Electrostatic Matching versus Close-Packing Molecular Arrangement

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J. Phys. Chem. B 2009, 113, 2436–2442

Electrostatic Matching versus Close-Packing Molecular Arrangement in Compressed Dimethyl Sulfoxide (DMSO) Polymorphs Roman Gajda and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: December 20, 2008

Single crystals of dimethyl sulfoxide (DMSO, (CH3)2SO) were in situ frozen at isochoric conditions in a diamond-anvil cell and their structures determined at 0.37, 0.56, and 2.4 GPa. At ambient pressure, DMSO freezes at 291 K in monoclinic phase R, space group P21/c, stable in all the temperature range from its melting point down to 2 K. On increasing the pressure, DMSO freezes at 140 MPa/296 K in phase R too, but above 540 MPa it collapses into a more compact triclinic phase β, space group P1j. The molecular aggregation in the crystal structure of DMSO is dominated by electrostatic attraction between negative and positive sites on the molecular surface and CH · · · O hydrogen bonds linking the molecules into dimers and chains. Most of this electrostatic matching and CH · · · O bonds present in phase R are preserved above 540 MPa, but the more tight packing in phase β is achieved at the cost of broken dipole-dipole interactions between antiparallel SdO groups and repulsing contacts between electropositive H-atoms squeezed to distances commensurate with the sum of van der Waals radii. The isostructural relation between phases R and β is strictly connected with the molecular aggregation governed by electrostatic matching of interatomic contacts. Introduction The polyfunctional molecule of dimethyl sulfoxide (DMSO), (CH3)2SO, with a highly polar SdO group and two hydrophobic CH3 groups (Figure 1), has the highest molecular dipole moment (4.3 D ) 14.34 × 10-30 Cm) and the highest dielectric constant, of 48 at 293 K, of any dipolar aprotic solvents.1 Owing to its remarkable properties, DMSO is one of the most often used solvents in chemical practice. The common usage of DMSO is the likely reason of the frequent occurrence of DMSO solvates, belonging to the largest class of crystal structures determined: there are 1265 such entries in the latest edition of Cambridge Structural Database (CSD). The molecular association of DMSO was studied by various methods: Raman spectroscopy,2 X-ray and neutron diffraction of the liquid,3 quantum-chemical calculations,3,4 and molecular dynamics (MD) simulations.3 According to the recent quantum-chemical calculations,3 dipole · · · dipole interactions are the main forces leading to the aggregation of molecules into dimers in vapor, whereas the MD, X-ray, and neutron diffraction results3 show that no dimers or chains of molecules interacting by dipole · · · dipole forces are formed in the liquid state, but only a slight preference for antiparallel orientation of SdO groups was observed. There is also evidence from the X-ray and neutron diffraction and from MD simulations of the existence of intermolecular hydrogen bonds CsH · · · O in liquid DMSO.3 The crystal structure of DMSO was first determined in 1966 independently by Viswamitra and Kannan at 213 K,5 and by Thomas et al. at 278 K.6 Recently, the structure of deuterated DMSO, perdeuterodimethyl sulfoxide (CD3)2SO, was determined by neutron powder diffraction at 2 and 100 K by Ibberson.7 No phase transitions were detected either in (CH3)2SO or in (CD3)2SO between their freezing temperature at 291 and 2 K. The structural studies of DMSO revealed very short CsH · · · O and CsD · · · O contacts, approaching 2.3 Å.7 The intermolecular interactions in this structure were intriguing in this respect that * Corresponding author. E-mail: [email protected].

all three shortest CsD · · · O bonds involved the D atoms of one methyl only, while those of the other methyl were much longer. This considerable anisotropy in the molecular environment was accompanied by a significant difference in covalent bonds, the S-C bond of the hydrogen-bonded methyl being by 0.040(4) Å longer than the other one. We have thus undertaken to investigate if the remarkable stability of the DMSO crystal over the whole temperature range at 0.1 MPa can be destabilized by elevated pressure, if the significant difference in intermolecular environment can be diminished by external force, and what is the role of CsH · · · O hydrogen bonding and electrostatic interactions in molecular association in the structure of pure DMSO and its solvates. Experimental Section The DMSO sample (purchased from Aldrich, without further purification) was in situ pressure frozen in a diamond-anvil cell

Figure 1. DMSO molecule and its electron-density envelope at 0.0004 au decorated with the electrostatic-potential color scale ranging from -186 kJ/mol (red) to 113 kJ/mol (purple).

10.1021/jp808987k CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

Molecular Arrangement in Compressed DMSO Polymorphs

Figure 2. Stages of DMSO R-phase single-crystal isochoric growth: (a) one seed of rounded shape at 366 K; (b) a needle at 356 K; (c) a plate, with shown Miller indices of its faces at 340 K; and (d) the single crystal fully filling the high-pressure chamber at 0.37 GPa/296 K. A ruby chip for calibrating pressure and some ruby powder are placed below and above the chamber center, respectively.

(DAC). A modified Merrill-Bassett DAC,8 with a gasket made of 0.3 mm steel foil with spark-eroded 0.5 mm in diameter hole, was used. The pressure was calibrated with a BETSA PRL spectrometer by the ruby-fluorescence method9 with a precision of 0.05 GPa. Pressure in the DAC chamber was gradually increased until the sample froze at isothermal conditions in the polycrystalline form. By increasing temperature, all crystal grains but one were melted (Figure 2a). Then temperature was slowly lowered until this single crystal grew at isochoric conditions to entirely fill the pressure chamber at room temperature (Figure 2b-d). The DAC with the DMSO sample was centered on a Kuma KM-4 CCD diffractometer by the gasket-shadow method.10 The Mo KR radiation from the sealed X-ray tube was graphite monochromated. The reflections intensities were collected in the ω-scan mode.10 The CrysAlis programs11 were used for data collection, unit-cell refinement, and initial data reduction. The data were corrected for the DAC absorption, gasket shadowing, and absorption of the sample itself.12,13 The structure was solved by direct methods using program SHELXS97 and refined with SHELXL97.14 The C, O, and S atoms were refined with anisotropic displacement parameters. Not all H atoms could be located in difference Fourier maps, and therefore the positions of H-atoms were calculated from molecular geometry [C-H ) 0.96 Å and Uiso(H) ) 1.2Ueq(C)] and refined as rigid groups allowed to rotate around the S-C bonds. Subsequently, DMSO single crystals were grown, X-ray diffraction data measured, and structures determined at 0.31, 0.37, 0.55, 0.56, 0.76, and 2.4 GPa. At 0.55 GPa and higher pressure the crystal habit of DMSO changed from needles to plates (Figure 3). The measurements at 0.37, 0.56, and 2.4 GPa yielded significantly better accuracy of atomic positions than measurements at 0.31, 0.55, and 0.76 GPa, which have been used for determining the unitcell dimensions only. Information about the X-ray diffraction measurements and of crystal structures at 0.37, 0.56, and 2.40 GPa are summarized in Table 1 and deposited in the CIF format as supplementary publications No. CCDC 703489 (phase R)

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Figure 3. Stages of the DMSO β-phase single-crystal isochoric growth in the DAC chamber leading to the sample at 0.56 GPa: (a) polycrystalline mass heated to 400 K; (b) few β-phase grains at 420 K; (c) a single seed left in the high-pressure chamber at 433 K; (d) the single crystal at 313 K with indexed faces before fully filling the chamber at 296 K/0.56 GPa.

and CCDC 703490, 703491 (phase β at 0.56 and 2.40 GPa, respectively) in the Cambridge Crystallographic Database Center. The molecular graphics were prepared by X-SEED.15 The compression of liquid and solid DMSO has been measured in a piston-cylinder high-pressure apparatus.16 Program Gaussian17 was used for calculating the electrostatic potential at the B3LYP/6-311++g(d, p) level of theory and program Moliso18 for mapping it onto the equi-potential molecular surface. The molecular volume was calculated by program MolVolAIR19 using the Monte Carlo method and van der Waals radii by Bondi.20 Results and Discussion At 0.54 GPa there is a clear discontinuity in the unit-cell compression of DMSO (Figure 4), showing that the crystal undergoes a first-order phase transition. The occurrence of this solid-solid phase transition is consistent with the distinctly different habit of single crystals grown below and above 0.54 GPa (compare Figures 2 and 3), with the measurement of DMSO compression (Figure 4) showing clearly anomalies at 0.14 GPa, associated with isothermal freezing, and at 0.54 GPa due to the R/β phase transition, and was finally confirmed by solving the structure of the crystal at 0.55, 0.56, 0.76, and 2.4 GPa in space group P1j. The freezing pressure of 0.14 GPa at 296 K has been measured by observing the anomalous compression of ∆VR ) 1.6% in the piston-cylinder apparatus. The DMSO solid between 0.14 and 0.54 GPa has been denoted as phase R, and this above 0.54 GPa as phase β. This single-crystal X-ray diffraction study showed that isochoric freezing of DMSO at 0.14 GPa/296 K leads to monoclinic phase R, space group P21/c, which can be also obtained on isobaric freezing at 0.1 MPa/ 291 K and is the only known DMSO structure at ambient pressure. When the crystal transforms into phase β above 0.54 GPa, its unit cell is halved (Z ) 2, Table 1) and the space-group symmetry is reduced to P1j. The abrupt volume change by ∆Vβ ) 2.3% (Figures 4 and

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Gajda and Katrusiak

TABLE 1: Experimental Data for the Selected High-Pressure DMSO Structure Determinations: Phase r at 0.37 GPa and Phase β at 0.56 and 2.4 GPa pressure (GPa) temperature (K) chemical formula Mr (g/mol) morphology crystal size (mm) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z, Dcalc (g cm-3) µ (mm-1), F(000) abs correction Tmin Tmax range of h, k, l reflns collected unique reflns observed reflns Rint no. of parameters GoF R1[I > 2σ(I)] wR2[I > 2σ(I)] R1 all data wR2 all data ∆Fmax, ∆Fmin (e Å-3)

phase R

phase β

phase β

0.37 296(2) C2H6SO 78.13 needle 0.45 × 0.42 × 0.20 monoclinic P21/c 5.291(2) 6.711(2) 11.453(6) 90 94.86(4) 90 405.3(3) 4, 1.281 0.58, 168 analytical 0.22 0.435 -6 e h e 6 -7 e k e 7 -5 e l e 5 2545 288 265 0.054 44 1.110 0.0432 0.1096 0.0488 0.1145 0.10, -0.16

0.56 296(2) C2H6SO 78.13 plate 0.46 × 0.40 × 0.20 triclinic P1j 5.308(3) 5.914(4) 7.243(2) 72.56(5) 84.35(4) 63.66(6) 194.26(18) 2, 1.336 0.61, 84 analytical 0.325 0.47 -6 e h e 6 -3 e k e 3 -8 e l e 8 936 132 130 0.039 29 1.15 0.0502 0.1111 0.0506 0.1113 0.23, -0.19

2.4 296(2) C2H6SO 78.13 plate 0.57 × 0.54 × 0.19 triclinic P1j 5.173(6) 5.636(6) 6.883(4) 71.51(8) 83.80(7) 63.07(11) 169.5(3) 2, 1.531 0.70, 84 analytical 0.33 0.47 -6 e h e 6 -4 e k e 4 -8 e l e 8 713 110 109 0.059 27 1.28 0.0579 0.1536 0.0583 0.1545 0.186, -0.189

5) at this phase transitions testifies that it is discontinuous (first order) in character. There are clear structural similarities between phases R and β, with the DMSO molecules arranged into characteristic motives of dimers and chains, and the lattice dimensions approximated by the matrix equation (subscripts R and β indicate the unit-cell vectors of phases R and β, respectively):

( ) ( )( ) aR 1 0 0 aβ bR ) 0 1 0 bβ cR 0 -1 2 cβ

For describing the molecular rearrangements at the R/β phase transition, the structure can be considered as built of dimers of CH · · · O bonded pairs of molecules, contained in the volume of one triclinic unit cell in phase β, and in the corresponding units in the structure of phase R indicated in Figure 6. The dimers are centrosymmetric. At the transition the dimers are dislocated within these units in this way that they rotate from the longer diagonal of the unit in phase R to the shorter diagonal in phase β. In phase R there are two orientations of the dimer units: in one orientation the edges of the units are parallel to direction [021], and in the other parallel to [021j]. In the β phase all the dimer units are arranged in the same manner. In the result of this transformation the β-structure acquires new inversion centers, but loses the screw axes and glide planes. It becomes apparent from Figure 6 that while the unit-cell vectors a and b clearly correspond between phases R and β, the third translation cβ rather corresponds to halved diagonal [011]R/2 than to cR/2. The correspondence of

[011]R/2 and cβ directions is also supported by a reduced strain between these directions compared to that between cR/2 and cβ (Figure 5): the length difference between [011]R/2 and cβ and angle between them are much smaller than those between cR/2 and cβ. Apart from the angular distortions of the unit-cell angles (Table 1), the lattice strain is most pronounced between cβ and [011]R/2 (Figure 5) and between bR and bβ, and a weak strain is generated by the transition along [x], coinciding with the shortest CH · · · O intermolecular contacts in the chains.

Figure 4. Molecular volume of DMSO compressed in a piston-andcylinder apparatus (blue squares) and in a DAC (red triangles). The vertical dashed lines mark the anomalies connected with freezing (m.p.—melting pressure) and R/β phase transition (pc) at 296 K.

Molecular Arrangement in Compressed DMSO Polymorphs

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Figure 5. Unit-cell dimensions a, b, and c of DMSO at 0.1 MPa/278 K (empty symbols) and their pressure dependence at 296 K (filled symbols). Also the halved [011] diagonal of the unit cell in phase R has been plotted. The freezing and R/β transition pressures are marked by the vertical dashed lines.

Figure 7. Autostereographic projections21 of DMSO molecules’ aggregation at 0.37 GPa (a) and 0.56 GPa (b). The CH · · · O bonded dimers are highlighted in blue, the CH · · · O bonded chains in pink, and the molecules interacting by dipole · · · dipole forces involving the SdO groups in green. In the β-phase structure every second of the dipole · · · dipole interactions has been broken, hence the empty green contour has been indicated only.

Figure 6. Projection of DMSO structure (a) in phase R at 0.37 GPa and (b) in phase β at 0.56 GPa. The volume units enclosing the CH · · · O bonded centrosymmetric dimers have been indicated with pink lines. The inversion centers, these common to phases R and the new ones in phase β, have been indicated in green.

Intermolecular Interactions. The isostructural relations between phases R and β reflect the intermolecular interactions in the DMSO structures. The main types of molecular aggregates, the CH · · · O bonded chains along [100] and centrosymmetric dimers (highlighted in Figure 7), are hardly affected by the R/β phase transition. The dimers are linked by a pair of CH · · · O bonds, and in the chains each translationally

repeated link is bounded by a pair of CH · · · O bonds between two methyl groups and one SdO acceptor. On the transition to phase β these aggregates are preserved; however, there are also other CH · · · O bonds which are broken and formed (Figure 8). The similarities in the molecular arrangements in phases R and β can be illustrated by the intermolecular distances mapped in a color scale on the Hirshfeld surfaces presented in Figure 9. The quantum chemical calculations on the association of DMSO molecules and configurations of their dimers suggested that the most stable dimers are bonded by SdO dipole-dipole interactions,3 with the S · · · O distance of ∼3.6 Å. Indeed, each SdO bond is involved by two such contacts in the DMSO phase R. The SdO bonds are approximately parallel to crystal axis [100] and are arranged in zigzag chains along axis [010] (see Figures 6 and 7). At 0.37 GPa each sulfur atom forms two S · · · O contacts, of 3.710(5) Å and 3.880(4) Å. At 0.56 GPa in phase β one of the dipole · · · dipole contacts is shortened to 3.502(15) Å, while the second one lengthens to 4.397(26) Å. Thus, in the β phase the SdO dipole · · · dipole interactions aggregate the molecules into dimers. It appears that this breaking of one of the SdO dipole · · · dipole contacts is a significant element of the transforming interactions when the R-structure collapses into the β-phase above 0.54 GPa. The packing coefficients of phase R at 0.37 GPa and phase β at 0.56 and 2.40 GPa are

2440 J. Phys. Chem. B, Vol. 113, No. 8, 2009

Gajda and Katrusiak TABLE 2: Geometry of Intermolecular Contacts (Å, deg) Involving Methyl H-Atoms in the DMSO Phases r and β at High Pressurea

Figure 8. Packing diagrams showing intermolecular hydrogen bonds around one central DMSO molecule: (a) in phase R at 0.37 GPa and (b) in phase β at 0.56 GPa. Short CH · · · O contacts have been indicated by the dotted lines. Those consistent between phases R and β are shown in green and those transforming in red.

DsH · · · A

H· · ·A

DsH · · · A

D· · ·A

C1sH1A · · · O1b C1sH1B · · · O1c C1sH1C · · · O1d C2sH2A · · · H1Ce C2sH2B · · · H1Bf C2sH2C · · · O1d

Phase 2.545 2.480 2.484 2.936 2.539 2.661

R at 0.37 GPa 170.63 3.497(12) 153.38 3.366(10) 155.77 3.383(5) 120.80 3.525 155.37 3.436 145.78 3.497(8)

C1sH1A · · · O1b C1sH1B · · · H2Cg C1sH1C · · · O1h C2sH2A · · · H1Ab C2sH2B · · · O1I C2sH2C · · · O1h C2sH2C · · · H1Bj

Phase 2.581 2.517 2.448 2.662 2.688 2.599 2.517

β at 0.56 GPa 162.98 3.509(39) 149.72 3.382 158.77 3.363(19) 127.42 3.335 138.21 3.465(13) 151.82 3.476(28) 125.64 3.175

C1sH1A · · · O1b C1sH1B · · · H2Cg C1sH1C · · · O1h C2sH2A · · · H2Ak C2sH2B · · · O1I C2sH2C · · · O1h C2sH2C · · · H1Bj

Phase 2.305 2.157 2.347 2.231 2.605 2.552 2.157

β at 2.4 GPA 171.39 3.258(26) 149.65 3.025 154.13 3.239(15) 132.82 2.966 127.16 3.275(13) 144.91 3.384(23) 131.56 2.887

C· · ·C

3.902(11) 4.117(7)

3.918(49) 4.160(22) 3.918(49)

3.634(43) 3.805(64) 3.634(43)

a

The H atoms were calculated from molecular geometry (C-H ) 0.96 Å) and allowed to rotate about the S-C bond. b 1 - x, -y, 1 - z. c 1 - x, y - 1/2, 1/2 - z. d x - 1, y, z. e -x, -y, 1 - z. f x, 1 + y, z. g x - 1, 1 + y, z. h 1 + x, y, z. I 1 - x, -y, -z. j 1 + x, y - 1, z. k 2 - x, -1 - y, 1 - z.

Figure 9. Hirshfeld surface for the DMSO molecule in the center of its coordination sphere embedded in the crystal structure at (a) 0.35 GPa and (b) 0.56 GPa. The color scale on the surface represents distances from the surface elements to the closest atoms renormalized by their van der Waals radii:22 red spots indicate the closest contacts associated with hydrogen bonds and electrostatic attraction. The Hirshfeld surface was plotted with program CrystalExplorer.23

0.668, 0.697, and 0.799, respectively; thus, it is plausible that the transformation to phase β at 0.54 GPa induces the closepacking arrangement. Some of the electrostatic interactions in the DMSO structure are repulsive, as they involve atoms with the same sign of their net atomic charges. In phase R there is one contact of this type only, H2B · · · H1B approaching 2.5 Å, although the shortest contact of H1B is to electronegative O1 (Table 2). In phase β there are three H · · · H contacts of about 2.5 Å per each DMSO molecule. At 2.4 GPa these H · · · H contacts are squeezed to about 2.2 Å. It is thus apparent that the phase R structure is governed by electrostatic interactions and that this structure is not closely packed,24 in the terms of tightly feeling the space. The close packing is achieved in phase β where the van der Waals surfaces of atoms, irrespective of their charges, are brought within the contact distance. This close packing does not induce any conformation changes of the methyl groups,

Figure 10. d-d plot comparing the closest intermolecular distances for each atom in one DMSO molecule in phase R at 0.37 GPa, with those in phase β at 0.57 GPa (full symbols) and at 2.4 GPa (empty symbols). The compression of contacts is measured in vertical shifts of the points from the y ) x dashed line.

although the bridgehead C · · · C distances at 2.4 GPa are considerably closer than the equilibrium contacts between methyl molecules calculated theoretically,25 and the orientation of the methyl groups does not correspond to the orientations assumed by the methyls located most closely (face-to-face, i.e., three H-atoms of one methyl staggered against three H-atoms of the methyl in contact). The closest contact observed at 2.4 GPa (Table 2) is intermediate between face-to-vertex and vertexto-vertex (the vertex denotes the C-H direction25). Despite this

Molecular Arrangement in Compressed DMSO Polymorphs

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TABLE 3: Bond Lengths (Å) and Angles (deg) in Phases r and β of Crystalline DMSO, Compared with Previous Neutron and X-ray Diffractiona pressure temp S1-O1 S1-C1 S1-C2 O1-S1-C1 O1-S1-C2 C1-S1-C2 a

0.35 GPab 296 K 1.499(4) 1.771(5) 1.797(10) 106.5(3) 106.5(3) 96.8(4)

0.56 GPab 296 K 1.497(7) 1.78(2) 1.78(5) 106.6(7) 105.8(14) 97.3(14)

2.4 GPab 296 K 1.519(6) 1.752(12) 1.91(4) 108.0(5) 102.8(12) 95.5(11) b

0.1 MPac 2K 1.495(2) 1.828(2) 1.818(2) 105.64(13) 106.55(13) 95.68(11) c

0.1 MPac 100 K 1.496(2) 1.838(3) 1.788(3) 105.21(16) 108.30(15) 96.37(12) d

0.1 MPad 213 K 1.471(8) 1.812(14) 1.801(10) 107.0(6) 107.4(6) 97.9(5)

0.1 MPae 278 K 1.513 [1.531(5)] 1.788 [1.798(10)] 106.7(4) 97.4(4) e

Values in brackets are corrected for thermal motion. This work. Ibberson, 2005. Viswamitra and Kannan, 1966. Thomas et al., 1966.

unfavorable orientation, no changes in the molecular conformation have been observed. The distance-distance plot26 in Figure 10 reveals intriguing dependence of intermolecular distances between DMSO in phases R and β: even at 2.4 GPa the closest S1 · · · S1 distance is longer than at 0.37 GPa in phase R. It is also apparent from this plot that the strongest compression of intermolecular contacts is observed for methyl C(2)H3, this one which does not form CH · · · O contacts in phase R. Consequently, new CH · · · O bonds are formed involving C(2)H3, and the interactions of the two methyl groups in phase β becomes similar. The more even distribution of contacts in phase β suggests that the electrostatic attraction between DMSO molecules plays a lesser role for the molecular aggregation, and the larger role is played by the molecular close packing at high pressure. Molecular Dimensions. The pyramidal shape of DMSO molecule has not been affected by pressure. There are no significant differences in the molecular dimensions involving the two methyl groups in any of the high-pressure determinations (Table 3). The SsC lengths and OdSsC angles are equal within errors, and also the HsCsSdO torsion angles are equal. The conformation of both methyls can be described as staggered with respect to the adjacent SdO and S-C bonds. Thus, the molecule is hardly distorted from the ideal Cs point-group symmetry, despite that in phase R the crystal environment is very asymmetric and short contacts CsH · · · O are formed to one methyl only. High pressure considerably moderates the differences around methyls, and phase β has two such short contact per each methyl compared to three vs one in the lowtemperature phase R (Figure 10, Table 3). The SdO bond lengths in the high-pressure structures agree within error with the neutron data7 and are slightly longer than the mean value of 1.492(1) Å,27 obtained by averaging a large number of accurate DMSO solvates and other pure sulfoxide structures. This is consistent with the observations that the SdO bond length slightly increases, both in free and coordinated sulfoxides, when the oxygen atom is involved in hydrogen bonding, while a marked lengthening was observed in the case of sulfoxide protonation.27 Conclusions In this high-pressure study of DMSO, we have measured its compression and freezing pressure, and determined its highpressure structures: phase R, consistent with that obtained by isobaric freezing, and a new triclinic phase β stable above 0.54 GPa. The mechanism of the solid-solid R/β phase transition triggered at 0.54 GPa can be rationalized by the competition between the specific CH · · · O hydrogen bonds and SdO dipole · · · dipole interactions, against the close-packing arrangement as a general feature of the structure exposed to high pressure. Both structures R and β are arranged in this manner, that the positive electrostatic-potential regions of the DMSO

molecular surface are located close to the negative electrostaticpotential regions of their neighbors. High pressure diminishes the role of the electrostatic forces and half of the dipole · · · dipole contacts between SdO groups are broken, and H · · · H contacts between electropositive atoms are formed in phase β. The molecules in phase R are arranged according to the electrostatic matching, whereas the arrangement in phase β is governed by electrostatic matching and close packing of molecules. Thus, in the low-pressure range (phase R) the intermolecular distances between similarly charged atoms are considerably longer than would be expected from their van der Waals radii,28 which is a consequence of the electrostatic repulsion between these atoms. The electrostatic interactions along with the steric hindrances in molecules can also considerably contribute to the nonisotropic van der Waals radii of atoms.29 The electrostatic matching of aggregating molecules in crystals has been also observed in other compounds: acetonitrile,30 dihalomethanes,31 and haloethanes.32,33 The electrostatic matching in DMSO is partly overwhelmed in the higher range of pressure above 0.54 GPa, where new contacts irrespective of the atomic charges appear in phase β and H · · · H distances become commensurate with the sum of traditional van der Waals radii;20 however, still this structure is evidently consistent with intermolecular electrostatic attraction. In phase β the intermolecular interactions involving the symmetry-independent methyl groups are more evenly distributed than in phase R. No difference between the molecular dimensions involving two methyl moieties have been confirmed in this high-pressure study either in phase R or in phase β. References and Notes (1) Higashigaki, Y.; Christensen, D.; Wang, C. J. Phys. Chem. 1981, 85, 2531–2535. (2) Czeslik, C.; Jonas, J. J. Phys. Chem. A 1999, 103, 3222–3227. (3) Onthog, U.; Megyes, T.; Bako´, I.; Radnai, T.; Gro´sz, T.; Hermansson, K.; Probst, M. Phys. Chem. Chem. Phys. 2004, 6, 2136–2144. (4) Jitariu, L. C.; Wilson, C.; Hirst, W. D. J. Mol. Struct. 1997, 391, 111–116. (5) Viswamitara, M. A.; Kannan, K. K. Nature 1966, 209, 1016–1017. (6) Thomas, R.; Shoemaker, C. B.; Eriks, K. Acta Crystallogr. 1966, 21, 12–20. (7) Ibberson, R. M. Acta Crystallogr. C 2005, 61, o571–o573. (8) Merrill, L.; Bassett, W. A. ReV. Sci. Instrum. 1974, 45, 290–294. (9) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, N. A. J. Appl. Phys. 1975, 46, 2774–2780. (10) Budzianowski, A.; Katrusiak, A. High-Pressure Crystallography; Katrusiak, A., McMillan, P. F., Eds.; Kluwer Academic Publishers: Dordrecht, 2004; pp 101-111. (11) Oxford Diffraction. CrysAlis CCD and CrysAlis RED, GUI Versions; Oxford Diffraction: Poland, 2003. (12) Katrusiak, A. REDSHAD; Adam Mickiewicz University: Poznan´, Poland, 2003. (13) Katrusiak, A. Z. Kristallogr. 2004, 219, 461–476. (14) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112–122. (15) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189–191. (16) Baranowski, B.; Moroz, A. Pol. J. Chem. 1982, 56, 379–391. (17) Frisch, M. J., et al. GAUSSIAN 03, reVision B.04; Gaussian Inc.: Pittsburgh, PA, 2003.

2442 J. Phys. Chem. B, Vol. 113, No. 8, 2009 (18) Hu¨bschle, C. B.; Luger, P. J. Appl. Crystallogr. 2006, 39, 901– 904. (19) Wro´blewski, M.; Katrusiak, A. MolVolAIR program for molecular Volume calculations; Faculty of Chemistry, Adam Mickiewicz University: Poznan´, Poland, 2008. (20) Bondi, A. J. Phys. Chem. 1964, 68, 441–451. (21) Katrusiak, A. J. Mol. Graph. Model. 2001, 19, 363–367. (22) McKinnon, J. J.; Jayatilaka, D.; Spackman, M. A. Chem. Commun. 2007, 3814–3816. (23) Wolff, S. K.; Grimwood, D. J.; McKinnon, J. J.; Jayatilaka, D.; Spackman, M. A. Crystal Explorer, Version 2.0; University of Western Australia, Australia, 2007. (24) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (25) Rowley, R. J.; Pakkanen, T. J. Chem. Phys. 1999, 110, 3368–3377.

Gajda and Katrusiak (26) Dziubek, K. F.; Katrusiak, A. Z. Kristallogr. 2004, 219, 1–11. (27) Calligaris, M. Coord. Chem. ReV. 2004, 248, 351–375. (28) Dance, I. New J. Chem. 2002, 27, 22–27. (29) Nyburg, S. C.; Faerman, C. H. Acta Crystallogr. B 1985, 41, 274– 279. (30) Olejniczak, A.; Katrusiak, A. J. Phys. Chem. B 2008, 112, 7183– 7190. (31) Podsiadło, M.; Katrusiak, A. Cryst. Eng. Commun. 2008, 10, 1436– 1442. (32) Bujak, M.; Podsiadło, M.; Katrusiak, A. Chem. Commun. 2008, 4439–4441. (33) Bujak, M.; Podsiadło, M.; Katrusiak, A. J. Phys. Chem. B 2008, 112, 1184–1188.

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