Atomic quadrupolar effect in intermolecular electrostatic interactions of chloroalkanes: the cases of chloroform and dichloromethane

The atomic quadrupolar effect in intermolecular electrostatic interactions is studied for chloroform and dichloromethane. From the fitting to the electrostatic potentials around these molecules obtained by ab initio molecular orbital (MO) calculations, atomic quadrupoles of the magnitude as large as Θ≅1.5ea02 are obtained for the chlorine atoms in these molecules. It is shown that a reasonably good fit to the electric field around the molecules can be obtained only by including those atomic quadrupoles, indicating that atomic quadrupoles are essential for correct representation of the intermolecular electrostatic interactions of these molecules. From the calculations of the radial distribution functions (rdfs) of liquid chloroform and dichloromethane by the Monte Carlo and molecular dynamics methods, it is shown that a significant atomic quadrupolar effect is seen in the rdfs of the C…C, C…H and H…H pairs. The intermolecular distances in the optimized structures of the dimers of chloroform and dichloromethane calculated by the ab initio MO method support the new features in the rdfs of the liquids that are seen upon including the effect of atomic quadrupoles. It is suggested that an atomic quadrupole has a significant effect on the angular (rather than radial) component of the location of the atoms of neighboring molecules.