Molecular Rotation and Dielectric Relaxation in a Crystal LatticeColin Clemett and Mansel Davies Citation: The Journal of Chemical Physics 32, 316 (1960); doi: 10.1063/1.1700941 View online: http://dx.doi.org/10.1063/1.1700941 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/32/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Deuteron spin–lattice relaxation from hindered rotations in molecular crystals J. Chem. Phys. 73, 2542 (1980); 10.1063/1.440488 Rotational relaxation of molecular fluorine J. Chem. Phys. 67, 1279 (1977); 10.1063/1.434943 Molecular theory of dielectric relaxation J. Chem. Phys. 62, 2130 (1975); 10.1063/1.430779 Rotational relaxation of molecular hydrogen J. Chem. Phys. 60, 3492 (1974); 10.1063/1.1681565 Rotational Relaxation of Molecular Nitrogen J. Chem. Phys. 46, 3418 (1967); 10.1063/1.1841233

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316 LETTERS TO THE EDITOR

low frequency (or at high pressure), Pen is the molar polarization at frequencies above the dispersive region (or at low pressure), and p.w is the pressure at the mid­point of the dispersion curve. The curve is plotted for the values Po=33.21 cc, P",=32.63 cc, and pM=114 mm. This corresponds to a line-broadening constant of 3.5 Mc/mm. No attempt has been made to evaluate the Cole-Cole parameter since the accuracy of the data when the entire dispersion covers a range of only 0.58 cc is inadequate.

Our low-pressure value of 32.63 cc means that the molar polarization measured at 9300 Mc should range from 32.63 to 32.77 cc as the pressure is varied be­tween zero and 1500 mm. Magnuson's value l of 32.97 cc is somewhat higher than this, but the difference is within the experimental error in the absolute value. Magnuson obtained a value of the distortion polariza­tion of 27.45 cc. If we assume that his value for the total polarization should have been higher by about 0.58 cc, the dipole momenc of 1,2-dichlorotetrafluoro~ ethane Lecomes 0.56 rather than the 0.S3 reported by Magnuson. For other gases the error caused by ig­noring the dispersion can be very much greater.

'" This work has been supported by Air Force Contract AF 33(616)-5581.

1 D. W. Magnuson, J. Chern. Phys. 24, 344 (1956). 2 Boggs, Crain, and Whiteford, J. Phys. Chern. 61. 482 (1957). S J. E. Boggs, J. Am. Chern. Soc. 80, 4235 (1958). 4 Boggs, Whiteford, and Thompson, J. Phys. Chern. 63, 713

(1959). 5 J. E. Boggs and A. P. Dearn, J. Phys. Chern. (to be published). 6 E. B. Wilson, Jr., J. Chern. Phys. 63, 1339 (1959). 7 W. Maier and H. K. Wirnrnel, Z. Physik 153, 297 (1958); ibid.

154, 133 (1959). .

Molecular Rotation and Dielectric Relaxa­tion in a Crystal Lattice

COLIN CLEMETT AND MANSEL DAVIES

Tlte Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Wales

(Received August 5, 1959)

PARTICULAR interest attaches to the process of molecular rotation in the solid state, especially in

those cases where the molecule would appear to be distinctly polar. Such behavior is clearly revealed by dielectric dispersion which allows the relaxation time and the energy and entropy factors for the process to be evaluated.

tOC ('0-""') 1012 T sec

TABLE I. Liquid succinonitrile.

59 51.8 18,5

65 50.5 16.,

74 48.4 14.3

tOC (oo-t",) 1012 T sec.

TABLE II. Solid succinotrile

25.5 62,1 61±4

39.8 57 .8

49±3

50.0 55. 0

43±2

We wish to report the findings for succinonitrile (NC·CH2 ·CH2 ·CN, m.p. 56.8°C) whose dielectric properties we have studied in both liquid and solid states up to frequencies of 900 Mcps. Low-frequency measurements were made in the Hartshorn-Ward apparatus and coaxial line measurements (from 250 to 910 Mcps) with the system already described.l

The liquid was examined at 59, 65, and 74°C, each ±0.5°C: l values were ± 1 percent, tano±2 percent. Within the accuracy of the data they fitted the simple (Debye) dispersion relations with the parameters given in Table I and E",=4.0. A previous precise value of Eo

is 56 . .15 at 57°C and fco is coincident with l for the solid below the rotational m.p.2 By writing

r=A exp(MJ/RT) ,

these data give MJ =3.9 kcal/g mole and A =4.8X 10-14

sec. This value of 11}] is precisely that deduced3 from the expression for the viscosity, 1) = A' exp(l1}] 1)/ RT). If one uses the expression

r= (!t/kT) exp[I1H*/RT-115*jR], (1)

one finds I1H*=3.2 kcal/g mole and 115*=0.0 cal/g moleoK.

Somewhat larger uncertainty pertains to the data for the solid: e' values were ±2% and tano±3% at 50° and these increased as the specimens were cooled in the line owing to the tendency for the homogeneous glass-like specimens to develop internal crystal faces and the possible formation of fissures. However, repeated determinations at 50.0, 39.8, and 25.5 (±O.S)OC pro­duced consistent results, the scatter in tano at 25° (where it is a maximum) being ±6%. Again, the data fit a single relaxation time at each temperature: the dispersion factors are summarized in Table II. Here f", was taken as 4.4, partIy on the basis of a measured value near - SO°C and 8.S kMcps. Morgan and White found 62.6 for EO in the solid at 37°C. The rotational freezing-point at which the molecular freedom is lost in the solid is defined by infrared observations as -43.7°C.4

In r=A exp(I1H/RT) the solid data give A =60X 10-14 sec. I1H = 2.7 kcal/ g mole: the values correspond­ing to Eq. (1) are I1H*=2.0g kcal/g mole; 115*= -4.8 cal/g moleoK.

It is particularly interesting to find that I1H is cer-. tainly less than for the relaxation in the liquid. The dielectric data make it clear that the rotation in the solid involves the whole molecule as the f' values lead to a dipole moment (3.SD) essentially the same as in the vapour (3.5D) or in dilute solution (3.8D).5

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LETTERS TO THE EDITOR 31i

The apparent negative entropy of activation in the solid recalls similar values where low !l.H terms ob­tain. 6 Its significance is presumably that the rotation in the solid requires cooperation between adjacent molecules to the extent of increasing the local order.

I Williams, J. Phys. Chern. 63, 534 (1959). 2 Morgan and White, J. Chern. Phys. 5, 655 (1937). 3 Timmermans, Physico-Chemical Constants of Pure Organic

Compounds (Elsevier Publishing Company, Inc., Amsterdam, 1950).

• Janz and Fitzgerald, J. Chern. Phys. 23, 1973 (1955). 5 Bloom and Sutton, J. Chern. Soc. 1941, 727. 6 Davies, Quart. Revs. (London) 8, 267 (1954).

NMR Studies on I-Methoxyvinyl Esters*

MARTIN SAUNDERS, JANIS PLOSTNIEKS, PETER S. WHARTON, AND HARRY H. WASSERMAN

Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut

(Received August 3, 1959)

WE have examined the NMR spectra of compounds of type I

H2 OCHa la, R=CH3

'" / 0 C=C II

/ '" C Ib, R C6H5

HI a~// ~ o RIc, R CFa

I

which have recently been prepared for the first time by the reactions of methoxyacetylene with carboxylic acids. I By determining the shifts of the methylene hydrogens we have obtained information concerning the relative spatial influences of the acyloxy groups, -CH3C02-, CSH 5C02-, and CF aC02- on these methylene hydrogens.

In Table I are listed the chemical shifts and coupling constants of the two methylene hydrogens (obtained from the typical AB quadruplet) as well as the chemical shifts of the methoxyl hydrogens for the three cases studied.

Substitution of a benzoyl group for an acetyl group shifted the methylene hydrogens 0.155 and 0.111 ppm downfield. This shift is probably largely the result of the magnetic anisotropy of the benzene ring, and is in the expected direction and roughly of the expected magnitude assuming that the benzene ring, ester group, and double bond are coplanar and using the theoretical curve of Johnson and Bovey.2

In the case of Ie, measurement of bond distances in the planar model indicates methylene hydrogen dis­tances of approximately 5 and 6 A from the trifluoro-

TABLE 1. NMR spectra of I-methoxyvinyl esters in dilute CC\. solution referred to benzene capillary, 40 Mc (all upfield).

R 01 (ppm) J li (cps) DOCR. (ppm)

CH3 2.81O±0.01 2.855±0.01 3.3 ±0.3 2.924±0.OO5 C,H.2.655±0.005 2.744±0.OO5 3.65±0.1 2.866±0.005 CF3 2.525±0.OO5 2.655±0.OO5 4.55±O.1 2.804±0.005

methyl group, the observed chemical shifts being 0.285 and 0.200 ppm downfield relative to Ia. The tri­fluoromethyl group thus appears to have a large anti­shielding effect compared to methyl.

The effect of benzoyl group is at least partly ac­counted for by its magnetic anisotropy. In the case of the trifluoromethyl group in addition to the magnetic anisotropy a strong polar effect would be expected to operate on the electrons of the intervening bonds so that they in turn would affect the chemical shift of the methylene hydrogens.

In exploring mechanistic aspects of the reaction of carboxylic acids with methoxyacetylene we studied the reaction of O-deutero benzoic acid (C6H 5C02D) with excess methoxyacetylene in the presence of mercuric ion.! The result was a mixture containing four species: la, both monodeutero adducts, and the dideutero ester. t This mixture most probably resulted from ex­change of the acetylenic hydrogen with the deutero acid, as shown by recovery of unreacted methoxy­acetylene containing deuterium. The amount of deuterium found by mass spectral analysis of the re­covered acetylene was that expected from statistical mixing.

The spectrum of this mixture (Fig. 1) contains six peaks in the olefinic hydrogen region (measured In

cps from center of peaks 3 and 4)

1, -4.54 2, -1.66 3, -1.00

4,1.00 5,2.68 6,4.i9.

The two peaks (Nos. 2 and 5) which are present in addition to those found in Ib are assigned to the two

-tolC:,..

FIG. 1. NMR Spectrum of reaction product of C,H.COOD with H-C=C-OCH, (neat).

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