Direct measurement of small differences in dipole moment and polarizability

Abstract

An instrument for the direct measurement of dielectric constant differences between gases of similar polarity is described. The method is based on the pressure dependence of the frequency ratio between two LC oscillators, the capacitive cell of each of which contains one of the gases. A sequence of measurements at different temperatures yields the dipole moment and polarizability differences. A check of the method with a pair of known gases, HCl and HBr, yielded: [µ(HCl)–µ(HBr)]= 0.284 D; [(P_E+P_A)(HCl)–(P_E+P_A)(HBr)]=–2.40 cm³/mol; [α(HCl)–α(HBr)]=–0.95 × 10^–24cm³/molecule. The error in the distortion polarization difference is 0.05 cm³/mol and the dipole moment difference is precise to 0.002 D.