J. Phys. Chem. B 2004, 108, 19089-19092
19089
Compression of Intermolecular Interactions in CS2 Crystal Kamil F. Dziubek and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´ , Poland ReceiVed: September 15, 2004; In Final Form: October 18, 2004
Carbon disulfide, CS2, mp ) 161.7 K, has been crystallized in situ in a diamond-anvil cell at 295 K and 1.26 GPa to give an orthorhombic Cmca structure. No phase transitions occur with further increasing pressure till the polymerization onset at 8 GPa. The crystal structure was determined by direct methods from singlecrystal X-ray diffraction at 295 K at two pressure points: 1.8 and 3.7 GPa (esd's in the lengths of CdS bond 0.0001 nm). The molecular rearrangements have been rationalized by the close packing and equidistant S‚‚‚S intermolecular interactions enforced by pressure. Although only slight lengthening of the covalent double CdS bond has been observed till 3.7 GPa, the intensified intermolecular S‚‚‚S and C‚‚‚S interactions reveal the possible reaction pathways of pressure-induced polymerization of CS2.
For decades high-pressure syntheses have been increasingly often applied for obtaining new materials recoverable to ambient conditions, many of them having unusual properties.1 Pressureinduced reactions can lead to new products of potential technological applications, such as superhard materials, wideband gap semiconductors, optical components, new superconductors, etc.2-5 High pressure is known to induce instabilities in unsaturated bonds, which often trigger the polymerization reaction. The details of such reaction mechanisms, and particularly the interdependence between the compression of intermolecular contacts and molecular deformations, are still not clear. Meanwhile, this information is crucial for understanding the structural features of the pressure-induced polymerization.6,7 Carbon disulfide is one of the first substances for which pressure-induced polymerization at high temperature was demonstrated in 1941.8,9 Since then, CS2 was thoroughly studied by various techniques at elevated pressure, e.g., by Raman,10-12 infrared,13 and Brillouin14 spectroscopy. It was also shown that the photopolymers obtained from irradiated CS2 vapors, and those synthesized at high pressure, exhibit the same properties.15 The photoinduced polymerization of CS2 has possible cosmochemical implications; for example, it is relevant to the chemistry of the dense Jovian atmosphere and CS2 polymers were detected in the spectra of the impact debris particles from the comet Shoemaker-Levy 9, which collided with Jupiter in July 1994.16 The phase diagram and chemical transformations of CS2 have been investigated between 0 and 300 °C, and from 0.1 MPa to 15 GPa.17 At room temperature the freezing pressure is 1.26 GPa,18 and no evidence of a solid-state phase transition was found till the black-polymer formation starting from about 8 GPa at room temperature.12,17 It was confirmed by singlecrystal diffraction study18 that till 8 GPa the crystal preserves the symmetry of the low-temperature phase;19,20 however, no atomic positions were determined. Structural studies of molecular transformations preceding the polymerization are usually hampered by phase transitions, very common for molecular crystals at elevated pressure. CS2 is exceptional in this respect that it preserves its symmetry till the polymerization, and it is convenient to study structural and molecular transformations prior to the chemical reaction. The compressibility of the lattice * Corresponding author. E-mail:
[email protected].
Figure 1. Single crystal of CS2 inside the high-pressure chamber of the DAC at 1.26 GPa. The ruby chip for pressure calibration is in the top left part of the chamber between the crystal and the gasket. Three black features protruding off the gasket to the crystal are filings of the tungsten foil.
parameters, measured by powder X-ray diffraction,21 was shown to be slightly anisotropic with the least compressible parameter b. Also the thermal expansion is anisotropic and ∂c/∂T is negative. In our study on the pressure-induced modifications in the CS2 structure, we have applied a precise method of high-pressure structural determinations recently developed for CCD-diffractometers.22,23 A four-pin diamond anvil cell (DAC) was loaded with liquid CS2 (BDH Chemicals Ltd., without further purification) using the method described previously.24,25 Pressure was measured using the ruby fluorescence method26 with an accuracy of 0.05 GPa. A gasket was made of tungsten foil, 0.2 mm thick, and pre-indented to ca. 0.1 mm. After the nucleation of several crystallites, the temperature was increased to melt the unwanted grains. On cooling, a round-shaped single crystal was obtained at 1.26 GPa/295 K (Figure 1), and the DAC screws were tightened till the crystal entirely filled the pressure chamber. Then the DAC was heated again and the screws tightened. Because the thermal expansion of CS2 crystal (coefficients of linear thermal expansion: Ra ) (1/a) (∂a/∂T) ≈ 4.7 × 10-4 K-1, Rb ) (1/b) (∂b/∂T) ≈ 4.9 × 10-4 K-1, Rc ) (1/c) (∂c/∂T)
10.1021/jp0458250 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004
19090 J. Phys. Chem. B, Vol. 108, No. 50, 2004 ≈ -7.6 × 10-4 K-1, assessed from temperature range between 77 and 150 K)20 is considerably larger than the linear thermal expansion of the DAC (approximately 1 × 10-5 K-1), some non-hydrostatic strain could be induced while the DAC was left to cool from 330 K, when the CS2 crystal fully filled the chamber, to room temperature. It is apparent that the thermal expansion is considerably reduced at high pressure: Bridgman’s measurements showed that the volume thermal expansion of CS2 decreases by a factor of 2.5 at 2.5 GPa,9 compared to the volume thermal expansion of CS2 crystal at 150 K/0.1 MPa.20 Nevertheless, at 1.8 GPa no sign of anisotropic strain could be detected by inspecting the reflections widths. It appears that in this soft molecular crystal dominated by van der Waals interactions, high temperature efficiently relaxes strains and activates the mobility of defects, so they diffuse to the edges of the high-pressure chamber. The pressure measured after cooling the DAC to 295 K was 1.8 GPa. The DAC was mounted on a KM4-CCD diffractometer equipped with a molybdenum X-ray tube, and centered by the gasket-shadow method.22 The diffraction data were collected using the ω-scan technique: 1° rotations and 40 s exposures. The reflection intensities were corrected for the DAC absorption and gasket shadowing,23 and the diamond-anvil reflections were eliminated. The structure was solved straightforwardly using direct methods (SHELXS-9727) and refined with SHELXL-97.28 After completing the measurement at 1.8 GPa, the high-pressure chamber was gently squeezed to increase pressure. After that, the crystal still diffracted, but a broadening of reflections indicated a partial damage and strains in the sample crystal. At 3.7 GPa we have “repaired” the crystal by annealing it at ca. 400 K for 2 h, which considerably sharpened the reflections. The annealing of the crystal at 3.7 GPa could cause anisotropic strain on cooling the DAC, as explained above. However, again, sharpening of the reflections testified to the absence of anisotropic strains and defects. The X-ray measurement was carried out, and the structural model successfully refined. Then the pressure was increased to 5.1 GPa; again reflections broadened, but at this pressure we did not succeed in “repairing” the crystal and only the unit-cell dimensions could be measured reliably. It shows that the temperature of 430 K (limited by the heat resistance of our DAC) is too low to activate diffusion of defects in the CS2 crystal at this pressure. We did not succeed in repairing this crystal by applying ultrasounds (44 kHz), either. The lattice parameters at 1.8 GPa are a ) 6.038(3), b ) 5.3510(12), and c ) 8.359(6) Å, at 3.7 GPa are a ) 5.815(4), b ) 5.215(2), and c ) 8.046(6) Å, and at 5.1 GPa are a ) 5.667(7), b ) 5.154(2), and c ) 7.820(11) Å, respectively. The crystal structure of CS2 can be considered as built of layers perpendicular to crystal direction [100] (Figure 2). The molecules forming one layer are staggered, each approximately perpendicular to its four neighbors. Each sulfur atom has three intermolecular contacts to other sulfur atoms within one layer, the two shortest of them are symmetry-dependent. The intramolecular S‚‚‚S interval completes the arrangement of four S atoms at the corners of the tetragon, indicated in Figure 2a. It can be observed in the space-filling model that the longest of the S‚‚‚S intermolecular contacts is longer than the sum of van der Waals radii. In consequence, a gap is left between the van der Waals spheres of the S atoms. At 1.8 GPa this void almost completely closes up, as can be seen in Figure 2b. The intermolecular contacts are further squeezed at 3.7 GPa. However, the most striking change at this pressure is a very small change in the CdS bond length, when the molecular
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Figure 2. One (001) crystal layer of CS2 molecules at 5.3 K (a) and 1.8 GPa (b). The intralayer S‚‚‚S contacts are marked by color lines: two symmetry dependent by green, one by red, and the intramolecular SdCdS distance by blue. The same color code has been used in Figure 3.
volume is compressed by almost 20% (compared to the crystal at 5.3 K/0.1 MPa).20 Our results clearly demonstrate that the pressure-induced structural changes in CS2 may be described in two stages. At the first stage the voids between the CS2 molecules are squeezed out, while the molecular geometry does not change. It can be observed in Figure 2a, that at ambient pressure (5.3 K/0.1 MPa)20 the arrangement of the molecules leaves the S‚‚‚Si contacts (superscript i denotes the symmetry code: -x, 1 - y, -z) longer by 0.364(3) Å than the sum of van der Waals radii.29 At 295 K/1.8 GPa this small void between the molecules is tightened to 0.091(2) Å (Figure 2b). Two other intraplanar S‚‚‚S distances within the (100) layer are commensurate with the van der Waals contactssthey are symmetry-dependent and equal 3.588(3) Å at 5.3 K/0.1 MPa.20 There are four short
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J. Phys. Chem. B, Vol. 108, No. 50, 2004 19091
Figure 3. Distance-distance plot comparing the structures of CS2 at low temperature (5.3 K/0.1 MPa,20 abscissae axis) with that at high pressure (ordinate axis): at 295 K/1.8 GPa (filled symbols) and at 295 K/3.7 GPa (open symbols). Circles represent the pairs of compared S‚‚‚S contacts, and the pairs of C‚‚‚S contacts are denoted with squares. The colors used in this plot correspond to the distances indicated in Figure 2 (e.g., the intramolecular S‚‚‚S contact is shown in blue), and the interlayer contacts are shown in black. The plotted points lying close to the dashed line (the intramolecular S‚‚‚S distance between the S atoms) have hardly changed, those below the dashed line have been compressed (the difference in contacts can be read along the ordinate axis), and that above the dashed line has expanded.
nonbonding van der Waals S‚‚‚S contacts between the (100) layers (two of them are symmetry-independent), and their lengths are intermediate between the S‚‚‚S distances within the layer. A distance-distance plot (d-d plot)30 illustrates the magnitudes of compressed contacts in CS2 (Figure 3). It shows that it is the S‚‚‚Si distance that is most squeezed within the (100) layer. Also the C‚‚‚S contacts are strongly shortened and molecules change their orientation to the point when the angles between the longest axes of close molecules approach 90° within the layers. Between 1.8 and 3.7 GPa the changes in molecular arrangement are much smaller than between 0.1 MPa and 1.8 GPa. The reason for this is that the compression of van der Waals forces is strongly nonlinear, and at 1.8 GPa the S‚‚‚S contacts have already been forced into nearly semi-equidistant positions, and the voids between the molecules have been eliminated. As illustrated in Figure 2, further transformations to equally spaced S atoms would require, apart from the compression of S‚‚‚S contacts, also an elongation of the intramolecular SdCdS distance. However, despite the high accuracy of our structural results, almost no significant change of the intermolecular CdS bond length has been observed. This bond is 1.546(1) Å at 5.3 K/0.1 MPa,20 1.540(1) Å at 295 K/1.8 GPa, and 1.550(2) Å at 295 K/3.7 GPa. It can be seen from the d-d plot (Figure 3), that a slight SdCdS elongation is the only negatively compressed distance in the structure. In principle, the observed trend of the CdS length changes is consistent with the intensified S‚‚‚S and new C‚‚‚S intermolecular interactions. It can be expected that the squeezed intermolecular contacts, and the C‚‚‚S contacts in particular, should lead to increased electron density between the interacting atoms, hence to the withdrawal of electrons from the CdS bonds and in consequence to their
Figure 4. Hirshfeld surfaces for the CS2 molecule embedded in the crystal structure at 5.3 K/0.1 MPa20 (top), 295 K/1.8 GPa, and 295 K/3.7 GPa (bottom). The color scale on the surface represents the shortest distances from the surface element to the closest atoms outside or inside the surface (red corresponds to the shortest contacts, blue to the longest). The CS2 molecule occupies the C2h-symmetric site in the crystal; thus each contact is doubled (these along C2 or Cs) or quadrupled. The Hirshfeld surfaces have been plotted with program Crystal Explorer.37
elongation. Owing to two new C‚‚‚S interactions of the carbon atom, its coordination increases from two in the CS2 molecule, and eventually becomes three or four in the polymerized structure. Thus the observed small CdS bond elongation is consistent with the theoretical expectations and can be considered as a molecular deformation preceding the polymerization. It thus appears that the observed elongation reflects the changes in electron density in the very first step leading to the polymerization. Also, the close packing of the S-atoms would additionally favor lengthening of the CdS bonds because the most efficient packing of spheres within one layer requires equal distances between the sulfur atoms. Of four intrasheet close S‚‚‚S distances indicated in Figure 2 (two of them symmetry-
19092 J. Phys. Chem. B, Vol. 108, No. 50, 2004 dependent) three are intermolecular, and the fourth is an intramolecular one. We have applied Hirshfeld surfaces for describing changes in the molecular environment with pressure.31,32 The nearest neighboring atomic contacts of the CS2 molecule at progressively elevated pressure have been presented on Hirshfeld surfaces in Figure 4. The most apparent molecular rearrangements in the compressed structure is the appearance of short C‚‚‚S distances, absent at ambient pressure. These close intermolecular C‚‚‚S contacts are a new feature characteristic of the high-pressure CS2 structure, which may be considered as an initial stage of the topochemical reaction of polymerization. The occurrence of the short C‚‚‚S distances at high pressure suggests that the possible reaction path to polymerization leads through the saturation of CdS bonds and formation of new CsS bonds. In this respect the high-pressure structural determination is consistent with the spectroscopic results identifying the CS2 polymerization products as [-C(dS)sS-]n linear or ring structures.15 The formation of CsC or CdC bonds in the pressure-induced polymerization has not been supported by our results. The patterns of shortest distances on the Hirshfeld surfaces also reflect the compression of S‚‚‚S contacts, as described above, which would be consistent with the formation of SsS bonds observed in the photopolymers. To our knowledge, this is the first precise X-ray diffraction determination of molecular dimensions in a compound undergoing pressure-induced polymerization. It shows that the molecular dimensions hardly change at the pressure approaching half of that required for the polymerization. Interestingly, the orthorhombic Cmca-symmetric structure of CS2 has been also observed for the analogous CO233 and CSe234 compounds. CO2 crystallizes in space group Pa3 at ambient pressure and transforms to the Cmca symmetry only when pressurized to 11 GPa.33 No further transitions at ambient temperature were found for CO2. This can be explained by the short CdO bond length and small van der Waals radius of the oxygen atom. Consequently, the C‚‚‚O interactions play a much more significant role than the O‚‚‚O interactions, compared to C‚‚‚S and S‚‚‚S contacts in CS2, respectively. In the highpressure/high-temperature phase of polymerized CO2 the structure resembles the SiO2 crystal, with the C atom tetrahedrally coordinated by oxygens.35,36 CSe2 with considerably larger Se atoms crystallizes in the Cmca-symmetric phase at 0.1 MPa below 227.7 K. This is consistent with the postulated role of the equidistant condition for the molecular arrangement, which increases with the van der Waals radius of X for the CX2 (X ) O, S, Se) compounds series. We have shown that this role also increases with pressure. Acknowledgment. This study was supported by the Polish Ministry of Scientific Research and Information Technology, Grant No. 3T09A18127. Supporting Information Available: The refinement details, atomic coordinates, anisotropic thermal parameters, and the plot of the pressure dependence of the unit-cell dimensions to 5.1 GPa. This material is available free of charge via the Internet at http://pubs.acs.org.
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