ISOTOPE EFFECTS ON THE STABILITY OF THE CARBON-MONOXIDE ACETYLENE VANDERWAALS MOLECULE AND THE HYDROGEN-FLUORIDE DIMER

Harmonic vibrational frequencies for the H-bonded and D-bonded isotopomers OC...HCCD and OC...DCCH and for DF...HF and HF...DF are calculated at the SCF and MP2 ab initio levels of theory, using large basis sets. The relative stabilities of the H-and D-bonded dimers are determined from differences in the zero-point energies. It is found that the D-bonded species are energetically favoured over the H-bonded species, in accordance with experiment. The energy of (OC...H 13CCH)-C-13 is computed to be 0.8 cm-1 lower than that of (OC...HC13CH)-C-13 at the MP2 level: combining this with an observed intensity ratio of 4:1 indicates a temperature of 0.8 K following the supersonic expansion.