Molecular Symmetry and Isostructural Relations in Crystal Phases of

Sep 10, 2009 - ... Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan, Poland ... phases, their symmetry, and site occupation factors of halogen...
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J. Phys. Chem. B 2009, 113, 13195–13201

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Molecular Symmetry and Isostructural Relations in Crystal Phases of Trihalomethanes CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 Kamil Dziubek, Marcin Podsiadło, and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed: June 29, 2009; ReVised Manuscript ReceiVed: August 14, 2009

Bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and their parent trihalomethanes, chloroform (CHCl3) and bromoform (CHBr3), form an intriguing series of isostructural crystal phases, the sequence of which depends on the Br/Cl substitution and thermodynamic conditions. The phase behavior of these compounds has been studied by isobaric calorimetry and isothermal compression, and the crystal structure of CHBrCl2 has been determined at 0.10 MPa/200 K, 0.73, 1.26, 2.53 GPa (all at 295 K), and that of CHBr2Cl at 0.43, 1.24 GPa (all at 295 K). CHBrCl2 frozen by isobaric cooling at 0.10 MPa crystallizes in space group P1j with Z ) 2, while its high-pressure polymorph in space group Pnma (Z ) 4) is stable at 295 K from its freezing pressure at 0.48 to at least 2.53 GPa. At the freezing pressure of 0.29 GPa, CHBr2Cl crystallizes in space group P63, with Z ) 2, and at 1.27 GPa, it transforms to the orthorhombic structure, space group Pnma (Z ) 4); CHCl3 has the identical symmetries, but their reverse sequence was observed. A subtle isostructural phase transition has been observed at 0.10 MPa and 214.9 K in CHBr2Cl. The relations between isostructural phases, their symmetry, and site occupation factors of halogen atoms observed in the low-temperature and high-pressure phases of trihalomethanes (CHCl3, CHBrCl2, CHBr2Cl, and CHBr3) have been explained by the directional character of electrostatic interactions between the molecules. A gradual ordering of the disordered Br and Cl atoms has been achieved in the compressed crystals, where the narrower volume of the atomic sites correlates with the increased occupancy of the smaller atom (chlorine). The molecular symmetry has been shown to control the molecular aggregation in the crystalline state, consistent with the crystal sitesymmetry and the balance of electrostatic matching and dispersion forces between molecules. Introduction A rapidly growing interest in the role of halogen bonding,1 and particularly halogen · · · halogen interactions,2 and C-H · · · halogen hydrogen bonding3 in crystal chemistry and engineering, as well as in practical applications, requires detailed information about the molecular aggregation at varied thermodynamical conditions. Therefore, this study on the temperature and pressure dependence of the crystals of the simplest halogenated compounds and their structure-property relations has been undertaken. Until now, several groups of halomethanes were studied, and their different crystal structures were associated with dissimilarities between the properties of halogen atoms, like atomic and van der Waals radii and electrostatic-potential distribution. The crystal structures of halomethanes and their transformations at varied temperature or pressure can be rationalized by analyzing the nature of cohesion forces and the molecular association. It was shown recently that of the dihalomethane structures with different halogen atoms in the molecule only in the crystal of CH2ClI the halogen atoms were ordered.4,5 In both CH2BrCl phases, the Br and Cl atoms are disordered 50:50 over two sites, even in the crystals of space group C2/c, where molecules are located in general positions.6 However, in isostructural CH2BrI, its only known crystal phase (space group C2/c), atoms Br and I are disordered at the 60:40 rate.7 Trihalomethanes CHCl3 and CHBr3 exhibit an intriguing polymorphic behavior. At low temperature, CHCl3 freezes in space group Pnma, Z ) 4 (phase R).8 On isothermal freezing, the same phase is formed,9,10 but it transforms to the polar phase * Corresponding author. E-mail: [email protected].

β (space group P63, Z ) 2) between 0.62 and 0.75 GPa.9 Polymorphism of CHBr3 is more complex: its isobaric freezing at 281 K leads to the disordered R phase (space group P63/m, Z ) 2) stable down to 268 K, when it converts to the ordered triclinic phase β (space group P1j, Z ) 2), whereas the rapid cooling of CHBr3 to the liquid-nitrogen temperature yields an ordered trigonal γ phase (space group P3j, Z ) 2).11 The isothermal freezing of CHBr3 at 295 K and 0.1 GPa leads to the polar phase δ (space group P63, Z ) 2), isostructural with β-CHCl3.9 The trihalomethanes containing both Br and Cl atoms, CHBrCl2 and CHBr2Cl, at variable temperature were studied by neutron powder diffraction12 and by Raman and infrared spectroscopic methods.13 Their low-temperature crystals are isostructural (hereafter phases R) and isostructural with β-CHBr3. The varied Br:Cl occupancy refinements of the average crystal structure were interpreted as consistent with the existence of three domains differing in the orientation of CHXY2 (X, Y ) Cl, Br) molecules about their pseudo C3 axis. This conclusion was consistent with the observation of the disorder at 5 K where dynamical reorientations of molecules were unlikely. At high pressure, we have crystallized both CHBrCl2 and CHBr2Cl in a diamond anvil cell (DAC) and studied their structures by single-crystal X-ray diffraction. At 0.73 GPa, a new CHBrCl2 phase β, isostructural to R-CHCl3, space group Pnma, has been revealed. In this phase β, the CHBrCl2 molecules lie on the mirror plane, consistent with their own Cs symmetry; however, the Br and Cl atoms are disordered. We further showed that this phase is stable at least up to 2.53 GPa, the highest pressure attained in this isochoric pressure-crys-

10.1021/jp906071z CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

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Figure 1. Stages of β-CHBrCl2 isothermal freezing inside the DAC chamber: (a) liquid:crystal equilibrium at 0.48 GPa/295 K; (b, c) crystal growth following the decrease of DAC chamber volume; (d) the single crystal filling entirely the chamber at 0.73 GPa/295 K. The ruby chips for pressure calibration are grouped near the edge of the chamber. The crystal faces have been indexed in Figure 1(a) and 1(b).

tallization study. Surprisingly, at 0.43 GPa, CHBr2Cl crystallized in space group P63 (hereafter denoted as CHBr2Cl phase β), isostructural to β-CHCl3 and δ-CHBr3. Owing to the Br:Cl 2/3: 1/3 disordering at each atomic site, the molecules acquire an average C3 symmetry consistent with their site symmetry in the crystal. However, at 1.24 GPa a new structure (CHBr2Cl phase γ), isostructural with β-CHBrCl2, has been observed also with the apparent Br:Cl disorder. A considerable volume change at the solid-solid β-γ phase transition in CHBr2Cl revealed by the compression measurement in a piston-and-cylinder apparatus is the first instance of the Br:Cl gradual ordering induced by pressure and temperature changes in all Br:Cl halomethane, haloethane, and Br;I dihalo- florinated ethane derivatives14-16 investigated so far. Moreover, calorimetric analyses of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 have been performed to verify the information on the temperature dependence of their phase behavior. Experimental Section Bromodichloromethane (mp 218 K) of g98% purity and dibromochloromethane (mp 251 K) of 98% purity, both from Sigma-Aldrich, were used without further purification in our high-pressure experiments. Each compound was loaded to the modified Merrill-Bassett DAC,17 sealed with the 0.1 mm thick gaskets made of carbon-steel foil, and in situ crystallized. The freezing pressures of CHBrCl2 and CHBr2Cl at 295 K are 0.48 and 0.29 GPa, respectively, as determined when the crystal and liquid coexisted in the DAC chamber and independently by compression measurements. The pressure was calibrated using the ruby-fluorescence method,18,19 with a BETSA PRL spectrometer (accuracy 50 MPa). The first single crystal of CHBrCl2 was grown isothermally: the high-pressure chamber volume was slowly increased, until all crystals but one of polycrystalline mass melted, and then decreased until the single crystal fully filled the high-pressure chamber at 0.73 GPa (Figure 1). After

Dziubek et al.

Figure 2. Isochoric freezing of β-CHBrCl2 crystal: (a) one crystal grain at ca. 450 K; (b, c) isochoric single-crystal growth when the DAC is cooled slowly; (d) the single crystal of β-CHBrCl2 at 1.26 GPa/295 K. The Miller indices of the crystal faces have been shown in Figure 2(a) and 2(c).

the X-ray diffraction measurement, the second single crystal was grown at isochoric conditions. The high-pressure chamber volume was slightly decreased causing crushing of the CHBrCl2 crystal. Then, the DAC was heated with a hot-air gun to ca. 450 K, and when all crystal grains except one melted, the DAC was slowly cooled and the single crystal grew to fill the whole volume of the chamber at 1.26 GPa/295 K. This single-crystal growth is shown in Figure 2. After collecting the diffraction data, a third high-pressure experiment was performed for the CHBrCl2 single crystal isothermally compressed to 2.53 GPa. Under isobaric conditions, CHBrCl2 contained in a glass capillary (0.3 mm in diameter) was in situ crystallized on a diffractometer, with an Oxford Diffraction Cryosystems CPC611 low-temperature attachment. While the liquid sample was cooled at the rate of 1 K min-1, the progress of crystallization was controlled visually through a microscope and by recording X-ray diffraction images. The sample froze at ca. 180 K giving characteristic powder-diffraction rings. Then the sample was repeatedly heated and cooled at a constant rate between 210 and 215 K to grow a single crystal suitable for the diffraction experiment. The single-crystal measurement was performed at 200 K. Two high-pressure experiments on CHBr2Cl were performed on single crystals grown at isochoric conditions. The polycrystalline mass of CHBr2Cl in the DAC was heated using the hotair gun to ca. 370 K, until all but one crystallite melted. Then it was slowly cooled to room temperature, when the single crystal filled the whole volume of the chamber at 0.43 GPa/ 295 K. The single crystal of CHBr2Cl growth and its self-healing process15 are illustrated in Figure 3. After collecting the diffraction data, the single crystal was heated to ca. 360 K, and the sample crystal melted to about 5% of its volume. Then the chamber volume was decreased, and the DAC was heated to 470 K and cooled slowly to room temperature. When the pressure settled at 1.24 GPa (Figure 4), the next diffraction data set was recorded. The reflection data for the CHBrCl2 and CHBr2Cl crystals were measured using a KM-4 CCD diffractometer and graphite-

Molecular Symmetry and Isostructural Relations

J. Phys. Chem. B, Vol. 113, No. 40, 2009 13197 molecule was located at a special position, were used). Selected crystal data and details of the refinements of CHBrCl2 and CHBr2Cl crystals are listed in Table 1. The compressibility measurements of CHBrCl2 and CHBr2Cl were performed up to ca. 2 GPa in a piston-and-cylinder apparatus,25 with an initial volume of ca. 5.4 mL. The differential scanning calorimetry (DSC) runs were recorded for the CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 samples enclosed in aluminum capsules on the DSC Q2000 V23.12 Build 103 apparatus. The DSC thermographs were performed in the 300-100 K range with a rate of 10 K · min-1 (Figures S1-S4 in the Supporting Information). Program GAUSSIAN0326 and a PC were used at the B3LYP/ 3-21G** level of theory for DFT calculations of the electrostatic potential on the molecular surfaces of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3. Electrostatic potential27 was mapped onto the molecular surfaces defined as the 0.001 au electron-density envelope.28 Discussion

Figure 3. Single-crystal growth stages of β CHBr2Cl: from the grain at 370 K (a) to the crystal filling the whole volume of the high-pressure chamber at 0.43 GPa/295 K (d). The three ruby chips for pressure calibration are grouped in the central part of the high-pressure chamber. The crystal faces have been indexed in Figure 3(a).

Figure 4. Growth stages of the γ-CHBr2Cl single crystal: (a) the highpressure chamber heated to ca. 470 K, (b) 400 K, and (c) 350 K; (d) the single crystal entirely filling the chamber at 1.24 GPa/295 K.

monochromated Mo KR radiation. The DAC was centered by the gasket-shadowing method.20 The CrysAlis programs21 were used for the data collections, determination of the UB-matrices, initial data reductions, and Lp corrections. The high-pressure data accounted for the effects of absorption of X-rays by the DAC, gasket shadowing, and absorption of the sample crystal itself.22,23 The structures were solved by direct methods using the program SHELXS-9724 and refined with anisotropic displacement parameters for non-H atoms by the program SHELXL97.24 The H atoms were located from the molecular geometry, and their Uiso values were 1.2Ueq of the C atom (either by instruction AFIX 13, or appropriate DFIX restrains when the

Molecular Symmetry. The molecular symmetry of mixedtrihalogen methanes, CHBrCl2 and CHBr2Cl, Cs, is lower than the C3V symmetry of the molecules of chloroform and bromoform. However, the molecular shape and electrostatic potential distribution on the molecular surface of molecules CHBrCl2 and CHBr2Cl (Figure 5) approximate the C3V symmetry. The similar size and electrostatic potential is the likely reason for the disorder of Br and Cl atoms substituted at chemically equivalent sites in molecules6,14 and for the well-known formation of solidstate mixed bromide and chloride salts. Other halogen atoms, like Br and I, also can mix readily in certain compounds.7,15 It is a general feature in many cases that the exchange of one or two halogen atoms into other ones leads to a disordered structure with gradually changing features. The CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 series belongs to the simplest organic compounds for studying the effects of the halogen-atom exchange. In the recently studied dihalomethane series, the different halogens are either disordered 50: 50 or ordered. The most striking effects of the Br/Cl exchange in the CHXY2 (X,Y ) Cl, Br) are the symmetry changes of the crystals and their phase transitions. It has been established in this study that CHBrCl2 is orthorhombic, space group Pnma, in all the pressure range between 0.48 (freezing pressure) and 2.53 GPa. No sign of transformation of this phase has been observed by the compressibility measurements to nearly 2.0 GPa (Figure 6). Structural Relations. The presently obtained results initially appeared contradictive to the neat picture of the isostructural relations between polymorphic phases of trihalomethanes. For the “end members” in the Br/Cl trihalomethanes, CHCl3 and CHBr3, it was found that there were several space group symmetries, Pnma, P1j, P3j, P63/m, and P63, of which space group Pnma was evidenced for CHCl3 in low temperature, and space groups P1j, P3j, and P63/m were observed for lowtemperature CHBr3 phases. Most recently, space group P63 was revealed as the prototypical isostructure common for both CHCl3 and CHBr3 at high pressure.9 It was established that the prototypical P63-symmetric structure was strongly dominated by electrostatic matching of molecules, arising from the high electrostatic potential on the molecular surface associated with the Br atoms, and somewhat lower values for the CHCl3 molecules. The crystal structures of CHBr3 in space groups P1j and P3j are two-dimensionally isostructural29 with that in space group P63, and the crystal in space group P63/m is its disordered

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TABLE 1: Crystal Data and Details of the Refinements of CHBrCl2 at 0.10 MPa/200 K, 0.73 GPa/295 K, 1.26 GPa/295 K, and 2.53 GPa/295 K and of CHBr2Cl at 0.43 GPa/295 K and 1.24 GPa/295 K CHBrCl2 phase R pressure/temperature formula weight crystal color crystal size (mm) crystal system space group unit cell dimensions (Å,°)

volume (Å3) Z Dx (g cm-3) wavelength (Å) absorption coefficient (mm-1) F(000) (e) 2θ max (°) min/max indices h,k,l reflections collected/unique Rint observed reflections (I > 4σ(I)) data/parameters goodness of fit on F2 final R1 indices (I > 4σ(I)) R1/wR2 indices (all data) ∆σmax, ∆σmin (eÅ-3) weighting scheme a absorption corrections DAC transmission min/max gasket shadowing min/max sample transmission min/max extinction method extinction coefficient a

CHBr2Cl phase β

CHBr2Cl phase γ

0.10 MPa/200 K

0.73 GPa/295 K

CHBrCl2 phase β 1.26 GPa/295 K

2.53 GPa/295 K

0.43 GPa/295 K

1.24 GPa/295 K

163.83 colorless 0.30 × 0.20 × 0.10 triclinic P1j a ) 6.0970(12)

163.83 colorless 0.43 × 0.43 × 0.08 orthorhombic Pnma a ) 7.4618(15)

163.83 colorless 0.42 × 0.42 × 0.07 orthorhombic Pnma a ) 7.3379(15)

163.83 colorless 0.41 × 0.41 × 0.06 orthorhombic Pnma a ) 7.1006(14)

208.29 colorless 0.51 × 0.51 × 0.09 hexagonal P63 a ) 6.1512(9)

208.29 colorless 0.50 × 0.50 × 0.08 orthorhombic Pnma a ) 7.4260(15)

b ) 6.0913(12) c ) 7.2502(15) R ) 81.11(3) β ) 82.13(3) γ ) 119.54(3) 224.22(8) 2 2.427 Mo KR, λ ) 0.71073 10.14

b ) 9.5064(19) c ) 5.8645(12) R ) 90 β ) 90 γ ) 90 416.00(15) 4 2.616 Mo KR, λ ) 0.71073 10.93

b ) 9.3006(19) c ) 5.7983(12) R ) 90 β ) 90 γ ) 90 395.72(14) 4 2.750 Mo KR, λ ) 0.71073 11.49

b ) 9.0022(18) c ) 5.6699(11) R ) 90 β ) 90 γ ) 90 362.43(12) 4 3.002 Mo KR, λ ) 0.71073 12.55

b ) 6.1512(9) c ) 7.0480(14) R ) 90 β ) 90 γ ) 120 230.95(7) 2 2.995 Mo KR, λ ) 0.71073 17.92

b ) 9.5830(19) c ) 5.9490(12) R ) 90 β ) 90 γ ) 90 423.35(15) 4 3.268 Mo KR, λ ) 0.71073 19.55

152 49.94 -7/6, -7/7, -8/7 907/647

304 58.48 -10/10, -3/3, -8/7 2884/183

304 58.64 -9/10, -5/5, -7/7 3039/259

304 58.64 -9/9, -2/2, -7/7 2858/169

188 57.86 -8/8, -8/8, -2/2 1832/144

376 58.84 -8/8, -13/12, -5/5 2967/262

0.017 577

0.059 162

0.102 227

0.110 148

0.066 134

0.126 225

647/41 1.070 0.0399

183/27 1.288 0.0462

259/26 1.186 0.0528

169/26 1.167 0.0569

144/15 1.133 0.0319

262/26 1.174 0.0643

0.0446/0.1155 0.0547/0.0712 0.0612/0.1294 0.0704/0.1145 0.0360/0.0677 0.0750/0.1775 0.51, -0.45 0.19, -0.28 0.41, -0.50 0.32, -0.33 0.21, -0.25 0.70, -0.75 x ) 0.0898; y ) 0.00 x ) 0.0034; y ) 1.52 x ) 0.0534; y ) 1.49 x ) 0.0365; y ) 3.01 x ) 0.0217; y ) 0.55 x ) 0.1066; y ) 0.00 0.10/0.36 SHELXL 0.155(19)

0.64/0.94 0.85/0.99 0.35/0.42 SHELXL 0.0053(15)

0.64/0.95 0.85/0.99 0.33/0.40 none

-

0.62/0.94 0.85/1.00 0.34/0.42 none

-

0.89/1.00 0.87/0.99 0.15/0.20 SHELXL 0.079(10)

0.87/1.00 0.87/1.00 0.15/0.21 none

-

w ) 1/(σ (Fo ) + x P + yP), where P ) (max(Fo , 0) + 2Fc )/3. 2

2

2 2

2

2

Figure 5. Electrostatic potential on the molecular surface, defined as 0.001 au of electron density, for CHCl3 (a), CHBrCl2 (b), CHBr2Cl (c), and CHBr3 (d). The electrostatic potential scale ranges from -0.015 au (red) to 0.015 au (blue).

version (with the half-occupied CH group residing on both sides of the Br3 triangle). It was thus concluded that space group Pnma was characteristic of the low-pressure CHCl3 structure, and stronger electrostatic interactions associated with the Br atoms and the crystals compressed in high pressure favor the layer

structures in space groups P1j, P3j, and P63. It was then expected that CHBrCl2 could behave similarly to CHCl3 and crystallize in space group Pnma. Indeed, this space group Pnma of CHBrCl2 has been observed (Table 1, Figures 6 and 7) for the pressure frozen crystals. However, CHBrCl2 crystallized at

Molecular Symmetry and Isostructural Relations

Figure 6. Compressibility of molecular volume of CHBrCl2 and CHBr2Cl in the liquid and solid states measured in the piston-andcylinder device and by X-ray diffraction.

isobaric conditions yielded the layered structure of phase R in space group P1j. Moreover, the space group Pnma of CHBrCl2 in phase β persisted to 2.53 GPa. It shows that CHBrCl2 exhibits a different preference than CHCl3, which is stable in space group Pnma in low temperature and in the low range of pressure only. Surprisingly, at 0.28 GPa/295 K, CHBr2Cl froze in the β phase of space group P63 symmetry, isostructural with the prototypical phase of CHCl3 and CHBr3 at high pressure. In still higher pressure, at 1.27 GPa, CHBr2Cl transformed to space group Pnma phase γ, characteristic of the lower-pressure range of CHCl3 (Table 1, Figures 6 and 8). Structural Disorder. The crystal symmetries of the CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 polymorphs at various thermodynamical conditions have been summarized in Table 2. It is characteristic that in all polymorphs of CHBr2Cl and CHBrCl2 the Br and Cl atoms are disordered. The Br/Cl disorder can affect the average symmetry of the CHBr2Cl and CHBrCl2 molecules in various ways. In this section, only these types of disorders where the halogen atoms occupy three sites, at 0°, 120° and 240° about the CH bond, which are relevant to the further discussion, will be considered. In all the trihalomethane structures, the molecules are either ordered or disordered with the common sites of the halogen atoms. Three disorder modes for the halogen atoms occupying three sites can be distinguished (Scheme 1). Generally, the average structure can be composed of the ordered molecules and various ratios of modes 1, 2, and 3 in many different manners. Mode 3 does not change occupancy of the Br and Cl sites, so we will exclude it from further consideration here. However, in mode 1 there is only one site fully occupied by Cl atoms, and it can occur in three orientations with respect to the Br site of the ordered structure. Thus, for the full description of a CHBrCl2 structure, the (i) ordered part, (ii) mode 1 in three orientations, and (iii) mode 2 have to be included (when necessary, mode 3 should be also included). This disorder of mode 3 was observed in β-chloroform and R-bromoform but was not detected in any trihalomethanes with mixed halogen atoms. The structure fully disordered according to mode 1 has the following occupancies of the halogen atom sites: Site 1: 1Cl, Site 2: 1/2Cl + 1/2Br, Site 3: 1/2Br + 1/2Cl. There are two remaining orientations possible for this mode 1 of disordering:

J. Phys. Chem. B, Vol. 113, No. 40, 2009 13199 Site 1: 1/2Cl + 1/2Br, Site 2: 1Cl, Site 3: 1/2Br + 1/2Cl, and Site 1: 1/2Cl + 1/2Br, Site 2: 1/2Br + 1/2Cl, Site 3: 1Cl. These three orientations of disorder mode 1 can be considered only in reference to a specified orientation in a crystal, for example, to the site of the Br atom in the ordered part of molecules. The structure fully disordered according to mode 2 has the halogen atom site occupancies: Site 1: 2/3Cl + 1/3Br, Site 2: 2/3Cl + 1/3Br, Site 3: 2/3Cl + 1/3Br. We have tried to connect these modes of disorder with specific molecular reorientations (about the C-H bond), but in principle, these modes of disordering are interdependent. For example, one can express each orientation in mode 1 by two others or mode 2 by a superposition of any two orientations in mode 1, hence the infinite number of possible combinations for many disordered cases, i.e., specific site occupation factors. An alternative way of describing the disorder in CHXY2 was applied by Torrie et al. (1999)12 for modeling the structures of CHBrCl2 and CHBr2Cl refined against their powder neutrondiffraction measurements. They used three component phases with identically oriented identical lattices but with the molecules rotated by 120° and 240° about the C-H bond. The mode 2 of disorder leads to the average C3V symmetry of CHXY2 molecules (type C3V symmetry with each site occupied on average by X1/3Y2/3 atoms). Mode 1 of disorder would result in average Cs molecular symmetry with site occupancies X1/2Y1/2, X1/2Y1/2, and Y1. The C3V-symmetric disordering is present only in the lower range of the highpressure CHBr2Cl phase β. At higher pressure, above 1.27 GPa, the CHBr2Cl molecules break the C3V symmetry and become Cs-symmetric. In all pressure ranges investigated, CHBrCl2 molecules are C1- or Cs-symmetric. High pressure is known to considerably reduce thermal vibrations and to eliminate disorder.30 It is thus plausible that it is the Cs molecular symmetry which is responsible for the Pnma high-pressure symmetries of CHBr2Cl and CHBrCl2. This factor of molecular symmetry overcomes the preference of molecules to aggregate by electrostatic forces into hexagonal layers. Even if this hexagonal arrangement is achieved for CHBr2Cl (because electrostatic interactions in CHBr2Cl are stronger than in CHBrCl2), on increasing pressure the average C3 symmetry of disordered CHBr2Cl molecules in space group P63 is broken, and the crystal transforms to space group Pnma with molecules located on the Cs-symmetric sites. Another aspect of the molecular versus crystal symmetry interplay is the pressure dependence of the unit cell dimensions in the CHBrCl2 orthorhombic phase β. The b/c ratio for this cell should become equal to ca. 3 (i.e., 1.73) for the crystal transformed to the hexagonal symmetry (see Figure 6 in Dziubek and Katrusiak, 20089). However, the b/c ratio in CHBrCl2 decreases with increasing pressure from 1.621 at 0.73 GPa to 1.558 at 2.53 GPa (Figure 9), which shows that the crystal becomes more distorted from the ideal hexagonal lattice with increasing pressure. These distortions correlate with the increasing difference in partial occupancy of the Br/Cl atoms distorting the average molecular structure from C3V symmetry. For example, the difference in the occupancy factors of Br in phase

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Figure 7. Projections of the polymorphic structures of CHBrCl2: (a) phase R at 200 K, space group P1j; (b) phase β at 0.73 GPa/295 K, space group Pnma; (c) phase β at 1.26 GPa; and (d) phase β at 2.53 GPa. The thermal ellipsoids have been shown at the 50% probability level, and the dashed lines indicate the shortest intermolecular contacts with their distances given in Å. The partial occupancy factors of the symmetry-independent Br and Cl atoms are listed by their labels (cf. Scheme S1 in Supporting Information).

SCHEME 1: Disordered Modes in CHBrCl2

Figure 8. Projections of the polymorphic structures of CHBr2Cl: (a) phase β at 0.43 GPa/295 K, space group P63; and (b) phase γ at 1.24 GPa/295 K, space group Pnma. The thermal ellipsoids have been shown at the 50% probability level, and the dashed lines indicate the shortest intermolecular contacts with their distances given in Å. The partial occupancy factors of the symmetry-independent Br and Cl atoms are listed by their labels (cf. Scheme S1 in Supporting Information).

TABLE 2: Symmetries of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 Polymorphs

β of CHBrCl2 is 0.10 at 0.73 GPa, 0.16 at 1.26 GPa, and 0.19 at 2.53 GPa. Interestingly, the R-to-β phase transition in CHBrCl2 is connected with a large jump in the partial occupancy factors of the halogens. These are the first structures among the halomethanes and haloethanes investigated so far where changes in the occupancies of the Cl and Br atoms have been observed at varied pressure conditions. In the orthorhombic

phase γ of CHBr2Cl at 1.24 GPa, i.e., just above the phase transition from the hexagonal phase β, the b/c ratio is equal to 1.611 (Figure 9). This value is somewhat larger but nearly fits to the regression line of (∂(b/c))/(∂p), which suggests that the stronger electrostatic interactions and heavier molecules of CHBr2Cl favor the hexagonal crystal symmetry. The magnitudes of the b/c ratio also confirm that the phase transition in CHBr2Cl is noncontinuous. Thermal Analysis. The DSC thermographs of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 samples have been performed at ambient pressure in the 300-100 K range with a rate of 10 K · min-1. Within this temperature range, no signs of solid-solid phase transitions were detected for CHCl3 and CHBrCl2 (Figures S1 and S2 in Supporting Information). Both in the cooling and heating DSC runs for CHBr2Cl, small signals appeared at 214.9 and 221.9 K, respectively, which can be interpreted as the solid-solid transition (Figure S3 in Supporting Information). The neutron powder-diffraction study of CHBr2Cl in the function of temperature did not report any phase transition for this compound,12 despite that one of the measurements was conducted at 230 K, i.e., between 221.9 K

Figure 9. b/c ratio values (full circles) plotted against pressure for the orthorhombic β-CHBrCl2 and their regression line. The b/c ratio estimate at p ) 0 is 1.647. The b/c ratio for γ-CHBr2Cl has also been plotted as the open circle.

Molecular Symmetry and Isostructural Relations and the mp at 251 K. In this paper, the structure of CHBr2Cl was considered as a superposition of three domains with different orientation of the molecules about the molecular pseudo C3 axis, and at 230 K, one of these domains disappears.12 We have reexamined the structural results in ref 12 (Torrie et al., 1999) and found that they are somewhat inconsistent with those in temperatures below 214.9 K: small molecular reorientations following the temperature changes have been reversed above 214.9 K, and one of the three composite domains disappears. These changes, however, are very small, and no symmetry change was detected in those studies. Therefore, it is plausible that the crystal undergoes a subtle displacive phase transition, which is isostructural and does not involve the symmetry change. The isostructural phase transitions are of the first order, according to the Erenfest classification, which is consistent with the observed hysteresis of 7 K of this transition between the cooling and heating runs. According to this reasoning, CHBr2Cl freezes into phase R below its melting point, and then below 214.9 K it transforms into isostructural phase R′. The cooling DSC run for CHBr3 (Figure S4 in Supporting Information) is consistent with the literature information on the thermal behavior of this compound.11 In this run below the melting point, a second-order phase transition follows at 247 K, with a long tail into the β phase. In the heating run, the analogous DSC signal of this phase transition is observed at the same temperature, although somewhat deformed, most probably due to the exothermic process of crystallization of an amorphous portion of the sample. At 265.3 K, there is another phase transition preceding the sample melting at 277.6 K, which most likely is a reversed transformation into phase R, previously observed at 26811 and 269.5 K.31 This interpretation would be supported by the magnitudes of the transition enthalpies derived from the DSC signal which are smaller for the cooling run (Supporting Information, Figure S4), as only a part of the sample crystallized when temperature was lowered, and higher for the heating run, when the amorphous portion of the sample crystallized. It appears that in the DSC run there is no trace of the γ phase, space group P3j, which can also be obtained from liquid CHBr3 on its rapid cooling from room to liquid-nitrogen temperature. Conclusions A consistent map of crystal symmetries has been drawn for the series of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3 halomethanes. The crystals of mixed-halogen methanes exhibit the reverse sequence of their space group symmetries, compared to the end members of this series, CHCl3 and CHBr3. This behavior has been rationalized by the molecular symmetry factor favoring the Pnma space group symmetry for the high-pressure structures of CHBrCl2 and CHBr2Cl, whereas owing to the C3V molecular symmetry of CHCl3 and CHBr3 these compounds transform to the phases of the P63 space group symmetry. It appears from the low-temperature and high-pressure studies on trihalomethanes that the phase behavior is complete within the studied range for CHCl3 and CHBrCl2; however, calorimetric results for CHBr2Cl and CHBr3 suggest that there are other, still not described, phases of these compounds. Acknowledgment. This study was supported by the Polish Ministry of Scientific Research, Grants No. N N204 1956 33

J. Phys. Chem. B, Vol. 113, No. 40, 2009 13201 and N N204 0547 35. Dr. Marcin Podsiadło acknowledges the receipt of the scholarships from the Foundation for Polish Science and the Adam Mickiewicz University Foundation in 2009. Supporting Information Available: Partial occupancy factors of the symmetry-independent Br and Cl atoms for the CHBrCl2 and CHBr2Cl (Scheme S1); differential scanning calorimetry (DSC) thermograph for CHCl3 (Figure S1); DSC thermograph for CHBrCl2 (Figure S2); DSC thermograph for CHBr2Cl (Figure S3); DSC thermograph for CHBr3 (Figure S4); atomic coordinates for CHBrCl2 and CHBr2Cl (Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114–6127. (2) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem.sEur. J. 2006, 12, 8952–8960. (3) van den Berg, J.-A.; Seddon, R. Cryst. Growth Des. 2003, 3, 643– 661. (4) Podsiadło, M.; Katrusiak, A. J. Phys. Chem. B. 2008, 112, 5355– 5362. (5) Podsiadło, M.; Katrusiak, A. CrystEngComm. 2009, 11, 1391–1395. (6) Podsiadło, M.; Katrusiak, A. Acta Crystallogr. 2007, B63, 903– 911. (7) Podsiadło, M.; Katrusiak, A. CrystEngComm. 2008, 10, 1436–1442. (8) Fourme, R.; Renaud, M. C. R. Acad. Sci. Paris 1966, 263, 69–72. (9) Dziubek, K.; Katrusiak, A. J. Phys. Chem. B 2008, 112, 12001– 12009. (10) Fourme, R. J. Appl. Crystallogr. 1968, 1, 23–30. (11) Myers, R.; Torrie, B. H.; Powell, B. M. J. Chem. Phys. 1983, 79, 1495–1504. (12) Torrie, B. H.; Binbrek, O. S.; Swainson, I. P.; Powell, B. M. Mol. Phys. 1999, 97, 581–586. (13) Anderson, A.; Lokat, G. A.; Smith, W. J. Raman Spectrosc. 1996, 27, 699–704. (14) Olejniczak, A.; Katrusiak, A.; Metrangolo, P.; Resnati, G. J. Fluorine Chem. 2009, 130, 248–253. (15) Olejniczak, A.; Katrusiak, A.; Vij, A. CrystEngComm. 2009, 11, 1073–1080. (16) Olejniczak, A.; Katrusiak, A.; Vij, A. CrystEngComm. 2009, 11, 1240–1244. (17) Merrill, L.; Bassett, W. A. ReV. Sci. Instrum. 1974, 45, 290–294. (18) Barnett, J. D.; Block, S.; Piermarini, G. J. ReV. Sci. Instrum. 1973, 44, 1–9. (19) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, R. A. J. Appl. Phys. 1975, 46, 2774–2780. (20) Budzianowski, A.; Katrusiak, A. High-Pressure Crystallography; Katrusiak, A.; McMillan, P. F.; Kluwer Academic Publishers: Dordrecht, 2004; pp 101-112. (21) Oxford Diffraction Xcalibur CCD system, CrysAlis Software System, Version 1.171; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2004. (22) Katrusiak, A. REDSHABS; Adam Mickiewicz University: Poznan´, Poland, 2003. (23) Katrusiak, A. Z. Kristallogr. 2004, 219, 461–467. (24) Sheldrick, G. M. SHELXS97 and SHELXL97; University of Go¨ttingen: Germany, 1997. (25) Baranowski, B.; Moroz, A. Pol. J. Chem. 1982, 56, 379–391. (26) Frisch, M. J. GAUSSIAN03, revision B.04; Gaussian, Inc.: Pittsburgh, PA, USA, 2003. (27) Murray, J. N.; Sen, K. D. Molecular Electrostatic Potential: Concepts and Applications; Elsevier: New York, 1996. (28) Bader, R. F. W.; Carroll, M. T.; Cheeseman, J. R.; Chang, C. J. Am. Chem. Soc. 1987, 109, 7968–7979. (29) Fa´bia´n, L.; Ka´lma´n, A. Acta Crystallogr. 2004, B60, 547–558. (30) Katrusiak, A. Cryst. Res. Technol. 1991, 26, 523–531. (31) Sharma, A. K.; Agarwal, V. K.; Mansingh, A. Chem. Phys. Lett. 1979, 68, 151–153.

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