Dipole induction and the solvent effect in dipole moment measurements

It has been conclusively proved in the last two years that there is a significant effect of the solvent on the measured value of dipole moment of a solute. It has also been shown that the effect is mainly determined by the dielectric constant of the medium, suggesting a relatively simple physical cause, but the precise law of relationship is still disputed. A list of the formulae which had been proposed at the time of the Faraday Society's discussion is collated by Goss. These all have the status of empirical formulae, since that of Raman and Krishnan contains two difficult accessible constants. The theory formula of Weigle is omitted from this list. It receives some discussion in the next section of this paper. These formulae agree in disregarding the properties of the solute, excepting its dipole moment. The solvent effect is considered to be proportionately the same for all polar substances. Müller makes an exception in suggesting a modified law for certain substances, mainly ketones. The hydrogen halides, with positive solvent effects, are considered entirely anomalous, and their increase of moment is attributed to an intramolecular change towards more ionic linkage.