QUADRUPOLE INTERACTIONS IN PURE NONDIPOLAR FLUORINATED OR METHYLATED BENZENES AND THEIR BINARY-MIXTURES

Quadrupole-quadrupole interaction energies for benzene (Bn), 1,3,5-trimethylbenzene or mesitylene (M), hexamethylbenzene (HMB), 1,3,5-trifluorobenzene (TFB) and hexafluorobenzene (HFB) and their binary mixtures have been calculated. The equations to obtain these energies, which were previously wrongly identified in terms of the pairwise interactions involved (N. M. D. Brown and F. L. Swinton, J. Chem. Soc., Chem. Commun., 1974, 770), are presented. Three molecular pair orientations were considered: face-to-face, edge-to-edge and mutually perpendicular. It is found that the preferred orientation for the pure components is that where both molecules are mutually perpendicular. For the equimolar mixtures the preferred orientation is determined by the sign of the product of the quadrupole moments of the two components. For the mixtures of HFB with Bn, M or HMB the quadrupole-quadrupole interaction energies are large and the preferred orientation is that where the molecules are oriented face-to-face. For the other mixtures the interaction energies are small and the stable orientation is either face-to-face or mutually perpendicular. The results are in agreement with available solid-liquid phase equilibria and X-ray studies which indicate the existence of 1 : 1 complexes for this type of mixture. It is concluded that quadrupole-quadrupole interactions are responsible for the existence and molecular orientation of such complexes.