A study of ethylene carbonate and ethylene monothiocarbonate as solutes in a thermotropic nematic phase

JOURNAL

OF MAGNETIC

RESONANCE

8,222-229

(1972)

Study of Ethylene Carbonate and Ethylene ~onothioca~~o~at~ Solutes iu a Therruotropic Nematic Phase M. A. RAZA Department

of Chemistry,

University

AND L. W. REEVES* of Waterloo,

Waterloo,

Ontario,

Canada

Received January 3 1,1972 The proton magnetic resonance spectra of ethylene carbonate (EC) and ethylene monothiocarbonate (EMTC) have been studied in the nematic solvent N-(p-methoxybenzylidene)-p-n-butyl-aniline (I). The results are compared with a previous study of the same molecules dissolved in a lyotropic liquid crystal phase (9). Considerable differences in the interproton distance ratios between thermotropic and lyotropic phases are explained in terms of significant changes in geometry. The differences in ring puckering between lyotropic and thermotropic phases are probably extreme because of the strong intermolecular force fields of the ion-dipole or hydrogen bonding type which are present in the lyotropic phase in contrast to the nonspecific van der Waals forces in thermotropic phases. INTRODUCTION A reasonably large number of studies of the nuclear magnetic resonance (NM spectra of molecules in nematic liquid crystals of the thermotropic type have been made

(I). Lyotropic nematic phases have been used in a few cases (Z-4), but only one solute, benzene, has been studied in both types of nematic solvent (4-8). The orientation of the benzene in the thermotropic nematic crystal is perpendicular to that in Iyotropic liquid crystals (4) and the degree of order is considerably smaller. The solubility of benzene in the lyotropic phase is less than 1% by weight. Lyotropic liquid crystals offer considerable advantages in regard to resolution of spectra, since sample spinning perpendicular to the principal magnetic field direction is possible without disturbing the oriented solute (2). We have recently studied ethylene carbonate (EC) and ethylene monothiocarbonate (EMTC) in the lyotropic phase described by Lawson and Flautt (2). The lyotropic phases are quite different in character to thermotropic phases in having different superlattice structures of oriented cylinders or of compact spheres (isotropic) and layer arrangements (smectic) (10,JI). We report here therefore studies of EC and EMTC in a thermotropic nematic phase for comparative purposes. EXPERIMENTAL

The solvent used for the present studies was N-(p-methoxybenzylidenej.p-n-butyl aniline (supplied by Distillation Products Division of Eastman Kodak Co.)

* This work was supported by the Defence Research Board and the National Research Council of Canada in grants to L.W.R. Copyright Q 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.

222

ORGANIC

CARBONATES

IN NEMATIC

PHASES

223

This solvent in the pure state has a crystalline to nematic transition temperature of 18°C and passes to the isotropic liquid at 41°C. Both EC and EMTC were maintained at 5 wt % concentration in the nematic solvent. Proton magnetic resonance spectra were observed in the HR mode of a Varian HA100 spectrometer and calibrated by audio side band techniques (12). Peak positions could be reproduced from 5 independent spectra to &l hertz. The line widths could be reduced to about 8 hertz with nonspinning samples and the ambient temperature of the NMR probe at 30.5”C. Uniformity of temperature throughout the sample were excellent as indicated by lack of deterioration in resolution at the extrema of the spectra. A spectral analysis program, for oriented solute molecules, called NMRFIT was used to optimize the fit of computed and experimental spectra (9). EC and EMTC was supplied by Eastman Organic Chemicals and purification was achieved as in our previous report (9). COMPUTATION

OF

INTERPROTON

DISTANCE

RATIOS

The arrangement of four proton spins in a plane with C,, symmetry has been treated previously and leads to values of the interproton distance ratios given below (17).

1

4 c

2

C 3

D

(5) =[(12!)2 + p!““‘-’ I43

+ 04gy3

- D13[(3

+ (g3”3~2

PI = 0

The dipolar coupling constants are derived from an analysis of the spectrum of the partially ordered solute molecule. The value of (ri4/yi2) was obtained by computing the function represented by Eq. [3] and obtaining the reasonable value, which makes the function zero. In DZh symmetry the four protons are arranged at the corners of a rectangle and the ratio (r3Jr12) becomes unity. Motional constants C,,,-,, and C+,,Z could be determined from the expression of Snyder (8) for Dij, the partially averaged dipole-dipole coupling constants. RESULTS

The spectrum of ethylene carbonate in the nematic solvent I is shown in the upper part of Fig. 1. The frequency markers refer to the center of the spectrum as an arbitrary zero. The inverted peaks in the spectrum are the second side bands of the other half of the spectrum from the modulation frequency in the Varian integrator of 2500 hertz.

224

RAZA

AND

REEVES

ORGANIC

CARBONATES

IN NEMATIC

PHASES

225

The center of the spectrum, which does not contain any transitions, is omitted for convenience of presentation. The lower part of Fig. 1 is the simulated spectrum from the best spectral parameters obtained by iteration using NMRFIT. The agreement between experimental and computed spectrum is shown in Table 1, the numbers of each TABLE

1

COMPUTEDAND

EXPERIMENTALLINEPOSITIONSFOR

THE SPECTRUM

OF EC IN THE NEMATIC SOLVENT I AT 30.5”C”

Line

Experimental position

1 2 3 4 5 6 I 8 9 10 11 12

-3062.0 -2915.0 -2678.0 -2657.0 -1775.0 -1688.0 1687.0 1774.0 2657.0 2678.0 2975.0 3062.0

PHASE OF

Theoretical Position*

Intensity

-3062.3 -2975.2 -2678.3 -2656.1 -1774.6 -1687.5 1687.5 1774.6 2656.1 2678.3 2975.2 3062.3

2.0 2.0 3.6 2.4 2.0 4.0 4.0 2.0 2.4 3.6 2.0 2.0

R.M.S. error = 0.5 hertz a The numbered transitions correspond identified in Fig. 1. ’ Positions are in hertz at 100 megahertz center of the spectrum. TABLE

to those to the

2

SPECTRAL PARAMETERS J(i, j) AND 0(&j) DERIVED FOR THE SPECTRUM OF EC IN THE THERMOTROPIC NEMATIC SOLVENT I AND CORRESPONDING VALUES REPORTED FOR THE.LYOTROPICNEMATIC PHASE (9) Chemical shifts 6, = 82 = s3 = 6, = 0 J(l, 2) = J(3,4) = -7.50 J(l, 3) = J(2,4) J(l, 4) = J(2, 3) = +8.77 Lyotropic D(1, 2) = D(3, 4) D(1, 3) = 0(2,4) D(1,4) = 0(2,3) c3z2-12

CX2L92 a Motional constants studies. Concentrations All measurements of megahertz.

-388.8 +47.8 +211.1 +0.02042 -0.02032

phase

= f7.10 Present

work

+3166.5 -58.1 -858.5 -0.1663 +0.03458

are also compared for the two in the lyotropic phase was 5 wt ‘/& frequencies are in hertz at 100

224

RAZA

AND

REEVES

transition corresponding to those in the figure. In Table 2, the spectral parameters are listed and compared with results in the lyotropic phase. The scalar couplings have been measured and were reported previously (9). The motional constants for the same molecule in the two phases are also given. The identification of the J’s and D's with the nuclei as, e.g., D(i,j) can be made by referring to the trapezium in the limit of a rectangle 9

Y d”

-2373.0

-1275.5

1056.1

FIG. 2. Proton magnetic resonance spectrum of EMTC in the nematic state of solvent I. The zero reference frequency is set in the center region of the spectrum which is omitted. The best fit computed spectrum is shown below. Transitions are numbered and listed in Table 3.

in the previous section. The 12 transitions can be interpreted in terms of a planar and rectangular array of 4 nuclei (AA’,“,“‘) in D,, symmetry with no chemical shift. In Fig. 2 the spectrum of EMTC (AA’BB’) oriented in solvent I is reproduced in a similar manner. Eighteen numbered transitions are observable and these were also optimized to the computed spectral fit shown in the lower part of the figure. The values in Table 3 show the agreement between experimental and theoretical line positions.

ORGANIC

CARBONATES

IN NEMATIC

TABLE

PHASES

3

EXPERIMENTAL AND THEORETICAL LINE POSITIONSIN THE NMR SPECTRUMOF EMTC IN THE NEMATIC PHASE OF SOLVENT(I)

Line

Theoretical

Experimental position”

1

-2196.0 -2155.0

2 3

-1999.0 -1880.0 -1741.0 -1646.0 -1434.0 -1389.0 1219.0 1294.0 1476.0 1676.0 1706.0

4 5 6 7 8 9

10 11 12 13 14 15 16 17 I8

1844.0 1963.0 2029.0 2088.0 2196.0

Position”

Intensity

-2196.9 -2155.3 -1998.9 -1879.8 -1741.3 -1646.0 -1433.5 -1388.4 1219.3 1295.2 1476.9 1675.0 1705.2 1843.7 1962.8 2027.9 2088.4 2196.6

2.0 1.9 3.2 2.1 0.6 0.5 1.9 3.4 3.0 2.0 0.9 1.7

1.0 0.5 2.8

0.4 2.0 1.6

R.M.S. Error = 0.7 hertz u Positions are in hertz at 100 megahertz relative to the center of the spectrum. TABLE

4

SPECTRAL PARAMETERSJ(i, j), D&j) AND MOTIONAL CONSTANTSFOR EMTC INTHENEMATICPHASEOFSOLVHVTI~ Chemical shifts

6, = S2, S3 = S4; S, - S3 = 98.8 (Lyotropic) = 83.9 (Present work) J(1, 2) = J(3, 4) = -7.50 J(1, 3) = J(2, 4) = +6.95 J(I,4) = J(2, 3) = +6.98 Lyotropic phase DU, 2) D(1, 3)= D(I, 4)= 0(3,4)

G.2LZ CLy2

D(2, D(2,

4) 3)

-357.9 +25.7 +114.9 -524.8

Present work t2102.6 -13.9 -459.3 +254.5.8

+0.01879

-0.1104

-0.00964

+0.01823

a Corresponding quantities for spectra in the lyotropic solvent are listed for comparative purposes (9). All measurements of frequencies are in hertz at 100 megahertz.

228

RAZA AND REEVES

The spectral parameters are listed in Table 4 and compared with the results from the Iyotropic phase (9) as are the motional constants C,,,-,, and C,,-,,,. In both cases EC and EMTC the corresponding dipolar coupling constants change sign between lyotropic and thermotropic phases as observed for benzene earlier (4). The larger geminal dipolar coupling D(3, 4) in EMTC is again associated (9) with the methylene group adjacent to the sulphur atom in the ring. The internal chemical shift between the two sets of methylene protons is considerably smaller in the thermotropic solvent (83.9 hertz) than in the lyotropic solvent (98.8 hertz). Some intermolecular interaction involving the ring sulphur is indicated by an upfield shift of 8.5 hertz for the adjacent protons in going from lyotropic phase to the neat compound (9), a further upfield shift is observed in the thermotropic nematic phase. Interproton are listed for values in the experimental

DISCUSSION distance ratios have been computed for EC using Eqs. [l] to [3] and these thermotropic solvent I used here and cornpared with the corresponding lyotropic solvent in Table 5 (9). The differences in these ratios are outside error estimated as hO.005. The systematic differences inherent in using

INTERPROTON

DISTANCE RATIOS LING CONSTANTS

TABLE 5 M EC AND EMTC LISTED

FROM DIPOLAR IN TABLES 2 AND 4.

EthyleneCarbonate Ratios of distances Presentwork Lyotropic Model I” 1.000 1.000 1.000 Yw/r,* 1.358" 1.389 1.358 r*4/r** 1.686 1.711 1.690 rf31r12 EthyleneMonothiocarbonate Lyotropic Ratios of distances Presentwork phase 0.938 0.880 r3.h 1.362 1.433 r141r12 1.672 1.713 rd12

COUP-

Model IIb 1.ooo 1.272 1.616

n Assuming a planar ring and angle H-C-H = 1ll”, and angle H-C-C = 114.6”. bAssuming a planar ring and the anglesH-C-H and H-C-C to be tetrahedral, 109.5”. c The error from random sourcesis ?tO.O05 in theseratios. two widely different media must reside in the intermolecular force field to which the solute molecule is subjected. In the lyotropic phase which contains water and sodium ions the hydrogen bonding at any of the ring oxygens or ion-dipole association with the sodium ions can alter geometry. In addition, any small ring puckering motion will be influenced by intermolecular force fields. These differences in apparent geometry between two orienting solvents are the first to be reported. It is of course entirely conceivable that small changes in geometry do occur when a gas molecule passes into a nonpolar solution; but the effect is much more drastic when the strong intermo1ecula.r force fields in lyotropic mesophases are considered (13-16).

ORGANIC

CARBONATES

IN NEMATIC

229

PHASES

Similar changes in interproton distance ratios are noted in Table 5 for EMTC. The rather small ratio 0.880 for (Y+JY~~)is changed considerably to a higher value 0.938 in the thermotropic nematic phase. These ratio changes all suggest that in both solute molecules the small ring puckering motions postulated before (9, 13-16) are even smaller in thermotropic nematic media. The nonspecific van der Waals forces are stronger in this phase (solvent I) while the planarity of the ring may also be perturbed by specific and strong associations with the polar ring atoms in the lyotropic phase. A discussion of the structures will proceed on the basis that EC and EMTC have virtually planar rings in the solvent used in this study. Referring to Table 5, Model I corresponds for EC to a planar five-membered ring c’s HCH = I1 I”, c’s HCC = 114.6”, distance C-H = 1.08A” and a C-C bond distance of 1.52A”. Model II refers to all tetrahedral angles at the carbon atoms, C-H bond lengths of 1.096A” and C-C bond length of 1.52A”. Model II is close to that taken for the hypothetical planar ring in the previous paper (9). In the present study an increase of the H-C-H angles of about 1.5” from a tetrahedral value gives interproton distance ratios in excellent agreement with experiment. There is no possibility of hydrogen bonding to ring oxygen atoms in the thermotropic nematic solvent (I) and the nonspecific van der Waals forces probably result in a more planar ring. While the uncertainty in interproton distance ratios because ofvibrational corrections probably approach 1ltl.5 %, the definite changes in these ratios in passing from one solvent to another exceed the extrema of these errors. No previous structural data for EMTC exists except our previous report in a lyotropic nematic solvent (9). The NMR studies reported here again clearly distinguish that the geminal H-C-H distance in the sulphur adjacent methylene is smaller than in the oxygen adjacent group. This ratio changes from 0.880 to 0.938 in going from the lyotropic to the thermotropic solvent. If the angle H,--C-H2 adjacent to the ring oxygen is assumed to be the same in EC and EMTC, namely, 1I 1O,then the ratio 0.938 enables the angle H,-C-H4 to be calculated as 102” in EMTC. REFERENCES

1. P. DIEHL AND C. L. KHETRAPAL, “NMR Basic Principles and Progress,” (P. Diehl, E. Fluck and R. Kosfeid, Eds.), Vol. 1, Chap. 1, Springer-Verlag, New York, 1969. 2. K. D. LAWSON AND T. J. FLAUTT, J. Amer. Chem. Sot. 89,5489 (1967). 3. P. J. BLACK, K. D. LAWSON, AND T. J. FLAUTT, Mol. Cryst. 7,201(1969). 4. P. J. BLACK, K. D. LAWSON, AND T. J. FLAUTT, J. Chem. Phys. 50,542 (1969). 5. A. SATJPE,Angew. Chem. (int. ed) 7,107 (1968). 6. E. SACKMANN, S. MEIBOOM, AND L. C. SNYDER, J. Amer. Chem. Sot. 89,5981(1967). 7. A. SA~PE, Z. Naturforsch. A 20, 572 (1965). 8. L. C. SNYDER AND E. W. ANDERSON, J. Chem. Phys. 42, 3336 (1965); J. Amer. Chem. 5023(1964). 9. S. A. BARTON, M. A. RAZA, AND L. W. REEVES, J. Mug. Res., in press. 10. V. LUZZATI, H. MUS-CACCHI, AND A. SKOULIS, Discuss. Faraday Sot. 25,43 (1958). 11. G. H. BROWN, J. W. DOANE, AND V. D. NEFF, “A Review of the Structure and Physical

Sot.

86,

Properties of Liquid Crystals,” C. R. C. Press, Cleveland, OH, 1971. 12. J. A. POPLE, W. G. SCHNEIDER, AND H. J. BERNSTEIN, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill, New York, 1959. 13. C. J. BROWN, Acta Crystallogr. 7,92 (1954). 14. C. L. ANGELL, Trans. Faraday Sot. 52,1178 (1956). 15. A. SIMON AND G. HEINTZ, Chem. Ber. 95,2333 (1962). 16. I. WANG, 17. P. DIEHL,

C. 0. BRITT, AND J. E. BOGGS, J. Amer. C. L. KHETRAPAL, AND U. LIENHARD,

Chem. Sot. 87,495O (1965). Can. J. Chem. 46,2645 (1968).