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The coupled cluster variant CCSD(T) has been employed to calculate potential energy and dipole moment surfaces for the hydrogen-bonded complexes NH 3 ···HF and NH 3 ···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 NH 3 ···HF, the umbrella bending vibration of this complex as well as for NH 3 ···HCN is found to be rather anharmonic. The familiar double harmonic approximation fails to yield reliable values for the IR intensities of the NH 3  (ND 3 ) symmetric stretching vibrations of either NH 3 ···HF or ND 3 ···DF. The wavenumbers of the intermolecular stretching vibrations are predicted as ν 4  = 277 cm −1  (NH 3 ···HF) and ν 5  = 154 cm −1  (NH 3 ···HCN). Owing to a large quadratic coupling term in the electric dipole moment function, the band ν 4 (NH 3 ···HF) has virtually zero intensity. The effects of deuterium substitution on wavenumbers and IR intensities have been studied for both complexes.

The Totally Symmetric Vibrations of NH 3 ···HF and NH 3 ···HCN: a CCSD(T) Study Including Anharmonicity Effects