Crystallographic and vibrational study of hexanitroethane

J. Phys. Chem. .%/ids Vol. 47. No. 12, pp. 11294137, Printed in Great Britain.

1986

0022-3697/86 Pngamon

CRYSTALLOGRAPHIC AND VIBRATIONAL OF HEXANITROETHANE

$3.00 + 0.00 Journals Ltd.

STUDY

D. BOUGEARDt, R. BOESE$, M. POLK$, B. WoosTt and B. ScmADERt TInstitut fiir Physikalische und Theoretische Chemie, Universitlt Essen, D 4300 Essen I, West Germany SInstitut fiir Anorganische Chemie der Universitat Essen, D 4300 Essen 1, West Germany (Received 16 December

1985; accepted 14 May 1986)

Abstract-The

crystal structure of hexanitroethane has been determined in the low-temperature ordered phase at 145 K (P2,/c: a = 10.152(2) A, b = 9.31 l(2) A, c = 10.251(2) A, p =97.54(l)“, V = 960.6(3) A’, 2 = 4 with 2 non-equivalent molecules in the unit cell). The far-infrared, infrared and Raman spectra of hexanitroethane in both low-temperature ordered and high-temperature disordered crystal phases were recorded. The internal modes are interpreted with the help of normal coordinate analysis and a conformational analysis with the MNDO method. The transition mechanism is discussed. Keywords: Crystal structure, powder spectra, IR spectra, Raman spectra, phase transition,

ethane, conformation,

hexanitro-

MNDO.

I. INTRODUCTION The study of hexanitroethane (HNE) was undertaken as part of a program about the phase transitions in hexasubstituted ethanes, C,X, (X = Cl, Br, CH3,

NO*) [l]. All these crystals undergo an orderdisorder transition to a modification known as “plastic” [2]. As the molecules in this phase are orientationally disordered the acronym ODIC (Orientational Disorder in Crystals) is often used [3]. Preliminary study and comparison with the literature showed some drastic discrepancies between our spectroscopic measurements and crystallographic data [4] concerning the number of molecules in the unit cell, and with the interpretation of Loewenschuss for the vibrational spectra concerning the molecule symmetry [5]. Further, a careful study of the crystallographic data in [4] revealed that the indexing of the reflexes was not compatible with the given orthorhombic face-centered Bravais lattice. The paper reports the results of a crystallographic and spectroscopic study of HNE. Experimental details are given in Section 2. The crystallographic results are developed in Section 3. Section 4 discusses the vibrational spectra while the spectroscopic study and the discussion of the phase transition follow in Section 5.)) 2. EXPERIMENTAL

Hexanitroethane was a commercial product and was recrystallized several times from n-hexane. A crystal with dimensions 0.28 x 0.32 x 0.21 mm3 was sublimed and selected at 0°C and cooled down

11 Table 6 which deals with Structure Factors of C,N,O,, is available as SUP 90129 (20 pages) from British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire, LS23 7BQ, U.K.

to - 128°C at a R3 Nicolet four circle diffractometer. The refinement of 25 reflexions (20” < 28 Q 25”), automatically centered and indexed, resulted in the cell dimensions of a = 10.152(2) ,& b = 9.31 l(2) A, c = 10.251(2) A, a=y=90”, p = 97.54(l)“, V = 960.6(3) A3 and P2, /c; the monoclinic symmetry was established by oscillation photos. 2 = 4, D, = 2.05 g cm3, Fooo= 600, p (Mo-Ka, Graphittemonochromator) = 2.03 cm-‘. The data collection of 3270 independent intensities (3” < 20 < 500) and 2723 unique observed reflexions [F, d 4.5 u (F)] was performed by the 28: o - 96 step-scan method with variable scanspeed (2-30” min-I), peak to background ratio 1:0.2 and subsequent profile fitting with a learnt profile [6,7]. The structure solution by direct methods, the structure refinement and plots were executed with SHELXTL-software [8] on a NOVA 3/ 12 (Data General). Anisotropic refinement of both independent molecules with the centre of symmetry at the midpoint of each C-C bond resulted in R = 0.033, R,,.= 0.037, w-’ = a2 (F) + 0.00044 F2 and a maximum residual electron density of 0.38 eA-’ at 0.5/O/O and l/0.5/0. No disorder could be detected by careful study of difference density maps. Raman spectra of polycrystalline samples sealed in glass tubes were excited by the 514.5 nm line of a Spectra-Physics Argon-ion laser and recorded between 1700 and 5 cm-’ with a Jobin-Yvon HG.2SSpectrometer. The infrared spectra of HNE in KBr and CsI pellets were recorded with a Perkin-Elmer 580 spectrometer between 1800 and 200cm-‘. The far infrared spectra of pellets of pure HNE between 400 and 20cm-’ were recorded with a Grubb-Parsons IS3 interferometer. The conformational study was done at the Hochschulrechenzentrum of the University of Essen with the MNDO-method (91; for the normal coordinate

1129

D. BOUGEARD et al.

1130

analysis the program package of Shimanouchi used [lo]. 3. CRYSTALLOGRAPHIC

3.1.

Crystal structure

was

Table

3 0 2

STUDY

and packing

-1

Powder spectra of the disordered room-temperature phase were interpreted as body-centered, cubic structures by Krien et a/.[41 and Noble et al.[l I]. Both the given data and assignments for a bodycentered cubic structure were in tolerable agreement with ours, nevertheless the latter calculated a density of 2.25 g cm-’ without regarding Avogardo’s number, which they assumed to be consistent with the experimental density of 1.88 g cm-j. The lowtemperature ordered phase powder spectra were only given by Krien et al. and assumed to be face-centered orthorhombic, but a very strong intensity was assigned to O/l/O which is not consistent with the necessary extinction rule h + k, k + 1, (I + h) = 2n. For this reason powder spectra of both phases are listed in Table 1. The assignments given for the ordered phase are based on the monoclinic cell, which was established by the single crystal data. Moreover, the powder spectrum was simulated from the atomic coordinates and cell data, given in Table 2 and the experimental Section. The intensities of the simulated powder spectrum proved to be in agreement with those listed by Noble et al. A transformation of the monoclinic cell to an orthorhombic C-centered unit cell by the transformation matrix lOO/Ol-l/O11 resulted in cell dimensions a=9.311 A, b=15.345 A, c=13.447 A, cr = 90.00”, fl = 90.00”, and y = 90.561”, which are

3 2 -2 1 4 -3 -1 0 -4 4 -2 -2 -4 0 -2 1

3.5434 3.4293 3.4140 3.3515 3.3373 3.1802 3.1589 3.1531 3.1224 2.9826 2.9576 2.9300 2.8744 2.8402 2.8152 2.7364

6.529 5.921 5.433 5.063 4.667 4.458 4.308 3.992 3.863 3.645 3.552 3.431

1 95 100 25 5 1 3 15 5 15 20

3.354

I

3.151

20

3.124

20

2.966

40

2.842

1

2.740

1

2 0 2 1 3 3 2 0 3 4 4 1 0 2 4 2 0 4 4 2 3

I

-4 2 4 2

2 1 3 3 2 4 4 3 3 1 0 0 3 0 4 4 2 1 2

I

I

3 0 3 4 4 4 1 2 4

2 3 3 1 1 3 1 3 4 4 3 2 2 0 1 4 3 5 3 2 1 2 3 1 4 5 3

Table

l(b).

The

-1 -3 -1 1 1 3 3 -3 2 -2 4 0 2 4 3 3 0

Table l(a). The low temperature modification 10.0547 5.9152 5.4456 5.0763 5.027 I 4.6512 4.4560 4.3097 3.9761 3.8238

0

2 3 3 3 2 1 2 3 0 1 1 1 1 1 3 0 0 4 1 1

-3 -5 -4 2 -1 -3 -3 -1

I

k

h

l(a) continued

2.7191 2.6460 2.6391 2.6032 2.5593 2.5493 2.5182 2.5170 2.5136 2.4636 2.4515 2.4486 2.4307 2.4266 2.4104 2.3958 2.3802 2.3255 2.3120 2.3127 2.3059 2.3030 2.2953 2.2782 2.2733 2.2392 2.2253 2.1975 2.1877 2.1798 2.1768 2.1647 2.1548 2.1543 2.1435 2.1142 2.1106 2.1106 2.1046 2.0174 1.9838 1.9717 I .9597 I .9548 1.9119 1.9090

25

2.572

15

2.520

3

2.469

10

2.333

35

2.247 2.220 2.194

5 15 IO

2.154

5

2.128

5

I

3

1.979

3

I.924

3

1.9037 1.890

3

2.02

1.8767 1.8705 I .8575

high temperature cation

1

1 2 2 2 3

1 2 1 2 1

0 0 1 0 0

3 4 330

2

1 0

(41:)

2.661

d<,b< 6.177 5.622 3.983 3.270 2.838 2.540 2.339 2.169 2.028 1.901 1.435 1.225 I.014

modifi-

I 1 100 1 15 5 5 1 <1

I <1
~rystaliographic

1131

and vibrational study of hexanitr~thane

Table 2. Atomic coordinates (multiplied by 104) and anisotropic thermal parameters (multiplied by IO’) of HNE. The temperature exponent has the form -2n* (n* u** U,, + . . . + Zhka*b*U,,) Atomic coordinates X

Wa) w4

Y

523(l) 801(l) 20031I) -38(l) 1079(I) 754(l) 3034(I) 1958(l)

5438(l) 6787(l) 4773(l) 5746(l) 6721(l) 7755(l) 5494(l) 3597(l) 6055(l) 568q 1) 9434(l) 8679( 1) 9937(l) 8396( 1) 8322( 1) 8488(l) 10892(1) 930311) 7241(t) 8857(I)

- .1295(l) 836(l) 5503( 1) 4912(l) 7010(l) 5743( 1) 3675( 1) 5740( 1) 7016(l) 8015(l) 5701(I) 6QWI)

Anisotropic thermal parameter of HNE 2

u,,

-337(l) SOS(l) -604(i) -1682(l) 1653(l) -69(l) -426(l) -1011(l) -1698(l) -2561(l) 145(l) 1243(l) 599( 1) -1075(l) 1090(l) 2107(l) 1441(l) 107(l) -892(I) -201(3(l)

15(l) 16(l) 220)

14(l) 15(l) 140) 21(l) 25(l) 27(l) 14(l) 2W) 20(l) 3041) 14(l) 2l(l) Wl) 17(l) 21(l) 31(l) 240) 16(l) 32(l) 29(l)

22jij

28(l) 17(l) 31(l) 20(l) 33(l) 37(l) 14(l) 17(l) 21(l) 18(l) 26(l) 33(l) 27(l) 31(l) 15(l) 27(l)

not identical to the cell dimensions given by Noble et al. and cannot be a~omplished because of the deviation from the 90” angle. Relations between the disordered phase bodycentered cubic cell and the primitive monoclinic ordered phase which might give some understanding for the phase transition could not be clearly derived. The structure solution revealed two independent molecules in the as~met~c unit, each with an inversion centre between the C-atoms at 0.5/O/O and l/0.5/0; the packing of the molecules is shown in Fig. 1, the positional and thermal parameters in Table 2. The closest intermolecular distance is 2.96 A from O(5) to O(2a). 3.2. Molecular structure The molecular parameters are listed in Table 3. Both molecules show a staggered conformation

cr,,

fJ23

15(l) 240) 18(l) 16(l) 20(l) 4lU) 36(l) 27(l) 25(l) 19(l) 16(l) 19(l) 220) 20(l) 29(l) 26(l) 24(i) 37(l) 32(l) 20(l)

O(I) 2(l) W) -I(l) -111) -O(1)

u22

3(l) 4(l) -4(l) 7(l) O(l) 20) -2(l) O(l) 2(l) -6(l) -4(l) l(1) -l(l) 6(l)

Crystal (- 128OC) Mol. 2 Mol. I C-N N-0(1,2,5) N-0(2,3,6) CCN CNO( I ,2,5) CN0(2,3,6) NCN ON0 NCCN CCNO( 1,2,5) Formation energy t Not calculated.

1.542 1.543 1.210 1.210 112.3 114.8 116.2 106.5 129.0 180.0 -47.7 -41.8 -44.4

u12

2(l) -f(l) X1) WI

--l(1) -31)

-4(l) 7(l) -2(l) -O(l) lo(l) 9(l) l(l) 4(l) 4(l) -2(l) 7(l) 13(l) -3(l) 2(l)

-4(l) -3(l) --5(l) 3(l) 2(l) l(l) --O(l) 00) -l(l) 2(l) -6(l) --O(l)

-O(l) o(l)

out

-21)

-2:; 3(i) -00)

the C-C bond. According to the site symmetry T(C) the dieder angles of the NO, groups are different. But it is noteworthy that ail these angles are inarangeofaboutV(-41.8to -5O.l”)withamean value of 44.6 and 47.8” for molecules 1 and 2, respectively. All other structural parameters are equal in the limit of the experimental precision. These results concerning the conformation are in contradiction with the conclusion of Loewenschuss who derived a & (3m) fete from the vibrational spectra of the free molecule. In fact, a careful reading shows that he only considered two conformations, a staggered (&,J and an eclipsed Du (fim2) one. But in fact he omitted the discussion of any lower s~rne~. By starting from DM the twisting of the NO2 groups, all in the same direction around the CN bonds, leads to S, (3) or 4 (32) symmetries depending on whether both sides of the

around

Table 3. Structural parameters of HNE. For free molecule parameters which are not statistically different in the crystal the mean values are reported in the crystal columns. Bond lengths in A and angles in degrees. Eds’s of X-ray structure dete~ination are 0.001 A for C-N and C-0,0.002 A for C-C, 0. lo for the angles

c-c

VU

1.542 1.543 1.211 1.211 112.3 114.7 116.3 106.4 128.9 180.0 -50.1 -47.2 -46.1

MNDO (free molecule) G,(3) &d(Jm ) &(32) 1.58 1.59 1.20

1.20 113.0 117.6 117.6 -_t 180.0 -43.9 682.2

1.59 1.59 1.20

1.20 112.9 118.0 118.7 180.0 180.0 742.9

I.58 1.59 1.20 1.20 113.4 117.8 117.8 148.7 -41.3 705.5 kJ mol-’

D. BOIJGEARD Ed al

1132

1

Fig. 1. (a) SHELXTL-plot

of Molecule one; (b) stereo-view of packing.

or antimolecule are twisted symmetrically symmetrically. In order to determine the most stable conformation we used the MNDO method [6, 71. The NOz torsion was fixed according to the S, or D, symmetry and all other structural parameters were allowed to relax, thus leading to two minima. The results are summarized in Table 3. The high formation energy is somewhat surprising, but it can be explained by two artifacts of the MNDO method. First, the parametrization of the method was done without including any reference substance with NO1 groups. As a result, a systematic error on the formation energy can be estimated in the order of 68.8 kJ mol-’ per NO, group (63.5 for CH,NO, and 74.0 for HONO by comparison with CH, and H,O, respectively) [9]. Thus this excess is of the order of 413 kJ mol-’ for six nitro groups. Further, MNDO has been shown to give systematic errors for overcrowded molecules; as a comparison we estimated a divergence of about 160 kJ mol-’ for 2,2,3,3-tetra-

methylbutane (“hexamethylethane”). By considering these two factors, the energy obtained for the minimum with S, symmetry agrees nicely with the experimental value of 119 KJ mol-’ obtained by Noble [ill. It is interesting to note that the S, minimum has a staggered carbon skeleton and that the C-C and C-N bond are longer than the usual values. The D, conformation leads to a relative minimum with a gauche conformation around the C-C bond. A more complete walk on the conformation surface (formation energy vs dieder angles NCCN and CCNO) shows that the structure with D,, symmetry is a saddle point between the S, and D, minima. Concerning the conformation the calculation reveals that the free molecules should have the same structure as in the crystal where packing forces are responsible for slight deviations of the dieder angles leading to a decrease of the symmetry from S, to C,. There is still a discrepancy concerning the C-C

and C-N bond lengths in crystai data on one hand and in the calculation and literature data on the other hand. The MNDO length of the C-C bond (1,576 A) is in agreement with similar values in overcrowded molecules: C!,(CH, ), : 1.58 A[1 2]; C,C16: 1.564A[13]; 1.57-1.64A for a series of molecules RIR2R3C-CR,R2R) in the solid state [14]. So far the only possible explanation lies in the nature of the molecule with 6 NOz groups which could interact differently as a free molecule and in the solid state. In this respect one should note the small difference between the calculated and measured CNO angles which couId aiso be due to the difference in the interactions between the free and solid state molecules according to [14]. Finally, from examination of space filling atom models, the external shape of the molecule with S6 symmetry turns out to be rather spherical. This fact is confirmed by a calculation of the ~larizability tensor with the MNDO method. The ellipsoid is nearly spherical for a .S, molecule

with z in direction of the C--C bond. It becomes somewhat less symmetric for the D, symmetry with lw iT = aYY= 1.71 x IO-*j cm3, ol,, = 1.82 x 1tl-23cm3, and very asymmetric for D,, with

This fact agrees well with the observation spherical molecules build plastic crystals.

that

4. VIBRATIONAL SPECTRA

The infrared, except the far-infrared region, and the Raman spectra of HNE in various solvents were obtained by Loewenschuss who proposed an assignment based on ~hara&te~sti~ group frequencies f5]. We have reproduced these solution spectra with frequency variations of only a few wavenumbers. The far-infrared spectrum of crystalline HNE was registered and is given in Fig. 2. The comparison of solution and powder spectra lead to the conclusion that all modes observed above lOOcm_’ which are present in all spectra can be assigned to intramolecular vibrations. The bands under 100 cm-’ are considered as lattice modes. With 4 molecules in the unit cell, one can expect that each band splits into 4 components, two of which are Raman active and

two IR active for the factor group CuI. The IR and Raman spectra of the crystal at room- and lowtemperature are given in Figs 2 and 3. Such a splitting is observed in the 40 K Raman spectrum for several bands. The frequency shift between solid state and solution spectra is always very small and of the order of the precision of the measurement. We can therefore conclude that, in agreement with the result in Section 3, the variations of the molecular parameters between the free molecule and the crystal must be very slight. Therefore, we have to reinterpret the spectra for a S, symmetry. By going from I& to S,, the A,, and A, species correlate with A, while A,, and .& correspond to A,. A, bands are Raman active while A, vibrations are active in the IR spectrum. In order to ascertain the assignment and to get a better insight into the vibrational behavior of the molecule a normal coordinate analysis was performed. As the free molecule was calculated we used the MNDO optimized S, structure given in Table 3 for this calculation. The starting values of tbe force constants were defined by taking into consideration the results obtained by Popov It5], Croner [f6] and Kuwae ff ?] on polynitroalkanes, nitroethane and nitrobenzene, respectively. The stretching forces of the NOz group were transferred from 1161according to the observation that the increase in the number of nitro substituents does not influence these forces [15]. The CN force constant was also transferred under consideration of the fact that by an increase of the number of NO, groups from CH,NO, to HC(NO,), the value of this constant decreases from 4.90+.46mdyn/,& 1151. The constant for the NO2 wagging coordinate was also transferred but under the addition of a F/F interaction which takes into account the vicinity of the NO, groups. Twenty force constants were defined, four being left constant, while the others were refined with the help of 27 observed frequencies. We cannot expect a very precise adjustment by using such a low rmmber of constants for a molecule with strong interactions and the final values of the force constants given in Table 4 are only indicative. A more reliable analysis should include i4N and t*O substituted molecules as well as some other polynitroaikanes. The assignment to the A, and E8 species is supported by the polarized Raman spectra; such meas~men~ are not a&able in the IR spectrum so that the assignment to A, or E, could be subject to inversion. Stifl, this rough first approach summarized in Table 4 reveals some interesting features. First of all, apart from some severe discrepancies the overall agreement is satisfactory if one considers the simplicity of the field used. Most of the bands have the same assignment as made by Loewenschuss. The main pattern all through the spectrum, and particularly under 1000cm-t, is determined by the fact that nearly all normal modes show strong

1134

D.

BOUGEARD

et

al.

Fig. 2. Infrared spectra of hexanitroethane. A: 4 mg/400 mg KBr; B: 4 mg/400 mg CsI; C,D: 200 mg as pure substance; Beamsplitter: C: 6.25 pm; D: 25 pm.

coupling of several internal coordinates so that the description in terms of group vibration is only qualitative. It is difficult to isolate a CN stretching frequency between 200 and 120Ocm-‘. Concerning the CN force constant, one can note a rather low value in good agreement with the observation of

T

300K * c

Popov about the decrease of the value with an increase in the number of NO, groups and with the very long CN bond calculated by MNDO (1.590 A by comparison with 1.50 8, for CzHSN02 [4]). The difference of 0.33 mdyn A- ’ in the value of the C-C force constant between nitroethane and hexanitro-

II

Fig. 3. Raman spectra of poiycrystaliine he~anitr~ethane. A: 150 mW laser input slidth l.Oem -‘; B: 50mW laser input slidth 1.2~m-~; C: 50mW laser input slid& 0.8cm ‘_

Crystallographic and vibrational study of hexanitroethane ethane can be caused by the repulsion of the NO* groups. This fact can be correlated with the 1.576 A long C-C bond obtained from the MNDO structure determination by comparison with 1.54A in CSHsN02 [14]. These strong interactions between NOz groups appear also in the high value of the CN/CN interaction constant for NO, groups located on the same carbon atom. Further, the fine structure of the splitting in the v, and v, NO2 region (1250-1650 cm-‘) is not completely reproduced by the calculation. These features could be improved by force constants between NO2 groups which would take into account the mutual interaction of the strong NO dipoles. Such an attempt was not done since the number of fitted parameters would be too high by comparison with the observed values. Due to the dieder angle ONCC with S, symmetry the out-of-plane NO, motions couple to some extent with other skeleton modes. Therefore, we have not enough observed frequencies of pure out-of-plane vibrations to accurately fit the corresponding force constants. This leads to some of the most drastic deviations (A,: 888, 774, 1582 cm-‘). As expected by going from Dsd to S, two bands are assigned to vibrations which should be. inactive for the higher Djd symmetry: 642cm-’ in the Raman and 774cm-’ in the IR spectrum.

vais cell [4]. Therefore the spectroscopic unit cell should contain only one molecule. Our spectroscopic results also indicate that the conclusions of Krien are wrong. The presence of one molecule in the spectroscopic unit cell would imply that only three librations are expected but at least five such bands are observed alone in the Raman spectrum (Fig. 3). The new structural data reported in Section 3 are in better agreement with the low frequency spectra. With Z = 4 we expect 21 lattice vibrations: 12 are Raman active (6 A, and 6 E,) and 9 are active in the IR spectrum (5 A, and 4 I$). With a site symmetry i (Ci) the librations and translations are expected in the g and u species, respectively. Seven frequencies are observed at 40 K: 28.5, 49.5, 49, 56 and 65 cm-’ in the Raman spectrum, 30 and 41 cm-’ in the infrared spectrum. Due to the high sublimation rate, reliable polarized Raman spectra could not be obtained so that an assignment in terms of symmetry is impossible; further, since the molecule is nearly spherical an assignment based on the inertial moments cannot be done and finally, because of the narrow frequency gap (100-4Ocm-‘) it cannot be excluded that couplings of internal coordinates with the external degrees of freedom exist.

4.2. Lattice vibrations Krien concluded from an X-ray study that the low-temperature form should be of the face-centered orthorhombic type with four molecules in the Bra-

5. PHASE TRANSITION There are two previous investigations of the polymorphism of HNE. Rosen [18] observed a phase transition at a temperature of 289.5 K. He found a cubic high-temperature form and an optically aniso-

Table 4. Force constants used for the normal coordinate analysis in mdyn A-‘, mdyn I%radm2 and mdyn rad-’ for stretching, bending and stretchbend constants, respectively. (a) see Ref. [14]; (b) see Ref. [13]; in this calculation local symmetry coordinates were defined which cannot he compared directly with internal coordinates; (c) see Ref. [12]; the force field is not completely defined in this paper; (d) these constants were not adiusted

This work

cc CN NO

NCC NCN

ONC ON0

l-NO2

rcc

rCN CCICN

CNjCN CN/NO NO/NO CC/NCC CN/NCC NO/ONC NO/ON0 NCC/ONC NCN/ONC rNO#NO,

4.38 2.12 8.28d 1.57 0.90 1.75 0.60 0.39* 0.30* 1.00 0.68 1.08 1.008* 0.37 -0.31 0.32 0.89 0.16 0.27

Parent molecules C,H,-NO,” 6.05 5.08 9.06 1.33 1.29 1.49 0.61 0.28-0.33 0.54 0.51 0.50 1.24 0.17 1.01 0.56 0.99 0.20

1135

CIH,-NOzb

Polynitroalkane’

4.11 3.05 8.28

4.90-4.46 8.90

0.391

0.80 1.24 0.35

0.46 1.008

0.32 0.38 0.58 0.25 0.38

D.

1136

BOUGEARD

et al.

Table 5. Results of the normal coordinate analysis Frequencies Expt. Ca1c.t 1627 1353 I143 858 375 335 113 1630 1268 I003 665 (391) 238 201 103 1621 1333 888 582 376 240 1639 1285 820 633 347 215 155 (642) (774) -

1617 1327 1116 806 435 326 136 1630 1266 995 666 421 361 223 206 104 1617 1347 936 632 408 231 1628 1249 805 652 383 355 215 131 92 746 219 813 219 44

Description Caract. Symmetry motions: S,.+ D,,, vaNOz vsN0, vcc P NOz vaNOz vsNOz &NO pN0; rNOz TCN

Potential energy distribution (PED)$ NO(90) NO(66) CC(87),CC/CN( - 32) OONC(22).N0(21) 0NC(79) CN(25),CN/CN(25), ONC(20) NCC(56),NCN(41) NO(88) NO(7O),~N(23) NCC(S4I,CN(34),NO(25) ON~(5S),ONO(~O),NCC(20) ONC(78),NCC(5O),NCC/ONC( - 48) I-NO,(33),NCC(26),NC(22) CN(75).rCN(32),CN/CN( - 30) rCN(50).CN(29) NCi(5i)

vaN0, vsNOz

vaN0, v sNOz VCN 6N0, rCN

FNOz zCN TCN TCC

NO(89) NO(60) NCC+NO+NCNfTNO,+ONC+CN ONC(59)JN0,(24) ONC(56),CN(21) NCC(41),NCN(30),NCC/ONC( - 20) NO(89) NO(79) ~~(62),ON~(28) ONC(6I),ONO(23),NCN(22~ rNO*(37),NCC(27),ONC(2S),NCN(Z2) CN(IOS),CN/CN( - 42),ONC(36) rCN(84) NCN(56),NCC(26),FNO,(20) NCC(83),NCC/ONC( - 32)JN02(373 r(71) rCN(99) r(23),NO+ONC+ONO rCN(99) rCC(99)

t The reported calculated frequencies are given for the calculation corresponding

to the structure with S, symmetry. Values in parentheses were not used in the fit; $The mode description is only given for such vibrations which can be unequivocally classiiied with the help of the PED and the vibration form; §Only PED terms greater than 20% are reported; for sums over 100% the balance is given by the addition of negative off-diagonal terms with participation smafier than 20%.

tropic low-temperature form. DSC (Differential Scanning Calorimetry) measurements and the X-ray investigations of Krien [4] showed the temperature of the phase transition to be between 289 and 292 K by heating and between 279 and 283 K when cooled. From Debye-Scherer patterns they deduced a cubic body-centered structure with two molecules in the unit cell for the Hugh-tem~rature form. The spectra of the disordered phase are given in Figs 2. and 3. A variation of the intensity with temperature was observed in the region IOO-120cm‘ ’ (Fig. 4). Both vibrations have been assigned to deformations of the NCCN skeleton. As the two bands at 391 and 103 cm-’ belong to the molecular E, species we can explain this effect as being due to the disappearance of degeneracy in the ordered phase. Thus, by increasing the temperature both components coalesce at 10°C by 390cm-’ and at around IOOcm-’ the component which overlapped with the A, band at

130 cm-’ shifts to a somewhat lower frequency (see shoulder by 115cm- I at 12’C) leading to an intensity decrease and to an increase of the band width at 130cm-’ (see Fig. 4). In good agreement with thermodynamic investigations [4] a hysteresis of about 9K was observed; this fact indicates a first order phase transition. With two molecules in the body-centered cell. the unit cell contains only one molecule. As a consequence all splittings of the internal mode disappear in the disordered phase. One expects also only three librations in the Raman spectrum and no IR active vibration. In fact all the lattice bands disappear at the transition (Figs 2 and 3). At the transition temperature no frequency shift could be observed for the internal modes. Therefore, we can conclude that the conformation of the molecule does not change very much at Tc and probably the molecular S, symmetry still exists above Tc fa test

Crystallographic

and vibrational study of hexanitroethane

1137

13.1 J K .mol, respectively, and are explained by the first two effects. A molecule with S, symmetry is not symmetric enough to be included on the main diagonal of a cube as required by the high temperature crystal structure. That means that a supplementary disorder is necessary to account for the crystallographic results and for the disappearance of the lattice modes; this disorder could explain part of the remaining 20 J (K . mol)-’ of the transition entropy. Possible motions causing such a disorder are rotations of the molecule around axes perpendicular to the C-C bands or rotations of the NOz groups around the C-N bands. Acknowledgedments-The

financial support of the Deutsche Forschungsgemeinschaft and of the Fonds der Chemischen Industrie is gratefully acknowledged.

REFERENCES 1. Woost B., Dissertation. University of Essen, West Germany (1983). Woost B. and Bougeard D., .I. c/rem. Phys. 84, 4810 (1986); J. Phys. Chem. Solids (in press). 2. Timmermanns J., J. Chim. Phys. 35, 331 (1938). 3. Sherwood J. N., The Plastically Crystalline StateOrientationally Disordered Crystals. John Wiley & Sons, New York 11979). 4. Krien G.,-Licht H. H. and Trimborn F., Exptosivstoffe

400

300

200

100 cm-t

Fig. 4. Temperature dependence of the Raman spectrum in the O-400 cn-’ range. Up and down arrows indicate heating and cooling processes, respectively.

of the normal mode for a molecule with L&, or D, symmetry reveals frequency shifts up to 15cm-’ which should be observed in the spectra). By comparison with other hexa-substituted ethane derivatives, the transition entropy is very high at 43.5 J (K .mol)-’ [4]. The entropy increase caused by the orientational disorder, the average crystal structure being a superposition of four molecules oriented along the main diagonal of a cube, is in the order of R In 4 = 11.6 J(K*mol)-t. Furthermore, the influence of a volume effect has to be considered [19], as in this crystal a volume variation between 6.1 and 19% was mentioned at the phase transition 14, 181. For CrCl, and C2Br6 the entropy variations are 23.5 and calculation

18, 203 (1970). 5. Loewenschuss A., Yellin N. and Gabai A., Spectrochim. Acta 3OA, 371 (1974). 6. Clegg W., Acta Cryst. 37A, 22 (1981). 7. Diamond R., A& Cry&. ZSA, 43 (1969). 8. Sheldrick G. M.. SHELXTL. A Complete Software Package for So&g Refining ‘and Disiiaying trystal Structures, Rev. 4.1 (1983). 9. Dewar M. J. S. and Thiel W., .I. Am. them. Sot. 99,4899 (1977); Thiel W., QCPE 11, 353 (1978). 10. Shimanouchi T., Computer Programs for Normal Coordinate Treatment of Polyatomic Molecules, University of

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