The Nature of Halogen···Halogen Interactions: A Model Derived from Experimental Charge-Density Analysis

摘要

That non-directional pair-wise atom-atom potentials containing only attractive (r~(-6) dispersion term) and repulsive (inverse-power or exponential term) contributions are insufficient to model Cl···Cl interactions in molecular crystals was appreciated 40 years ago. The layered orthorhombic crystal structures (space group Cmca) of the halogens (Cl2, Br2, I2) cannot be anticipated with isotropic potentials (which would predict, for example, cubic packings, such as the Pa3 structure as seen for N2, NO, and CO), and consideration of previously postulated quadrupole-quadrupole interactions did not resolve this issue. Two possibilities have commonly been invoked to explain these anomalous halogen-atom contacts.