Polarized FTIR spectra and low temperature structure of adamantanol derivatives

Journal of Molecular Structure, 222 (1990) 305-317 Elsevier Science Publishers B.V., Amsterdam - Printed

305 in The Netherlands

POLARIZED FTIR SPECTRA AND LOW TEMPERATURE STRUCTURE OF ADAMANTANOL DERIVATIVES Part I. 2-(2,4,6-Trimethylphenyl)adamantan-2-01

J. BARAN’, J.A. KANTERS’, M. WIERZEJEWSKA-HNAT’

E.T.G. LUTZ3, J.H. VAN DER MAAS3, A. SCHOUTEN’

and

‘Institute of Chemistry, Wrociaw University, Joliot-Curie 14, 50-383 Wroclaw (Poland) 2Laboratory for Crystal and Structural Chemistry, University of Utrecht, Padualaan 8,3584 CH Utrecht (The Netherlands) 3Laboratory for Analytical Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht (The Netherlands) (Received 25 May 1989)

ABSTRACT Results of X-ray structure and FTIR investigations for 2- (2,4,6_trimethylphenyl)-adamantan2-01 are prese$ed. &,H,,O, M,=270.41, monoclinic, C2/c, a=35.458(5), b=8.2142(9), $=10.0701(6) A,P=97.928(9)“, V=2905.0(6) A3,Z=8, D,=1.237gcmp3, MoKcu,1=0.71073 A, .u=O.7 cm-‘, F(000) = 1184, T= 150 K, R=0.042 for 2015 observed reflections. The methyl groups ortho to the ring junction cause severe geometric distortions. The phenyl ring deviates from hexagonal symmetry and planarity. The ortho methyl groups display large angular deformations._There are several very short intramolecular H---H contacts ranging from 2.00(3) to 2.29(3) A. The hydroxyl group is not involved in any O-H***0 type hydrogen bond; however the FTIR results (the position and dichroic ratio of the v(OH) band) indicate that a weak intermolecular 0-H.-en type interaction is present in the crystal.

INTRODUCTION

There are numerous papers in the literature concerning IR studies of different alcohols in solutions (see for instance refs. l-9). Thanks to a high sensitivity of the OH vibration to local surroundings important structural information can be derived from the OH stretching frequencies. Among them the presence of an intramolecular O-H*** K interaction was demonstrated for several groups of alcohols [ 3,7-g]. The structure and hydrogen bonding of solid alcohols have drawn considerable attention as well. They have been studied mostly by means of X-ray diffraction [lo-181 and also by IR spectroscopy [ 19-241. According to our knowledge only two papers have been published on polarized IR spectra of

0022-2860/90/$03.50

0 1990 Elsevier Science Publishers

B.V.

306

single crystals of alcohols [ 19,211. The results of only one of them have been discussed in relation to the crystal structure [ 191. We have undertaken X-ray structure and polarized IR studies of some adamantanol derivatives for which, due to large bulky substituents, different degrees of self association should be expected. By combining the X-ray and IR methods we should like to reveal O-H***0 and/or O-H.**n intra- and intermolecular hydrogen bond frames and to relate the structure to the spectroscopic data. As a first part of these investigations the results on 2- (2,4,6-trimethylphenyl) -adamantan-2-01 (3MPA2) are presented and discussed. EXPERIMENTAL

Crystals were grown by slow evaporation of a solution of the compound in ethanol. The crystals have the shape of thin plates (parallel to the bc plane) elongated along the b direction. X-ray measurements Data were measured on a crystal of dimensions 0.12 x 0.22 x 0.30 mm on an Enraf-Nonius CAD-4 diffractometer with Zr filtered MO KCYradiation. Lattice constants were derived from the setting angles of 25 reflections in the range 14.14 < 8< 18.00”. 2550 intensities were collected at low temperature (150 K) with the o-28 scan mode, Ao= (0.60 + 0.35tan0) ‘, 20,, = 50” and h, k, 1range -42+41,0+9,0+12 respectively, of which 2015 reflections with 122.5 o(l) were considered observed. Two standard reflections (422 and 1002)) measured every hour showed a r.m.s. deviation of 2.3 and 3.4%, respectively. Intensities were corrected for Lorentz and polarization effects, but not for absorption. The structure was solved by routine application of SHELXS86 [25] and all H atoms were located from difference maps. The structure was refined by full-matrix least-squares procedures with the SHELX76 program [ 261. The number of refined parameters was 285, including scale factor, anisotropic thermal parameters for non H-atoms, individual isotropic thermal parameters for H atoms and positional parameters for all atoms. The refinement converged at Rz0.042 and R,=0.039 with w = l/a” (FO). The average shift/error ratio is 0.0011, the maximum shift/error ratio is 0.022 and S= 1.53. The atomic scattering factors were taken from International Tables for Crystallography [ 271. The final difference map had maximum and minimum densities of 0.20 and -0.26 A-“. The program package EUCLID [ 281 was used for the calculation of geometries and preparation of illustrations. FTIR polarized measurements Crystals (of ca. 0.5 mm diameter) of a good quality were selected. Their orientation was determined by means of a polarizing microscope. The third

307

mean line of an indicatrix is parallel to the b axis. The thinnest sheet was used for the measurement. It was stuck onto a disk with a pinhole of diameter less than 0.5 mm in the center. The IR spectra were measured with a Perkin-Elmer 1800 FTIR spectrometer with a resolution of 2 cm-‘. A 4~ beam condenser was used. A wire grid ( AgBr) polarizer was inserted behind the sample. The spectra were taken with the polarizer set parallel (A,) and perpendicular (B,) to the b monoclinic direction. DISCUSSION

Crystal structure

The atomic coordinates together with equivalent isotropic thermal parameters (C and 0 atoms) and isotropic thermal parameters (H atoms) are listed in Table 1. Bond distances and bond angles involving non-H atoms are given in Table 2. A perspective view of the molecule with atom numbering and a projection of the unit cell on the uc plane are shown in Figs. 1 and 2, respectively. The C-C distances in the adamantane fragment range from 1.526(3) to 1.560(3) A with an average of 1.54( 1) A. Two distances are relatively long: C(l)-C(2) (1.560(3) A) andC(2)-C(3) (1.551(3) A).Thebondanglesrange from 105.0(2) to 111.8(2) ‘, with an average of 109.5(16) ‘. Two angles are small,C(l)-C(2)-C(3) (105.0(2)“) andC(4)-C(5)-C(10) (106.9(2)“).The bond angles involving bridging C (2 ) deviate from tetrahedral values as was also observed in 2-phenyl-adamantan-2-01 [ 131 and in 1-phenylcyclohexanol [ 111. The four six-membered rings all have nearly ideal chair conformations as follows from the puckering parameters 8 of Cremer and Pople [ 291 which amount to 2.8 (2 ) ,0.5 (2 ) ,0.8(2 ) and 1.0 (2 ) ‘, respectively. The phenyl ring is appreciably distorted from hexagonal symmetry as is demonstrated by the variation of distances and angles which have ranges of 1.381(3) to 1.429(2) A 115.8(2) to 123.7(2)“, respectively, and also by the endocyclic torsion angles which range from -8.2 (3) to 8.8(3) ‘, the largest ones involving the C-C bonds adjacent to the ring junction. The phenyl ring also deviates significantly from planarity as follows from the oplanevalue of 0.051(2) A. The C atoms of the methyl groups deviate from the phenyl plane: C(17) (0.29(l) A), C(18) (-0.15(l) A) and C(19) (0.18(l) A). The two methyl groups ortho to the ring junction have deviating angular geometries. The angular distortion which is clearly visible in Figs. 1 and 3 is exemplified by the C-C-C angles facing the adamantane cage: C( 17)-C( 12)-C( 11) (127.2(2)“) andC(19)-C(16)-C(ll) (125.9(2)“),whichareenlargedatthe expense of C(17)-C(12)-C(13) (112.7(2)“) and C(19)-C(16)-C(15) (112.9 (2 ) o ). The angles involving the para methyl group have normal values. The distortion of the phenyl ring and particularly the angular deformation of the ortho methyl groups are caused by internal crowding. Though this strain

308 TABLE 1 Fractional coordinates and equivalent isotropic thermal parameters with their e.s.d.s in parentheses for 2-(2,4,6-trimethylphenyl)-adamantan-2-01

O(l) C(l)

C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(l0) C(l1) C(12) C(13) C(l4) C(l5) C(l6) C(l7) C(18) C(l9)

n

Y

z

U(w)”

0.14194(4) 0.09910(6) 0.12140(6) 0.09026 (6) 0.06324 (6) 0.04221(6) 0.01876(6) 0.04538(6) 0.07530 (6) 0.06611(6) 0.07245(6) 0.15203(5) 0.15763(5) 0.18493 (6) 0.20851(5) 0.20592 (6) 0.17907(6) 0.13945(7) 0.23590(7) 0.18314(7)

0.1837(2) 0.1531(3) 0.2787(2) 0.3779(3) 0.4640(3) 0.3372(3) 0.2240(3) 0.1377(3) 0.0401(3) 0.2655(3) 0.2409(3) 0.3746(2) 0.5469(2) 0.6180(3) 0.5302(3) 0.3618(3) 0.2841(2) 0.6686(3) 0.6125(3) 0.0991(3)

0.7029 (2) 0.4968(2) 0.5939(2) 0.6521(2) 0.5410 (2) 0.4468 (2) 0.5239 (2) 0.6350(2) 0.5736(2) 0.7292(2) 0.3862(2) 0.5288(2) 0.5343(2) 0.4647(2) 0.3930(2) 0.4008(2) 0.4669 (2) 0.6200(3) 0.3126(3) 0.4678(3)

0.0177(5) 0.0152(7) 0.0142(6) 0.0147(6) 0.0173(7) 0.0181(7) 0.0187(7) 0.0175(7) 0.0175(7) 0.0178(7) 0.0176(7) 0.0133 (6) 0.0144(6) 0.0177(7) 0.0172(7) 0.0181(7) 0.0158(6) 0.0206(s) 0.0249 (8) 0.0201(7) U(is0)

H(01) H(C1) H(C3) H(C5) H(C7) H(C13) H(C15) H(C41) H(C42) H(C61) H(C62) H(C81) H (C82) H(C91) H(C92) H(C101) H(C102) H(C171) H(C172) H(C173) H(C181) H(C182) H(C183) H(C191) H(C192) H(C193)

0.1535(g) 0.1171(5) 0.1034(5) 0.0256(5) 0.0299(5) 0.1886(6) 0.2234(5) 0.0785(5) 0.0454(5) -0.0015(6) 0.0049(6) 0.0628(5) 0.0932 (5) 0.0825(6) 0.0474(6) 0.0883 (6) 0.0609(5) 0.1426(6) 0.1134(6) 0.1531(6) 0.2258(7) 0.2448(g) 0.2597(7) 0.2112(6) 0.1707(6) 0.1734(5)

0.246(3) 0.087(2) 0.458(2) 0.395(2) 0.065(2) 0.739(3) 0.296(2) 0.532(2) 0.529(2) 0.286(2) 0.142(2) .0.042(2) - .0.020(2) 0.211(2) 0.334(2) 0.317(2) 0.160(2) 0.636(3) 0.686(3) 0.772(3) 0.621(3) 0.725(4) 0.550(3) 0.073(3) 0.050(3) 0.047 (2)

“U(eq)=1/3sum(i)sum(j)U(&x(i)*a(j)*a(i).aCj).

0.757(3) 0.452(2) 0.719(2) 0.377(2) 0.687 (2) 0.474(2) 0.358(2) 0.486 (2) 0.585 (2) 0.564(2) 0.463(2) 0.511(2) 0.647(2) 0.808(2) 0.768(2) 0.334(2) 0.323(2) 0.719(2) 0.597 (2) 0.618(2) 0.220(3) 0.349 (3) 0.306(2) 0.466(2) 0.391(2) 0.549(2)

0.07 (1) 0.018(6) 0.016(5) 0.019(6) 0.013(5) 0.021(6) O.Oll(5) 0.016(6) 0.021(6) 0.016(5) 0.022 (6) 0.017(6) 0.021(6) 0.028(6) 0.023 (6) 0.020(6) 0.018(6) 0.033(7) 0.025(6) 0.038(7) 0.057(9) 0.09 (1) 0.058(g) 0.030(6) 0.022 (6) 0.020(6)

309 TABLE 2 Bond distances (A) and bond angles (deg) for 2- (2,4,6-trimethylphenyl)adamantan-2-01 D

Angle

Value (deg)

Bond

Distance (A)

0(1)-C(2) C(l)-C(2)

1.457(2)

C(2)-C(l)-C(8)

110.5(2)

1.560(3)

C(2)-C(l)-C(10)

C(l)-C(8)

1.534(3)

C(l)-C(l0) c(2)-C(3)

1.537 (3) 1.551(3)

C(S)-C(l)-C(l0) O(l)-C(2)-C(1)

110.6(2) 108.9(2)

C(2)-C(l1)

1.558(3) 1.541(3)

O(l)-C(2)-C(3) O(l)-C(2)-C(l1)

106.0( 1) 107.7(2) 106.4(2)

C(l)-C(2)-C(3)

105.0(2)

1.540(3)

C(l)-C(2)-C(l1)

113.2(2)

C(4)-C(5) C(5)-C(6) C(5)-C(10)

1.532(3) 1.529(3)

C(3)-C(2)-C(l1)

117.9(2)

C(2)-C(3)-C(4)

111.8(2)

1.526(3)

C(6)-C(7)

1.534(3) 1.527(3)

C(2)-C(3)-C(9) C(4)-C(3)-C(9)

110.5(2) 107.9(2)

C(3)-C(4) C(3)-C(9)

C(7)-C(8) C(7)-C(9) C(ll)-C(12)

C(3)-C(4)-C(5) C(4)-C(5)-C(6)

109.8(2)

C(4)-C(5)-C(10)

110.6(2) 106.9(2)

C(ll)-C(16)

1.423(3)

C(6)-C(5)-C(l0)

110.4(2)

C(12)-C(13) C(12)-C(17)

1.399(3) 1.521(3)

C(5)-C(6)-C(7)

109.2(2)

C(13)-C(14) C(14)-C(S)

1.381(3)

C(S)-C(7)-C(8) C(6)-C(7)-C(9)

109.8(2) 109.2(2)

C(8)-C(7)-C(9)

108.0 (2)

C(l)-C(8)-C(7) C(3)-C(9)-C(7)

110.3(2) 110.6(2)

C(l)-C(lO)-C(5) C(2)-C(ll)-C(12)

110.4(2)

C(2)-C(ll)-C(16)

118.1(2)

C(12)-C(ll)-C(16)

115.8(2) 119.8(2)

C(14)-C(18) C(15)-C(16)

C(16)-C(19)

1.533(3) 1.429(2)

1.389(4) 1.508(3) 1.390(3) 1.526(3)

C(U)-C(12)-C(13)

125.9(2)

C(ll)-C(12)-C(17) C(13)-C(12)-C(17)

127.3(2)

C(12)-C(13)-C(14)

123.7(2) 116.2(2)

C(13)-C(14)-C(15) C(13)-C(14)-C(18) C(E)-C(14)-C(18) C(14)-C(15)-C(16) C(ll)-C(16)-C(15) C(ll)-C(16)-C(19)

C(15)-C(16)-C(19)

112.8(2)

121.9(2) 122.0(2) 122.6(2) 121.2(2) 125.9(2) 112.9(2)

is relieved by bending of the ortho methyl groups, there are still very short intramolecular H---H contacts. These contacts involve H atoms bonded to C(17) and C(3) (contact distances 2.02(3) and 2.29(3) A) and C(17) and C(4) (2.00(3) A) andHatomsbondedtoC(19) andC(1) (2.10(3) and2.12(3) A). It is noteworthy that the distances involving atoms of the C (2)-C (11)

Fig. 1. Molecular structure

Fig. 2. Projection

with atom numbering of the title compound.

of the unit cell on the UCplane.

junction are enlarged and this also holds for the C (1 )-C (2)-C (11) and C (3)C (2 )-C (11) junction angles (Table 2 ). The disposition of the phenyl ring with respect to the adamantane cage is given by the torsion angle 0-C(2)-C(ll)C (12) which amounts to - 111.4 (2) ‘. It is curious to note the deviation from the ideal value of - 90’ which would distribute the internal strain equally over both ortho methyl groups. In 2-phenyl-adamantan-2-01 where very short H---H contacts have also been observed betweenH atoms in ortho positionof the phenyl ring and those of the adamantane cage, the torsion angle is equal to -98.1(2)” [13]. The hydroxyl group is not involved in hydrogen bonding of the O-H.**0 type. The H-O-C (2 )-C (11) torsion angle is 61(2 ) o which implies that the H

311

c

Fig. 3. Geometry of the 0-H..*n chain projected on the bc plane. The adamantane fragment and mesitylene protons are omitted for clarity (I) x, y, z; (II) x, 1 -y, l/2 + z; (III) x, y, I+ z.

atom is in the favourable gauche position which is indicative of the absence of an intramolecular O-H***x interaction. Infrared results The v(OH) stretching vibration The polarized IR spectra of the 3MPA2 single crystal for the bc sample are presented in Fig. 4 and Fig. 5 and the powder spectrum in Fig. 6. The wavenumbers, relative intensities and tentative assignment of the bands are given in Table 3. In the O-H stretching region one sharp band is observed at ca. 3549 cm-’ with a shoulder at 3520 cm-‘. For a free OH group of alcohols a position of the stretching vibration is expected to be situated slightly above 3600 cm-‘. For the monomeric alcohols in low temperature matrices this absorption appears in between 3600 and 3660 cm-’ [ 30-321. A similar range for this absorption was found for solutions of alcohols in non-polar solvents [33, 341. For tritertbutylcarbinol, the only example of a monomeric alcohol in the solid state known so far, the v (OH) band is observed at 3608 cm-’ [ 201. In dilute solution in Ccl, the OH stretching vibration of the isolated 3MPA2 molecule is observed at 3614.5 cm-l. The lower position of the OH stretching vibration observed in the studied spectra for 3MPA2 (3549 cm-l) is characteristic for the presence of a weak intermolecular H bond in the crystal and indicates that the hydroxyl group is involved in a weak intermolecular O-H... x bond. This conclusion is supported by the crystal structure since there is a close approach between the OH group and the midpoint M of the C (13)-C (14) bond (at x, 1 --y, l/2 + z) of the phenyl ring. The geometry of this O-H---M configuration with O---M, H-*-M distances and 0-H***M angle of 3.47(2), 2.61(2)

312 2-(2,4,6-trimethylphenyl)adamantan-2-01

single

crystal

t P

L

4000

-

!. . . . . 3500

3000

.Lich.-. Ellb

Pl

.-LI

2500

2000

Wavenumber/(cm-‘)

Fig. 4. FTIR spectra of the 3MPA2 single crystal for the bc sample in the 4000-2000 cm-’ range: without polarizer (upper), polarized parallel to the c axis (middle) and parallel to the b axis (bottom).

and 177(2) “, respectively, is indicative of a weak intermolecular O-H-.*x interaction. Similar O-H--*x interactions with comparable geometries have been reported for a number of 17-hydroxypregnene steroids [ 351. Figure 3 presents a scheme of the 0-H*-*x hydrogen bonds which link the molecules to form chains running parallel to the c axis. The repeating unit of one chain contains two molecules related by a c glide plane. The v (OH) stretching band is observed in the polarized spectra measured in both directions (see Fig. 4) with a dichroic ratio R,,, equal to 0.58. To reA

313 Z-(2,4,6-trlmethylphenyi) odomanton-Z-01

uthout

2000

angle

crystal

Polarizer

1000

I

1600

1400

1200 Wovenumber

1000

BOO

600

4

/(Cm')

Fig. 5. FTIR spectra of the 3MPA2 single crystal for the bc sample in the 2000-400 cm-’ range: without polarizer (upper), polarized parallel to the c axis (middle) and parallel to the b axis (bottom).

2-12,4,6-tnnethenylpenyl)

I

\

k

00

Fig. 6. FTIR spectrum of 3MPA2 in KBr pellet.

odamoton-2-01

powder

spectrum

(in KBr)

314 TABLE

3

Infrared frequencies

(cm-‘),

relative intensities”

and assignment

for 3MPA2

KBr pellet

Single crystal

3546 s

3549 vs 3520 wsh

3549 vs 3520 vwsh

3026 VW

3026 m 2966 vs

3027 vs 2966 vs

2953 m 2935 m

2953 vs

2953 vs

2937 vs

2936 vs

A

2912 s

2919 vs 2895 vs

2923 vs 2893 vs

A A

2882 vs 2854 vs

2882 vssh 2857 vs

M A

2736 VW

2736 VW

2708 VW

2706 VW

2687 VW

2680 vw 2654 VW

2968 w

2897 s 2882 vs 2853 s 2735 VW 2708 VW 2670 VW

2660 VW

Assignmentb

4OH) M M A

2500 VW 2412 vw 1905 VW

1905 VW

1765 VW 1740 VW

1740 VW

1765 VW 1740 VW

1607 w

1608 s 1560 VW

1607 m 1557 w

1558 VW 1480 w

1476 m

1467 vwsh 1459 w

1460 s

1447 w

1447 vs

1458 s 1449 vs 1435 vssh

1382 vwsh 1376 VW 1367 VW 1354 VW 1343 w 1329 VW 1321 vwsh 1310 vwsh 1299 VW 1281 VW 1260 1245 1236 1223 1197

vw vw VW VW VW

1383 w 1374 w 1367 w 1354 m 1343 s 1329 s 1321 1310 1298 1281 1258 1245 1236

m w w w w w w

1416 w 1387 wsh 1376 s

I

1367 m

M

Y(C-C)+G(CCH)

M

&,(CH,) V(C-C)

M

6(CH,)

A

&CH,) &(CH,)

A

M

M A

G(CCH)

1354 m &CCH)

1331 m 1325 wsh 1304 m i

1241 vw 1223 w

1197 VW

overtones and combinations

&CH)

>

+p,(CH,)

A

v(C-C)+p,(CH,)

A

S(COH)

?

+v(C-0)

S(CCH)

M

p,(CH,)+S(CCH)

A

315 TABLE 3 (continued)

1172 VW

Assignmentb

Single crystal

KBr pellet

E II b

E II c

1171 s

1173 m 1170 vwsh

u(C-C)

M

1098 w 1081 w 1049 w

S(CCH) v(C-C) +pt(CHz) p,(CH,)?

A A M

1030 VW 994 vs 976 m 965 VW 932 m 914 w 903 m

Y(C-c)+G(ccc) v(C-0) +6(COH)

M ?

PACK) +u(C-C)

A

~r(cH,)

A

&CH)

M

u(C-c!)

A

G(CCC) S(CCH) +S(CCC)

A M

G(CCC)

A

u(C-C) +S(CCH)

M

S(CCH)

M

S(CCC)

A

1162 wsh 1132 VW 1125 VW 1108 VW 1098 VW 1081 VW 1046 w 1037 vwsh 1029 VW 992 m 973 VW 961 VW 931 w 914 VW 903 VW 885 VW 875 VW 861 w 846 w 840 vwsh 811 VW 781 VW 745 vw 685 VW 658 VW 634 VW 600 VW 582 VW 520 VW 472 VW 463 VW

1

1125 VW 1108 w 1099 VW 1081 m 1045 s 1029 vs 993 vs 975 vwsh 961 w 931 vs 914 m 903 m 884 w 879 w 874 w 861 vs 847 w 839 m 812 VW 782 VW 146 w 684 VW 658 w 634 w 601 VW 582 w 575 vwsh 518w 464 s 439 VW 416 VW 409 VW

876 w 851 VW 847 VW 838 VW 811 m

I

)

I

745 VW 685 VW 659 w 602 VW 575 VW 520 VW 472 w 441 vw

>

“Relative intensities: vs, very strong; s, strong; m, medium; w, weak; VW,very weak; sh, shoulder. b~, stretching; 6, bending; p,.,,wagging; pt, twisting; pr, rocking; M, A, mesitylene and adamantane fragments, respectively.

316

produce the experimental value of the dichroic ratio one may consider several possibilities of the transition dipole moment (t.d.m.) orientation: ( 1) the t.d.m. of the v (OH) vibration is directed parallel to the O-H bond; (2) due to the O-H-*.x interaction the t.d.m. is directed along, (a) O(l)***C(13) (x, l-y, l/2+2), (b) O(l)*-sC(14) (x, l-y, l/2+2) directions; (3 ) an intermediate situation is present with the t.d.m. being oriented along a line in between those mentioned above. Theoretical values of the dichroic ratio Rcjb calculated from the directional cosines for the cases (l), (2a) and (2b) are equal to 0.91, 2.22 and 0.46, respectively. Thus, on the basis of the IR results (Rcib= 0.58) it follows that the t.d.m. of the v (OH) vibration is oriented along a line lying in between the ( 1) and (2b) directions. It is worthwhile to note that the position of the v (OH) band is exactly the same for both directions measured, which indicates that a correlation field (Davydov) splitting is not observed for this vibration in the studied crystal.

Other vibrations A group of intense bands is observed at about 3000 cm-l which can be easily attributed to the asymmetric and symmetric stretching vibrations of CH, CH, and CH, groups. Several bands in the 1750-1480 cm-l region are characteristic for the benzene ring vibrations [ 361. The assignment of bands below 1500 cm-’ is less straightforward for at least two reasons: (1) the mixed character of many of the vibrations in this range; (2) complex forms of the vibrations which make it impossible to foresee the transition dipole moment directions and thus, the polarization features of the bands. Because of this the descriptions of the bands presented in Table 3 are for the greater part rather approximate. The assignment of vibrations of the adamantane fragment are based on spectroscopic studies and normal coordinate analysis made by Bailey [ 371 for adamantane C,,H,, and its deuterated analogue. Modes of the mesitylene moiety are interpreted on the basis of the IR investigation of 2,4,6_trimethylbenzene [ 381. Both stretching v (C-O) and in-plane deformation 6(OH) vibrations are expected to appear between 1300 and 1000 cm- ‘. The complexity of this region makes the assignment of these modes rather dubious. The out-of-plane deformation 7( OH) is situated beyond the measured range.

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