Neutron powder diffraction studies of di­methyl sulfoxide

The crystal structure of per­deutero­di­methyl sulfoxide, (CD₃)₂SO, has been refined at 2 and 100 K, and characterized as a function of temperature up to 275 K against high-resolution neutron powder diffraction data. The structure determined previously by Thomas, Shoemaker & Eriks [Acta Cryst. (1966), 21, 12–20] (T = 278 K) is shown to remain down to 2 K. At 2 K, the S—O bond distance is 1.495 (2) Å. The fact that the mol­ecule is distorted from ideal C_s point symmetry may be explained by the short D...O contacts of the respective methyl groups.

S1. Comment

The coordination chemistry of sulfoxides boasts an extensive literature covering structural, spectroscopic and quantum chemical studies [see, for example, Caligaris (2004) and references therein]. However, in this recent review on metal sulfoxide complexes, it is noted that the reference value for the S—O bond length in pure dimethyl sulfoxide (DMSO) dates back to limited X-ray single-crystal studies by Thomas et al. (1966) and Viswamitra & Kannan (1966). The present study, using modern high-resolution neutron powder diffraction techniques, aims to provide a more accurate reference structure at low temperature.

The asymmetric unit of (I) at 2 K is presented in Fig. 1, and plots of the Rietveld refinement results at 2 and 100 K are shown in Fig. 2 and Fig.3, respectively. The molecular dimensions derived at both temperatures are unexceptional. The S—O bond length of 1.495 (2) Å (2 K) is in excellent agreement with the average value of 1.492 (2) Å reported by Calligaris (2004) for uncoordinated sulfoxides. [Calligaris also notes a recent unpublished study by Geremia at 164 K, quoting a distance of 1.480 (2) Å and 1.499 Å on correction for thermal motion effects.] The deviations from ideal C_s point symmety of the molecule are small but significant. For example, at 2 K, the methyl groups are eclipsed and rotated only 2.75 (13)° (C1) and 1.50 (13)° (C2) away from the ideal orientation. No signifcant differences are observed in S—C bond distances at this temperature.

Fig. 4 gives a representation of the packing in this structure. The molecules are arranged in layers parallel to the ab plane with the S—O bond dipoles directed into each sheet. The coupling between molecules within a sheet is indicated by the selected intermolecular distances given in Table 1. Each D atom of atom C1 makes a short D···O contact, whereas there is only one D···O contact associated with the C2 methyl group. This contrast in short contacts is reflected by the smaller displacement parameters observed for atom C1 compared with those of C2 and accounts in large part for the observed deviations from ideal molecular symmetry which are seen to increase with temperature. At 100 K, significant differences are observed in the S—C bond lengths, which are 1.828 (3) and 1.788 (3) Å for C1 and C2, respectively. This reflects the anisotropic environment of the molecule in the crystal, which is also observed in the thermal expansion.

The variation of unit-cell volume as a function of temperature is shown in Fig. 5. The data are well fitted using a Debye model (Sayetat et al., 1998) with the refined parameters V_{0 K} = 392.936 (1) ų and θ_D = 251.8 (4) K. The relative thermal expansion along each of the unit cell axes is shown in Fig. 6. The expansion along c is markedly greater than for a and b, reflecting the stronger intermolecular contacts within the layers of molecules in this plane.

S2. Experimental

Deuterated DMSO melts at 291 K. A 99.99% deuterated sample supplied by Sigma Chemical Ltd was hand ground at liquid nitrogen temperatures. A sample (approximately 2 g) of the fine powder obtained was then sealed in a 11 mm-diameter vanadium sample can. The sample was loaded in to a standard vanadium tailed `orange' cryostat (Brochier, 1977) and the sample was annealed at 100 K for 2 h. Data were recorded at 100 K for a period of 112 µAh (ca 3 h). The sample was then cooled to 2 K and measurements made for a period of 150 µAh (ca 4 h). The sample was then heated to 10 K and then up to 275 K in 10 K steps, recording data for 9 µAh (5 min) following a temperature equilibration period of 2 min. Data were recorded using the high-resolution powder diffractometer (HRPD) at the ISIS Facility, Rutherford Appleton Laboratory, England, at backscattering < 2θ > = 168 ° over the time-of-flight range 30–130 ms corresponding to a d-spacing range of some 0.6–2.6 Å.

Computing details

For both compounds, data collection: ISIS instrument control program (ICP); cell refinement: GSAS (Larson & von Dreele, 1994); data reduction: Standard HRPD normalization routines; program(s) used to refine structure: GSAS (Larson & von Dreele, 1994); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 1990); software used to prepare material for publication: GSAS (Larson & von Dreele, 1994).

Figures

	Fig. 1. The asymmetric unit of (I) at 2 K, with displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. The final Rietveld plot of (I) at 2 K, showing the calculated (line), observed (o) and difference (lower) profiles. Vertical bar markers indicate calculated Bragg peak positions. The equivalent d-spacing range corresponds to 0.7–2.4 Å (0.7–1.2 Å inset).
	Fig. 3. The final Rietveld plot of (I) at 100 K, showing the calculated (line), observed (o) and difference (lower) profiles. Vertical bar markers indicate calculated Bragg peak positions.
	Fig. 4. Packing diagram showing selected intermolecular contacts (see also Table 1). [Symmetry codes: (a) −x, −y, −z; (b) −x, y − 1/2, 1/2 − z; (c) 1 + x, y, z.]
	Fig. 5. Variation in unit-cell volume (circles) and Debye fit (see text) with temperature on heating for (I). Values from the previous single-crystal studies of Thomas et al. (1966) (triangle) and Viswamitra & Kannan (1966) (square) are shown for comparison.
	Fig. 6. Thermal expansion of the a (circle), b (square) and c (triangle) lattice constants relative to their values at 2 K.

(2K) dimethyl sulfoxide

Crystal data

C2D6OS	Z = 4
Mr = 84.17	Dx = 1.418 Mg m^−3
Monoclinic, P21/c	Melting point: 291 K
a = 5.2213 (1) Å	Time-of-flight neutron radiation, λ = 1.24-5.36 Å
b = 6.7525 (1) Å	T = 2 K
c = 11.2121 (1) Å	Particle morphology: irregular powder
β = 94.8460 (3)°	white
V = 393.88 (1) Å^3	cylinder, 25 × 11 mm

Data collection

HRPD diffractometer	Scan method: tof 30-130 ms
Specimen mounting: standard cylindrical vanadium sample holder

Refinement

Least-squares matrix: full	Profile function: GSAS TOF Profile function number 2
Rp = 0.037	93 parameters
Rwp = 0.042	0 restraints
Rexp = 0.030	0 constraints
R(F^2) = 0.07004	Weighting scheme based on measured s.u.'s
χ^2 = 1.904	(Δ/σ)max = 0.04
12485 data points	Background function: Shifted Chebyshev function (10 terms)
Excluded region(s): None	Preferred orientation correction: None

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
S	0.1815 (4)	0.1472 (3)	0.19629 (19)	0.0177 (8)
O	−0.10281 (18)	0.13025 (17)	0.16809 (10)	0.0198 (4)
C1	0.3199 (2)	−0.06760 (15)	0.12605 (9)	0.0162 (4)
C2	0.2938 (2)	0.32852 (15)	0.09285 (9)	0.0210 (5)
D11	0.23813 (18)	−0.07637 (13)	0.03411 (9)	0.0315 (5)
D21	0.25820 (18)	−0.19343 (14)	0.17760 (8)	0.0325 (5)
D31	0.5238 (2)	−0.05021 (14)	0.13491 (8)	0.0345 (5)
D12	0.21332 (18)	0.29407 (14)	0.00258 (9)	0.0321 (5)
D22	0.21954 (19)	0.47009 (15)	0.12351 (9)	0.0375 (5)
D32	0.50049 (19)	0.32540 (13)	0.10063 (8)	0.0356 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S	0.0178 (13)	0.0263 (16)	0.0102 (14)	−0.0037 (12)	0.0083 (12)	0.0010 (14)
O	0.0192 (7)	0.0223 (7)	0.0185 (7)	−0.0028 (6)	0.0046 (6)	0.0004 (7)
C1	0.0097 (7)	0.0211 (8)	0.0180 (7)	0.0007 (6)	0.0024 (5)	0.0001 (6)
C2	0.0214 (8)	0.0187 (8)	0.0238 (8)	0.0002 (6)	0.0072 (6)	−0.0009 (6)
D11	0.0291 (8)	0.0330 (10)	0.0314 (9)	−0.0063 (7)	−0.0029 (7)	0.0020 (7)
D21	0.0409 (10)	0.0245 (8)	0.0325 (8)	0.0021 (7)	0.0058 (7)	−0.0010 (8)
D31	0.0224 (9)	0.0341 (9)	0.0459 (10)	−0.0034 (7)	−0.0030 (7)	−0.0003 (7)
D12	0.0333 (8)	0.0325 (8)	0.0300 (8)	0.0010 (7)	−0.0009 (7)	0.0003 (7)
D22	0.0431 (10)	0.0276 (8)	0.0426 (9)	−0.0020 (7)	0.0092 (7)	0.0043 (8)
D32	0.0225 (8)	0.0391 (11)	0.0450 (9)	0.0059 (7)	0.0020 (7)	0.0019 (7)

Geometric parameters (Å, º)

S—O	1.495 (2)	C1—D21	1.0914 (14)
S—C1	1.828 (2)	C1—D31	1.0673 (15)
S—C2	1.818 (2)	C2—D12	1.0878 (14)
S—D11	2.401 (2)	C2—D22	1.0967 (14)
S—D21	2.347 (2)	C2—D32	1.0755 (14)
S—D31	2.378 (2)	D11—D21	1.7876 (13)
S—D12	2.406 (2)	D11—D31	1.803 (2)
S—D22	2.342 (2)	D21—D11	1.7876 (13)
S—D32	2.382 (2)	D21—D31	1.7888 (14)
C1—D11	1.0835 (14)
D11···O	2.3453 (15)	D31···O^ii	2.3025 (12)
D21···O^i	2.3035 (14)	D32···O^ii	2.5167 (14)
O—S—C1	105.64 (13)	D21—C1—D31	111.91 (11)
O—S—C2	106.55 (13)	S—C2—D12	109.24 (11)
C1—S—C2	95.68 (11)	S—C2—D22	104.19 (11)
S—C1—D11	108.34 (11)	S—C2—D32	108.11 (11)
S—C1—D21	104.14 (11)	D12—C2—D22	111.02 (11)
S—C1—D31	107.48 (11)	D12—C2—D32	112.17 (11)
D11—C1—D21	110.55 (11)	D22—C2—D32	111.73 (11)
D11—C1—D31	113.85 (11)

Symmetry codes: (i) −x, y−1/2, −z+1/2; (ii) x+1, y, z.

(100K) dimethyl sulfoxide

Crystal data

C2D6OS	Z = 4
Mr = 84.17	Dx = 1.406 Mg m^−3
Monoclinic, P21/c	Melting point: 291 K
a = 5.2390 (1) Å	Time-of-flight neutron radiation, λ = 1.24-5.36 Å
b = 6.7581 (1) Å	T = 100 K
c = 11.2696 (1) Å	Particle morphology: irregular powder
β = 94.8053 (3)°	white
V = 397.60 (1) Å^3	cylinder, 25 × 11 mm

Data collection

HRPD diffractometer	Scan method: tof 30-130 ms
Specimen mounting: standard cylindrical vanadium sample holder

Refinement

Least-squares matrix: full	Profile function: GSAS TOF Profile function number 2
Rp = 0.038	93 parameters
Rwp = 0.040	0 restraints
Rexp = 0.034	0 constraints
R(F^2) = 0.1192	Weighting scheme based on measured s.u.'s
χ^2 = 1.369	(Δ/σ)max = 0.04
12802 data points	Background function: Shifted Chebyshev function (10 terms)
Excluded region(s): None	Preferred orientation correction: None

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
S	0.1799 (4)	0.1534 (4)	0.1936 (2)	0.0293 (11)
O	−0.1036 (2)	0.1331 (2)	0.16757 (12)	0.0367 (6)
C1	0.3174 (3)	−0.06637 (18)	0.12695 (12)	0.0305 (6)
C2	0.2937 (3)	0.32905 (18)	0.09175 (11)	0.0383 (7)
D11	0.2376 (2)	−0.07372 (16)	0.03554 (11)	0.0434 (7)
D21	0.2558 (2)	−0.18948 (17)	0.17796 (9)	0.0497 (8)
D31	0.5219 (2)	−0.04672 (17)	0.13624 (10)	0.0453 (7)
D12	0.2133 (2)	0.29255 (17)	0.00228 (10)	0.0513 (7)
D22	0.2175 (2)	0.46976 (18)	0.12154 (10)	0.0659 (9)
D32	0.4992 (2)	0.32583 (15)	0.09989 (9)	0.0532 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S	0.0264 (18)	0.042 (2)	0.021 (2)	−0.0071 (17)	0.0117 (17)	−0.0098 (19)
O	0.0297 (10)	0.0433 (11)	0.0380 (10)	−0.0111 (9)	0.0090 (10)	−0.0048 (10)
C1	0.0220 (10)	0.0372 (12)	0.0325 (11)	0.0031 (10)	0.0040 (9)	−0.0022 (9)
C2	0.0319 (13)	0.0376 (12)	0.0456 (13)	−0.0032 (10)	0.0054 (10)	0.0007 (10)
D11	0.0429 (12)	0.0416 (13)	0.0447 (12)	−0.0092 (10)	−0.0023 (10)	0.0033 (10)
D21	0.0631 (15)	0.0367 (12)	0.0511 (13)	0.0111 (10)	0.0163 (11)	0.0007 (12)
D31	0.0341 (12)	0.0457 (12)	0.0565 (13)	0.0012 (9)	0.0058 (10)	−0.0048 (10)
D12	0.0536 (13)	0.0510 (13)	0.0485 (12)	0.0103 (10)	0.0000 (12)	−0.0008 (11)
D22	0.0734 (18)	0.0443 (13)	0.0830 (16)	−0.0041 (12)	0.0236 (14)	0.0068 (12)
D32	0.0338 (11)	0.0587 (15)	0.0671 (14)	0.0064 (11)	0.0044 (11)	−0.0044 (11)

Geometric parameters (Å, º)

S—O	1.496 (2)	C1—D21	1.0761 (17)
S—C1	1.838 (3)	C1—D31	1.0759 (19)
S—C2	1.788 (3)	C2—D12	1.0880 (17)
S—D11	2.390 (3)	C2—D22	1.0949 (17)
S—D21	2.360 (3)	C2—D32	1.0731 (19)
S—D31	2.377 (3)	D11—D21	1.7809 (16)
S—D12	2.372 (3)	D11—D31	1.8046 (16)
S—D22	2.301 (3)	D21—D11	1.7809 (16)
S—D32	2.360 (3)	D21—D31	1.7903 (15)
C1—D11	1.0800 (18)
D11···O	2.3715 (18)	D31···O^ii	2.3097 (16)
D21···O^i	2.3101 (17)	D32···O^ii	2.5184 (16)
O—S—C1	105.21 (16)	D21—C1—D31	112.59 (15)
O—S—C2	108.30 (15)	S—C2—D12	108.69 (13)
C1—S—C2	96.37 (12)	S—C2—D22	103.30 (14)
S—C1—D11	107.05 (13)	S—C2—D32	108.53 (13)
S—C1—D21	105.13 (14)	D12—C2—D22	110.99 (14)
S—C1—D31	106.36 (13)	D12—C2—D32	112.48 (14)
D11—C1—D21	111.38 (14)	D22—C2—D32	112.36 (14)
D11—C1—D31	113.66 (15)

Symmetry codes: (i) −x, y−1/2, −z+1/2; (ii) x+1, y, z.