The Electric Moment of Gaseous Molecules of Halogen Hydrides

Dielectric constant of the hydrogen halides HCl, HBr and HI to 300°C.—An improved heterodyne null method of measurement was used in which the beats between two high frequency oscillations from separate electron tube generators are adjusted to the frequency of a tuning fork, the small change of capacity due to the introduction of the gas being compensated by a large change in a large capacity in series with the gas condenser. Measurements were made at atmospheric pressure, using carefully purified gases, over a range of nearly 400° from just above the liquefaction point. The results are well represented by the Debye equation $(\epsilon -1)vT=AT+B$, where $v$ is the specific volume and $T$ the absolute temperature. [The values found for $A$ are.001040 (HCl),.001212 (HBr) and.001856 (HI); and for $B$ are.895(HCl),.52(HBr) and.12(HI)]. This agreement lends support to the Debye classical theory of electric polarization due to fixed moments in the molecule. The values obtained for the electric moment of the molecules are, in ${10}^{-18\mathrm{}}$ c.g.s.u., 1.034(HCl), 0.79(HBr) and 0.38(HI). The Pauli quantum theory gives an equation of the same form as Debye's but leads to smaller values for the electric moment. The upper limit for the moment given by infra-red absorption data for HCl, however, is 6 times the classical value and 13 times the quantum value and hence does not decide between the two theories.