Theoretical study of the interaction between monohalogenated ethylenes and water

The complexes between monohalogenoethylenes (X = F, Cl, Br) and water have been studied theoretically at the MP2/6-311++G(3df,2p) level. The calculations include the optimized geometries, the interaction energies along with a natural bond orbital (NBO) analysis. Stable linear and cyclic structures are formed. In the linear structures, the CH…O hydrogen bonds are rather weak, the binding energies ranging from 2.4 to 4.9 kJ mol−1 and being ordered as CH2CHF < CH2CHCl < CH2CHBr. As indicated by the changes of the CH bond length and the ν(CH) stretching frequencies, the complexes are at the border between blue- and red-shifted hydrogen bonds. In the closed structures, the molecules are held together by CH…O and OH…X hydrogen bonds. These structures are more stable than the linear ones, the binding energies ranging from 6 to 7.7 kJ mol−1. A contraction of the CH bond of the CHX group and a blue shift of its stretching frequency is predicted when the X atom participates in the formation of the closed structure. A NBO analysis shows that the governing factor for the decrease of the σ∗(CH) occupancies is the decrease of the intramolecular hyperconjugation energy rather than the intermolecular charge transfer.