A theoretical study of the hydrogen bonding between the vic-, cis- and trans-C2H2F2 isomers and hydrogen fluoride

MP2/6-31++G(d,p) and B3LYP/6-31++G(d,p) theoretical calculations have been employed to investigate the hydrogen bonding formation involving the vic-, cis- and trans-C2H2F2 isomers and hydrogen fluoride. Our calculations have revealed for each isomer the preferential existence of two possible hydrogen-bonded complexes: a non-cyclic complex and a cyclic complex. For all the three isomers the binding energies for the non-cyclic and cyclic hydrogen complexes are essentially equal using both the MP2 and B3LYP calculations, being that the cyclic structure is slightly more stable. For instance, the binding energies including BSSE and ZPE corrections for the non-cyclic and cyclic structures of cis-C2H2F center dot center dot center dot HF are 8.7 and 9.0 kJ mol(-1), respectively. using B3LYP calculations. The cyclic complex formation reduces the polarity, in contrast to what occurs with the non-cyclic complex. This result is more accentuated in vic-C2H2F2 center dot center dot center dot HF. In this latter, Delta mu(cyclic) is -3.07 D, whereas Delta mu(non-cyclic) is +1.92 D using B3LYP calculations. Their corresponding MP2 values are +0.44 D and -1.89 D, respectively. As expected, the complexation produces an H-F stretching frequency downward shift, whereas its IR intensity is enhanced. On the other hand, the vibrational modes of the vic-, cis- and trans-C2H2F2 isomers are little affected by complexation. The new vibrational modes due to hydrogen bonding formation show several interesting features, in particular the HF bending modes which are pure rotations in the free molecule. (C) 2008 Elsevier B.V. All rights reserved.