[CONTRIl3UTION FROM THE
FRICK CHEMICAL
LABORATORY, P R I N C E T O N U N I V E R S I T Y ]
Microwave Absorption and Molecular Structure in Liquids. XXIII. The Dielectric Relaxation Times and Dipole Moments of Several Aryl Isocyanates and Related Compounds1 BY BRIANR.
JOLLIFFE AND
CHARLES P. SMYTII
RECEIVED SEPTEMBER 16, 1957 Dielectric constant and loss measurements at wave lengths of 577 m . and 10.0, 3.22 and 1.25 cm. have been carried out a t 20, 40 and 60" upon dilute benzene solutions of phenyl isocyanate, phenyl isothiocyaiiate, p-tolyl isocyanate, tolylene-2,ddiisocyanate, 1-naphthyl isocyanate and 2-biphetiylene isocyanate. The densities, viscosities and refractive indices of tlie solutions have also been measured. The molecular dipole inonients calculated from these data indicate t h a t the direction of the bond from the ring to the hiear isocyanate group is close to, if not coincident with, that of tlie axis of the group. The dielectric data have been used to construct arc plots, from u hich tlie dielectric relaxation times and their almost negligibly small distribution parameters have been calculated. The rather large molecular relaxation times resulting from the considerable protrusion of the polar group from thc ring are found to vary i n tlie expected maimer with the direction of the molecular dipole relative to the molecular axes.
In previous papers of this series relaxation times of polar molecules have been measured both in the pure liquid and in solution in non-polar solvents, and the results discussed in terms of the viscosities of the liquids and the geometry of the molecules. In the present work, the influence of molecular shape on relaxation time has been investigated further in benzene solutions of phenyl isocyanate, p-tolyl isocyanate, tolylene-2,4-diisocyanate, 1naphthyl isocyanate, 2-biphenylyl isocyanate and phenyl isothiocyanate.
niolecular dipole moments by meuis of the equation of Halverstadt and Kumler.8 The m o ~ ~ i e rvalues it thus obtained are given in Table 111, where, in the cases of the three substances previously measured, they are seen to agree with the literature values within the probable error of the latter. Table I1 gives the calculated critical wave lengths and the corresponding relaxation times and, it1 addition, the slopes of the straight lines obtained by plotting the measured viscosities of the solutions it1 ceritipoises against the mole fractions of the polar solutes. Table I1 also gives the heats of activation for dipole relaxation calculated from the slopes of the plots of the logarithm of the critical wave length against the reciprocal of the absolute temperature.
Discussion of Dipole Moments Although the determination of the dipole nioThe solutes were obtained from Brothers Chemical Company and Eastinan Kodak Company. Preliminary meas- ments of these compounds was incidental t o the inurements were made using tlie substames without purifica- vestigation of the molecular relaxation times, the tion, and final determinatioiis were made vvith qmlples moment values in Table I11 have some interest as which had been distilled under reduced pressure. C .P." evidence of structure and thus some significance in grade berizeiie was used tlirougliout the wol-k u-itliout any the interpretation of the values of the relaxation further treatment after xiieasureinents of the static dielectimes. Electron diffraction7 and microwave and tric constant and loss iu the microwave region had shown that, for use iri these sulutions, it wits indistiiiguishabie from infrared have shown the isocyanate and benzene which llad been distilled after drying nritii sodium isothiocyanate groups t o be linear with an angle hydride. 125" t o 132" between the group axis and the It-N Thc techniques used for measuring e ' , the dielectric cunbond in CH3NC0, HNCO, H S C S and DSCS. statit, and E", the loss, at wave lengths ol l 2 Z and 3.22 em. have been described in earlier papers,* as has the method Earlier dipole nioiiient ~iieasurenientson $-substiby which eo, the "static" dielectric constant is measured a t tuted phenyl isothiocyanates'O showed the angle t o a wave length of 5 i 7 meters. The reatliiigs xt 10.0 cm. were made with a resoriatit cavity appar:itus coirstructetl be hardly distinguishable from 1SO" in these aroby M r . D. A . Pitt.3 A t least tliree solutioi;s of each sub- matic compounds, and the niotnent calculated for stance were measured at each frequeucy a t te~nperaturcsof p-chlorophenyl isocyanatell on the assumption of 20, 40 and 60" in tlie concentration range 0.005 to 0.04 mole complete linearity, t h a t is, an aiigle of lSOo, diffraction. The slope a of the hicar plot of E' or e" agaiiist the weight fraction was used as i i i previous to coni- fered from the observed value by no inore than the possible experimental error. The inomelit value pute the positioii of points 011 Cole-Cole arc plots,5 from which the critical wave length aud relasation time for e:icli 2.68 of p-tolyl isocyanate in Table 111 is indistincompound were obtained. guishable from the sum of the phenyl isocyanate Table I shows the results obtained at the wave leugtli moment 2.20 in Table 111 and the toluene moment X~ a t which the measurement was carried o u t . The valuc at ,577 m. wave length is the slope of tlie so-called static di- 0.37 in the vapor state and 0.4 in solution,12 indic n ~ is . tlie electric constaut and the value at 5.89 X cating an angle of lSOo. Calculation of t h e 1110slope of the square of tlie I-ciractive iilries for the sotliurn-r> merit of tolyleiie-2,4-diisocyanate as the vector sum Materials and Experimental Methods
line. These data, together lvitli the measured specific volumes of tlie solutioiis, iiave beeii used to calculate the
(1) T h i s research was supported by the United States Air Force through t h e Office of Scientific Resrarch of t h e Air Research a n d D e velopment Command under contract No. A F 18 (600) 1331 Iceproduction. translation, publication, use o r disposal iu whole cir in part try or for the United S t a t e s Government is p e r m i t t e d . ( 2 ) TV. LI. Iieston. J r . , E.J . Iienne!!y and 0.P.S m y t h , Txils J O U R 11. 1,. l,aqi!er an,! C J'. S n l y t h , rbztl , 70, 4(J07 N ~ T . 7, 0 , 4093 (1948); (1
