J. Phys. Chem. B 2008, 112, 12001–12009
12001
Polar Symmetry in New High-Pressure Phases of Chloroform and Bromoform Kamil F. Dziubek and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: May 6, 2008
Polar ordering has been induced by pressure in solid chloroform (trichloromethane), CHCl3, and bromoform (tribromomethane), CHBr3, obtained by isochoric and isothermal freezing in a diamond anvil cell. Structures of these new polymorphs have been determined by single-crystal X-ray diffraction, CHCl3 at 0.62 and 0.75 GPa and CHBr3 at 0.20 and 0.35 GPa. Despite different centrosymmetric structures of all low-temperature phases of CHCl3 (space group Pbcn) and CHBr3 (P63/m, P1j, and P3j), the high-pressure phases are isostructural in space group P63. The polar phase of CHBr3 is formed at 295 K, already at the freezing pressure of ∼0.1 GPa, while CHCl3 transforms from the Pbcn phase into the P63 phase between 0.62 and 0.75 GPa. It has been demonstrated that the electrostatic contribution to halogen · · · halogen and H · · · halogen interactions in the CHCl3 and CHBr3 molecular crystals is favorable for the polar aggregation and that this effect intensifies with increasing pressure. 1. Introduction The origin of polar aggregation of molecules and ions in crystals is of particular interest in chemistry, physics, and materials sciences in general.1 The polar and noncentrosymmetric crystals are required2 for nonlinear optics, ferro-, piezo-, and pyroelectric materials, optoelectronic transducers, actuators, and other applications. However, in most cases, achiral molecules crystallize in nonpolar space groups.3,4 For better understanding of the rules governing the symmetry of crystal phases, we have investigated homologous prototypical compounds of chloroform and bromoform. They belong to the simplest halogenated alkanes, and like other halomethanes,5,6 their crystal structures are of particular interest. Chloroform is often used in chemical practice, mainly as a solvent and an extractant, and was one of the first anesthetics.7 Bromoform, as a liquid of relatively high density of 2.89 g mL-1 at 298 K, is applied for separating mixture components having different specific gravities and for determination of the density of solids.8 The anesthetic potency of chloroform and bromoform is reportedly associated with the acidic hydrogen in their molecules and the resulting ability to reversibly perturb hydrogen bonds in target biomolecules.9–11 Such acidic hydrogen-containing molecules, prone to act as weak proton donors forming hydrogen-bonded complexes with O or N bases, are therefore described as “CH acids”.12 Apart from considerable industrial production of chloroform and bromoform, they both occur in much larger quantities naturally, albeit in small concentrations in oceans, soil, and the atmosphere.13–16 The molecules of chloroform and bromoform are C3Vsymmetric, pyramidal, and polar (dipole moments in the gas phase: 3.37 × 10-30 and 3.30 × 10-30 C · m, respectively17), and in this respect, they resemble pyramidal molecules or ions, such as the IO3- anions in the R-LiIO3 pyroelectric and piezoelectric crystal. The polar symmetry of the R-LiIO3 crystals is usually connected with the pyramidal shape of the IO3anions.18,19 Meanwhile, all of the ambient pressure phases of chloroformandbromoformarecentrosymmetric.Bothchloroform20–24 * To whom correspondence should be addressed. E-mail: katran@ amu.edu.pl.
and bromoform23,25,26 were studied at high pressure in detail by infrared and Raman spectroscopy. According to these reports, at room temperature, CHCl3 freezes between 0.60 and 0.79 GPa and CHBr3 between 0.10 and 0.33 GPa (the solidification pressure of chloroform estimated very roughly by counting the number of screw rotations to be 3 GPa21 significantly disagrees with the other results). The spectroscopic data suggested the existence of solid-solid phase transitions observed at different pressure values by various authors, one in chloroform, either at 1.26,20 4.6,22 or 6 GPa,23,24 and two in bromoform, the first at 1.025 or 0.8 GPa23 and the second at 4.225 or 5.15 GPa.23 Moreover, infrared spectra of CHCl3 suggested that the flash freezing of a superpressurized sample led to metastable polycrystalline mass, detected by a single peak in the spectrum.24 If pressure was then either decreased or increased, an irreversible phase transition took place. More recently, different crystal habits of chloroform compressed in the DAC were observed, depending on the history of the pressure variation.27 The unidirectional long thin hexagonal plates appeared when the pressure was increased in several cycles within a short time, while the longer compression period favored the formation of bulky polyhedrons. Although in most of the temperature-dependent studies of chloroform at ambient pressure28,29 no solid-state phase transitions were evidenced, there were two exceptions. The differential scanning calorimetry measurements revealed the existence of a new metastable phase of chloroform being formed at about 70 K below the melting point of stable modification; the melting point of this metastable phase is higher by about 10 K than that of the stable form R.30 Raman investigations revealed a 1 K-wide premelting region, for which the existence of a new disordered phase was postulated.31 The ambient pressure structure of crystalline chloroform was determined at 185 K by Fourme and Renaud;32 it is orthorhombic, in space group Pnma. This phase will be denoted as R-CHCl3. Soon after, Fourme pressure-froze chloroform and studied it by the single-crystal X-ray diffraction. At 0.7 GPa/ 298 K, he established that the space group and unit-cell dimensions are consistent with these of the R phase33 and reported the reliability factor. However, to our knowledge, no
10.1021/jp8020134 CCC: $40.75 2008 American Chemical Society Published on Web 07/31/2008
12002 J. Phys. Chem. B, Vol. 112, No. 38, 2008 high-pressure structural data on chloroform or bromoform have been reported. The crystal structure of bromoform determined by an X-ray diffraction study at 253 K/0.1 MPa (R-CHBr3)34 was found to be disordered in space group P63/m with respect to the mirror plane perpendicular to the [001] crystallographic direction and the molecular C3V axis. The ambient pressure study on the temperature dependence of deuterated bromoform, CDBr3, by neutron powder diffraction35 revealed that the disordered R phase exists between the melting point at 281 K down to 268 K. Below 268 K, R-bromoform converts to the ordered triclinic β phase, space group P1j. Rapid cooling of CDBr3 to liquid-nitrogen temperature yields an ordered trigonal γ phase, space group P3j. It transforms irreversibly to the β phase upon annealing above about 193 K. Remarkably, the triclinic β-phase unit cell35 can be transformed by matrix (-1,-1,0; 1,-1,0; 0,0,1) to a twice larger pseudomonoclinic cell; however, absence of any symmetry elements other than the inversion center confirms the reported triclinic space group. Thepressuredependencesofthefreezingpointsofchloroform36–38 and bromoform39 were investigated by Bridgman. From these data, it was concluded that the ratio of the temperature derivative of the volume change to the volume change itself is constant along the melting curve and that the slope of this relationship for chloroform and bromoform is very similar.40 The compressibility study of chloroform was performed to ∼4.9 GPa38,41 and that for liquid bromoform up to its freezing pressure (0.15 GPa at 323 K and 0.34 GPa at 368 K).42 Bridgman reported the anomalous compression of chloroform at high temperature, which could be attributed to the presence of the metastable crystal form.38 Theoretical attempts were also made to elucidate the character of disorder and phase transitions in solid bromoform. In an early study, a statistical model describing the orientational disorder in R-CHBr3, its only crystal phase known at that time, was postulated.43 The authors concluded that the correlation in orientation of adjacent molecules depends on whether the molecules are in the same layer (weak correlation) or in adjacent layers (strong correlation). Later, group theory was applied for systematically describing the R, β, and γ phases of bromoform,44 and a phase diagram was obtained, minimizing their free energy. In another paper, a description of phase transitions using simple intermolecular potentials was proposed,45 and two other possible crystal structures were associated with the local minima, both in the space group R3j. The main aim of the present study of ours was to determine if the intriguing preference of CHCl3 and CHBr3 to form centrosymmetric structures applies only to ambient pressure and to explore the role of halogen · · · halogen and hydrogen · · · halogen interactions for the molecular association in crystalline state. 2. Experimental Methods 2.1. Crystal Growth. The single crystals of the title compounds were grown according to the procedure described previously.5,6,28 Liquid chloroform (p.a. grade from POCh, used without further purification) and bromoform (p.a. grade from POCh, distilled freshly before the experiment) were loaded into a four-pin diamond anvil cell (DAC) furnished with 0.8 mm culet diamond anvils and beryllium backing disks and sealed with steel gaskets, similarly for both compounds. Small ruby chips were placed inside of the experimental chamber for pressure calibration by the ruby fluorescence method,46 using a BETSA PRL spectrometer; the accuracy of calibration was 0.05 GPa. The single crystals of both compounds were grown in a
Dziubek and Katrusiak
Figure 1. Isochoric growth stages of single crystals of (a-c) chloroform at 0.62 GPa and (d-f) bromoform at 0.2 GPa in the highpressure chamber. Both crystals are aligned with their [001] axes along the DAC axis (down the viewing direction), and the CHBr3 crystal clearly exhibits its hexagonal habit. Small ruby chips for pressure calibration are located in the upper part (a-c) and in the central part (d-f).
similar manner (Figure 1); the DAC chamber was squeezed to increase pressure, which resulted in the sample freezing as a polycrystalline mass, and subsequently, the DAC was heated until all but one crystallite melted. Upon cooling, this single crystal grew continuously until it entirely filled the DAC chamber. To achieve higher pressure, the obtained crystal was melted to the size of a small grain and left to nucleate the isochoric growth of the single crystal after reducing the DAC chamber volume, following the procedure described above. The single crystals could be successfully grown to 0.35 GPa of pure CHBr3 and to 0.75 GPa of pure CHCl3, the pressure limits for which the samples could be melted below 500 K, the temperature at which beryllium backing plates of the diamond anvils in the DAC soften. 2.2. X-Ray Diffraction, Data Reduction, and Refinement. Diffraction data were collected at room temperature with the graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) on a KM-4 CCD diffractometer. The DACs were centered using a gasket-shadow method.47 The diffraction frames were recorded in the ω-scan mode with 1° rotations and 30 s exposures.47 Data collections, determinations of the UB matrices, initial data reductions, and Lp corrections were carried out with CrysAlis programs.48 The reflection intensities were corrected for the DAC absorption, gasket shadowing, and absorption of the samples themselves,49 and the diamond anvil reflections were rejected. For the hexagonal phases of CHCl3 and CHBr3 the E2 distribution indicated a noncentrosymmetric space group, and the systematic absences showed a diffraction class of P63/m (the DAC allowed access to the reflection with the Miller index range -9 e l e 9 for the CHCl3 crystal at 0.75 GPa, -3 e l e 3 for the CHBr3 crystal at 0.20 GPa, and -2 e l e 2 for the CHBr3 crystal at 0.35 GPa). The structures were solved straightforwardly by direct methods (SHELXS-97) and refined against |F|2 using the SHELXL-97 program50 in space group P63. The crystal data of the high-pressure phases are summarized in Table 1. In the CHCl3 structure, a prominent difference Fourier peak appeared at the location consistent with the alternative position of the methine group, CH. Thus, the refinement has been repeated in a higher-symmetry space group P63/m; however, a considerable increase of the R factors indicated a partial disorder of the methine moiety in space group P63 rather than the crystal higher-symmetry space group P63/m. Hence, the refinement was resumed in space group P63
Polar phases of CHCl3 and CHBr3
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TABLE 1: Crystallographic Data for Pressure-Frozen r- and β-Chloroform and δ-Bromoforma phase temperature, K pressure, GPa fw, g/mol crystal size, mm crystal system space group, Z Z a, Å b, Å c, Å V, Å3 Fcalc, g/cm3 µ, mm-1 θ range, deg index ranges reflns collected completeness, % Rint data [I > 2σ(I)] data/parameters GOF on F2 R1 [I > 2σ(I)] R1 (all data)b wR2 (all data)b exti coeff largest diff peak, e/Å3 largest diff hole, e/Å3
R-CHCl3 295(2) 0.62(5) 119.37 0.43 × 0.43 × 0.20 orthorhombic Pnma, 4 4 7.408(1) 9.403(1) 5.754(3) 400.8(2) 1.978 2.04 4.33 - 29.95 -9 e h e 9 -12 e k e 12 -4 e l e 4 2685 54.9 0.1845 272 339/25 0.917 0.0355 0.0467 0.1067 0.34(4) 0.276 -0.299
β-CHCl3 295(2) 0.75(5) 119.37 0.43 × 0.43 × 0.18 hexagonal P63, 2 2 5.886(2) )a 6.654(1) 199.7(1) 1.985 2.05 5.04 - 29.71 -4 e h e 5 -4 e k e 4 -9 e l e 9 1094 65.6 0.0578 199 238/17 0.813 0.0295 0.0346 0.0804 0.78(7) 0.284 -0.197
δ-CHBr3 295(2) 0.20(5) 252.75 0.09 × 0.42 × 0.42 hexagonal P63, 2 2 6.323(2) )a 7.285(14) 252.2(5) 3.328 23.80 3.72 - 29.36 -8 e h e 8 -8 e k e 8 -3 e l e 3 2068 47.8 0.1884 145 196/13 0.907 0.0407 0.0611 0.1156 0.15(2) 0.506 -0.462
δ-CHBr3 295(2) 0.35(5) 252.75 0.07 × 0.42 × 0.42 hexagonal P63, 2 2 6.312(1) )a 7.151(16) 246.7(6) 3.402 24.32 3.73 - 29.39 -8 e h e 8 -8 e k e 8 -2 e l e 2 2179 37.3 0.0665 136 148/14 0.764 0.0253 0.0304 0.0761 0.15(2) 0.257 -0.298
a All crystals were colorless disks (they adopted a shape of a high-pressure chamber). Full listings of refinement details and atomic positions are given in CIF files in Supporting Information. b R1 ) ∑|Fo| - |Fc|/Fo|; wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2; w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) (Fo2 + 2Fc2)/3.
for the structure model including the methine group partially disordered in two sites, with the occupation factor (SOF) refined for one of them and equal (1-SOF) for the other. This considerably reduced the final R factors for CHCl3 at 0.75 GPa, and the SOF value converged to 0.74(5). No signs of CH disorder were observed in the difference Fourier maps for CHBr3, and attempts to refine the disordered P63/m symmetric model significantly increased the R value.51 This new polar phase of CHBr3 has been labeled as phase δ, in sequence with previously described phases R, β, and γ. The H atoms in all of the structures were initially located in the difference Fourier maps but subsequently refined with the constrained C-H bond length and Uiso set at 1.2 times Ueq of the C atom carrier. The final crystallographic data and refinement details are summarized in Table 1. 2.3. Molecular Electrostatic Potential Calculations. The GAUSSIAN03 program suite52 and a PC were used for calculating the electrostatic potential on the molecular surface of CHCl3 and CHBr3. The DFT calculations were carried out at the B3LYP/6-311++G(d,p) level of theory. The electrostatic potential was mapped onto the molecular surface defined as an electron isodensity envelope.53 The pictures of the molecular electrostatic potential surfaces were generated using software Moliso.54 2.4. Compressibility Measurements. The measurements on the compression of chloroform and bromoform have been performed in a piston-and-cylinder apparatus,55 with the initial sample volume of 9.8 mL. 3. Results and Discussion 3.1. Polar Symmetry of CHCl3 and CHBr3, High-Pressure Phases. This single-crystal X-ray diffraction study confirmed that chloroform freezes in the orthorhombic R phase at 296 K
and 0.62 GPa. When compressed to 0.75 GPa, chloroform undergoes a solid-state transition to a hexagonal and polar phase β, space group P63. In the β phase, the structure is partially disordered; about 3/4 of the molecular dipoles point in one sense along the [001] axis, and the remaining 1/4 of the molecules assume the opposite orientation. Bromoform freezes at 0.2 GPa in the δ phase, space group P63, isostructural with β-chloroform, but no orientational disorder of the molecules was detected. Thus, despite different ambient pressure phases of chloroform and bromoform, their high-pressure phases have the same polar symmetry and very similar structures; the molecules are located on the 63 axes, and only small molecular rotations distort the structures from the higher space group symmetry P63cm (see Figure 2). Space group P63cm belongs to the least frequent ones among organic and organometallic crystals, and only 15 structures of this symmetry were deposited by 2008 in the Cambridge Structural Database.56 There are over 21 times more structures of the P63 symmetry (317 entries) and nearly 40 times more (566 entries) of symmetry P63/m. 3.2. Intermolecular Interactions in CHCl3 and CHBr3. The two main factors determining the molecular association are the structure of molecules and their intermolecular interactions. The halogen · · · halogen and H · · · halogen contacts dominating intermolecular interactions in chloroform and bromoform can be straightforwardly described by a conclusion from the Hellmann-Feynman theorem,57–59 stating that noncovalent interactions are primarily electrostatic in nature since they originate from “the attraction of each nucleus for the distorted charge distribution of its own electrons”.58 It was shown recently, that the electrostatic potential plotted onto molecular surfaces of halogenated aliphatic6 or aromatic60 hydrocarbons is consistent with the electrostatic attraction61 leading to the
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Figure 2. Molecular arrangement shown along (top) and perpendicular (bottom) to layers (001) in β-chloroform (a) and δ-bromoform (b). The molecular surfaces are 0.00385 au electron density envelopes; their size was adjusted so that the surfaces were in touch but did not interpenetrate. They are mapped with the electrostatic potential color scale ranging from -19 (red) to 247 kJ/mol (blue).
molecular aggregation. The layout of molecular electrostatic potential surfaces of chloroform and bromoform in the crystal structure illustrates the origin of attractive forces, as these are the oppositely charged molecular surface regions which are in contact (Figure 2). In β-CHCl3 and δ-CHBr3 phases, each CHX3 molecule (here, X denotes Cl or Br atoms) is involved in six triangular motives (one on each side of each halogen atom), three of which are significantly tighter than the other three. It can be seen in Figure 2 that the C-X bonds are inclined to the crystal (100), (010), and (11j0) planes by a small but hardly pressure-dependent angle ε (Table 2). If these inclination angles were reduced to zero, the six triangular motives about the molecule would become equal, and the crystal would acquire the symmetry of space group P63cm. This relatively small deviation from the highersymmetry space group indicates that the ε inclination is very significant and characteristic of halogen · · · halogen interactions. It can be observed (Figure 2) that if the molecules rotated to the angle of the ε ) 0° position, the positive region around the halogen atom cap would approach the positive “bay” between two halogens of the neighboring molecules, and also, its negative rim would be moved closer to the negative rims of these neighboring halogens (see Figure 2). Owing to these electrostatic interactions in the energetically favored position, each halogen atom favors one short contact involving its positive (pole-cap) region and one contact involving its negative (atomic rim) region. These close contacts are maximized within the smaller triangular motives (Figure 2). Owing to the ε angle rotation, each halogen atom involved in the smaller triangular motif forms
no other short contacts in the layer; therefore, each smaller triangular motive is surrounded by three larger gaps (Figure 2). The P63 symmetric phases of CHCl3 and CHBr3 can be considered as built of layers of molecules perpendicular to [001], interacting by a cooperative cyclic motif of halogen · · · halogen bridges forming equilateral triangles. Such an energetically favorable triangular arrangement was recognized in molecular crystals of halogenated compounds,62 extensively exploited in crystal engineering63–67 and investigated theoretically.68 The geometry of this motif is consistent with the anisotropy of charge distribution on the surface of halogen atoms and with the trigonal symmetry of chloroform and bromoform molecules. The dimensions of the short halogen · · · halogen contacts in the ambient and high-pressure phases of chloroform and bromoform have been compared in Table 2. The arrangement of molecules into oriented layers (with all of the CH groups directed to one side of the layer) is common to the β phase of CHCl3 and to phases β, γ, and δ of CHBr3; also, it is likely that very small oriented regions of the layers exist in disordered phase R of CHBr3. The energy difference between the parallel and antiparallel arrangement of the neighboring molecules in the layer can be assessed from the molecular electrostatic potential values mapped on the density isosurfaces. The molecular electrostatic potential energy difference between two arrangements of two molecules oriented either parallel or antiparallel (in both cases, with the halogen atoms lying in one plane) only very slightly favors the antiparallel association.
Polar phases of CHCl3 and CHBr3
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TABLE 2: Distances and Angles Characterizing Halogen · · · Halogen Interactions structure R-CHCl3 (185 K, 0.1 MPa)
32
R-CHCl3 (295 K, 0.62 GPa) β-CHCl3 (295 K, 0.75 GPa) R-CDBr3 (273 K, 0.1 MPa)35 R-CHBr3 (253 K, 0.1 MPa)34 β-CDBr3 (220 K, 0.1 MPa)35
distance X · · · X, Å
angle C-X · · · X, deg
Cl1 · · · Cl1 3.596 Cl1 · · · Cl2 3.687 Cl1 · · · Cl1 3.539 Cl1 · · · Cl2 3.647 3.494 3.751 3.760 Br1 · · · Br2 3.785 Br2 · · · Br3 3.813 Br3 · · · Br1 3.789
C-Cl1 · · · Cl1 148.30 C-Cl1 · · · Cl2 164.95 C-Cl1 · · · Cl1 149.12 C-Cl1 · · · Cl2 163.48 103.71/156.50 100.12/159.58 102.23/159.51 C-Br1 · · · Br2 98.56 C-Br1 · · · Br3 156.23 C-Br2 · · · Br1 152.44 C-Br2 · · · Br3 99.65 C-Br3 · · · Br1 97.30 C-Br3 · · · Br2 150.86 C-Br1 · · · Br2 98.03 C-Br1 · · · Br3 155.17 C-Br2 · · · Br1 152.04 C-Br2 · · · Br3 98.75 C-Br3 · · · Br1 97.20 C-Br3 · · · Br2 151.28 100.79/155.29 101.93/154.79 102.08/155.64
β-CDBr3 (14 K, 0.1 MPa)35
Br1 · · · Br2 3.707 Br2 · · · Br3 3.737 Br3 · · · Br1 3.710
γ-CDBr3 (14 K, 0.1 MPa)35 δ-CHBr3 (295 K, 0.20 GPa) δ-CHBr3 (295 K, 0.35 GPa)
3.678 3.736 3.724
3.3. Polar Symmetry of CHCl3 and CHBr3. In the trigonal CHCl3 and CHBr3 crystals, a hypothetical ordered layer with alternating positions of the CH groups (i.e., their sense up and down [001]) cannot be reconciled with the trigonal periodic symmetry; the alternating positions of the dipoles along two directions, for example [100] and [010], imply their identical orientations along direction [1j1j0], which should be symmetry equivalent with [100] and [010], as illustrated in Figure 3. Thus, in the disordered structure, both the opposite positions of CH groups within one layer and polytypism69 of the layers are possible. The neighboring layers in the β-chloroform and δ-bromoform are shifted by vector [-1/3, 1/3, 1/2] and stacked in the alternating ABAB manner. Layers A and B are polarized in the same sense along [001]; the polar molecules have the same orientation with the H atoms pointing up the [001] axis in δ-CHBr3, and about 3/4 of molecules are oriented parallel in β-CHCl3. The positively charged H atoms point at the negatively charged equatorial region of the halogen atoms (Figure 2). In the R-bromoform and δ-bromoform structures, the ABAB sequence of layers is the same; however, in the R phase, the molecular dipoles are disordered. Moreover, the low-temperature
Figure 3. A schematic illustration of the hexagonal symmetry broken by alternative arrangement of chloroform or bromoform molecules; the molecules with the H-atoms directed up and down have been shown by full and open circles, respectively.
angle ε, deg
4.92 4.33 4.03 1.88/2.03/2.17
2.00/2.05/2.28
3.76 4.21 4.29
β- and γ-bromoform phases are also built of molecular polar layers, but in these structures, the consecutive layers are antiparallel (Figure 4). 3.4. Electrostatic Potential on Molecular Surface. At ambient pressure, chloroform does not freeze into the layered structure with molecules arranged into the trigonal pattern. The calculations of electrostatic potential showed that the positive electrostatic potential caps on the extensions of the C-X bonds (X ) Cl, Br, and I) are considerably larger and more positive for atoms Br and I than that of the Cl atom.61,70,71 For example, the minimum and maximum molecular electrostatic potentials mapped on the 0.00385 au molecular envelope around the halogen atom in CHCl3 (i.e., on the ring around the halogen atom and on the end cap, respectively) are -19.1 and 127.7 kJ/mol, compared to -17.8 and 173.3 kJ/mol in CHBr3. This is consistent with the role of electrostatic interactions between the halogen atoms for the formation of the molecular layers in the high-pressure β-CHCl3 and in all phases of CHBr3. According to the earlier studies on the variation of the nature of homohalogen interactions, the Cl · · · Cl is the weakest of the interhalogen bondings.61 Thus, the observation of the polar β-CHCl3 testifies that there are significant intermolecular interactions capable of compensating the energetic cost of the dipolar ordering within domains. It appears that these interactions depend strongly on intermolecular distances, as suggested by the antiparallel ordering of layers in low-temperature bromoform phases β and γ, as opposed to the parallel ordering when elevated pressure is applied. The distribution of smaller and larger triangular gaps within the layers is also clearly connected with the intermolecular interactions between the layers. In all of the hexagonal and triclinic phases, the CH groups protrude toward the larger gaps between the molecules of the neighboring layer. The larger gap gives a better access of the electropositive hydrogen to the electronegative rim of the halogen atoms. Thus, it is plausible that these C-H · · · halogen interactions between the neighboring layers are the key factors for the formation of the polar structure. This interaction is electrostatically attractive, and for the polar structure, it appears to be the main cohesion force along [001]. A reverse orientation of one of the layers would lead to different interactions of this layer on both sides. On one side, the
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Figure 4. Schematic drawings of CHX3 molecules in (a) β-CHCl3 or δ-CHBr3, space group P63; (b) R-CHBr3, space group P63/m; (c) γ-CHBr3, space group P3j; (d) β-CHBr3, space group P1j; (e) R-CHCl3, space group Pnma; and (f) an alternative arrangement of molecules within one row, impossible for an ordered (pseudo)hexagonal layer.
neighboring layers would have their CH groups directed one toward the other, and on the other side, the layers would interact with the CX3 groups, like in the structure observed in the centrosymmetric ambient pressure β-and γ phases of CHBr3 (see Figure 4c and d). Owing to the electrostatic potential distribution (Figure 2), the interactions X3C-H · · · X3C-H · · · X3C-H between positive and negative regions are more favorable than the H-CX3 · · · X3C-H interactions. In the ordered polar CHBr3 structures, between the sheets, only the C-H · · · halogen contacts are shorter than the sums of van der Waals radii, and in the ordered centrosymmetric phases, the sheets have contacts shorter than van der Waals radii on one side only (Table 3). Apart from the parallel and antiparallel arrangements of polar sheets, also polar and antiparallel arrangements of molecules within a single sheet should be considered. However, although the nonpolar arrangement of molecules within one layer is
slightly more favored energetically, the alternative orientation of every second molecule directed to the opposite side of the layer cannot be reconciled with the hexagonal symmetry of the layer, as explained above. 3.5. (Pseudo)symmetry and Character of the r f β Transition in CHCl3. Our detailed compressibility measurements repeated for chloroform and bromoform at 295 K to ∼1.1 GPa have not revealed any anomalies, which could be associated with a solid-solid phase transition (Figure 5). The absence of a detectable volume change associated with the R f β phase transition in chloroform suggests that the transition may be continuous in character. Although no group-subgroup relation exists between symmetry classes mmm and 6, class 6 in β-CHCl3 and δ-CHCl3 is only slightly distorted from class 6m2 (the ε rotations of molecules discusses above), and the 6/mmm symmetry is broken by the polar arrangement of the molecules
Polar phases of CHCl3 and CHBr3
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TABLE 3: Interlayer Br · · · Br and H(D) · · · Br Distances in the Structures of Bromoforma structure R-CDBr3 (273 K, 0.1 MPa) R-CHBr3 (253 K, 0.1 MPa)34 β-CDBr3 (220 K, 0.1 MPa)35 35
β-CDBr3 (14 K, 0.1 MPa)35 γ-CDBr3 (14 K, 0.1 MPa)35 δ-CHBr3 (295 K, 0.20 GPa) δ-CHBr3 (295 K, 0.35 GPa)
distance Br · · · Br, Å
distance H(D) · · · Br, Å
4.206 4.210 3.975/4.092/4.185 (within one bilayer); 4.058/4.163/4.236 between bilayers) 3.968/4.052/4.102 (within one bilayer); 3.935/4.021/4.116 between bilayers) 4.007 (within one bilayer); 4.075 (between bilayers) 4.229 4.172
3.216 3.232 3.013 2.909 2.922 3.073 3.053
a The bilayers are defined as two layers of CHBr3 molecules linked by CH · · · Br hydrogen bonds in centrosymmetric phases β and γ. For the R-CHBr3 phase, a local polar ordering has been assumed for calculating the Br · · · Br distance.
Figure 5. Compression of chloroform (green points) and bromoform (brown points) determined in a piston-and-cylinder apparatus. Bridgman’s data on chloroform41 have been indicated as blue points.
Figure 6. The orthorhombic unit cell (solid lines) and atomic positions of R-CHCl3 and the pseudohexagonal unit cells (dashed lines). Full and open circles denote molecules of the upper and lower sheets, respectively.
along [001]. Moreover, there is a close relation of the orthorhombic unit cell to a hexagonal one (Figure 6). It is remarkable that the orthorhombic lattice of R-CHCl3 can be transformed into a monoclinic setting, where half of the unit cell corresponds to the pseudohexagonal unit cell with the following dimensions: aph ) 5.575 Å, bph ) 5.841 Å, cph ) 7.485 Å, and γ ) 121.59° (calculated from the low-temperature data32). The transformation matrix from the orthorhombic to pseudohexagonal lattice is (0,0.5,0.5; 0,0,1; 1,0,0). However, the positions of atoms are clearly orthorhombic.
3.6. Isostructural CHX3, CFX3, NX3, and PX3 (X ) Cl, Br, I) Compounds. The results obtained in this study for chloroform and bromoform can be also applied for predicting properties and structures of other analogous compounds. For example, there is some resemblance between the crystalline phases of bromoform and iodoform, CHI3. The early ambient pressure room-temperature X-ray diffraction studies72,73 suggested space group P63, which was subsequently revised to P63/m based on the neutron-diffraction data.74 A later temperature-dependent Raman study revealed a solid-solid phase transition at 260 K.75 When assuming the domain structure of solid iodoform, the crystal below 260 K consists primarily of domains with parallel molecules (local symmetry P63, like in δ-bromoform), while above the transition temperature, the domains with antiparallel orientation of molecules prevail (in domains with local symmetry P3j, like in γ-bromoform).31,75 The averaged symmetry of the crystal as a whole is, in both cases, P63/m. Interestingly, both dibromochloromethane, CHBr2Cl, and dichlorobromomethane, CHBrCl2, crystallize at low temperatures in triclinic P1j phases isostructural with β-bromoform and exhibit no phase transitions down to 5 K.76 This results from the fact that although CHBr2Cl and CHBrCl2 structures are statistically disordered, the Br and Cl atoms favor certain sites about the pseudo C3 axis over the others. It breaks the threefold symmetry of the molecular sites and excludes the hexagonal crystal symmetry. Other analogous compounds are trichlorofluoromethane, CFCl3, at low temperatures and orthorhombic77 in space group Pbca, and tribromofluoromethane, CFBr3, crystallizing at low temperatures78 in the space group Pnma in the structure similar (homeostructural) to R-CHCl3. Although the highly electronegative fluorine atom attached to the ipso carbon strengthens the halogen · · · halogen interaction, the structure is governed by hetero (F · · · Cl, F · · · Br) rather than by homo halogen · · · halogen interactions. The CFBr3 crystal homeostructural to CHCl3 suggests that the F substitution leads to the similar balance of F/H · · · X and X · · · X interactions in these two compounds. Finally, trihalomethanes are somewhat related to the trihaloderivatives of nitrogen and phosphorus. The structure of NCl3 may be regarded as a polytype of the R-CHCl3 structure with layers perpendicular to [001] packed differently.79 Phosphorus trihalides adopt the structures isostructural to trihalomethanes; PCl380,81 and PBr382 crystallize in space group Pnma, while PI383 crystallizes in P63. 4. Conclusions This high-pressure study revealed new isostructural and polar phases of chloroform and bromoform. The polar arrangement of molecules in the high-pressure structure is consistent with the electrostatic interactions involving halogen atoms. It has been
12008 J. Phys. Chem. B, Vol. 112, No. 38, 2008 clarified that chloroform and bromoform can form polar phases. The polar symmetry of the prototypic R-LiIO3 crystal is commonly connected with the pyramidal shape of IO3- anions.18,19 The energy of electrostatic interactions between the CHX3 molecules assembled within the layers is similar for the parallel and antiparallel orientations. However, the alternating dipole orientation within one layer is inconsistent with the hexagonal symmetry (which may be the origin of the disordered R phase of CHBr3). The interlayer X3C-H · · · X3C-H interactions are also electrostatic in nature and can be considered as weak C-H · · · X hydrogen bonds. They are considerably favored compared to the H-CX3 · · · X3C-H interactions required for the antiparallel arrangement of polar layers. Pressure intensifies the intermolecular interactions because the intermolecular distances are reduced. In particular, electrostatic forces are increased, which favors the parallel arrangements of the polar layers. Pressure also eliminates disorder and counteracts the nonpolar arrangement of the molecules within one sheet, consistent with the observation of the polar phases of chloroform and bromoform. The halogen · · · halogen interactions of Br are considerably stronger than those between Cl, and therefore, the polar CHBr3 δ phase is formed at a lower pressure than the polar phase of CHCl3, and a partial disorder still persists in the high-pressure phase of chloroform. Acknowledgment. The authors are grateful to Dr. Christian B. Hu¨bschle of Freie Universita¨t Berlin for his support with the MolIso computer program and for writing an ad hoc Perl script for calculating the local molecular electrostatic potential extrema on the electron density isosurface. This work was supported by Polish Ministry of Science and Higher Education, Grant No. 3T09A18127. Supporting Information Available: Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Curtin, D. Y.; Paul, I. C. Chem. ReV. 1981, 81, 525–541. (2) Zhang, H.; Wang, X.; Zhang, K.; Boon, K. T. J. Solid State Chem. 2000, 152, 191–198. (3) Pidcock, E. Chem. Commun. 2005, 3457–3459. (4) Saha, B. K.; Nangia, A.; Nicoud, J.-F. Cryst. Growth Des. 2006, 6, 1278–1281. (5) Podsiadło, M.; Dziubek, K.; Katrusiak, A. Acta Crystallogr., Sect. B 2005, 61, 595–600. (6) Podsiadło, M.; Dziubek, K.; Szafran´ski, M.; Katrusiak, A. Acta Crystallogr., Sect. B 2006, 62, 1090–1098. (7) Stratmann, L. Chloroform. The Quest for ObliVion; Sutton Publishing: Stroud, U.K., 2003. (8) Richards, F. M.; Lindley, P. F. International Tables for Crystallography; Kluwer Academic Publisher: Dordrecht, The Netherlands, 2004; Vol. C, pp 156-159. (9) Davies, R. H.; Bagnall, R. D.; Jones, W. G. M. Int. J. Quantum Chem. Quantum Biol. Symp. 1974, 1, 201–212. (10) Davies, R. H.; Bagnall, R. D.; Bell, W.; Jones, W. G. M. Int. J. Quantum Chem. Quantum Biol. Symp. 1976, 3, 171–185. (11) Sandorfy, C. Anesthesiology 2004, 101, 1225–1227. (12) Reutov, O. A.; Beletskaya, L. P.; Butin, K. P. CH-Acids; Pergamon Press: Oxford, U.K., 1978. (13) Laturnus, F.; Haselmann, K. F.; Borch, T.; Grøn, C. Biogeochemistry 2002, 60, 121–139. (14) Carpenter, L. J.; Liss, P. S. J. Geophys. Res. 2000, 105, 20539– 20547. (15) Hoekstra, E. J.; de Leer, E. W. B.; Brinkman, U. A. T. EnViron. Sci. Technol. 1998, 32, 3724–3729. (16) Quack, B.; Wallace, D. W. R. Global Biogeochem. Cycles 2003, 17, 23-1–23-27. (17) Nelson, R. D.; Lide, D. R.; Maryott, A. A. Selected Values of Electric Dipole Moments for Molecules in the Gas Phase, In National Standard Reference Data Series; National Bureau of Standards: Boulder, CO, 1967; Vol. 10.
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