J . Phys. Chem. 1988, 92, 243-241 SCHEME I
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pK=6.5
, ( C r 02-4
1
chromium(V1) reduced remains equal to 3 all along the photoreaction. For solution of lower acidity, the deprotonation of the complex (HCrO,--acrylonitrile) takes place, resulting in the shift toward 370 nm and providing the H+ ions required for the reduction. This result accounts for the apparent decrease of the ratio of H + consumed per chromium(V1) reduced. In addition, we checked the two forms (protonated and deprotonated) to be photoactive with respect to the photoreduction of chromium(V1)
243
into chromium(III), so that the reaction can be achieved. The overall mechanism of the photoredox phenomenon between chromium(V1) and acrylonitrile in aqueous solution is summarized in Scheme I. It does not appear any complexation occurs in the ground state between HCr04- and acrylonitrile. When excited, HCr0; reacts with acrylonitrile to form a complex. The complex (HCr04--. acrylonitrile) is only formed photochemically and is characterized through its pK properties. Such evidence has never been obtained in our previous investigations on HCr04--a amino acidi2 and HCr0;-acrylamide9 systems. Unfortunately, because of the very low concentration and/or the relative unstability, any attempt to isolate the complex failed. By a charge-transfer reaction in the complex, chromium(V) and an organic radical initiator of polymerization are formed. Henning et a1.I' already described the intervention of chromium(V) in the photooxidation of alcohol by HCrO,; in the case of alcohol, it appeared a reaction in the ground state between HCr0,- and alcohol, giving rise to the chromic ester. Chromium(V), whose reactivity as an oxidant has been well establi~hed,'~-'~ reacts with acrylonitrile to pursue the oxidation process, the final photoproducts being chromium(II1) and polyacrylonitrile. Registry No. (NH4)2Cr20,,7789-09-5; H,C=CHCN,
107-13-1.
(17) Henning, H.; Scheibler, P.; Wagener, R.; Thomas, P.; Rehorek, D. J . Prakt. Chem. 1982, 324, 279-291. (18) Krumpolc, M.; RoTek, J. J . Am. Chem. SOC.1979,101, 3206-3209. (19) Krumpolc, M.; RoEek, J. Inorg. Chem. 1985, 24, 617-621.
Conformatlon of Ethylene Glycol and Phase Change in Silica Sodalite J. W. Richardson, Jr.,+ J. J. Pluth, J. V. Smith,* W. J. Dytrych, Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637
and D. M. Bibby Chemistry Division, DSIR, Petone, New Zealand (Received: June 15, 1987)
The crystal structure of silica sodalite, including possible locations for encapsulated ethylene glycol, was determined at room temperature by using a combined single-crystal X-ray and powder neutron diffraction analysis. Unit cell composition: Si,2024.2C2H4(OH)2, M , = 845, cubic, Im3m, a = 8.830 (1) A (X-ray), a = 8.8273 (1) A (neutron). These refinements reveal that the correct space group for silica sodalite is Im3m (rather than 143m) and, therefore, that the sodalite framework is fully expanded. At room temperature, each sodalite cage contains one ethylene glycol molecule which has a range of geometrical positions. Intramolecular distances for the ethylene glycol molecule are (X-ray) C-C 1.78 (4), C-0 1.30 (6) A, (neutron) C-C 1.70 (3), C-O 1.25 (3) A. The shortest distances (3.4 A) between oxygens of the framework and molecule are consistent with weak hydrogen bonding. From the neutron diffraction data it was found that a sluggish phase change from cubic to lower symmetry occurs upon cooling below 200 K. At 1 0 K, silica sodalite appears to be monoclinic with approximate cell parameters a = 12.250 (8) A, b = 12.471 (8) A, c = 8.512 (6) A, p = 91.37 ( 6 ) O , based on an indexing of 12 peaks, but the precise symmetry is as yet unknown.
Introduction The geometrical relationship between a simple inorganic framework such as sodalite and an encapsulated organic species such as ethylene glycol is important because it may provide a clue to the templating action' or other roles played by organic species in the synthesis of microporous materials from gels. In some materials, the interaction between the organic species and the framework is strong enough to indicate a structure-directing influence? In others, the organic species is geometrically disordered and its role in the synthesis is less well u n d e r ~ t o o d . ~ , ~ Most natural and synthetic members of the sodalite family contain two tetrahedral species (e.g., A1 and Si) and both elect Also Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, IL 60439.
0022-365418812092-0243$01SO10
trostatically coupled extraframework ions (e.g., Na') and encapsulated neutral species (e.g., NaC1). Structure determination has been difficult because of various types of pseudosymmetry related to geometrical and chemical change^.^^^ The synthesis of pure silica sodalite from systems containing only ethylene glycol or 1-propanol' is important because the tetrahedral site is occupied only by Si and because the encapsulated organic species may be (1) Flanigen, E. M. Adu. Chem. Ser. 1973, 121, 119. (2) Price, G. D.; Pluth, J. J.; Smith, J. V.; Araki, T.; Bennett, J. M. Nature 1981, 292, 818. (3) Baerlocher, Ch.; Meier, W. M. Helu. Chim. Acta 1969, 52, 1853. (4) Bennett, J. M; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J.; Smith, J. V . ACS Symp. Ser. 1983, 218, 109. (5) Dempsey, M. J.; Taylor, D. Phys. Chem. Mineral. 1980, 6, 197. (6) Depmeier, W. Acta. Crystallogr., Sect. B 1984, B40, 185. ( 7 ) Bibby, D. M.; Dale, M. P. Nature 1985, 317, 157.
0 1 9 8 8 American Chemical Society
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acting both as a template and a solvent. We report crystal structure determinations by X-rays and neutrons of the disordered ethylene glycol complex of silica sodalite (EGSS) at room temperature and demonstrate that a phase change to low symmetry (possibly monoclinic) occurs upon cooling to 10 K. A discussion of the possible conformations of the ethylene glycol molecule and its relationship to the framework will be given.
Experimental Section Single-Crystal X-ray Data Collection. The silica sodalite crystals exist as small -30 Fm truncated octahedra. A representative crystal was mounted on an automatic Picker-Krisel 4-circle diffractometer using a 12-kW rotating anode X-ray generator (Cu radiation) as a source. Refinement, using the 20 values for 20 reflections (53' < 20 < SO0), each the average of eight possible settings, gave the cubic cell parameter a = 8.8382 (8) A. A total of 359 intensities was measured. Averaging resulted = 0.02, sin 0/Ain the retention of 78 intensities with Rmerge(F) (max) = 0.58, index ranges 0 Ih I10,O Ik I10,O II IIO. All observed diffractions were consistent with body-centered cubic symmetry (Im3m or 143m). No absorption correction was made owing to the insignificant absorption coefficient and the isotropic shape of the crystal. Powder Neutron Data Collection. Four grams of the sample was sealed in a cylindrical vanadium can ( 7 / 1 6 in. diameter, 2'14 in. in length, 0.5 mm wall thickness) in a helium atmosphere, and cooled by a two-stage, closed-cycle Displex refrigerator. Data were taken on the Special Environment Powder Diffractometer at IPNS at controlled temperatures at 295 and 10 K and several temperatures in between. Diffraction data from the f 150' detector banks were used for refinement of the 295 K data set, in the range 0.70 Id 5 3.12 A, which included 126 reflections. The refined unit cell parameter was a = 8.8273 (1) A. Because the cell dimensions from the single-crystal X-ray data (8.8382 (8) A) and the powder neutron data (8.8273 ( I ) A) disagreed, an X-ray diffractometer pattern was taken for a mixed powder of silica sodalite and a reference spinel ( a = 8.0833 %.) calibrated against pure silicon ( a = 5.43088 A). Use of an internal standard eliminates any systematic error, and the cell dimension, a = 8.830 (1) A, from the X-ray powder data demonstrates that there is a systematic error of -0.1% in the cell dimension from the single-crystal X-ray data. The cell dimensions from the X-ray and neutron powder data (8.830 (1) and 8.8273 (1)) agree to 1 part in 3000. This is adequate for the present purpose, but it seems desirable to make a detailed study of the possibility of a small systematic error in the cell dimension from the neutron powder data. The neutron data were analyzed with the Rietveld refinement technique,* modified for use with time-of-flight data from a pulsed neutron s o u ~ c e . ~ Background *'~ fitting was done using a refinable six-parameter analytical function." The eight refinable peakshape parameters of the resolution function'* were determined previously from a Si standard. The peaks from silica sodalite and spinel in the X-ray powder diffractometer pattern showed a similar ratio. Because the ratio is independent of Bragg angle. it was concluded that some type of inherent structural disorder is present in the silica sodalite. The alternative cause of broad peaks, namely small particle size, would have generated a dependence of breadth on Bragg angle. All peaks were found to have the same shape with Gaussian full widths about 1.5 times that of the Si standard. Neutron scattering lengths (in cm-12) were as follows: Si, 0.41 5: (8) Rietveld, H . M . J . Appl. Crystallogr. 1969, 2, 65. (9) Jorgensen, J. D.; Rotella, F. J. J . Appl. Crystallogr. 1982, 15, 27. (10) Von Dreele, R. B.; Jorgensen. J. D.; Windsor, C. G. J . Appl. Crystallogr. 1982, 15. 581. (1 I ) Rotella, F. J. "Users Manual for Rietveld Analysis of Time-of-flight Neutron Powder Diffraction Data at TPNS": (Argonne National Laborator), unpublished), 1986. (12) Carpenter, J. M.; Mueller, M.H.; Beyerlein, R. A,; Worlton, T. G.; Skold, K.; Pelizzari, C. A,; Peterson, S. W.; Jorgensen, J. D.; Brun, T. 0.; Watanabe, N.; Kimura, M.; Gunning, J. E. Proc. Neutron Diffraction Conf.. Petten, August 5 - 6 Reactor Centrum Nederland. 1975; RCN-234. pp 192-208.
0 7 0 8 0 818
0928
1038
1148
1258
1368
1478
1588
1198
1808
1918
2028
d SPACING (A)
Figure 1. Neutron diffraction profile fit for silica sodalite in the lower range of d values Pluses are the raw data, and the continuous line is the calculated profile A difference curve appears at the bottom of the figure Tick marks indicate the positions of allowed reflections Background has been removed before plotting
TABLE I: Atomic Coordinates and Equivalent Displacement Parameters for Silica Sodalite from (a) X-ray Refinement and (b) Neutron Refinementa atom site
OCCUD.
x
Y
z
Si 0(1) O(2) C H
12d 24h 12e 16f 961
1.0 1.0 1/3 1/4 1/12
(a) X-ray Refinement 0.2500 0.5000 0.0000 0.0000 0.6474 (3) 0.6474 (3) 0.500 0.680 (7) 0.500 0.558 (3) 0.558 (3) 0.558 (3) 0.586 0.674 0.533
Si 0(1) O(2) C H
12d 24h
1.0 1.0 1/3 1/4 1/12
(b) Neutron Refinement 0.2500 0.5000 0.0000 0.0000 0.6475 (1) 0.6475 (1) 0.674 (5) 0.500 0.500 0.555 ( I ) 0.555 (1) 0.555 (1) 0.542 0675 0.533
12e
16f 961
U*"b 0.163 (4) 0.0369 (9) 0.34 (3) 0.17 (4) 0.33 (8) 0.0118 (12) 0.0395 (10) 0.21 (2) 0.12 (1) 0.23
'Random experimental error (1 a) in brackets to same decimal position (e.g., 0.6474 i 0.0003). Uw is defined as '/,~:I=,@=,U~,u,*u,*-
(ais,).
0, 0.580; C, 0.665; H, -0.374. It was necessary to remove a noticeable noncrystalline scattering component using Fourier filtering'3 for these Rietveld refinements.
Structure Refinement Least-squares refinement of the X-ray intensities and Rietveld profile refinement of the neutron powder pattern yielded precise positions for the framework Si and 0 atoms. A difference Fourier map, calculated from the X-ray amplitudes, revealed distinct but broad regions of electron density inside the sodalite cage. A peak at f ( x , x , x : x I= 0.55) and symmetry-related positions, approximately 0.77 A from the center of the cage, was assigned to a carbon atom. A second peak at & ( X , ' / ~ , ' / ~ : X= 0.70), approximately 1.47 from the carbon atom, was identified as an oxygen atom. X-ray refinement shifted the carbon atom significantly away from the center of the cage resulting in a longer C-C distance, whereas the neutron refinement produced only a small shift. Addition of ethylene hydrogens, based on the model to be described below, resulted in shifts of C back toward ( ' / 2 , 1 / 2 , 1 / 2 ) in both refinements. The final X-ray least-squares refinementsI4 minimized hF with u ( F ) computed from u(Z) = [total counts + (2% of total counts)*]]/*and with w = uF-2. R = 0.024, wR = 0.028, S = 3.0, maximum A/u = 0.00; maximum and minimum heights of the final difference-Fourier synthesis +0.24 and -0.23 e (13) Richardson, J. W., Jr.; Faber, J., Jr. Advances in X-ray Analysis; Plenum: New York, 1985; Vol. 29, p p 143-152. (14) Sheldrick, G. M. 1976;, S H E L X ~ ~A: Program for Crysral Structure Determination; Cambridge University: London, 1976.
Crystal Structure of Silica Sodalite
The Journal of Physical Chemistry, Vol. 92, No. 1. 1988
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Figure 2. Stereoplot of the unit cell of silica sodalite showing only the topological linkage of tetrahedral Si atoms.
Figure 3. Stereoplot of a representative configuration of the ethylene glycol molecule in the sodalite cage of silica sodalite. Framework Si and 0 atoms are represented by ellipsoids, while the ethylene glycol atoms are represented by spheres: smallest H, intermediate C, largest 0. TABLE II: Interatomic Distances and Angles for Silica Sodalite distance, A angles, deg atoms X-rav” neutron atoms X-rav neutron Si-O(1) 1.587 (2) 1.586 (1) O(l)-%-O(l) 110.3 (2) 110.4 (1) O(2)-C 1.30 (6) 1.25 (3) O(1)-Si-O(1) 109.0 (1) 109.0 (1) c-c 1.77 (4) 1.70 (3) Si-O(1)-Si 159.7 (2) 159.6 (1) C-H 1.08 (4) 1.08 O(2)-C-C 91 (2) 92 (1) 98 (2) 114 O(1)-0(2) 3.38 (5) 3.42 (3) O(2)-C-H O(1)-0(2) 3.47 (3) 3.49 (2) C-C-H 124(2) 113 H-C-H 109 (3) 109 “Based on a = 8.830 A.
The final Rietveld refinement converged with R(p)= 0.078 and R(wp) = 0.013. Figure 1 gives the profile fit. Final atomic positional and equivalent displacement parameters for both refinements are given in Table I, interatomic distances and angles
in Table 11, and anisotropic displacement parameters in Table 111.
Discussion Figure 2 shows the topological linkage of tetrahedral Si atoms in silica sodalite. The framework is made up entirely of truncated octahedra (sodalite units) sharing 4- and 6-rings to form a body-centered arrangement. There is one ethylene glycol molecule in each cage. The ethylene glycol molecule cannot conform to the m3m point symmetry at the center of the sodalite unit, thus ruling out any single structural arrangement. However, some conformational information can be derived from our crystallographic results. Space Group at Room Temperature. Because the symmetry of the sodalite framework depends on both the occupancy of the tetrahedral sites and the amount of collapse from the fully expanded position, it was necessary to check the space group symmetry carefully. Whereas the maximum topological symmetry
TABLE III: Anisotropic Displacement Parameters” for Silica Sodalite from (a) X-ray Refinement and (b) Neutron Refinement
0.0178 (6) 0.0277 (11) 0.40 (6)
H
0.0135 (8) 0.055 (2) 0.21 (6) 0.17 (4) 0.33 (8)
U(3,3) U(1,2) (a) X-ray Refinement 0.0178 (6) 0.0000 0.0000 0.0277 (1 1) 0.40 ( 6 ) 0.0000
Si O(1) O(2) C H
0.0027 (13) 0.0632 (12) 0.08 (4) 0.120 (7) 0.228
0.0164 (12) 0.0276 (8) 0.162 (15) 0.120 (7)
(b) Neutron Refinement 0.0164 (12) 0.0000 0.0276 (8) 0.0000 0.162 (15) 0.0000 0.120 (7) -0.035 (5)
atom
Si O(1)
O(2) C
a
U(1,1)
Defined as exp(-2r2E;=,
U(2,2)
x$,Uijai*aj*hihj),
U(1,3)
0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 -0.035 (5)
u(2,3)
0.0000 0.0135 (14) 0.0000
0.0000 0.0123 (7) 0.0000 -0.035 (5)
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Richardson et al.
of Im3m is reduced to Pm3m for alternating AI and Si atoms of typical aluminosilicate members of the sodalite family, this reduction is not applicable to silica sodalite. Collapse of the framework reduces the space group symmetry to P43n for A1,Si alternation and 143m for a pure SiOz framework. A decision between Im3m and 143m can be obtained from the value of the x coordinate of oxygen O( l ) , which would lie in position (Oyy) for the former and (xyy) for the latter. Typically x = 0.05 for %,AI sodalites. Refinements of the present X-ray data for (xyy) gave x 0.005 f 0.001. Because this value is so close to zero, it appears that the framework is fully expanded, or nearly so. Consequently, all further refinements were made in Im3m. 2.02 and 2.14 A in the neutron However, weak peaks at d powder pattern are indexable as (331) and (410), (322) on the , . I 8.8-A cubic cell, and some deviation from body-centering may 3.41 3.45 3.49 3.53 3.57 3.61 3.65 occur. Such a deviation might result from cooperative positioning d-SPACING (Angstroms) of EG molecules in adjacent sodalite cages, or from twisting of the silica framework, or both. Conformation of Ethylene Glycol. We have investigated how intramolecular hydrogen bonding between the terminal oxygen atoms of the ethylene glycol molecule, and hydrogen bonding from the terminal oxygens to framework oxygens, would be affected by changes in position and conformation of the EG molecule within the wide latitude allowed by the electron-density peaks. Our refinements were based on a disordered model with carbon atoms lying on 3-fold axes f(x,x,x), f(-x,x,x), f(x,-x,x), and f(x,x,-x) with x = 0.56 and oxygen atoms lying on 4-fold axes f ( ~ ’ , ’ / ~ , ~ / ~ ) , =k(‘/2,x’,1/2)and f(’/211/2,~’) with x’= 0.68. The backbone of the EG molecule is defined by choosing a pair of carbon atoms, e . g i f(x,x,x), and two oxygen atoms; the first, for instance, (x , / 2 , 1 / 2 ) and the other chosen from (‘/z,l/z-x?, (‘/2,-~’,’/~), and ( - x ’ , ’ / ~ , ~ / ~This ) . produces interatomic distances and angles 4.130 4.205 4.280 4.555 4.430 4.505 4 (from the neutron refinement) C-C = 1.70 (3) A, C-0 = 1.25 d-SPACING (Angstroms) (3) A, C-C-0 = 92 (l)’, and 0-C-C-0 = f60’ (gauche) or 0 180° (trans). Ethylene glycol in the gauche conformation would s be expected to exhibit internal hydrogen bonding, while the trans conformation would show hydrogen bonding to the framework only. Although the diffraction results do not differentiate between the gauche and trans conformations, they do rule out conformations with 0-C-C-0 = 0’ or f 1 2 0 ° . From ab initio calculation^,'^+^^ the most stable conformations of ethylene glycol, labeled tGg’ and gGg’, are stabilized by intemal hydrogen bonding. This labeling system identifies the local conformations defined by angles H-0-C-C, 0-C-C-0, and C-C-0-H’ in ethylene glycol. The conformation labeled tGg’ has one hydrogen trans to the EG backbone and one gauche (with internal hydrogen bonding), while the gGg’ conformation has both hydrogens nearly gauche to the backbone (with double internal hydrogen bonding). The conformation labeled tTt’, with the backbone and both hydrogens trans, has an energy -2.5-3.5 5.84 5.94 6.04 6.14 6.24 6.34 I kcal/mol higher than tGg’.16 Because additional energy terms d-SPACING (Angstroms) will arise from weak hydrogen and van der Waals bonding to Figure 4. Comparison of 295 and 10 K data. Dashed lines represent framework oxygens, it is necessary to use with caution the ab initio observed data and the solid lines are the best fits with multiplets of peaks calculations for an isolated molecule. In fact, Raman spectroscopic whose shapes are determined by the instrumental resolution function. (a) investigations” reveal that dilution in DMSO stabilizes the trans The (211)reflection is fit (150’ data bank) with a single peak at d = conformer. In addition, a conformational isomerization from 3.605 A at 295 K. At 10 K the reflection is fit with five peaks at d = gauche to trans can be induced by IR irradiation.]* 3.454,3.502,3.518,3.559,and 3.585 A, respectively. The monoclinic Figure 3 is a stereoplot of a representative conformation which cell described in the text generates seven reflections at d = 3.456,3.502, easily satisfies the experimental evidence and theoretical expec3.512,3.525,3.555,3.564,and 3.583A. (b) The (200) reflection is fit (60° data bank) with a single peak at d = 4.408 8, at 295 K, and two tations for chemical bonding. The conformation represented in peaks at d = 4.254and 4.369 A at 10 K. The monoclinic cell generates Figure 3, labeled tGg’, has considerable tolerance for the protons two reflections at d = 4.249 and 4.370A. (c) The (1 10)reflection is fit of the hydroxyl groups, especially when displacements of the (60”data bank) with a single peak at d 6.240 8, at 295 K, and three framework oxygens are needed to explain the high value of the peaks at d = 6.043,6.134,and 6.241 A. The monoclinic cell generates displacement factor U, (Table I). No attempt was made to model four reflections at d = 6.047,6.121,6.138,and 6.240 A. the hydroxyl hydrogens, although they would be expected to be directed at nearby framework oxygens. Positions for the ethylene hydrogens used in the X-ray and neutron refinements were calculated assuming 0-C-C-O= 60’. As noted in the section above, (15) Van Alsenoy, C.; Van den Enden, L.; Shafer, L. J . Mol. Strucr. 1984, inclusion of ethylene hydrogen atoms was important. When the 108, 121. significant disorder of the molecule is taken into consideration, (16) Almlof, J.; Stymne, H. Chem. Phys. Lett. 1975, 33, (l),118, these hydrogens represent a shell of density about 1.7 A from the (17) Pruettiangkura, P.;Ho, S.;Schwartz, M. Spectrosc. Left. 1979, 12(9), center of the sodalite cage. With X-ray diffraction, this represents 619. (18) Takeuchi, H.;Tasumi, M. Chem. Phys. 1983, 77, 21. a small, but nonnegligible, positive electron-density contribution,
-
-
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,
C=
1
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Crystal Structure of Silica Sodalite
The Journal of Physical Chemistry, Vol. 92, No. I , 1988 241
while for the neutron diffraction, this represents a negative scattering-density contribution. X-ray refinement without the inclusion of these atoms forced the carbon atoms toward these locations in an attempt to account for the electron density, and the resulting C-C distance of 2.02 A is chemically impossible. The neutron refinement was relatively unaffected by neglect of H atoms. 29SiN M R Results. The observed Si-0-Si angle of 159.7(2)’ at rmm temperature is consistent with the angle of 159.5Odeduced from the measured position (1 17.1 ppm)19 of the 29Si isotropic chemical shift with respect to the relationship proposed in ref 20. Splitting of the peak into three components at -116.79,-1 17.13, and -1 17.37ppm indicates some kind of structural distortion not resolved by our diffraction techniques. All the peaks in the time-of-flight pulsed-neutron diffraction pattern have the same width. Similarly all the peaks in the powder and single-crystal X-ray patterns have the same width when account is taken of the al,a2 doublet. The X-ray diffraction peaks are about one-half broader than those for well-crystallized quartz and aluminophosphate crystals. N o superstructure lines were detected in Weissenberg and rotation X-ray photographs, but unidentified lines were found in the X-ray powder diffractometer pattern. It seems necessary to appeal to some sort of local distortion from cubic symmetry which is averaged out by the diffraction process but not by the nuclear magnetic interaction. Low-Temperature Structure. Cooling of the sample to 10 K produced changes in the crystal structure evidenced by multiple splitting of each cubic diffraction (Figure 4). A sequence of preliminary measurements by pulsed-neutron diffraction showed the following: temperature = 285 K,time = 0.5 h, symmetry = cubic; 250 K,0.5 h, cubic; 200 K, 1.0h, cubic; 150 K,cubic noncubic in first few minutes and all noncubic in 1 h; 175, 185, and 195 K, 1 h, noncubic; 205 K,7-8 h, noncubic. These data indicate that the transition is sluggish, that it may show hysteresis, and that the noncubic phase is probably stable to a temperature at least as high as 205 K but probably lower than -220 K. Ordering of the glycol molecules, associated with crinkling of the silica framework (as in other silica polymorphs), is a possible cause of the phase transition. Figure 4 shows the splitting patterns for the (21l), (200),and (1 10) cubic reflections. The peaks were fit as shown by using an assumed peak shape based on the room temperature data. The (200) reflection is split into two peaks with an approximate intensity ratio of 2:l.This is consistent with a tetragonal distortion. The splitting patterns of the (211) and (1 10) reflections, however, are too complex for this. A face-centered monoclinic cell (a’ = a - b, b’ = a + b, c’ = c), retaining one of the diagonal twofold axes of the cubic cell, and thus the doublet splitting of the (200) reflection, was developed by using the measured d-spacings of the 10 K data. The cell parameters given in the abstract were obtained from least-squares refinement of a monoclinic cell using the measured d-spacings. This monoclinic cell can be transformed into a pseudo-cubic-triclinic-like body-centered cell with a = b = 8.741,c = 8.512A, a = 180 - p = 89.04*,y = 88.98’. Many additional peaks, consistent with the monoclinic unit cell, but not the face-centering, are present in the 10 K data. There is not enough resolvable intensity in the monoclinic pattern to unambiguously determine the space group symmetry, but the indications
+
(19) Meinhold, R. H.; Bibby, D. M. Zeolites 1986, 6, 427. (20) Smith, J. V.; Blackwell, C. S. Nature 1983, 303, 223.
are that the lattice may be primitive, possibly due to cooperative ordering of the EG molecules. Ethylene glycol in the trans conformation can have point symmetry as high as CZh,while the highest gauche conformation point symmetry is C,. If there is nearly complete ordering of the EG molecules at 10 K,the space group symmetry of the structure may unambiguously define the conformation, or vice versa.
Related Structures A number of structural studies of the basic aluminosilicate series Nag[AlSi04]6(0H)2.nH20have been undertaken. These show varying degrees of hydrogen bonding between encapsulated OHand H 2 0 and the framework. Hydroxysodalite, Nag[AISi04J6(OH),, was found to be cubic at 8 K with an oxygen at the center of each sodalite cage and a hydrogen randomly distributed over four sites at 1.09A from the oxygen and 2.28 A from a sodium atom.” Two members of the series Na6[AlSiO&.nH,O exhibited hydrogen bonding between nonframework (H,O) constituents and framework oxygens, although no phase transitions were detected.22923 Hydrosodalite, Nag[AlSiO4],(OH),.2H~0, is orthorhombic (P222)at 1 1 3 K24with lattice parameters a = 8.925 (6) A, b = 8.909 (6)A, c = 8.870(6)A. This distortion of the cubic ( a = 8.888 (1)A) lattice is similar to that observed for lowtemperature silica sodalite, with a = b. The tetramethylammonium ion encapsulated in synthetic TMA sodalite, (CH3),N~AlSi,01z,does not conform to the cubic symmetry of the ideal framework3 presumably because of asymmetric hydrogen bonding, but the deviations were not sufficient to produce an observable distortion of the cubic lattice. Conclusions We conclude that the high-temperature structure of silica sodalite with encapsulated ethylene glycol is essentially cubic from the viewpoint of coherent diffraction, and that there is no coherency between the orientations of the EG molecules in adjacent sodalite cages. We propose that there is coherency between the orientations of the EG molecules in the low-temperature structure, and we shall attempt to obtain single-crystal X-ray diffraction as a test. Further measurements are needed to provide information on the temperature coefficient of the apparent hysteresis of the phase change. This should be relatable to the activation energy for a change of conformation of an EG molecule in a sodalite cage. Acknowledgment. We thank N S F for grants C H E 84-05167, C H E 86-18041,and DMR 82-16892. J.W.R. is supported half-time by the Intense Pulsed Neutron Source (DOE contract W-3 1-109-ENG-38). Registry No. EG silica sodalite, 110797-70-1.
Supplementary Material Available: Observed and calculated structure factors for silica sodalite (1 page); powder neutron diffraction data (1 1 pages). Ordering information is given on any current masthead page. (21) Luger, S.; Felsche, J.; Fischer, P. Acta Crystallogr., Sect. C 1987, c 4 3 , 1. (22) Baerlocher, C.; Felsche, J.; Fischer, P.; Luger, S. Zeolites 1986, 6, 367. ( 2 3 ) Felsche, J.; Fischer, P.; Luger, S. Acta Crystallogr., submitted for publication. ( 2 4 ) Bonderava, 0.S . ; Malinovskii, Yu.A. Sou. Phys.-Crystallogr. 1983, 28, 213-216.
