Dimethyl sulfoxide and dimethyl sulfone. Heat capacities, enthalpies of

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PROPERTIES OF DIMETHYL SULFOXIDE AND DIMETHYL SULFONE

1309

Dimethyl Sulfoxide and Dimethyl Sulfone. Heat Capacities, Enthalpies of Fusion, and Thermodynamic Properties1 by H. Lawrence Clever and Edgar F. Westrum, Jr.2 Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48104 (Received March 19,1969)

Heat capacities of dimethyl sulfoxide (DMSO) from 5 to 350°K and of dimethyl sulfone (DMSO2)from 5 to 420°K together with the enthalpy increments for melting [3434 cal/mol at 291.67"K for dimethyl sulfoxide 4374 cal/mol at 382.01"K for dimethyl sulfone] were determined by adiabatic calorimetry. Values at 298.15"K of C,, So, (H" - Hoo)/T,and - (Go - H"o)/T are 36.61, 45.12, 27.74, and - 17.39 for DMSO(1) and 29.96 34.77, 17.11, and -17.66 for DMSOz(c). The potential energy barriers to methyl group rotation in both substances were estimated both by analysis solely of the low-temperatureheat capacity of the solid and from a thirdlaw entropy cycle involving the ideal gas evaluated from spectroscopic data and molecular parameters. The latter method gives a barrier 2.8 and 3.4 kcal/mol for each methyl group in DMSO and DMS02. Chemical thermodynamic properties of the ideal gases are evaluated to 1000°K. No corroborative evidence was found for the dielectric anomaly previously reported for DMSO.

Introduction Chemical thermodynamic research on a substance with as dramatic a chemical and pharmaceutical nature as that of dimethyl sulfoxide, (CH3)B0 or DMSO, hardly needs further justification. Despite the systematic penetration of the skin, remarkable dipolar aprotic solvent characteristics, unique chemical reactivity, and association into molecular chains in the liquid phase, this compound, known for more than a century, has been extensively investigated only in recent years. Since relatively few thermochemical and thermal data exist on dimethyl sulfoxide and dimethyl sulfone, (CH3)2S02or DMSO2, this study provides precise cryogenic thermal properties of both compounds, answers to relevant structural questions, and resolution of the considerable discord among the few thermodynamic properties reported or estimated. Thermal data on compounds containing methyl groups are interesting since they permit determination of potential barriers hindering internal rotation of these groups. Moreover, many methyl containing compounds have solid-solid transitions ; e.g., methan01,~" acetone, 3b methylamine, methylamine hydrochloride, 3d methyl mercaptan, methyl bromide, 3f and neopentane.3e Two of these, methanol and neopentane, have entropies of melting less than the 5 cal/(mol OK) characteristic of a plastic crystaL4 Dimethyl sulfoxide, (CHs)dO, and dimethyl sulfone, (CH3)2S02, show some properties suggestive of plastically crystalline behavior. The report of a small entropy of melting,6 AS,, of only 5.4 cal/(mol OK) for dimethyl sulfoxide based upon cryoscopic depression suggested that this substance might also be a plastic crystal. However, subsequently reported determinations indicated values of 10.4s and 11.1' based upon cryoscopy and 11.4 f 0.2 cal/(mol based upon direct calorimetric determination.

The heat capacities (C,) for the solid below the melting point [19.8 cal/(mol "1.01and for the liquid above the melting point (33.4) reported by the latter investigators* are so at variance both with our determinations and with another calorimetrically determined value of 46 for the liquid near 305'KD that reservations seemed appropriate. The trend of the dielectric loss factor us. temperaturelo for solid dimethyl sulfoxide has a maximum between 165 and 180°K. Dimethyl sulfone has a relatively high melting point (109OC) in comparison with the melting points of compounds such as SOzC12 and SOzF2, which are similar in structure and molecular weight. Mackle and O'Hare" made a statistical calculation of the ideal gas thermodynamic properties (1) This work was supported in part by the Division of Research of the United States Atomic Energy Commission. (2) To whom correspondence concerning this work should be addressed. (3) (a) 1%.G. Carlson, U. 8. Atomic Energy Commission Reports, TID-15153,1962; (b) K. K. Kelley, J. Amer. Chem. SOC.,51, 1145 (1929); (c) J. G.Aston, C. W. Siller, and G. H. Messerly, ibid., 59, 1743 (1937); (d) J. G. Aston and C. W. Ziemer, ibid., 68, 1405 (1946); (e) H.R. Russell, Jr., D. W. Osborne, and D. M. Yost, ibid., 64, 165 (1942); (f) C.J. Egan and J. D. Kemp, {bid., 60, 2097 (1938); (g) J. G.Aston and G. H. Messerly, ibid., 58,2354 (1936). (4) J. Timmermans, J. Phys. Chem. Solids, 18, 1 (1961). (5) H. L. Schlafer and W. Schaffernicht, Angew. Chem., 72, 618 (1960); cf, H. Prfickner, Erddl Kohle, 16, 188 (1963) and W. 0. Ranky and D. C. Nelson in "Organic Sulfur Compounds," Vol. 1, N. Kharasch, Ed., Pergamon Press, London, 1961, p 170. (6) J. J. Lindberg, J. Kenttamaa, and A. Nissema, Suomen KemC

stilehti, 34B, 98 (1961). (7) E.F. Weaver and W. Keim, Proc. Indiana Acad. Sci., 70, 123 (1960). (8) T. Skerlak and B. Ninkov, Glasnik Dru&a HemiEara Technol. 8.R . Bosne Hercegouine, 11, 43 (1962); c f . Chem. Abstr., 61, 2496d (1964)* (9) Y. Murakami and T. Yamada, Kagaku Kogaku, 26, 865 (1962). (10) M. Freymann, Compt. Rend., 253, 2061 (1961). (11) H.Mackle and P. A. G. O'Hare, Trans. Faraday Soc., 58, 1912 (1962). Volume 74, Number 6 March 19,1070

1310

H. LAWRENCE CLEVER AND EDQAR F. WESTRUM, JR.

of both dimethyl sulfoxide and dimethyl sulfone from spectral and molecular data then available. S a n d P reported the dimethyl sulfone crystal structure. A review by Lindberg13 and two more recent revie~s'~v~6 summarizing the structural and physicochemical properties of dimethyl sulfoxide have appeared.

I: 0

100

OK.

200

300

dnn

20 'K.

30

40

Experimental Section Samples. Both the dimethyl sulfoxide and dimethyl sulfone were supplied by the Crown Zellerbach Corporation, Camus, Wash. The dimethyl sulfoxide was specially purified by multistage contact with activated charcoal and subsequently distilled through a 20plate glass-packed column, under 8 Torr pressure, a head temperature of 78", and a reflux ratio of 4 : l . Approximately 5% forerun and residue were discarded. Light transmission of the product was greater than 100% at 350 mp compared to water. At 275 mp the absorbance was 0.20, at 300 mp 0.03, and 1.0 at 261 mp. The freezing point was 18.51". Maximum volatile impurities determined by gas chromatography were less than 0.04%. It was used as received after thorough degassing through several cycles of freezing, evacuating, and thawing. It was then poured through a closed glass-metal system under a low pressure of helium gas into a gold-plated copper calorimeter (laboratory designation W-2416). Helium gas was added as conduction medium and the calorimeter was sealed with a goldgasketed demountable valve. The dimethyl sulfone was purified by dissolution of 100 g of solute in 66 g of boiling water. Ammonium hydroxide was added to the hot solution until pH 9, and the solution was filtered. Crystallization was spontaneous on slight cooling and performed with omtinuous stirring to 3". The filtered product was washed with water at 0" and air dried at 50", and repeatedly sublimed under vacuum. The dimethyl sulfone melts a t 108.85' and is neutral. No volatile impurities could be detected by gas phase chromatography. The sample was melted and poured into a silver calorimeter (laboratory designation W-22) in the nitrogen atmosphere of a drybox for the heat capacity measurement,s from 300 to 420°K. The gold-plated copper calorimeter (laboratory designation W-17A) was then filled in a similar manner for the low temperature heat capacity determinations. Sample mass (g, in vacuo), gram molecular weights (C-12 basis), and pressure (Torr) of conduction helium gas were respectively dimethyl sulfoxide (5350°K) 86.517, 78.134, 219; dimethyl sulfone (300-420°K) 89.612, 94.133, 123; dimethyl sulfone (5-340°K) 72.007,94.133,269. Cryogenic Apparatus. The Mark I11 c r y o ~ t a t ' ~ with capsule-t ype platinum-resis t ance thermometer A-3 was used for the low temperature heat capacity determinations by the quasi-adiabatic technique.'* The intermediate temperature heat capacities were determined in the iWark IV'# thermostat with the The Journal of Physical ChemtktTy

IO

X

Figure 1. The heat capacity of dimethyl sulfoxide arid dimethyl sulfone showing melting at T,.

platinum-resistance thermometer A-7. The data were converted to thermodynamic functions by the computer program described earlier.20 The heat capacity of the calorimeter-heater-thermometer assembly was determined by a separate series of measurements with small adjustments applied as needed for slight differences in the quantities of helium and thermal conductivity grease between determinations with and without sample. The sample heat capacity as a percentage of the sample-calorimeterheater-thermometer total was : for dimethyl sulfoxide; a maximum of 93% at 1O"K, minimum 68% at 100"K, 75% at 275'K, and 79% in the liquid region; for dimethyl sulfone (low temperature), SO% at IOO"K, increasing to 86% at 340"K, and (intermediate temperature) greater than 78% above 300°K. Manual shield control was used below 80°K; at higher temperatures three separate channels of record(12) D.E. Sands, 2. Krist., 119, 245 (1963). (13) J. J. Lindberg, Finska Kemistsamfundets Medd., 70, 33 (1961). (14) D.Martin, A. Weise, and H. J. Niclas, Angew. Chem., Int. Ed. Engl., 6,818 (1967). (15) W.S. MacGregor, Ann. N . Y . Acad. Sci., 141, 3 (1967); C f . Chem. Abstr., 67, 14797k. (16) E. T. Chang and E. F. Westrum, Jr., submitted to J . PhW. Chem. (17) E.F. Westrum, Jr., J . Chem. Educ., 39, 443 (1962). (18) E.D.West and E. F. Westrum, Jr., in "Experimental Thermodynamics," Vol. 1, J. P. MoCullough and D. W. Scott, Ed., Butterworth and Co., Ltd., London, 1968,p 333. (19) J. C.Trowbridge and E. F. Westrum, Jr., J . Phy8. Chem., 67, 2381 (1963). (20) B. H.Justice, Appendix to Ph.D. Dissertation, University Of Michigan; cf. U. 8. Atomic Energy Commission Report TID-12722, 1961.

PROPERTIES OF DIMETHYL SULFOXIDE AND DIMETHYL SULFONE

1311

Table I: Heat Capacity of Dimethyl Sulfoxide and Dimethyl Sulfone" Dimethyl Sulfoxide T -Series

CP

I-

270.50 27.33 275.42 28.74 282.76 32.47 288.69 73.3 291.15 308 291.41 960 291.52 3,420 291.53 7,080 291.55 8,720 291.55 12,400 291.58 6,890 291.59 5,730 291.61 4,680 292.33 190.7 ----Series II226.06 235.10 243.90 252.47 260.81 268.89 276.67 -Series 76.88 84.47 92.20 100.27 108.95 117.68 125.98 134.09 142.45 151.20 159.64 167.83 175.79 183.60 191.18 198.57

0

T

CP

T

205.80 214.11 223.45

21.54 22.15 22.93

61.88 70.18

A H , Detns. A

298.11 301.38

-

I

-

111

10 63 11.49 12.28 13.05 13.95 14.67 15.40 16.08 16.78 17.47 18.12 18.74 19.32 19.90 20.48 21.10 I

36.49 36.61

Series IV

285.22 291.41 291.51 291.55 291 .57 291.58 293.00 -Series

23.10 23.87 24 66 25.51 26.42 27.50 29.07

Dimethyl Sulfonea

5.50 6.28 6.92 7.63 8.28 8.94 9.63 10.33 11.12 12.03 13.05 14.33 15.79 17.36 19.05 20.90 22.94 25.40 28.36 31.64 35.24 39.25 43.85 49.10 55.05

-

86.7 4,200 13,000 24,800 36,800 16,450 80.7 V-0.037 0.062 0.095

- --0.137

0.195 0.243 0.318 0.410 0.498 0.611 0.766 0.977 1.237 1.526 1.853 2.224 2.639 3.139 3.729 4.371 5.042 5.738 6.468 7.246 8,048

--

T

CP

8.920 9.881

Series VI

61.79 69.19 77.70 86.28

-

Series I 116.40

8.913 9 * 774 10.72 11.69

-

I

36.63 36.73 36.89 37.00 37.15 37.30

287.53 289.51 292.48 295.77 299.05 -Series 191.58 194.75 197.89 200.99 204.07 207.13

--

36.39 36.42 36.44 36.55 36.65

IX

-

20.48 20.71 20.95 21.18 21.42 21.65

10.75 11.63 12.49 13.28 14.08 14.92 15.80 16.67 17.52 18.32

Series I11 6.33 7.21 7.99 8.75 9.47

0.036 0.058 0.078 0.132 0.185

Series IV 6.01 0.028

37.37 37.50 37.59 37.68

Series VIII

15.35

Series I1 73 20 80.61 88.05 ' 95.71 103.62 112.05 121.06 130.26 139.51 148.85

-Series VII-Enthalpy Detn. E AHm Detns. F Enthalpy Detn. G 331.68 337.55 343.38 348.02

T

CP

T

CP

Low-Temperature Series

Enthalpy Detn. B Enthalpy Detn. C AHm Detns. D 298.78 304.43 310.23 316.01 321.75 327.48

CP

6.99 7.82 8.71 9.58 10.51 11.59 12.77 14.10 15.59 17.27 19.19 21.42 24.07 27.21 30.79 34.38 37.89 41.66 45.84 50.54 55-87 62.16 69.07 76.52

0.052 0.079 0.121 0.189 0.269 0.358 0.510 0.700 0.944 1.253 1.645 2.129 2.729 3.444 4.241 5.004 5.704 6.379 7.083 7.809 8.565 9.411 10.259 11.130

11.333 78.24 Enthalpy Detn. A 148.50 18.28 19.05 157.58 19.82 166.94 20.57 176.37 21.32 185.95 22.05 195.53 205.06 22 79 214.75 23.53 224.44 24.26 234.06 25.02 243.62 25.74 253.11 26.45 262.76 27.20 272.45 27.96 281.91 28.70 29.41 291.14 30.11 299.84 30.77 308.67 317.30 31.49 325.77 32.24 334.08 32.92 342.24 33.51 I

Intermediate-Temperature Series Series I '

303.21 311.59 321.53 331.88 342.01 351.97

30 45 31.08 31.81 32,71 33.45 34.04 ~

Series I1 355.42 34.49 365.44 35.28 375.57 36.41 381.27 314.8 381.99 141,800 381.9 43,600 381.99 142,900 381.9 64,700 381 .99 69,600

381.99 295,200 381.99 60,300 382.01 36,500 382.99 220.1 386.07 42.89 Series 111 362.68 35.09 372.82 35.90 380.07 173.4 381.99 46,790 381.99 96,800 381.99 131,270 381.99 160,190 382.00 206 ,900 385.49 90.66 Series IV 299.52 29.99

Enthalpy Detn. A 365.05 35.22 Enthalpy Detn. B 394.22 43.08 398.93 ' 43.35 Series V 285.59 28.86 296.26 29.75 Enthalpy Detn. C Enthalpy Detn. D 388.52 42.83 393.39 42.98 398.27 43.17 403.27 43.30 408.49 43.54 413.95 43.83

Units: cal, mol, OK.

ing electronic circuitry provided with proportional, rate, and reset control actions maintained the temperature of the adiabatic shield within approximately a millidegree of the calorimeter temperature. Thus the energy exchange between calorimeter and surroundings was reduced so that it was negligible in comparison with other sources of error. All measurements of temperature, time, potential, resistance, and mass were referred to standardizations or calibrations of the National Bureau of Standards.

Results and Discussion Heat Capacity. The experimentally measured heat capacities are shown in Figure 1 as a function of temperature and are presented in Table I in chronological order so that temperature increments across individual determinations in a series may be estimated from adjacent mean temperatures. The data are stated in terms of the defined thermochemical calorie exactly equal to 4.1840 J and an ice point of 273.15'K. The low-temperature and intermediate-temperature heat capacities Volume 74, Nunabar 6 March 10, 1070

H. LAWRENCE CLEVERAND EDGAR F. WEBTRUM, JR.

1312 of dimethyl sulfone accord within 0.15% across the overlap region between 290 and 350°K. Two heat capacity measurements in the dimethyl sulfoxide undercooled liquid range were obtained. No vaporization adjustment was made because the vapor pressure data on dimethyl sulfoxide of Douglasz1indicated the adjustment was less than 0.02% Long enthalpy-type determinations extending over relatively large ranges of temperature (Enthalpy Detns. B, C, E, and G for dimethyl sulfoxide, A and C for dimethyl sulfone; cf. Table I) required enthalpy increments which correlated well (*0.05%) with those evaluated by integration of the smooth curves over the same ranges. Such determinations also served to establish that no change in phase occurred, for example, during cooling for determinations in the helium region. Melting. In the course of heat capacity measurements five sets of determinations were made through the dimethyl sulfoxide melting region and four through that of the dimethyl sulfone (cf. Table 11). The enTable I1 : Enthalpy of Melting' Number

Designation

of

detns.

-

H T ~ HT,~

AHrndta

Dimethyl Sulfoxide Series I Detns. A Series IV Detns. D Detns. F

14 2 7 3 2

3963.30 3969.6 3968.7O 3967.4 3960.8

3431 3438 3437 3435 3429 3434 f 3

Dimethyl Sulfone Series I1 Series 111 Detn. B Detn. D

12 8 1 1

4766.60 4750. gC 4753.9 4754.2

4384 4368 4371 4371 - . 4374 f 5

Units: cal, mol, O K . * Tz and TI are 300 and 280' for dimethyl sulfoxide, 385 and 375" for dimethyl sulfone. e Corrected for drift. d Lattice is 577 for dimethyl sulfoxide, 383 for dimethyl sulfone. * Additional 45 cal excess added for 220280°K for the dimethyl sulfoxide.

thalpy of melting for dimethyl sulfoxide is AHm = 3434 3 cal/mol; that for dimethyl sulfone is AHm = 4374 5. The corresponding entropies of melting are ASm = 11.78 f 0.01 and AS, = 11.45 f 0.01 cal/(mol OK), respectively. These values indicate that neither compound is plastically crystalline. The purity and melting point are defined by series I and IV for dimethyl sulfoxide and series I1 and I11 for dimethyl sulfone. The amount of liquid-soluble, solid-insoluble impurity can be estimated from a plot of the apparent melting temperature, T , against the frac-

*

The Journal of Physical Chemistru

tion melted, 1/F. The temperature corresponding to 1/F = 1 is the triple point, T I , of the calorimetric sample. The temperature corresponding to 1/F = 0 is the triple point, To,of the pure sample. The mole fraction impurity, Nz, is given by the expression Nz = AHm(T0 - Tm)/RTo2. Values of T I , To, and Nz, respectively, are for dimethyl sulfoxide 291.59, 291.67, 0.00159, and for dimethyl sulfone, 382.004, 382.01, and 0.00009. Melting points of 18.55,5 18.42,6 and 18.45OZ2have been reported for the sulfoxide; the value 1O9Oz3for dimethyl sulfone is representative of the literature. Thermodynamic Functions. The molal values of the heat capacity, entropy, enthalpy increment, and Gibbs energy function are listed in Table I11 at selected temperatures. The values of the derived thermal properties have been calculated with a high-speed digital computer by integration of a least-squares polynomial fitted through the data points.20 Below 5OK the heat capacity data were extrapolated by means of the Debye T a limiting law. Nuclear spin and isotopic mixing contributions have not been included in the Gibbs energy functions and entropy. Estimated probable errors in the thermodynamic functions are less than 0.1% above 100OK. Barriers to Internal Rotation The magnitudes of the potential energy barriers which hinder internal rotation of the methyl groups in dimethyl sulfoxide and in dimethyl sulfone were estimated by two independent methods. WuZ$'s Method. This approachz4 is based upon an analysis of the contributions to the experimental heat capacity (C,), i e . , the lattice contribution (CVL),that of the internal vibrations (CUI), and internal rotation (CQR),and the difference (C, - C,). The lattice heat capacity is taken as a sum of Debye or Debye and Einstein functions fitted to the experimental C, which has been corrected for C,I and (C, - C,) at relatively low temperatures, The internal vibrations are taken as Zg,E(BE,/T) with the room temperature spectral assignments (assumed to be nearly temperature independent). The (C, - C,) difference is calculated from the quasi-thermodynamic equation of Lord,25 (C, C,) [aCOL bCV1]2T. An Einstein function is chosen to express the torsional oscillations of the methyl groups. After fitting CVLand C,I the contributions (C, - C,) and CUR are empirically adjusted to obtain the best agreement between the observed and calculated C,. The barrier height can be calculated from the equation Vo = 0.049351, X 1040(BE/n)2 cal/mol in which IPis the

+

(21) T. B. Douglas, J . Amer. Chem. Soc., 70, 2001 (1948). (22) T. Smedslund, Nord Kemistmdtet (Nelsingjors), 7, 199 (1950). (23) W. R. Feairheller, Jr., and J. E. Katon, Speckrochim. Acta, 20, 1099 (1964). (24) C. A. Wulff, J. Chem. Phys., 39, 1227 (1963). (25) R. C. Lord, Jr., ibid., 9, 700 (1941).

PROPERTIES OF DIMETHYL SULFOXIDE AND DIMETHYL SULFONE

1313

Table I11 : Thermodynamic Functions of Dimethyl Sulfoxide and Dimethyl Sulfone" Dimethyl Sulfoxide T

5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 273.15 280 291.67

c?J 0.027 0.358 1.092 2.047 3.060 4.051 4.994 5.860 6.651 7.376 8.682 9.870 10.989 12.05 13 05 14.00 14.89 15.74 16.56 17.36 18.15 18.91 19.66 20.38 21.10 21.83 22.61 23.45 24.33 25.26 26 29 27,67 28.27 30.07 I

I

...

SO

Crystal 0.009 0.100 0 374 0.816 1.381 2,072 2.723 3.448 4.184 4.923 6.386 7.814 9.206 10.562 11.884 13.174 14.430 15.656 16.852 18.022 19.168 20.291 21 394 22.476 23.54 24.59 25.62 26.64 27.66 28.67 29.68 30.70 31.02 31.74 32. 56b I

I

Ha

Diimethyl Sulfa)ne

- HOo

0.03 0.78 4.28 12.08 24.84 42.63 65.28 92.44 123.8 158.8 239.3 332.1 436.4 551.7 677.3 812.6 957.1 1110.2 1272 1441 1619 1804 1997 2197 2405 2619 2842 3072 3311 3559 3816 4085 4173 4373 4602b

-(Go

-

Hoo)/T

T

CP

0.002 0.022 0.088 0.212 0.388 0.606 0.858 1.137 1.434 1.746 2.398 3.070 3.751 4.432 5.112 5.786 6.454 7.115 7.768 8.413 9.049 9.678 10.298 10.911 11.52 12.11 12.70 13.29 13.87 14.44 15.00 15,57 15.74 16.13 16.7gb

5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 273.15 280 290 298.15 300 350 382.01b

0.016 0.210 0.843 1.817 2.940 4.070 5.128 6.087 6.949 7.730 9.120 10.37 11.54 12.66 13.71 14.73 15.70 16.64 17.55 18.42 19.26 20.07 20.85 21.63 22.40 23.17 23.93 24.70 25.46 26.22 26.99 27.76 28.00 28.54 29.32 29.96 30.10 34.07

382. O l b 390 400 410

...

Liquid 291.67 298.15 300 350

a

Units:

...

44.34b 45.12 45.35 51.09

36.61 36.65 37.74

cal, mol,

OK.

b

8036b 8270 8339 10198

16.7gb 17.39 17.56 21.95

...

42.94 43.20 43.66

s o

Crystal 0.005 0.056 0.246 0.616 1.141 1.778 2.486 3.234 4.002 4.775 6.130 7.811 9.274 10.698 12.087 13,442 14.765 16.059 17.326 18.566 19.782 20.974 22.14 23.29 24.42 25.53 26.63 27.71 28.77 29.83 30.87 31.91 32.23 32.93 33.94 34.77 34.95 39.89 42 97 I

Liquid 54.42 55 31 56.40 57.47 I

X o - Hoo

0.02 0.44 2.89 9.44 21.31 38.85 61.89 89.97 122.6 159.3 243.7 341.3 450.9 571.9 703.8 846.1 998.2 1160.0 1331 1511 1699 1896 2100 2313 2533 2761 2996 3239 3490 3749 4015 4288 4376 4570 4859 5101 5156 6760 7891 12265 12606 13036 13470

-(Go

-

H"o)/T

0.001 0.012 0.054 0.144 0.289 0.483 0.718 0.985 1.278 1.589 2.248 2.936 3.637 4.343 5.048 5.750 6.446 7.136 7.819 8.494 9.162 9.822 10.47 11.12 11.76 12.39 13.01 13.62 14.23 14.84 15.43 16.02 16.21 16.61 17.19 17.66 17.76 20.57 22.31 22.31 22.99 23.81' 24.62

Assuming enthalpy of melting isothermal a t T,.

reduced moment of inertia of the methyl rotation, OE is the Einstein temperature obtained from the fitted C,R curve, and n is the number of maxima encountered in one complete revolution of the internal rotator. The Of the heat capacity are given in Table IV. For dimethyl sulfoxide the spectral assignments of Horrocks and Cottonz6confirmed by Carter, Freeman, and H'enshal127were used. The lattice heat capacity Of the flattened dimethyl sulfoxide molecule28-a1is well represented by

a combination of Debye and Einstein terms since it has a molecular weight large relative to its moment of (26) W. D. Horrocks, Jr., and F. A . Cotton, Spectrochim. Acta, 17, 134 (1961). (27) J. H. Carter, J. M. Freeman, and T. Henshall, J . Mol. Spectros., 20, 402 (1966). (28) 0. Bastiansen and H. Viervoll, Acta Chem. Scand., 2 , 702 (1948). (29) H. Dreizler and G. Dendl, Z . Naturforsch., 19a, 512 (1964). (30) R . Thomas, C. B. Shoemaker, and K. Eriks, Acta Cryst., 21, 12 (1966).

Volume 74, Number 6 March 10, 1970

H. LAWRENCE CLEVWR AND EDGAR F. WESTRUM, JR.

1314.

Table IV : Estimation of Magnitude of Barrier Heights by Wulff's Method Dimethyl sulfoxide

+

CSL

CVR

a b n Barrier height Torsional libration

Dimethyl sulfone

+

4D(130/T) 2D(265/T) 2E(385/T) 0.0050 0.0050 3 4 . 1 kcal/mol

3D(130/T) 3D(197/T) 2E (380/T) 0.0064 0.0016 3 4 . 2 kcal/mol

268 cm-'

264 cm-1

+

inertia; CVL = 4D(130/T) 2D(265/T) gives a methyl rotation barrier of 4.1 kcal/mol. Analysis of the distorted tetrahedral dimethyl sulfone molecule12 with the spectral assignment of Feairheller and K a t ~ nand ~ ~two Debye functions to represent the lattice contribution i.e., COL = 3 0 . 3D(197/T) corresponds to a methyl (130/T) rotation barrier of 4.2 kcal/mol. Third-Law Method. The height of the potential barrier was also obtained by comparison of the experimental and statistically calculated values of the vaporphase third-law entropies for both compounds. The third-law, ideal-gas entropy of dimethyl sulfoxide was calculated from the liquid entropy with an enthalpy of vaporization and a compression correction deduced from the vapor-pressure equation of Douglas log P (Torr) = 26.49558 - 3538,32/T - 6.0000 log T and A H , = 16,196 - 11.922T. The resulting third-law ideal-gas entropy of 73.32 cal/(mol OK) at 2 9 8 ~ 5 ° K was matched exactly by the spectroscopic entropy (cf. next section) provided assignment of a barrier of 2.8 kcal/ mol (222 cm-l) was made for each of the two methyl groups. The third-law, ideal-gas entropy of dimethyl sulfone was calculated from the liquid thermal entropy with the vapor pressure equation of Bushfield, Ivin, Mackle, and O'Hare:a2 log P (Torr) = 7.624 - 2241/ (t 223.0) and A H , = (2.303)(2241)RT2(t 223.0)-2. The third-law entropy of 81.05 cal/(mol OK) at 385°K was matched by the statistical entropy (cf. next section) upon assignment of a methyl rotation potential barrier of 3.4 kcal/mol(237 cm-l) for each methyl rotation. Microwave spectroscopic evaluation for the internal rotation barrier in dimethyl sulfoxide yields a value of 3.07 kcal/rn01.~~For dimethyl sulfone a spectroscopic interpretation gives 3.74 kcal/m01.~~Both of these spectral values accord better with thermal values derived by the third-law method than with those based solely on heat capacity data on the crystalline state. The two unobserved torsional modes of dimethyl sulfoxide were assigned by Horrocks and Cottonz6 to 223 It 20 and 197 20 cm-l by analogy with trimethylphosphine. Mackle and O'Harel' assumed that each

+

+

+

*

The Journal of Physical Chemistry

methyl group experiences an identical restriction to internal rotation and assigned an intermediate value of 210 cm-' to the S-CHa torsional libration. This S-CHa corresponds to a barrier of 2.6 kcal mol-'. The values from the present research with new molecular parameters and guidance from the third-law values to evaluate barrier heights (222 cm-l, 2.8 kcal/ mol) in conjunction with the tables of Pitzer and Gwinn96 are in reasonable accord with these earlier estimates and direct spectroscopic determination. For dimethyl sulfone, our rotational barrier and torsional modes (3.4 kcal and 237 cm-l) are in good accord with the value determined of Fujimori, but are considerably higher than the admittedly tentative assignment of 106 cm-' suggested by Feairheller and Katon.2a Thermal Properties from Molecular Data. The idealgas, rigid-rotator, harmonic-oscillator, statistical-thermodynamic properties of dimethyl sulfoxide were calculated with the vibrational assignments of Horrocks and Cottonz6 and the structural parameters from an analysis of the microwave spectra by Dreizler and Dend12Qgiven in Table V. The corresponding properties for dimethyl sulfone were calculated with the vibrational assignments of Feairheller and K a t ~ nand ~ ~ the X-ray crystal structure parameters of Sands12 also given in Table V. As usual, in both instances the effects of vibrational anharmonicity, rotation-vibration interaction, centrifugal stretching, etc., have been neglected. It has been assumed further that the barriers are of simple cosine shape, and that the Pitzer-Gwinn tables for internal rotation apply. On the basis of the above vibrational assignments, moments, reduced moments of inertia, and potential barriers to internal rotation discussed in previous sections, values for the molal thermodynamic functions for both compounds were computed by standard procedures and are compiled in Table VI. Significant differences appear between these values and those devised earlier on less definitive data." Comparison of the third-law and spectroscopic entropies is shown at three selected temperatures for both compounds in Table VII. The temperature dependence of the values is reasonable. Gibbs Energies of Formation. Douglasa6 made a calorimetric study of the direct oxidation of dimethyl sulfide to dimethyl sulfoxide and of dimethyl sulfoxide to dimethyl sulfone at 291°K. For the reaction

(31) M. A. Viswamitra and K. K. Kannan, Nature, 209, 1016 (1966). (32) W. K. Busfield, K. J. Ivin, H. Mackle, and P. A . G. O'Hare, Trans. Faraday SOC.,57, 1058 (1961). (33) H. Dreieler and G. Dendl, 2. Naturforsch., 20% 1431 (1965). (34) K. Fujimori, Bull. Chem. SOC.Jap,, 3 2 , 1374 (1959). (36) K. S. Pitzer and W. D. Gwinn, J . Chem. Phys., 10, 428 (1948). (36) T. B. Douglas, J . Amer. Chem. Soc., 6 8 , 1072 (1946).

PROPERTIES OF DIMETHYL SULFOXIDE AND DIMETHYL SULFONE

1315

Table V: Fundamental Vibrations and Structural Parameters Used in the Calculation of Ideal Gas Statistical Thermodynamic Properties

Fundamental vibrations (in cm-1) Structural parameters

Dimethyl Sulfoxide Reference 26 2973 (4), 2908 (2), 1455, 1440, 1419, 1405, 1319,1304, 1102,1016,1006, 929, 915, 689, 672, 382, 333, 308 Reference 29

Dimethyl Sulfone Reference 23 3008 (2), 2965 (2), 2922 (2), 1423 (2), 1407 (2), 1343, 1329, 1291, 1135, 1114, 1008, 995, 940, 756, 700, 502, 468, 383, 320, 292 Reference 12

1.810 d 1.477 d 1.095 d 96" 23' 106' 43' 107" 31'

1.778 A 1.445 A 1.095 A 103" 0' 108' 48'

c-s s-0 C-H L csc L cso L SCH L os0

..,

...

117" 54' 5.26 x 10-40 7356.9 x

5.08 x 10-40 2880.0 X 10-117

(int. rot.) D = IAIBIC

p

Table VI : Thermodynamic Properties of Dimethyl Sulfoxide and Dimethyl Sulfone in the Ideal Gas State

liquid dimethyl sulfoxide a t the same temperature. For formation of ideal gas as defined by the reaction 2C(c, graphite)

Dimethyl Sulfoxide 273.15 298.15 400 500 600 700 800 900 1000

71.51 73.32 80.24 86.39 92.01 97.17 101.99 106.45 110.63

20.21 21.39 25.86 29.37 32.32 34.81 36.97 38.83 40.46

58.22 59.40 63.69 67.75 71.33 74.57 77.79 80.73 83.52

13.29 13.92 16.55 18.64 20.68 22.60 24.20 25.72 27.11

(1

72.34 74 I47 82.20 89.17 95.64 101.41 106.90 111.97 116.70

58.49 59.75 64.50 68.75 72.68 76.34 79.85 83.15 86.27

13.85 14.72 17.70 20.42 22.96 25.07 27.05 28.82 30.43

2C(c, graphite)

+ 3H&) + O&) + S(rhomb) = (CH&S02(c)

+ 3Hdg) +

This value was obtained after revision of Do~glas'3~ value for AHf"of dimethyl sulfone by adjustment of the enthalpy increment for the hydrogen iodide oxidation,37 use of the most recent AHtO of dimethyl sulfide,38 and adjustment of the temperature to 298.15"K. The result compares well with the (adjusted) value of AHfO of (- 107.45 i 0.7) kcal/mol from heat of combustion on dimethyl sulfone. The studies of Busfield, et corresponding A&" and AGP" values are -118.2 cal/ (mol OK) and -72.2 kcal/mol. Busfield, et ~ 1 . , * give ~ an (adjusted) value of AHrO for the ideal gas state

1/202(g)

2C(c, graphite)

22.25 23.68 29.04 33.34 36.81 39.61 42.01 44.04 45.84

Units: cal, mol, OK.

2C(c, graphite)

= (CH,),SO(g)

the AHfO and AGfO are (-51.56 i 0.15) and (-33.49 i 0.15) kcal/mol, respectively. These values involve Douglas'21 vapor pressure equation and the thermodynamics of the 2S(rhomb) = Sz(g) ~ u b l i m a t i o n . ~ ~ A AHf" of -107.31 kcal/mol at 298.15"K was obtained for the reaction

Dimethyl Sulfone 273,15 298.15 400 500 600 700 800 900 1000

+ 3Hz(g) + '/zOz(g) + '/zSz(g)

+ S(rhombic) = (CH&30(1)

AH*' at 298.15OK is (-48.86 i 0.1) kcal/mol for liquid dimethyl sulfoxide after we updated the enthalpy increment of the hydrogen peroxide decomposition used by Douglas with recent critically evaluated data,37 employed the recent AH~Oof dimethyl sulfide reported by McCullough, et U Z . , ~ * and adjusted t o 298.15OK. Use of the dimethyl sulfoxide entropy gives A&" = -83.35 cal/(mol OK) and AGf" = -24.01 kcal/mol for

+ 3Hz(g) + Odg) + S(rhomb) = (CH&SO,(g>

(37) D. D.Wagman, W. H. Evans, V. B. Parker, I. Halow, S. M. Bailey, and R. H. Schumm, "Technical Note 270-3," National Bureau of Standards, Washington, D. C. 20234. (38) J. P. McCullough, W. N. Hubbard, F. R. Frow, I. A. Hossenlopp, and G. Waddington, J . Amer. Chem. Soc., 79, 561 (1957). (39) W. K.Busfield, H. Mackle, and P. A. G. O'Hare, Trans.Fara-

day SOC.,57, 1054 (1961).

Volume 7.4, Number 6 March 10,io70

H. LAWRENCE CLEVERAND EDGAR F. WESTRUM, JR.

1316

-

Table VI1 : The Third-Law and Spectroscopic Entropies of Dimethyl Sulfoxide and Dimethyl Sulfone at Several Temperaturesa T o r AT

Property

Dimethyl Sulfoxide

at 288.W K

Debye Ta extrapolation Numerical quadrature of IC, dT/T, crystal AS," = AH,"/Tm, melting

0-5°K 5" - T m 291.67"K

-8 or ASat 320'K

0.01 32.55 11.78 7

Numerical quadrature of IC, dT/T, liquid So(liquid) A S v = AH,/T, vaporization R In P,compression to 1 atm Ideal gas correction (assumed) S"(idea1 gas) third law entropy

Tm-T

T

-

0.78

3.40

45.12 42.39 -14.19 0.00 73.32

47.74 38.69 -11.33 0.00 75.10

51.09 34.35 -8.06 0.00 77.38

73.32

74.82

76.92

S"(idea1 gas) spectroscopic entropy (cf. text)

T

at

350'K

~

6.75

Dimethyl Sulfone at 385'K

382.01

11.45 A

7

Numerical quadrature of I C , dT/T, liquid So(liquid) AS, = AH,/T, vaporization R In P,compression to 1 atm Ideal gas correction (assumed) #"(ideal gas) third law entropy

Tm-T T

S o(ideal gas) spectroscopic entropy (cf. text) a

Units: cal (mol

of (-89.1 reaction

f.

2C(c, graphite)

0.33 54.75 35.22 -8.92

1.98 56.40 33.53 -7.61

0.14

82.32 f 0.18

3__ .58 58.00 31.98 -6.40 0.00 83.58 f 0.24

81.06

82.21

83.31

0.00

KKz

~

0.00

OK).

0.7) kcal/mol at 298.15"K. For the

+ 3H&) + Oz(g) + l/zSz(g)

=

(CHa)zSOz(g)

AH^" and AGf" are - 104.4 and -75.1 kcal/mol, respectively, a t 298.15"K if use is made of the recent enthalpy increment3' for 2S(rhomb) = Sz(g). In calculating AHt" from enthalpy of combustion data, the values were adjusted to the average value of two recent determinations of the enthalpy of formation of aqueous sulfuric Their values of AH" at 298.15"K for the reaction S(rhomb)

at 415'K

0.01 42.96

Debye T3 extrapolation Numerical quadrature of I C a dT/T, crystal AS," = AH,"/T,, melting

0-5°K 5" - Tm

at 400'K

+ Hdg) + 20&) +

115H20(1) =

[H2S04y

115H201(1)

-212.17 and 212.24 kcal/mol are averaged as (-212.20 i 0.04) kcal/m01.~~ The J O U Tof~Physical Chmistrg

The Dielectric Anomaly. No evidence for the dielectric anomaly observed by Freymann'O was noted in either the regular heat capacity determinations or in series IX heat capacity determinations with small temperature intervals. Thermal Properties of the Liquid Phase. As pointed out by Lindberg13 the high polarity and pyramidal structure of the dimethyl sulfoxide molecule occasions organization in the liquid state which is reflected in the relatively high freezing and boiling points and high enthalpy of vaporization. As may be seen in Figure 1, the heat capacity of the liquid is about 9 cal/(mol "I