Dielectric Relaxation of Dihalobenzenes. III. Pure Liquids

The real and imaginary components of the complex relative dielectric permittivity have been measured at 100 kHz and at several microwave frequencies for pure liquid ortho‐ and meta‐dihalobenzenes at several temperatures in the range 0–80°C. The data for the difluorobenzenes, for the dichlorobenzenes and for the dibromobenzenes essentially obey the simple Debye theory while the data for the diiodobenzenes lead to the Cole–Cole arc plots. The derived values of the macroscopic relaxation times decrease as the temperature increases in all cases and the value for the ortho isomer is always greater than that for the meta isomer at the same temperature except for diiodobenzene. In addition, the relaxation times increase as the mass of the halogen substituent increases. However, the microscopic relaxation times calculated from the dielectric data are not consistent with the corresponding values calculated from the molecular volumes, temperatures, and viscosities. In all cases, the extrapolated value $ε∞$ is significantly greater than the square of the index of refraction and the difference is approximately proportional to the square of the electric dipole moment. Use of the Onsager relation allows the calculation of the dipole moment from either $ε∞$ or $nD2$⁠. The calculated values increase as the temperature increases but the ratio of the value calculated from $nD2$ to that calculated from $ε∞$ varies inappreciably with temperature. Also the ratio of the dipole moment for the meta isomer to that for the corresponding ortho isomer is almost independent of temperature.