Influence of Isotopologue Dipole Moments on Precision Dielectric-Constant Measurements

Measurements of the relative permittivity (static dielectric constant) of fluids such as methane have been interpreted with the assumption of zero dipole moment. This assumption is not strictly true, due to the presence of isotopologues with small, nonzero dipole moments. We investigate the significance of this effect by analyzing the effect of the dipole of CH3D on the static dielectric constant of methane. It is found that the isotopologue effect is more than two orders of magnitude smaller than the uncertainty of the best existing measurements. Similar estimates for other compounds such as H-2 and CO2 produce even smaller effects. Therefore, the interpretation of these measurements with a dipole moment of zero remains valid.