Quadrupole moments of benzene, hexafluorobenzene and other non-dipolar aromatic molecules

The electric field-gradient birefringence method for determining molecular quadrupole moments, originally developed and applied to gases by Buckingham, has been extended to the study of nondipolar molecules in solution. Equipment developed for this purpose is described and infinite-dilution molar field-gradient birefringence constants at 298 K and 632.8 nm are reported for benzene, 1,3,5-trimethylbenzene, hexamethylbenzene, 1,3,5-trifluorobenzene and hexafluorobenzene as solutes in carbon tetrachloride. Analysis of results yields the molecular quadrupole moments, which are discussed in relation to the charge distributions in these molecules. The prediction that the quadrupole moments of benzene and hexafluorobenzene are opposite in sign is verified and the nature of molecular interactions involving these molecules is considered.