A theoretical study of the molecular structures and vibrational spectra of the N2O•••(HF)2

Theoretical calculations using both the MP2 and B3LYP levels of calculation with a 6-311++G(3df,3pd) basis set have been performed to determine stable structures and molecular properties for the H-bonded complexes involving nitrous oxide (N2O) and two HF molecules. Five complex have been characterized as minima since no imaginary frequency was found. Three complex are predicted to be relatively more stable with binding energies varying from 14 kJ mol(-1) to 23 kJ mol-1 after BSSE and ZPE corrections. Our calculations have revealed that the second complexation with HF preferably occurs with the first complexed HF molecule, i.e., forming the X center dot center dot center dot H-F center dot center dot center dot H-F skeleton with X = O or N instead the F-H center dot center dot center dot N-N-O center dot center dot center dot H-F one. As expected, the H-F chemical bonds are increased after complexation due to intermolecular charge transfer from n isolated pair of the X atom (X = N, O or F) to the sigma anti-bonding orbital of HF. For the strongly bounded complex, the doubly complexed HF molecule acts as a bridge between the two end molecules while transferring electrons from N2O to HF. Both possess the same amount of residual charge but with opposite signs. The H-F stretching frequency of the monoprotic acid is shifted downward after complexation whereas its IR intensity is much enhanced. This increase has been adequately interpreted in terms of equilibrium hydrogen charge and charge-flux associated to the H-F stretching using the CCFOM model for infrared intensities. This procedure has also allowed to analyze the new vibrational modes arising upon H-bond formation, especially those associated with the out-of-plane and in-plane HF bending modes, which are pure rotations in the HF isolated molecule. (C) 2011 Elsevier B.V. All rights reserved.