A New Ethane Polymorph - Crystal Growth & Design (ACS Publications)

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A New Ethane Polymorph Marcin Podsiadlo, Anna Olejniczak, and Andrzej Katrusiak Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01474 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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Crystal Growth & Design

A New Ethane Polymorph Marcin Podsiadło,* Anna Olejniczak and Andrzej Katrusiak** Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland

ABSTRACT: A new high-pressure crystal form of ethane has been revealed at 2.46 GPa/295 K. This polymorph IV is tetragonal, of space group P42/mnm, with the H-atom disordered due to the rotations of molecules about the C−C bond. The crystal structure has been determined by singlecrystal X-ray diffraction up to 5.90 GPa. The structure of polymorph IV resembles the previously determined low-temperature plastic phase I (stable below 90.32 K), of cubic space group Im3m, metastable phase II observed between 89.78 and 89.68 K, as well as monoclinic phase III (space group P21/n) stable below 89.68 K. In phases III and IV the molecules are arranged according to the weak electrostatic complementarily between neighbours.

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INTRODUCTION Ethane (C2H6) is the simplest of hydrocarbons containing a single C–C bond, the most common building blocks in most of organic molecules. The ethane has the simplest molecule capable of transforming its conformation by changing the positions of terminal methyl groups relatively easily rotated around the single C−C bond. Along with methane, ethane constitutes the largest natural deposit of organic carbon and hydrogen on Earth and it is one of the most common fuels. Ethane is mainly concomitant with methane in underground deposits, however relatively little is known about the high-pressure behaviour of ethane. Three low-temperature solid-ethane phases were reported at ambient pressure.1 Liquid ethane freezes at 90.32 K into plastic phase I with molecules orientationally disordered in the cubic structure of space group Im3m.2 Phase II, with the molecules orientationally ordered, was found in the extremely narrow range from 89.78 K to 89.68 K,1,3 below which the crystal transforms into phase III of monoclinic space group P21/n and molecules ordered.2 The phase diagram of ethane was determined according to calorimetric (DSC)1 and NMR3 measurements up to 1.3 GPa in the temperature range from 210 K to 89 K. In this diagram the plastic phase I extends to the triple point at about 1.3 GPa and 217 K. High pressure Raman scattering performed in a diamond-anvil cell (DAC) up to 8 GPa at 300 K performed by Shimizu et al., 19894 revealed the transition of liquid ethane to solid phase II transition at 2.5 GPa/300 K and the phase II-phase III boundary at 3.3 GPa.4 Recent X-ray ambient pressure investigations of polycrystalline ethane in the temperature range 6-90 K confirmed the existence and symmetry of phases I and III and determined the unit cell parameters of orthorhombic phase II. It was suggested on the basis of the lack of reproducibility in the heating and cooling regimes that phase II is metastable.5 High pressure efficiently modifies the crystal structure and molecular interactions of organic compounds.6-13 Presently we have revealed new information about phase II, we found a new ethane phase IV and determined its structure by


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single-crystal X-ray diffraction. We have measured the compression of phase IV up to 5.90(2) GPa and updated the P/T phase diagram in this range (Figure 1). These new data confirm the metastable character of phase II.

Figure 1. Ethane P/T phase diagram, with insets magnifying the gas-liquid and solid phases equilibria. The literature data2-4,14 incorporated in the diagram are explained in the Supporting Information.

EXPERIMENTAL SECTION Ethane, 99.5% pure, from Linde Gaz Polska was compressed and loaded into a modified highpressure Merrill-Bassett15 diamond-anvil cell (DAC) at cryogenic conditions and in situ crystallized. At 295 K ethane froze at 2.46 GPa in the form of polycrystalline mass filling the whole volume of the high-pressure chamber. Single-crystals of ethane were obtained in isochoric conditions (Figure 2): the DAC with the polycrystalline mass was heated with a hot-air gun until all but one grain melted. Then the DAC was slowly cooled to room temperature and the single


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crystal grew and eventually filled the entire volume of the chamber. Ethane showed exceptionally beautiful intense colours in the polarized light, owing to high optical anisotropy of its crystals. This feature allowed the observations of fine effects of liquid convection in the diamond-anvil chamber. The convection flow results in four parts of the single crystal growing on the bottom anvil thinner than the crystal beneath the separations of the convection regions (Figure 2d,e). The convection was activated by temperature difference between the gasket and central part of the chamber only at the lowest pressure, and above 3.0 GPa the increased viscosity stopped the convection. The experimental details and progress in growing the single crystals of ethane are shown in Figures S1-S5 in the Supporting Information.

Figure 2. Stages of the ethane single-crystal growth in the DAC chamber viewed with polarizedlight: first experiment (a−f): (a) polycrystal grown isothermally at 295 K; (b) polycrystal-liquid equilibrium at 335 K; (c) one crystal seed at 335 K; (d, e) at 330 K; (f) the single-crystal filling the DAC chamber at 295 K and 2.70 GPa; second experiment (g−i): (g) polycrystal grown isothermally at 295 K; (h) one crystal seed at 413 K; and (i) the single-crystal filling the DAC


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chamber at 365 K and 3.75 GPa. The ruby chip for pressure calibration lies in the right part of the DAC chamber. Diffractometers KUMA KM4-CCD and Xcalibur EOS were used for high-pressure studies. The DAC was centred by the gasket-shadow method.16 The CrysAlisPro program suite was used for data collections, determination of the UB-matrices and initial data reduction.17 The intensity of reflections have been accounted for the absorption of X-rays by the DAC, shadowing of the beams by the gasket edges, and absorption of the sample crystal itself.18,19 The crystal structure of ethane was solved by direct methods and the H-atoms were located according to the crystal symmetry (i.e. the special position of the molecule, a in Wyckoff's notation, and of one of Hatoms on a mirror plane, Wyckoff's position j) and the molecular geometry (C−H distance 0.96 Å)20. This site symmetry of the molecule implies the disorder of its H-atoms in halfoccupied positions at 60° around the C−C direction. The structure of ethane phase IV has been determined at eight pressure points between 2.70 and 5.90 GPa. The crystal data of ethane phase IV are summarized in Table 1. Other crystal data and experimental details are listed in Table S1 in the Supporting Information. Program CrystalExplorer21 was used for calculating the electrostatic potential;22 it was mapped onto the molecular surfaces defined as 0.001 a.u. electron-density envelope.23 Table 1. Selected crystal data of ethane C2H6. phase I2

phase II5

phase III2

phase IV

phase IV

0.0001

0.0001

0.0001

2.70(2)

5.90(2)

90

89.7

85

295(2)

295(2)

Crystal system

cubic

orthorhombic

monoclinic

tetragonal

tetragonal

Space group

Im3m

−

P21/n

P42/mnm

P42/mnm

5.304(2)

4.289

4.226(3)

5.3027(3)

5.092(2)

P (GPa) T (K)

a (Å)


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b (Å)

5.304(2)

5.660

5.623(4)

5.3027(3)

5.092(2)

c (Å)

5.304(2)

5.865

5.845(4)

3.9429(5)

3.675(18)

β (°)

90

90

90.41(6)

90

90

149.214

142.377

138.890

110.869(17)

95.3(5)

2, −

−, −

2, 0.5

2, 0.125

2, 0.125

0.669

0.699

0.719

0.901

1.048

−

−

−

0.0501

0.0263

0.0260

−

0.0520

0.0537

0.0540

Volume (Å3) Z, Z' Dx (g cm-3) R1 (I > 2σ(I)) R1 (all data)

DISCUSSION All our attempts to obtain the crystal of ethane phase II between the high-pressure freezing at about 2.5 and up to 3.3 GPa failed and only the crystal of phase IV was obtained instead. Moreover, we did not observe any other form of ethane during the isochoric crystallization. These observations are consistent with the metastable nature of phase II, as suggested by Klimenko et al., 2008.5 The tetragonal symmetry of ethane phase IV with the unit cell similar to that of phase III (Z = 2, see Table 1) is most remarkable, as it implies that molecules are disordered orientationally about the C−C bond, as illustrated in Figure 3. In majority of crystals high temperature induces the orientational disorder,24 where it is eliminated by high pressure, i.e. the ∂Tc

dp

is positive.25-27 There are few exceptions, such as the KH2PO4 ferroelectrics. Ethane

phase IV is evidently related in the unit-cell dimensions and in the positions of molecules with monoclinic phase III, where the H-atoms are ordered (cf. Figures 1 and 3). The lattice relation between ethane phases III and IV implies that parameter aIII corresponds to cIV, bIII to aIV and cIII to bIV (Fig. 4; the subscripts indicate the phases). The structural relation between phases III and


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IV is further confirmed by the monoclinic angle βIII being close to the right angle (Table 1). The ordered positions of H-atoms in phase III are connected to the intermolecular interactions responsible for the molecular arrangement. The electrostatic potential has been mapped on the molecular surfaces of ethane molecules in sheet (011) for phase III and in sheet (110) for phase IV (Figure 3). The lowest magnitude of electrostatic potential is on the C-atoms surface, and the highest potential is on the surface of H-atoms at the extension of C−H bonds. In phase III the molecules are oriented in this way, that the electropositive H atoms point at the electronegative region of C-atoms in neighbouring molecules. This orientation is electrostatically favoured, as the lowest electrostatic potential regions match the highest positive ones. On the other hand such a matching in this case is equivalent to directional interactions between the molecules. In other words, the matching of opposite potentials fixes the direction and orientation of the molecules in contact. According to our observation that in high pressure phase IV the molecules rotate, it appears that the arrangement of molecules in phase III is somewhat diverted from that of closely-packed molecules, and small voids exist in the structure when the molecules are in electrostatically favoured positions. Furthermore, the electrostatic matching of molecules in phase III induces a monoclinic strain. In phase III, the electrostatically matched molecules are inclined by angle 12.59°, measured between the C−C bonds (symmetry code: x, −1+y, 1+z). In phase IV this angle changes to 0°, which increases the symmetry to the tetragonal system. The deviation of the C−C···C'−C' angle from 0° can be considered as an order parameter of the transition and it also translates to the monoclinic strain of the crystal. The phase transition between phases III and IV is ferroelastic, where phase IV is the prototypical phase, and phase III is its orientational state. Thus the order parameter of this phase transition can be referred to the lattice strain. Difference (βIII − 90°) would be an order parameter describing the shear strain of


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the crystal lattice and difference (bIV − aIV) (corresponding to parameters cIII and bIII, respectively, of the monoclinic cell) as an order parameter related to the direct linear strain. The monoclinic strain of phase III is eliminated in phase IV in this way it reduces the barrier stabilizing the orientation of molecules about their C−C bonds, and consequently facilitates the activation of this type of disorder. The Boltzmann energy of thermal vibrations at 295 K can be used for estimating the activation energy of molecular rotations, although it can be considerably lower, even below kB 85 K (1.75 kJmol-1) when assuming a negative dTc/dp (kB is the Boltzmann constant and Tc the critical temperature of the transition). The axial disorder of molecules in phase IV preserves the main electrostatic component of the cohesion forces, i.e. the attracting contacts between electronegative methyl tips with the electropositive rims about the disordered cylindrical side (Figure 3). The short contacts mapped on the Hirshfeld surfaces of molecules (Figure 5) show that molecules interact much stronger in phase IV than in phase III.

Figure 3. Crystal structures of ethane with calculated electrostatic potential in the colour scale ranging from –0.024 a.u. (red) to 0.121 a.u. (blue) mapped onto the surfaces of 3 central molecules: (a) in phase III at 0.1 MPa/85 K; and (b) in phase IV at 5.90 GPa/295 K. In phase IV (b) two positions of disordered molecules and their surfaces are superimposed.


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Figure 4. The unit-cell volume (Å3) and parameters a, b and c (Å) of ethane phases III (at 85 K) and IV at 295 K plotted as a function of pressure. The dashed lines joining the points of different phases, temperature and pressure is for guiding the eye only.

Figure 5. The Hirshfeld surface21 of the ethane molecule decorated with the colour scale of intermolecular distances normalized against the van der Waals radii28 (in close neighbours Catoms are balls and H-capped sticks) at: (a) 0.1 MPa/85 K; (b) 2.70 GPa/295 K; and (c) 5.90 GPa/295 K. The overlapped surfaces are red, just touching white and separated blue. The colour scale ranges from −0.292 to 1.000 a.u.21


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Although the disorder induced by high pressure may appear counterintuitive, due to considerably increased intermolecular interactions stronger binding the molecules and compressed space required for molecular movements, in ethane the intermolecular interactions are relatively weak and the molecular shape has a relatively high symmetry of axis C3i along the C−C bond. It is also important that the temperature of our experiments is considerably higher than that where phase III was determined. The Gibbs-energy entropy component connected to the two sides disorder ∆ST, where ∆S=Rln2, R is the gas constant, is equal to 1.21 kJmol-1, which is apparently sufficient to activate the rotations, even when the effect of tighter packing is considered. The pressure effect on the work component of the crystal free energy between the 2.5 GPa

low-temperature (0.1 MPa/85 K) and high-pressure (2.5 GPa/295 K) can be estimated as

∫ Vdp 0.1MPa

and it is equal to 11 kJmol-1, significantly more than ∆ST.

CONCLUSIONS We have established that despite the considerable pressure of 5.9 GPa the molecules of ethane are disordered in a new tetragonal phase IV. This phase is most relevant to the natural deposits of ethane below 70 km under Earth surface. Despite the electrostatic-matching effect stabilizing the molecular orientation in low-temperature phase III, in the structurally very similar phase IV the molecules are rotationally disordered, due to the thermal activation and due to a possible reduction of the Ep barrier for the molecular rotations by pressure. In these terms, ethane belongs to the group of simple molecules most common in Nature, which are disordered at relatively high pressure. For example, at 295 K the crystals of methane are disordered to at least 10 GPa,29,30 of ammonia to 3.6 GPa,31 of water H2O (at 300 K) to 62 GPa.32 Orientational disorder of substituents, molecules and ions much heavier than ethane is quite common in crystals, and about


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24% of the Cambridge Structural Database deposits are flagged with descriptor ‘disordered’. Further studies are still required in order to determine the boundary between phases III and IV in ethane and to understand all factors responsible for the disorder of molecules in this compound.

ASSOCIATED CONTENT Supporting Information: detailed experiment and structures description. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] **E-mail: [email protected] ACKNOWLEDGMENT Financial support received from the Polish Ministry of Higher Education and Science (Project Iuventus Plus No. IP2014 037873) is gratefully acknowledged. REFERENCES (1)

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For Table of Contents Use Only A New Ethane Polymorph Marcin Podsiadło,* Anna Olejniczak and Andrzej Katrusiak**

At 2.46 GPa ethane crystallizes in a new tetragonal phase IV with rotating molecules. Spectacular convection of liquid ethane above the single-crystal surface has been observed.


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A New Ethane Polymorph - Crystal Growth & Design (ACS Publications)

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