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VOLUME 72, NUMBER 5 MAY 15, 1968
The Tempierature Variation of the Dipole Moment of o-Dimethoxybenzene by Louis M. DiBello, Helen M. McDevitt, and Dominic M. Robert? Chemistry Department, Villanova University, Villanova, Pennsylvania 19086
(Received January 2, 1968)
The dipole moment of o-dimethoxybeiizene has been measured over a temperature range of from 25 to 160" for liquid o-dimethoxybenzene and over a total range of from -20 to 165" for solutions of o-dimethoxybenzene in benzene, decalin, and paraffin oil. The dipole moment increases with temperature with nearly the same slope in all four series of measurements. The dipole moments calculated from measurements on decalin and paraffin oil solut,ions become constant above 90" a t a value below that expected for free rotation. The exclusion of certain rotational positions because of steric factors may account for the difference. The dipole moments of m- and p-dimethoxybenzene, measured on solutions in decalin, were found to remain constant over the temperature range from 20 to 130".
Previous studies of o-dimethoxybenzene show that, unlike m- and p-dimethoxybenzene, its electric dipole moment varies with temperature. Mizushimaza reported values of from 1.19 to 1.33 D for solutions of odimethoxybenzene in an unspecified solvent from 10 to 40", and Jatkar2b found values from 1.15 to 1.31 D for the liquid froin 25 to 85". I n addition, while all three isomers show shortened dielectric relaxation times, interpreted af; indicating rotational orientation of the methoxy groups, only the ortho isomer showed two separable dispersion regions.a By choosing solvents with a long liquid range, it has been possible to extend the study of the dipole moments of the dimethoxybenzenes to higher temperatures. This study reveals a change in behavior just beyond the range studied in previous works, i.e., a levelling off of the dipole moment to a constant value at higher temperatures for o-dimethoxybenzene in both decalin and paraffin oil. Additional measurements were made on liquid odimethoxyberizene, solutions of o-dimethoxybenzene in benzene, and Eiolutions of m- and p-dimethoxybenzene in decalin, all over a range of temperatures.
Experimental Section o-Dimethoxybenzene was distilled over calcium hydride, and the fraction boiling a t 206-207" was taken, giving a refractive index of 1.5319 at 25". m-
Dimethoxybenzene was distilled over calcium hydride, and the fraction boiling at 216-218" was taken, giving a refractive index of 1.5250 at 20". p-Dimethoxybenzene was purified by distillation, giving a melting point of 56.5-57.0". Paraffin oil, from Fisher Scientific Co. , was used as a solvent without further purification. All measurements made use of oil from a single batch, giving a refractive index of 1.4793 at 25". Benzene was purified by fractional distillation over calcium hydride. Decalin was fractionally distilled over calcium hydride. The fraction boiling from 188 to 189", with a density of 0.88165 a t 25" and a refractive index of 1.4763 a t 20", was analyzed by gas chromatography and found to consist of approximately 60% cis- and 4001, trans-decalin. Dielectric constants were measured at 1.0 hlHa using a General Radio 716-CS1 capacitance bridge with a General Radio 1211-C unit oscillator as the signal source and an Allied Radio R-100A communications receiver, with an S meter, as the null detector. The dielectric cell, similar to that described by S m ~ t h , ~ (1) Address correspondence t o this author a t St. Joseph's College, Philadelphia, Pa. 19131. (2) (a) S. Mizushima, Y. Morino, and H. Okasaki, Sei. Papers Inst. Phys. Chem. Besearch (Tokyo), 34, 1147 (1938); (b) 8. K.K. Jatkar and C. M. Deshpande, J. Indian Chem. Soc., 37, 1 (1960). (3) D. M. Roberti and C. P. Smyth, J. Amer. Chem. Soc., 82, 2108 (1960).
1405
1406 consists of concentric rhodium-plated brass cylinders enclosed in glass. It is estimated that the apparatus yields a precision of 0.1% in the dielectric constant. General Electric No. 10 insulation oil was used in the constant-temperature bath. Low temperatures were attained by immersing a bulb containing Dry Ice and acetone into the bath. Measurements of density, using an Ostwald pycnometer, and refractive index, with an Abbe refractometer, were made for the liquid and for each solution over a range of temperatures from 25 to 50". Extrapolated values were used in calculation of dipole moments at other temperatures. The dipole moment of the liquid was calculated with the Onsager equation,6 using n D 2 for E , . The dipole moments in nonpolar solvents were calculated by the method of Halverstadt and Kurnlerj6 using five solutions and calculating the slopes by a least-squares analysis. The solutions had the following ranges of weight fractions of o-dimethoxybenzene for each solvent: benzene, 0.01-0.16; paraffin oil, 0.003-0.05; decalin, 0.01-0.06. The ranges of weight fractions in decalin were from 0.024 to 0.118 for m-dimethoxybenzene and from 0.0057 to 0.069 for p-dimethoxybenzene.
Results and Discussion Values of the dipole moment of o-dimethoxybenzene are plotted against temperatures in Figure 1. Measurements in benzene give values of 1.30 D at 20" and 1.32 D at 25", identical with values reported by Klages' and TiVeissberger,8respectively. I n all four series the dipole moment increases with temperature at approximately the same rate. The slopes for the range between 25 and 60" are 0.0039, 0.0038, 0.0043, and 0.0038 for the liquid, benzene, paraffin oil, and decalin, respectively. While there may be some error in the slopes for the solutions owing to variation of the solvent effect with t e m p e r a t ~ r e the , ~ similarity of the slope for the solutions and for the liquid seems to eliminate a role for the solvent as an explanation of the temperature variation. The dipole moment reaches a constant value above 90" at 1.58 D in paraffin oil and 1.64 D in decalin. The dipole moment of the liquid has not yet levelled off a t the highest temperature measured, 160", where its value is 1.63 D. The dipole moments of m- and p-dimethoxybenzene in decalin were found to be constant over the range from 20 to 130". Values from measurements made on solutions in decalin have not previously been reported, but the values of 1.48 D for the meta and 1.84 D for the para isomer compare reasonably well with values reported for the other solvents.2a~7i10~11 The temperature variation of a dipole moment, for example, of 1,2-dichloroethane, l 2 has been interpreted as resulting from the effect of temperature on the relative numbers of several conformers of different polarity. The Journal of Physical Chemistry
L. A I . DIBELLO,H. NI. MCDEVITT,AND D. M. ROBERTI
-20
0
20
40
60
80
100
120
140
160
Temp, O C .
Figure 1. The dependence of the dipole moment of o-dimethoxybenzene on temperature for measurements of liquid o-dimethoxybenzene (open circles) and solutions of o-dimethoxybenzene in benzene (closed circles), paraffin oil (diamonds), and decalin (squares).
The observed dipole moment would be the statistical average of the dipole moments of the conformers. At sufficiently high temperatures, the potential energy barriers hindering interconversion of the conformers are surmounted, and a state of essentially free rotation results. The observed dipole moment will then remain constant with further increase in temperature. The free-rotation value of the dipole moment may be calculated by the method of Eyring.I3 For o-dimethoxybenzene, the calculated free-rotation value14 is 1.93 D, still above the maximum values observed in this study. The results of the present study indicate that there are two separate phenomena to be considered for o-dimethoxybenzene. There is first a temperature-dependent restriction on rotation which reduces the contribution of the more polar conformers at lower temperatures. Since this temperature dependence is not observed with the meta and para isomers, it presumably involves relatively short-range interactions, possibly electrostatic interactions between the adjacent methoxy groups. An additional restriction on rotation must be assumed to account for the observation that the maxi-
s. 0. Morgan, J . Amer. Chem. Soc., 50, 1547 (1928). (5) L.Onsager, ibid., 5 8 , 1486 (1936). (6) I. F. Halverstadt and W. K. Kumler, ibid., 64, 29SS (1942). (7) G. Klages and E. Klopping, 2. Elektrochem., 57, 369 (1953). (8) A. Weissberger and R. Sangewald, Physik. Z., 30, 792 (1929). (9) H. 0. Jenkins, Trans. Faraday Soc., 30, 739 (1934). (10) G. Klages and A. Zentek, 2.Naturforsch.. 16a, 1016 (1961). (11) M . Aroney, R. J. W. Le Fevre, and 9. Chang, J . Chem. Soc., 3173 (1980). (12) C . P. Smyth, J . Amer. Chem. SOC.,46, 2151 (1924). (13) H. Eyring, Phys. Rev., 39, 746 (1932). (14) C. P. Smyth, "Dielectric Behavior and Structure," McGraw-Hill Book Co., Ino., New York, N. Y., 1955, p 371. (4) C. P. Smyth and
1407
THECALCULATION OF COHESIVE AND ADHESIVE ENERGIES mum value of the dipole moment observed for odimethoxybenxene in the two solutions which showed a levelling was still considerably smaller than the value calculated for free rotation. Examination of molecular scale models shows that certain rotational positions are restricted 13terically. The cis-cis position in which both methoxy carbons are adjacent and coplanar with the benzene ring is most severely restricted. I n addition, other positions in which the methoxy carbons
are coplanar with the ring are somewhat restricted by the ring hydrogens. Reduction of the contribution of these rotational positions may account for the difference between the observed maximum dipole moment and the calculated free rotation value.
Acknowledgment. The authors wish to acknowledge the contribution of Villanova University in providing financial support for carrying out this study.
The Calculation of Cohesive and Adhesive Energies from Intermolecular Forces at a Surface
by J. F. Padday and N. D. Uffindell Research Laboratories, Kodak Ltd., Wealdstone, Harrow, Middlesex, England
(Received November 16, 1967)
Surface tensions of the n-alkanes and interfacial tensions between the %-alkanesand water have been calculated. The calculations use a modified form of the Moelwyn-Hughes’ equation for the dispersion interaction between two particles, the integration method of Hamaker to derive the total interaction across a plane surface, the geometric mean relationship of Good and Girifalco for the interaction of two dissimilar phases, and an assumption that the entropy of surface formation equals the difference between the interaction energy so calculated and the total internal energy of surface formation. The calculated suiface tensions of the nalkanes are compared with and agree well with experimentally determined values; also, some of their calculated interfacial-tension, contact-angle, and spreading-coefficient measurements with water all agree with the corresponding experimental values. For other systems, calculations are limited to the contribution of the dispersion forces to the total interaction of the system.
Introduction
WlZ = Wlzd
The work of cohesion, W,, and the work of adhesion, W,, are defined by Young1 and Duprb2 by the equations
w,
=
2YL
w* = YL + Ys + YSL
(1)
(2) W, and W, are both surface free energy terms of an idealized system and in practice their values have been obtained by substituting appropriate surface and interfacial tensionfi into eq 1 and Z3 Such substitution is unjustified for almost all real solid-liquid systems because the surface tension is likely to vary from one part of the surface to another and because some elastic deformation of the bulk solid is inevitable. These calculations are confined to pure liquids and to lowenergy hydrocarbon surfaces where such effects, although present, are unlikely to produce significant errors. The interaction energy W I ~arising from physical or van der Waals forces between two particles is given by
+ WIZD + WIZK
(3)
where W12d is the dispersion contribution and W1ZD and WIZK the respective Debye and Keesom contributions to the total interaction energy. In this treatment, the calculations are restricted to nonpolar systems in which the Debye and Keesom forces are zero. The dispersion interaction energy, W12d, between two particles is the decrease in their potential energy arising from only dispersion forces when bringing them from an infinite distance of separation to a distance T apart and is given by Moelwyn-Hughes4 as (1) T. Young, “Collected Works,” Vol. 1, G. Peacock, Ed., John Murray, London, 1855. (2) A. Dupr6, “Theorie Mecanique de la Chaleur,” Gauthier-Villars, Paris, 1869,p 369. (3) N.K. Adam, “The Physics and Chemistry of Surfaces,” 3rd ed, Oxford University Press, London, 1941,p 2; F. 0. Koenig, 2.Elektrochem., 57, 361 (1953); R. E. Johnson, J. Phys. Chem., 63, 1656 (1959). (4) E. A. Moelwyn-Hughes, “Physical Chemistry,” 2nd ed, Pergamon Press Ltd., London, 1961,p 392.
Volume 76,Number 6 M a y 1968
