH3HF)-H-..., the umbrella bending vibration of this complex as well as for (NH3HCN)-H-... 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 (NH3HF)-H-... or (ND3DF)-D-.... The wavenumbers of the intermolecular stretching vibrations are predicted as v(4) = 277 cm(-1) ((NH3HF)-H-...) and v(5) = 154 cm(-1) ((NH3HCN)-H-...). Owing to a large quadratic coupling term in the electric dipole moment function, the band v(4)((NH3HF)-H-...) has virtually zero intensity. The effects of deuterium substitution on wavenumbers and IR intensities have been studied for both complexes.