Molecular electrostatics. Part 4.—Determination of disordered-multipole internal electric fields in liquids from the infrared intensity of formally forbidden bands : studies on diphenylacetylene and tetrachloroethylene

Abstract

The fluctuating electric field experienced by a molecule in a fluid due to the randomly changing dispositions of its fellows induces in it a varying dipole moment which has a non-zero mean square value. This results in weak infrared absorption by formally forbidden vibrational modes. If the electric field dependence of the infrared intensity of such a mode is known, the RMS “internal field” of a liquid acting along the relevant axis of an absorbing molecule can be calculated from the observed liquid phase infrared intensity arising from it. This has been done for the symmetric multiple bond CC stretching modes of the title compounds. The infrared intensity of this mode in diphenylacetylene varies with liquid environment and is ascribed to induction by the liquid internal field; that of the corresponding mode in tetrachloroethylene has a constant intensity which arises predominantly from some other effect. The RMS internal field along the CC axis of diphenylacetylene at infinite dilution is (3.2 ± 0.3)× 10⁸ V m^–1 in chloroform and (1.7 ± 0.2)× 10⁸ V m^–1 in carbon tetrachloride, in good agreement with values calculated from dipole and octopole moments, respectively, for these molecules.