Electrostatic Energies in the 1,4-Dichlorobenzene Polymorph Crystals: the Role of Charge Density Overlap Effects in Crystal Packing Analysis

Abstract

Electrostatic and polarization energies for the three known polymorphic crystal structures of 1,4-dichlorobenzene, as well as for one particularly stable virtual crystal structure generated by computer search, were calculated by a new accurate numerical integration method over static molecular charge densities obtained from high level ab initio molecular-orbital calculations. Results are compared with those from standard empirical atom-atom force fields. The new electrostatic energies, which include charge density overlap (penetration) effects, are seen to be much larger than and sometimes of opposite sign to those derived from point-charge models. None of the four polymorphs is substantially more stable than the others on electrostatic-energy grounds. Molecule-molecule electrostatic energies have been calculated for the more important molecular pairs in each of the four structures; trends are found to be very different from those indicated by point-charge energies or by total energies estimated with a parametric atom-atom force field. Conclusions based exclusively on analysis of intermolecular atom contacts and point-charge electrostatics may need to be modified in the light of the new kind of calculation.