Competing Patterns of Weak Directional Forces in Pressure-Frozen

The isothermal growth of CH2ClI single crystal is presented in Figure 2. .... Electrostatic potential was mapped onto the molecular surface defined as...
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J. Phys. Chem. B 2008, 112, 5355-5362

5355

Competing Patterns of Weak Directional Forces in Pressure-Frozen CH2ClI and CH2I2 Marcin Podsiadło and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´ , Poland ReceiVed: January 23, 2008; In Final Form: February 19, 2008

Isostructural relations and phase transitions of dihalomethanes have been rationalized by the competing patterns of CH‚‚‚halogen hydrogen bonds and halogen‚‚‚halogen interactions, the common weak directional interactions in soft organic matter. Pressure-frozen chloroiodomethane, CH2ClI, at 295 K and 0.72 GPa forms centrosymmetric phase III, which at ca. 400 K and 1.6 GPa disproportionates into CH2Cl2 and CH2I2. The directional character of intermolecular contacts between halogen atoms results from the characteristic anisotropic charge distribution on molecular surface.

1. Introduction

TABLE 1: Space-Group Symmetries of Dihalomethane Crystals

Modern technologies increasingly rely on versatile properties of soft materials. In common use are various organic and silicate substances, such as polymers, liquid crystals, pigments, dyes, pharmaceutical drugs, or magnetic liquids. These materials are light, contain no metals, can dissolve in water, and are environmentally friendly. Their properties depend on the structure of molecules and intermolecular interactions, which are weak compared to electrostatic forces in inorganic ionic crystals. However their contribution is often essential for the formation of polymorphs, their transformations, and properties, such as dielectric constant, optical activity, conductivity, thermal expansion, or solubility. The significance of soft materials in polymers, pharmaceuticals, pigments, and electronic industries revoked intense studies on the nature of weak interactions, their types, and role in the molecular aggregation.1-14 Along with hydrogen bonds, halogen‚‚‚halogen interactions are considered to be the strongest of cohesion forces in molecular crystals, but their role for molecular aggregation is still disputable. Apart from the evidence of molecular attraction to the halogen atoms,14-16 there are also arguments for the repulsion of the halogens, e.g., chlorophobic interactions.17-21 Although abandoned in Nature, the experimental observation of halogen‚‚‚ halogen interactions is often blurred by other types of interactions and close-packing requirement for the association of large and irregularly shaped molecules in crystals.22 Hence we have studied a series of simple halomethanes, the smallest organic molecules arranging according to hardly disturbed halogen‚‚‚ halogen and hydrogen‚‚‚halogen forces, which often have decisive role for the crystal symmetry and properties.23-25 Dihalomethanes were intensely studied in low temperatures by powder diffraction26-28 and spectroscopic methods,29-36 but no information about most of their structures at varied temperature and pressure have been reported. We have determined these unknown crystal structures and found that dihalomethanes exhibit characteristic phase transitions and systematic isostructural relations, summarized in Table 1. For example, space group Pbcn is specific of CH2Cl2 and CH2BrCl, where halogen‚‚‚ halogen interactions are weak, while CH2I2sowing to strongly interacting iodine atomsscrystallizes in space group Fmm2. * To whom correspondence should be addressed. E-mail: katran@ amu.edu.pl.

a

Disordered phases.

Space group C2/c of isostructural compounds, in the central part of Table 1, is common for all dihalomethanes, except of CH2ClI (with halogens considerably different in atomic volume and weight) and CH2Cl2. The Raman and far-IR spectroscopic studies of CH2ClI and CD2ClI showed37 that each of them has four different crystalline phases: plastic phase I, partly ordered phase III, and fully ordered phases II and IV. The plastic phase I exists between the melting point at 245 and 195 K. When slowly cooled or quenched in liquid nitrogen, the sample transforms near 180170 K to phase III, which can be retained to liquid-helium temperatures. Depending on the sample history, phase III can also transform to phases II and IV. The sample of rapidly quenched phase III transforms to phase II near 165 K, while this cooled slowly transforms back to phase I near 190 K or to phase IV near 187 K. The spectroscopic analysis suggested centrosymmetric structures with Z g 4 for phases II and III and a noncentrosymmetric structure with Z g 8 for phase IV. The structure of CD2ClI phase III determined at 13 K by neutron powder diffraction38 was consistent with the spectroscopic results: the crystal is orthorhombic, space group Pnma with Z ) 4, but no disorder was observed. We have determined the high-pressure structure of CH2ClI, in situ pressure frozen in a diamond anvil cell, by X-ray singlecrystal diffraction at 1.10, 1.29, and 2.33 GPa. It is apparent from the literature that CH2ClI is sensitive to the thermal conditions of crystallization. This dependence was also observed for other compounds.39 Therefore we undertook to crystallize CH2ClI in isothermal conditions. We have shown that purely isothermal single-crystal crystallization can be performed in a Merrill-Bassett diamond-anvil cell. The isothermally com-

10.1021/jp8006574 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/11/2008

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Figure 2. Stages of isothermal growth of a single crystal of CH2ClI from its mixture with methanol: (a) the two liquids separated at high pressure, and a border between them is clearly visible (methanol-rich mixture to the right), and in the CH2ClI-rich liquid (to the left side), three small crystal grains are located one at the center, one at 8, and one at 12 h; (b, c) isothermal growth of the grain at 8 h following the reduction of the high-pressure chamber volume; (d) the single crystal at 1.29 GPa/295 K filling ca. half of the chamber volume. The ruby chip is at the right edge of the high-pressure chamber.

Figure 1. Stages of CH2ClI isothermal freezing inside the DAC chamber: (a) polycrystalline mass at 1.10 GPa/295 K; (b) liquid: polycrystal equilibrium at 0.72(5) GPa/295 K; (c) one crystal grain at 0.72 GPa/295 K; (d, e) crystal growth following the decrease of DAC chamber volume; (f) the single crystal filling nearly entirely the chamber at 1.10 GPa/295 K. The ruby chips for pressure calibration are grouped near the right edge of the chamber, and one long thin ruby lies close to the center.

pressed CH2ClI freezes in the orthorhombic space group Pnma, isostructural to the low-temperature phase III of the deutered analogue, CD2ClI.38 Below we report the molecular association, crystal structure, and properties of CH2ClI in the function of pressure. 2. Experimental Methods Chloroiodomethane (mp 245 K at 0.1 MPa) of 97% purity from Sigma-Aldrich was used: at 295 K it was loaded to two modified Merril & Bassett40 diamond-anvil cells (DAC) and was in situ crystallized. The gaskets used for the high-pressure experiment were made of 0.1 mm thick carbon-steel foil with the initial hole diameter of 0.5 mm. After the indentation, the holes were 0.07 mm high and 0.36 mm in diameter (1.10 GPa) and 0.08 mm in height and 0.45 mm in diameter (2.33 GPa)s these were the final dimensions of the pressure-frozen crystals. Pressure calibration by the ruby-fluorescence method41,42 with a BETSA PRL spectrometer afforded the accuracy of 50 MPa. The freezing pressure of CH2ClI at 295 K, 0.72 GPa, has been measured by isothermal compression, when crystals and liquid coexisted inside the DAC chamber (see Figure 1b). Then highpressure chamber volume was slowly increased till all crystals but one melted and decreased till the single crystal fully filled the high-pressure chamber. Then pressure was increased to 1.10 GPa, to secure stable conditions during the X-ray diffraction measurements. The isothermal growth of the CH2ClI crystal is

illustrated in Figure 1. In the other DAC analogically another single crystal of CH2ClI was obtainedsit was squeezed to 2.33 GPa without heating the sample. The diffraction measurements were carried out several days after the crystallization. Then one of the DACs was heated to ca. 400 K, and a solid-solid transition was observed. While the Pnma-symmetric CH2ClI single crystal melted, new grains appeared, and their crystal habit was different from those of the low-pressure phase. Despite efforts, one single crystal of this new phase inside the high-pressure chamber was not obtained, but lots of small crystal grains grew simultaneously, even if the DAC was cooled very slowly. For exploring higher-pressure regions of the CH2ClI phase diagram, the DAC was loaded with the mixture of chloroiodomethane and methanol in 1:1 volume ratio. The chamber in carbon-steel foil was 0.42 mm in diameter and 0.08 mm high (1.29 GPa), and 0.41 mm in diameter and 0.07 mm high (1.56 GPa). When CH2ClI crystallized as a polycrystalline mass, the chamber volume was slowly increased till all crystal grains but one melted. Then pressure was increased to 1.29 GPa/295 K to ensure stable conditions of the single crystal during diffraction experiments. The isothermal growth of CH2ClI single crystal is presented in Figure 2. After collecting the diffraction data the high-pressure chamber volume was slightly decreased and the DAC was heated to ca. 490 K. When the single crystal was melting, new rounded grains appeared. One of them was allowed to grow till 295 K, when pressure stabilized to 1.56 GPa. After collecting the diffraction data it turned out that the chemical reaction occurred according to the formula: 2CH2ClI f CH2Cl2+CH2I2. The isochoric growing process of the CH2I2 crystal is illustrated in Figure 3. The disproportionation of CH2ClI and crystallization of CH2I2 was verified in the following experiment. Pure CH2ClI was loaded into the DAC and crystallized. The single-crystal of Pnma-symmetric phase of CH2ClI was squeezed to ca. 3 GPa.

Competing Patterns in Pressure-Frozen CH2ClI and CH2I2

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Figure 3. Isochoric freezing of Fmm2-symmetric CH2I2 crystal from the reacting CH2ClI:methanol mixture viewed in polarized light: (a) one crystal grain at ca. 490 K, liquid methanol is separated in the form of small droplets; (b, c) isochoric single-crystal growth when the DAC is cooled slowly; (d) the single crystal of CH2I2 at 1.56 GPa/295 K filling ca. half of the high-pressure chamber volume.

Then the DAC was heated, and at ca. 420 K characteristic features of the mixture, illustrated in Figure 4 and later associated with the CH2ClI disproportionation, occurred. After melting the whole single crystal of CH2ClI the DAC was cooled to 295 K. Then pressure inside the DAC chamber was reduced to melt the whole sample. After increasing pressure new singlecrystal was obtained isothermally at 0.62 GPa/295 K. This pressure is lower than the freezing pressure of CH2ClI observed in previous experiments and higher than the freezing-pressure of CH2I2 at 0.16 GPa/295 K;24 however it is consistent with the freezing of CH2I2 dissolved in CH2Cl2. The unit-cell dimensions and symmetry of the single crystal measured by X-ray diffraction confirmed the disproportionation of CH2ClI. The reflections data collections of the CH2ClI crystals at 1.10(5), 1.29(5), and 2.33(5) GPa and of the CH2I2 crystals at 0.62(5) and 1.56(5), all at 295(2) K, were performed using a KM-4 CCD diffractometer with the graphite-monochromated Mo KR radiation. The centering of the DAC was performed by the gasket-shadowing method.43 The reflections were collected with the ω-scan technique, 0.80° frame widths and 35 s exposures at 0.62 GPa, 0.85° frame widths and 35 s exposures at 1.10 GPa, 0.80° ω-frames and 25 s exposures at 1.29 GPa, 0.70° frame widths and 25 s exposures at 1.56 GPa, and 0.80° ω-frames and 35 s exposures at 2.33 GPa. The CrysAlisCCD and CrysAlisRED programs44 have been used for collecting data, UB matrices determination, initial data reductions, and Lp corrections. In all measurements the reflection intensities have been corrected for the effects of absorption of X-rays by the DAC, shadowing of the beams by the gasket edges and absorption of the sample crystals themselves.45,46 The unit-cell dimensions have been corrected for the effect of reflections profiles shifts due to the crystal absorption and gasket shadowing. The CH2ClI and CH2I2 crystal structures were solved by direct methods using the program SHELXS-97 and refined with atoms C1, Cl1, and I1 anisotropic (except for atom C1 in the CH2I2 structure at 1.56 GPa/295 K) by program SHELXL-97.47 For Pnma-symmetric phase of CH2ClI restrains were applied to obtain the C-H bond length of 1.05 Å, and Uiso of H1 was

Figure 4. The disproportionation of CH2ClI to CH2Cl2 and CH2I2: (a) Pnma-symmetric single crystal of CH2ClI at 0.72 GPa/295 K; (b) this single crystal squeezed to 2.85 GPa/295 K; (c-e) the disproportionation of CH2ClI to CH2Cl2 and CH2I2 at ca. 420 K; during the melting process of the CH2ClI single crystal, new crystal grains of separately crystallized CH2I2 and CH2Cl2 appeared; (f) polycrystalline mass of separately crystallized CH2I2 and CH2Cl2 at 2.85 GPa/295 K. The ruby chip is at the upper right edge of the high-pressure chamber.

constrained to 1.2 Ueq of C1. In the Fmm2-symmetric phase of CH2I2 the position of the H atom was restrained to obtain the C-H bond length of 1.01 Å and its isotropic displacement parameter set at 1.3 times Ueq of the carbon atom. Selected details of the structures refinements and crystal data are listed in Table 2. Program GAUSSIAN0348 and a PC were used at the B3LYP/ 3-21G** level of theory for density functional theory calculations of the electrostatic potential on the molecular surfaces of diiodomethane and chloroiodomethane. Electrostatic potential was mapped onto the molecular surface defined as 0.001 au. electron-density envelope.49 The compressibility measurements of CH2ClI and CH2I2 have been performed in a cylinder-and-piston apparatus, described previously by Baranowski & Moroz,50 with an initial volume of 9.8 mL. The crystal structures were drawn by program XP.51 3. Results and Discussion 3.1. The Disproportionation of CH2ClI to CH2Cl2 and CH2I2. Thermochemical study of the liquid-phase equilibrium reaction of dihalomethanes was performed by Davalos et al.52 by NMR spectroscopy. They concluded that the reaction among dihalomethanes (2CH2BrI f CH2Br2 + CH2I2) is dominated by entropy effects. The heats of vaporization of the three

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TABLE 2: Crystal Data and Details of the Refinements of CH2ClI and CH2I2 Structures pressure/temperature formula weight crystal color crystal size (mm) crystal system space group a (Å) b (Å) c (Å) volume (Å3) Z Dx (g cm-3) wavelength (Å) no. reflections for cell parameters µ (mm-1) F(000) (e) 2θ max (degrees) 2θ range (degrees) Limiting indices

reflections collected/unique Rint observed reflections (I > 4σ(I)) data/parameters goodness on fit on F2 final R indices (I > 4σ(I)) R indices (all data) ∆σmax, ∆σmin (eÅ-3) weighting scheme

absorption corrections DAC transmission min/max gasket transmission min/max sample transmission min/max extinction coefficient extinction method

CH2ClI phase III

CH2ClI phase III

CH2ClI phase III

CH2I2 phase II

CH2I2 phase II

1.10(5) GPa/295 K 176.38 colorless 0.36 × 0.36 × 0.07 orthorhombic Pnma 6.3150(13) 6.5616(13) 8.6926(17) 360.19(12) 4 3.252 Mo KR, λ ) 0.71073 2588 9.35 312 59.34 8-59 -5 e h e 5 -8 e k e 8 -11 e l e 11 2678/347 0.0482 298 347/22 1.101 R1 ) 0.0207 R1 ) 0.0256 wR2 ) 0.0533 0.87, -0.51 1/(σ2(F02) + (0.03 41P)2), where P ) (max(F02,0) + 2Fc2)/3

1.29(5) GPa/295 K 176.38 colorless 0.39 × 0.34 × 0.08 orthorhombic Pnma 6.3026(13) 6.5131(13) 8.6599(17) 355.48(12) 4 3.296 Mo KR, λ ) 0.71073 2077 9.48 312 59.10 8-59 -3 e h e 3 -8 e k e 8 -11 e l e 11 2325/212 0.0477 204 212/23 1.139 R1 ) 0.0320 R1 ) 0.0330 wR2 ) 0.0852 1.03, -1.50 1/(σ2(F02) + (0.0593P)2 + 0.53P), where P ) (Max(F02,0) + 2Fc2)/3 DAC, gasket, and sample crystal 0.64/0.95 0.85/0.99 0.41/0.47 0.031(6) SHELXL

2.33(5) GPa/295 K 176.38 colorless 0.45 × 0.45 × 0.08 orthorhombic Pnma 6.2105(12) 6.3402(13) 8.5189(17) 335.44(12) 4 3.492 Mo KR, λ ) 0.71073 3493 10.04 312 60.00 8-60 -7 e h e 8 -7 e k e 7 -9 e l e 9 2444/334 0.0849 302 334/22 1.099 R1 ) 0.0357 R1 ) 0.0385 wR2 ) 0.0877 0.86, -0.82 1/(σ2(F02) + (0.0488P)2 + 0.65P), where P) (Max(F02,0) + 2Fc2)/3 DAC, gasket, and sample crystal 0.63/0.92 0.85/0.99 0.38/0.48 none

0.62(5) GPa/295 K 267.83 colorless 0.42 × 0.42 × 0.20 orthorhombic Fmm2 7.1819(14) 12.912(3) 4.6930(9) 435.19(15) 4 4.088 Mo KR, λ ) 0.71073 873 14.221 456 59.32 11-59 -9 e h e 10 -8 e k e 8 -6 e l e 6 919/154 0.0296 150 154/14 1.155 R1 ) 0.0154 R1 ) 0.0161 wR2 ) 0.0372 0.31, -0.61 1/(σ2(F02) + (0.0193P)2 + 1.13P), where P ) (Max(F02,0) + 2Fc2)/3 DAC, gasket, and sample crystal 0.87/1.00 0.63/0.96 0.16/0.21 0.010(1) SHELXL

1.56(5) GPa/295 K 267.83 colorless 0.41 × 0.41 × 0.07 orthorhombic Fmm2 6.9208(14) 12.695(3) 4.5338(9) 398.34(14) 4 4.466 Mo KR, λ ) 0.71073 631 15.54 456 57.98 11-58 -9 e h e 9 -4 e k e 4 -6 e l e 6 853/94 0.0923 92 94/12 1.174 R1 ) 0.0171 R1 ) 0.0187 wR2 ) 0.0212 0.37, -0.21 1/(σ2(F02) + (0.00 89P)2), where P ) (Max(F02,0) + 2Fc2)/3

DAC, gasket, and sample crystal 0.64/0.91 0.85/0.99 0.46/0.52 none

compounds are such that the solution phase heat of reaction should be equal to the gas-phase heat of reaction. The disproportionation of CH2ClI and formation of CH2I2 have been thoroughly confirmed (see Experimental section). Besides, the molecular volume measured for the obtained crystal at 1.56 GPa agrees very well with the compressibility of CH2I2 (Figure 5), and the crystal symmetry Fmm2 is identical to that of CH2I2 measured at 0.16 GPa.24 Second, after decreasing pressure to 0.62 GPa, the single-crystal remained intact, and another diffraction measurement could be performed. This pressure is lower by ca. 0.1 GPa than the freezing pressure of CH2ClI. CH2Cl2 freezes at 1.33 GPa,23 thus at 0.62 GPa the single crystal of CH2I2 froze of its solution in CH2Cl2. 3.2. Pressure-Frozen Structures of CH2ClI and CH2I2. The structure of phase III of CH2ClI can be considered as built of layers, governed by halogen‚‚‚halogen interactions. In this Pnma-symmetric phase III the molecules lay on the mirror plane, where short intermolecular I‚‚‚I, Cl‚‚‚I, and CH‚‚‚Cl contacts bind the molecules into sheets, shown in Figure 6. Between these layers, and along the most compressible direction b, the molecules interact via short Cl‚‚‚Cl and CH‚‚‚I contacts (see Table 3). The electrostatic potential mapped on the surfaces of molecules in phase III, shown in Figure 6b, illustrates the electrostatic origin of the cohesion forces: the positive and negative regions of molecular surface are in contact on neighboring molecules. Only the interlayer Cl‚‚‚Cl interactions in phase III are electrostatically unfavorable; the negative regions

DAC, gasket, and sample crystal 0.64/0.94 0.86/1.00 0.49/0.56 0.002(1) SHELXL

Figure 5. Molecular volumes of CH2ClI (red) and CH2I2 (purple) in the function of pressure. Freezing pressures are marked by isochoric pressure jumps at 0.70 GPa for CH2ClI and at 0.10 GPa for CH2I2. Red and purple circles represent molecular volume measured in the cylinder-and-piston press, and triangles those from high-pressure X-ray single-crystal diffraction.

of the Cl atoms surface are in contact. The charge density of Cl atoms is much lower compared to H and I atoms in CH2ClI, which explains the existence of Cl‚‚‚Cl contacts in phase III. At ca. 400 K and 1.6 GPa, CH2ClI disproportionates to CH2Cl2 and CH2I2, of which CH2I2 crystallizes in noncentrosymmetric Fmm2 space group. In this phase II the CH2I2 molecules

Competing Patterns in Pressure-Frozen CH2ClI and CH2I2

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Figure 6. The pressure-frozen crystal structure of CH2ClI phase III at 1.10 GPa/295 K: (a) two layers of CH2ClI molecules perpendicular to b (full bonds in the upper layer), shortest intermolecular I‚‚‚I and Cl‚‚‚Cl distances are presented by dashed lines and indicated for the structure compressed to 1.10(5) GPa (red), 1.29(5) GPa (green) and 2.33(5) GPa (blue); (b) one of these layers (Cl‚‚‚I contacts also marked). The major and minor radii of the iodine and chlorine atoms (red and blue sections of ellipsoids drawn about the atoms, respectively) correlate with the negative and positive electrostatic potential on the molecular surface indicated for 4 molecules; the red-to-blue color scale ranges from -58.66 to 118.56 kJ/mol, respectively. The molecular and van der Waals dimensions53 have been represented to scale. The displacement ellipsoids are drawn at the 50% probability level.

TABLE 3: Main Interatomic Distances (Ångstroms) and Angles (Degrees) Observed in Low-Temperature38 and Pressure-Frozen Chloroiodomethane Phase IIIa,b CH2ClI

0.10M Pa/13 Kb

C1-H1 (Å) C1-Cl1 (Å) C1-I1 (Å) Cl1‚‚‚I1 (Å) ∠H1-C1-H1A (°) ∠H1-C1-Cl1 (°) ∠H1-C1-I1 (°) ∠Cl1-C1-I1 (°)

1.064(1) 1.744(2) 2.147(2) 3.262(2) 110.5(2) 109.8(1) 106.6(1) 113.5(1)

I1‚‚‚I1i (Å) I1‚‚‚I1j (Å) Cl1‚‚‚Cl1k (Å) I1‚‚‚Cl1l (Å) I1‚‚‚Cl1m (Å) H1‚‚‚I1n (Å) H1‚‚‚I1o (Å) H1‚‚‚Cl1p (Å) H1‚‚‚Cl1q (Å) ∠C1-I1-I1i (°) ∠C1-I1-I1i (°) ∠C1-Cl1-Cl1k (°) ∠C1-I1‚‚‚Cl1l (°) ∠C1-Cl1‚‚‚I1r (°) ∠C1-I1‚‚‚Cl1m (°) ∠C1-Cl1‚‚‚I1s (°) a

3.931(2) 4.192(2) 3.531(1) 3.769(2) 3.774(2) 3.385 3.355 3.151 3.221 91.85 159.58 80.05 150.38 104.59 100.93 167.40

1.10 GPa/295 K 1.05 1.747(7) 2.120(6) 3.253(2) 108.8(33) 108.5(15) 108.3(4) 114.2(3)

1.29 GPa/295 K

2.33 GPa/295 K

1.05 1.824(20) 2.114(12) 3.268(6) 122.1(62) 102.5(34) 108.7(8) 112.0(6)

1.05 1.749(8) 2.119(13) 3.263(3) 105.4(69) 109.8(29) 108.4(8) 114.7(7)

Intermolecular Distances 3.8515(7) 3.8356(9) 4.1347(7) 4.1087(12) 3.4820(13) 3.4632(32) 3.7222(16) 3.7025(31) 3.7478(21) 3.7219(63) 3.287(10) 3.231(14) 3.302(5) 3.247(19) 3.022(28) 3.073(58) 3.197(30) 3.089(59) Intermolecular Angles 91.83(17) 93.39(52) 158.04(17) 156.12(52) 79.56(8) 79.34(24) 151.12(17) 152.53(56) 103.09(19) 103.29(33) 99.40(18) 97.48(51) 165.16(19) 165.50(32)

3.7636(9) 4.0208(9) 3.3952(19) 3.6462(26) 3.6479(26) 3.169(23) 3.175(10) 2.885(55) 3.135(62)

ΣvdW (Å)

3.944 4.214 3.547 3.626 3.714

91.17(26) 157.64(26) 78.40(14) 150.13(26) 101.89(44) 98.72(27) 164.02(45)

For calculating the sums of van der Waals radii, ΣvdW, elliptical van der Waals surfaces have been assumed, and the radii have been obtained

according to the formula: r ) xrs2cos2φ+rl2sin2φ, where φ is the C-halogen‚‚‚halogen angle, and rs and rl are the minor and major halogen radii.53 b Low-temperature structure of CD2ClI. c Symmetry codes: (i) -0.5 + x, y, -0.5 - z; 0.5 + x, y, -0.5 - z; (j) 1 - x, -y, -z; 1 - x, 1 - y, -z; (k) -x, -y, -z; -x, 1 - y, -z; (l) 0.5 + x, y, -0.5 - z; (m) 1 + x, y, z; (n) 0.5 - x, 1 - y, 0.5 + z; (o) 1 - x, 1 - y, -z; (p) 0.5 + x, y, 0.5 - z; (q) -x, 1 - y, -z; (r) -0.5 + x, y, -0.5 - z; (s) x - 1, y, z.

occupy the special C2V(mm)-symmetric sites at the nodes of the F lattice. The molecules are all oriented in the same sense along 001 and arranged into the layers perpendicular to a with short

halogen‚‚‚halogen and CH‚‚‚halogen contacts (see Figure 7, Table 424). Unlike in phase III of CH2ClI, there are no short halogen‚‚‚halogen contacts between the layers in CH2I2 phase

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Figure 7. The pressure-frozen crystal structures of CH2I2 phase II at 1.56 GPa/295 K; one layer of molecules perpendicular to a with short intermolecular halogen‚‚‚halogen distances (dashed lines). The molecular and van der Waals dimensions have been represented to scale. The displacement ellipsoids are marked at the 50% probability level. Electrostatic potential mapped on the molecular surfaces ranges from -46.87 (red color) to 112.44 kJ/mol (blue color).

TABLE 4: Selected Bond Lengths (Ångstroms), Angles (Degrees), and Intermolecular Contacts (Ångstroms) Compared for Pressure-Frozen Structures of CH2I2a CH2I2 at C-H (Å) C-I (Å) ∠I-C-I (°) ∠H-C-H (°) I‚‚‚I (Å) I‚‚‚Ii (Å) H‚‚‚Ij (Å) H‚‚‚Ik (Å)

0.16 GPa/295 K24 0.62G Pa/295 K 1.56 GPa/295 K Molecular Dimensions 1.01 1.01 2.129(1) 2.117(1) 113.27(4) 113.13(4) 109.6(2) 106.1(2) 3.556(1) 3.533(1)

1.01 2.136(1) 110.82(5) 114.1(2) 3.517(1)

Intermolecular Contacts 3.810(1) 3.748(1) 3.400(1) 3.346(1) 3.578(1) 3.507(1)

3.626(1) 3.190(1) 3.390(1)

Symmetry codes: (i) -x, 0.5 - y, 0.5 + z; (j) 0.5 + x, y, -0.5 + z; (k) x, y, z - 1. a

II; all contacts are formed between opposite charges of halogen and H atoms. Thus electrostatically favorable interactions form in all crystallographic directions. It can be shown that the directional character of halogen‚‚‚ halogen forces originate from the spatial distribution of electrostatic potential, Vs(r), on the molecular surface. In Figure 8, the potential Vs has been plotted as a function of angle C-X-s (X ) halogen atom Cl or I; s ) molecular surface point). According to Bader et al.,49 the 0.001 electrons/bohr3 contours of the molecular electronic densities best represent molecular surfaces. The plots confirm that the directional character of the intermolecular contacts correlate with the preferential orientation of the molecules in aggregates. Short intermolecular halogen‚ ‚‚halogen contacts observed in the crystal structures (marked as crosses in Figure 8) all fall in the regions of opposite electrostatic potential, compatible with the electrostatic attraction between molecules. Deviations of the observed C-halogen‚‚‚ halogen angles from the highest and lowest electrostatic

Figure 8. Electrostatic potential on the molecular surfaces of CH2ClI and CH2I2 plotted for the section comprising heavy atoms (X-C-X) in the function of angles C-X-s (in degrees) (see the molecular contours in the insets; X ) halogen atom Cl (green) or I (purple), s ) the molecular surface point). The directions of contact points observed in the structures of CH2ClI phase III (a) and CH2I2 phase II (b) are shown as crosses in purple (contacts of I) and in green (Cl).

potentials, optimum for each halogen‚‚‚halogen electrostatic attraction, result from the close-packing requirements and are much smaller in CH2I2 phase II than in CH2ClI phase III. In this phase the difference between electrostatic potentials in I‚‚‚I contact is 122.98 (kJ/mol), while in Cl‚‚‚I it is 100.90 (kJ/ mol). In the structure of CH2I2 phase II the difference between electrostatic potentials of iodine atoms in contacts is 115.41 (kJ/ mol). Also, these are the most negative regions on the halogen atoms in CH2I2 phase II that play the role of H acceptors of weak CH‚‚‚halogen hydrogen bonds (the most positive potential is on the H atoms). Space group Pnma of phase III of CH2ClI is unique for dihalomethane crystals investigated so far. The first-investigated crystal structures of mixed dihalomethanes-bromochloromethane, CH2BrCl, are disordered, isostructural with the Pbcn-symmetric CH2Cl2 and C2/c-symmetric CH2Br2 crystals.25 Also disordered are pressure- and temperature-frozen phases of CH2BrI isostructural with the C2/c-symmetric CH2Br2. However it can be noted that there is a clear relation between the CH2ClI and CH2I2 structures. They both are built of molecular sheets and can be transformed into very similar arrangements by rotating the CH2ClI molecules within the sheets by ca. 60°, so the bisector of the Cl-C-I angle overlayers with the [x] axis. This imaginary

Competing Patterns in Pressure-Frozen CH2ClI and CH2I2 transformation, illustrated by an animation in the Supporting Information, testifies that these crystals are to some extent isostructural. 4. Conclusions The isostructural relations between the crystalline dihalomethanes fully confirm the dominant role of halogen‚‚‚halogen interactions for the molecular arrangements in these structures. It has been shown for these simple model compounds that the halogen‚‚‚halogen and CH‚‚‚halogen intermolecular interactions are clearly directional. This directional character of the halogen‚ ‚‚halogen interactions can be illustrated by the hypothetical transformation between the CH2ClI and CH2I2 structures, where the molecules are differently oriented to use alternative types of halogen‚‚‚halogen bonds. The electrostatic potential on the surfaces of the halogen atoms is strongly anisotropic, but the potential values strongly depend on the halogen atom and increases in the Cl < Br < I sequence. Hence, the role of halogen interactions increase in this order, too, which explains the specific molecular aggregation types in the dihalomethanes series. Owing to the directional character of the halogen‚‚‚ halogen interactions, their attraction can be easily upset when other factors dominate the molecular arrangement and place the halogen atoms in the orientations where the directional conditions for attraction are not fulfilled. This crystal-packing effect can be responsible for the chlorophobic effects postulated for organic crystals.17-21 However, small halomethanes can easily adjust their orientation to minimize the interaction energy with local environment, which explains their easy association as guest molecules in many crystal structures. For example, dichloromethane is the second most frequent solvate in organic crystals, after water. Acknowledgment. This study was supported by the Polish Ministry of Scientific Research, Grant No. N N204 1956 33. Supporting Information Available: Unit cell dimensions of chloroiodomethane phase III as a function of pressure (Figure S1); the shortest intermolecular contacts as a function of pressure in the CH2ClI phase III (Figure S2); unit cell contents with atomic labeling for chloroiodomethane phase III (Figure S3); crystallographic data in the CIF format; Hirshfeld surfaces for CH2ClI and CH2I2 (Figure S4); stages of isothermal growth of CH2I2 at 0.62 GPa/295 K (Figure S5); atomic coordinates for CH2ClI and CH2I2 (Table S1); animation of imaginary transformation from CH2ClI to CH2I2. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University Press, 2002. (2) Boldyreva, E. V. Cryst. Eng. 2003, 6, 235-254. (3) Boldyreva, E. V. Acta Cryst. 2008, A64, 218-231. (4) Boldyreva, E. V.; Ivashevskaya, S. N.; Sowa, H.; Ahsbahs, H.; Weber, H.-P. Z. Kristallogr. 2005, 220, 50-57. (5) Boldyreva, E. V.; Naumov, D. Yu.; Ahsbahs, H. Acta Cryst. 1998, B54, 798-808. (6) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1, 277-290. (7) Budzianowski, A.; Katrusiak, A. J. Phys. Chem. 2006, B110, 97559758. (8) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725-8726. (9) Dziubek, K. F.; Katrusiak, A. J. Phys. Chem. 2004, B108, 1908919092. (10) Kolesov, B. N.; Boldyreva, E. V. J. Phys. Chem. 2007, B111, 14387-14397.

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