Structure and bonding studies of ClX-C₂H₄ (X = H or Br) van der Waals complexes

High-quality ab initio calculations at the MPn (n = 2, 3 or 4) levels for the π-donor complex formed by HCl and ethene, and the recently characterized ClBr-ethene complex are presented. Interaction energies were calculated with the inclusion of both basis-set superposition error and zero-point energy corrections, resulting in values of about 1 kcal mol^-1 for both complexes. The total charge densities for both complexes yielded molecular graphs indicative of the weak binding in these molecules, and correspondingly, calculated charges showed that the XCl moiety has an overall slightly negative charge, while the ethene moiety is slightly positive. Analysis of the Laplacian of the charge density showed that the geometry of the ClBr- C₂H₄ complex may be understood in terms of the VSEPR model as an example of an AXYE₃ molecule.