Noncovalent Interactions in Complexes Involving the Cyclic C2H2X (X = O, S, Se) Molecules and the Lewis Acids YF (Y = F, Cl, Br, H)

A computational study of the cyclic Lewis bases C2H2X (X = O, S, Se) interacting with the Lewis acids YF (Y = F, Cl, Br, H) in model C2H2X...YF dyads was undertaken. It was shown that the noncovalent X...Y interaction in these binary complexes increases with increasing YF dipole moment for the C2H2O complexes, but not for the C2H2S and C2H2Se complexes, where instead the X...Y interaction increases with increasing X/Y electronegativity difference. This suggests that electrostatic forces dominate the binding in the former, whereas polarization effects dominate in the latter complexes. These explanations were supported by natural bond orbital (NBO) and atoms in molecules (AIM) analyses of the electron density, as well as interaction energy decomposition analyses. A subsequent study of the cooperative effect of an additional hydrogen bond in the model triads, FH...C2H2X...YF and C2H2X...YF...HF, found that the X...Y interaction in the dyads is weakened in the former and strengthened in the latter, with the trends identified in the dyads persisting in both triads.