An ab initio MP2 study of HCN-HX hydrogen bonded complexes

An ab initio MP2/6-311++G* * study has been performed to obtain geometries, binding energies and vibrational properties of HCN-HX H-bonded complexes with X = F, Cl, NC, CN and CCH. These MP2/6-311++G** results have revealed that: (i) the calculated H-bond lengths are in very good agreement with the experimental ones; (ii) the H-bond strength is associated with the intermolecular charge transfer and follows the order: HCN-HNC approximate to HCN-HF > HCN-HCl approximate to HCN-HCN > HCN-HCCH; (iii) BSSE correction introduces an average reduction of 2.4 kJ/mol on the MP2/6-311++G** binding energies, i.e. 11% of the uncorrected binding energy; (iv) the calculated zero-point energies reduce the stability of these complexes and show a good agreement with the available experimental values; (v) the H-X stretching frequency is shifted downward upon H-bond formation. This displacement is associated with the H-bond length; (vi) The more pronounced effect on the infrared intensities occurs with the H-X stretching intensity. It is much enhanced after complexation due to the charge-flux term; (vii) the calculated intermolecular stretching frequencies are in very good agreement with the experimental ones; and, finally, (viii) the results obtained for the HCN-HX complexes follow the same profile as those found for the acetylene-HX series but, in the latter case, the effects on the properties of the free molecules due to complexation are less pronounced than those in KCN-HX.