July, 1958
DIELECTRIC RELAXATION TIMES OF SOME AMINES
773
and entropy of vaporization of W03, together with we can calculate the dissociation energies given in the calculated free energy change at 1368”K., Table IX. are given in Table VIII. The agreement among TABLE IX the three authors is good on AFISe8, whereas a disHEATSOF DISSOCIATION crepancy exists between Berkowitz’sZ0values on the one hand and Ueno’sz and ours on the other for A@, AHT and A&. Reaction kcal./mole Combining our data with the heats of vaporiza284.6 i 1.7 MoOi(g) +MO(g) 20 tion of Moe and the dissociation energy of O ! Z , ~ ~ Mo08(g) +Mo(g) + 30 410.3 ==! 2 . 4
+
(23) P. Brix and G. Herzberg, Can. J . Phys., 32, 110 (1954).
220.9 f 7 . 5
(MOO8)8(g) +3MoOdg)
THE DIELECTRIC RELAXATION TIMES OF SOME AMINES, DIMETHYLTHIANTHRENE, DIBENZOTHIOPHENE, TRIPHENYLPHOSPHINE AND TRIPHENYLARSINE BY A. H. PRICE‘ The Edward Davies Cheinistry Laboratories, University College of Wales, Aberystwyth, Wales Received August 13, 1957
The effective dielectric relaxation time of a polar solute will vary markedly from the value for an equivalent rigid structure if the molecule contains internal relaxation mechanisms arising from rotating groups., inversion phenomena (as in ammonia), or flap ing (as in thianthrene), provided the associated frequencies are not too different from those used in the measurements. &udies of a number of different model structures have been made in this way from 3 X lo4 to 108 c.!s. With the exception of tribenzylamine the molecules studied ap ear to be essentially rigid. The influence of the viscosity of the medium upon the relaxation times and the adequacy of t t e Hill treatment of this factor is also considered.
The frequency dependence of the dielectric properties of materials was first studied by Debye. He related the macroscopic dielectric properties with the molecular structure, assuming a rigid spherical molecule having a single relaxation time proportional to the macroscopic viscosity of the medium and the molecular volume. Further extensions of the theory have been made t o inchdemon-spherical molecules, distributions of relaxation times and the correct form of the viscosity factor.2 The work described here is a survey of the dielectric absorptions of a series of molecules in the pure state and in solution, with the particular aim of assessing whether they behave as rigid structures or not. The influence of non-rigidity is very apparent for many polar solutes recently studied. Experimental
measurable loss tangent) of viscosity 29 c.P., had a loss tangent of 0.60 X at the same frequency. Initially, this change in solvent loss tangent was not fully ap reciated and led us erroneously to postulate a resonance aisorption for triethylamine.6 The other solvents behaved normally. The xylene properties have been described elsewhere,* while tetrachloroethylene showed no loss tangent in the range of the apparatus. The relaxation time ( T ) is calculated for the pure media and for the concentrated solutions from the Debye equation l
The dielectric properties of the materials were studied in the frequency range 3 X lo4to 108 c./sec. using a Hartshorn-Ward apparatus.’ The general characteristics and limits of accuracy found for this apparatus have been discussed by Aihara and Davies.8 Briefly, the loss tangent (tan 6) could be measured to an accuracy of h 3 % or h0.06 X 10-3 (whichever was the greater) and the permittivity to hl%. The systems studied were either ure liquids or solutions in Nujol, xylene or tetrachloroethyfene. For solutions the loss tangent was taken as the difference between the solution and solvent loss tangent, but a correction had to be applied to the solvent loss with Nujol as the solution viscosity was different from the viscosity of the pure solvent. Thus, Nujol (viscosity approximately 200 c.P.) had a loss tangent of 0.93 X at 70 Mc. sec., while a solution of Nujol in tetrachloroethylene (which showed no
where c , No, T, k and are the molar concentration, Avogadro’s number, the absolute temperature, Boltzmann’s coqtant and the dipole moment, respectively. Both equations 1 and 2 may be expressed in the form
( 1 ) Atomic Weapons Research Establishment, Aldermaston, Berks, England. (2) (a) C. J. F. Bottoher, “Theory of Electric Polarization,” Elsevier Publishing Co., Amsterdam, 1953; (b) M. Davies, Quart. Revs. London, 8 , 250 (1954). (3) A. Aihara and M. Davies, J . Colt. Sci., 11,671 (1956). (4) L. Hartshorn and W. H. Ward, J.I.E.E., 79, 597 (1936).
tan S =
(e.
- n2)
w7
+
1 O)T* where cs is the static dielectric constant, n2 the square of the refractive index and E’ the permittivity a t frequency f (where 0 = 2 ~ f ) .For dilute solutions where (ea - n2) is very small equation 2 is used E
(3) Thus a plot of w/tan 8 against w a should be linear with
