The conformation and vibrational spectra of trans-1,4-dicyanocyclohexane

Journal Elsevier

of Molecular Structure, 95 (1982) 117-129 Scientific Publishing Company, Amsterdam

-Printed

THE CONFORMATION AND VIBRATIONAL DICYANOCYCLOHEXANE

0. H. ELLESTAD*, Department

P. KLAEBOE

of Chemistry,

(Received

19 February

in The Netherlands

SPECTRA

OF TRA NS-1,4-

and T. WOLDBAEK

University

of Oslo, Oslo 3 (Norway)

1982)

ABSTRACT The IR spectra of trans-1,4-dicyanocyclohexane as a melt, as a solute in various solvents, as KI and polyethylene pellets and as amorphous and annealed crystalline solids at 90 K have been recorded in the region 4000-50 cm-‘. Additional spectra at high pressures (l-50 kbar) have been recorded and the dichroic ratios of oriented polycrystalline films are obtained above 200 cm-‘. Raman spectra of the compound as a melt, as an amorphous and crystalline solid at 90 K and dissolved in acetone, chloroform and benzene have also been recorded. The compound exists as an equilibrium mixture of ee and aa conformers in solution, in the melt and in the amorphous solid at 90 K, but as one conformer only, apparently the ee form, in the crystalline state. Unlike the corresponding dihalocyclohexanes, trans-1,4dicyanocyclohexane cannot be converted to an “au crystal” either by exposure to high pressure or by annealing to a metastable crystal. The fundamental frequencies of both conformers have been interpreted in terms of CPn molecular symmetry and the assignments supported with a force constant calculation by the overlay technique transferring force constants from various related molecules.

INTRODUCTION

The conformations and vibrational spectra of six truns-1,4dihalocyclohexanes [l-5] having chloro, bromo and iodo substituents were recently reported. Three of these (dichloro, dibromo and diiodo) had Czh symmetry while three (bromochloro, chloroiodo and bromoiodo) had C, symmetry. These compounds had various features in common: (1) equilibria between ee and aa conformers were present in the vapour, in solution and in the melt, (2) the equilibria were shifted towards the ee form in polar solvents [ 61, (3) the molecules were all in the ee conformation in the stable crystalline state, (4) progressively more au conformer was present in the crystals before melting, (5) metastable low-temperature crystals could be formed with molecules in the aa conformer, (6) compressing the solids at room temperature to ca. 50 kbar gave crystals with au conformer molecules and (7) when compressed to ca. 10 kbar the conformational equilibria [ 71 of truns-1,4dichloroand dibromo-cyclohexane dissolved in CS, were shifted towards the au form. *Present

address:

Statoil,

0022-2860/82/0000-0000/$02.75

N-4001

Stavanger, 0 1982

Norway. Elsevier

Scientific

Publishing

Company

118

Due to the symmetry of these molecules (CZhor C,) and their unique conformational features in the solid state, the IR and Raman spectra were assigned with considerable certainty. A substantial amount of work was therefore spent on the development of reliable force fields [ 51 for these compounds. Similar force fields were also adopted for fluorocyclohexane [8] , chloro-, bromo- and iodo-cyclohexane [9] . It was decided to extend these studies to include trans-1,4dicyanocyclohexane (DCNC), which is believed to be very similar to the dihalocyclohexanes, and the results are presented in this paper. EXPERIMENTAL

DCNC was a commercial product from K & K Laboratories which was recrystallized from benzene and further purified by sublimation in vacua. The IR spectrometers for mid-IR and far-IR spectroscopy, the Fouriertransform spectrometer, the Raman spectrometer, the cryostats for IR and Raman and the high-pressure diamond anvil cell have been described in recent papers [2, 3,8] . RESULTS

AND

DISCUSSION

The IR spectrum of DCNC as a melt at ca. 400 K is shown in Fig. 1. An amorphous solid spectrum at 90 K, obtained by shock-freezing the vapour on a CsI window is presented in Fig. 2, while the spectrum of the same sample after annealing to room temperature and retooling to 90 K is shown in Fig. 3. The dichroism of a partly oriented polycrystalline film is illustrated in Fig. 4. Far-IR spectra of DCNC as a saturated solution in benzene and as a polyethylene pellet are shown in Figs, 5 and 6, respectively. Finally, the Raman spectrum of DCNC as a solid at room temperature is shown in Fig. 7. It is immediately apparent from Tables 1 and 2 that there is mutual exclusion between the IR and Raman bands in the various states of aggregation, although a certain number of coincidences do occur. This is in agreement with the structure of DCNC since both the ee and au conformers should have Czh symmetry.

Fig. 1. Infrared

spectrum

of trans-1,4-dicyanocyclohexane

(DCNC)

as a melt at ca. 410 K.

119

Fig. 2. Infrared

spectrum

of DCNC as an amorphous

Fig. 3. Infrared

spectrum

of DCNC as a crystalline

solid at 90 K.

solid at 90 K, annealed

at 270

K.

Frequency (Cm-') Fig. 4. Infrared

dichroism

of a partly

oriented

polycrystalline

sample of DCNC.

Conforma tional equilibria

It can be seen from the IR curves that the melt (Fig. 1) and the amorphous solid (Fig. 2) have a number of bands which vanish in the crystalline solid at ambient (Fig. 4) or at low (Fig. 3) temperature. A correspondingly large number of bands were observed in solution (C6H6, CHCl,, CDCl,, CH,CN) both in IR and in Raman. The obvious conclusion is that DCNC exists as a mixture of ee and aa conformers in the melt, in solution and in the amorphous state at 90 K, but that only one conformer is present in the crystal.

Fig. 5. Far-infrared

spectrum

of DCNC dissolved

Fig. 6. Far-infrared

spectrum

of DCNC

in benzene.

in a polyethylene

pellet.

A cm-l

Fig. 7. Raman

spectrum

of DCNC as a crystalline

solid at ambient

temperature.

For the six trans-1,4dihalocyclohexanes it was clear from X-ray crystallography [lo] and from empirical C-halogen stretching correlations [ 11,121 (cf. Table 7 of ref. 3) that the ee conformer is present in the “ordinary” crystal. For DCNC no X-ray studies are reported and the corresponding spectral correlations are not known. However, a number of arguments strongly support the conclusion that DCNC, like the corresponding trans-1,4dihalocyclohexanes, crystallizes in the ee conformer. The force constant calculations agree much better with the assumption of the ee rather than the au conformer in the crystal. Thus, as one example, the polarized Raman band at 522 cm-’ also present in the crystal, should belong to the ee form since the calculated value was 516 cm-’ (Table 3). The closest au bands of species ug are calculated at 631 and 391 cm-‘. The bands which are present in the crystal are enhanced and those that vanish in the crystal diminish in intensity when DCNC is dissolved in polar solvents. The reverse is true in non-polar solvents. Similar features were observed [l-5,11] for tram-1,4dihalocyclohexanes and explained as a

121

stabilization of the ee conformer in polar solvents due to larger quadrupole moments [6]. Two bands (an IR band at 667 cm-’ and a Raman band at 616 cm-‘) in the region 600-700 cm-’ assigned to the au conformer agree with the “spectral indicator” suggested for a number of alkyl cyclohexanes as recently discussed by Zhizhin and Sterin [ 131 . Since the mono [8,9,13] and trans-1,4disubstituted [l-5,11] cyclohexanes with halogen and alkyl substituents generally crystallize in the e (ee) conformer, it was expected that DCNC follows the same pattern. However, it is surprising that monocyanocyclohexane is an exception to this rule and is shown to crystallize in the a conformer in the high-pressure [ 141 as well as in the low-temperature [ 151 crystal. It has been argued [6] that the conformational equilibrium of the trans1,4dihalocyclohexanes in the vapour and in non-polar solvents is shifted more towards the au form than that in the corresponding monohalocyclohexanes. In DCNC, however, the equilibrium is nearly the same as in cyanocyclohexane. Many of the strongest bands (1331,1202,1008,828 and 278 cm-’ in IR, 765 and 620 cm-’ in Raman) observed in the melt and in solution are assigned as au bands, Since the au bands are nearly as intense as neighbouring ee bands, the conformational equilibrium is probably close to 60% ee and 40% au in solution and in the melt. The highest relative intensity of the au bands was observed in the amorphous solid (obtained by shockcooling the vapour) for which the vapour equilibrium is maintained. Here the au conformer seems slightly more abundant than the ee. For the truns-1,4dihalocyclohexanes too, the conformational equilibrium is more shifted towards the au form in the vapour [6] than in non-polar solvents [l-6,11] . Low-temperature

crystal

Very careful annealing experiments were carried out independently in IR and in Raman cryostats of the amorphous solid formed by shock-freezing the vapour to 90 K (Fig. 2). Slow heating to ca. 130 K did not cause any significant spectral changes. At ca. 140 K the ee bands slowly increased and the au bands decreased in intensity, presumably because the thermodynamic conformational equilibrium for the amorphous solid was established. Whereas the truns-1,4dihalocyclohexanes started to form apparently metastable crystals containing the uu conformer in the 165-195 K range [l-4] DCNC gradually formed the stable ee crystal. After approximately 1 h at 170 K the au bands had practically vanished and factor group splitting revealed a crystalline sample (Fig. 3). No further significant changes were detected after annealing to ambient temperature and subsequent cooling to 90 K. Apart from sharper bands and more pronounced factor group splitting, the annealed crystalline sample retooled to 90 K exhibited IR and Raman spectra identical to those of the “ordinary” crystals recorded at room temperature.

122 TABLE

1

Infrared

spectral

dataa for tram-1,4-dicyanocyclohexane

Solution CDCl,

Melt

2950

-2940sbd

SC

2932s

Amorphous 90K

2925w

2895m

2895m

2862s

2863s

2240sd

sh

2870s

2243s 2227msh

1466wsh 1460ssh

1463msh

1438msh -142Owsh

1451s 1445msh 1440msh 1425wsh

1454s 1445msh 1437wsh 143owsh

-1365~ 1354m 1349m

1355m 1350m

137ow 1357msh 1352m

1331s 1307w 1297m

1331s 1305w 1298m

1450sbd

1281~ 1254~ 1233m 1217x7 1202s 1156~ 1119m

1050s -1030vw 1008s 979s

942

1334m 1299m

1282~

1284~

1270~~

1272w 1261~ 1237~ 1223~ 1202s 1192vwsh 1169w 1122m 1106~ 1093w 1055m 1035w 1013s 981m

1233~ 1201s

1120m

1051s 1034"W 1011s 980s

w

ee

300Kb

aa

-2940vsbd

sh

225Ows.h 2235s

Interpretation

crystanine 90K 2960ssh 2956s

2957s

2935s

(DCNC)

2918m 2911m 2884s 2865s 2848~~ 1 2292w 2271~ 2250ws.h 2241s 2229m sh 2222wsh 2215~ 2202w 1466wsh 1463s 1457msh 1

2919s

2870s 111 2856~11 2263~ shI1 2239m 2232s

sh 1

“43

II

bu

comb.

2206~ 1467ws.h 1459s 1456s 1 I1

comb.

"wbu 1443s * 1430wsh 1371m * * 1343wsh 1341m 1 * 1314m * 1286111sh 1278m * 1260~~ 1245m 1229w 1202vw * * * 1099w 1081~ 1057s 1043m 1015w *

1441s III * 1426~ 1398vwI~ 1368m 111 * * 1338m * 1309 m * 1281m 1276m *

"30%

111 111 comb. 111 sh 111

“13 +

comb. 1240m 11 1224~11 * * * * 1095 w 111 1077 w IfI 1054SIl 1037m 1015WIl *

“34%

comb.A, "aebu comb. ",s+"szBu

967w

966wIl

956vw 936vw 93ovw

955WIl

"35% "2s + "a,% "27 + ",I&

93OWshIl

"14 + "a& "wbu

929w

931wd

932w

918m

919md

919w

922SIl

903s

901sd

904m

;;;r sh Ill I 870wshIl

“,,&I

123 TABLE

1 (continued)

Melt

Solution CDCl,

862 m sh 856 s 828s 810 w

829 s

829s 814 w 791 VW 762~ 752~ 666 w 640 VW 612~ 599 VW 566 w

556s

557 w

515w 488m

522vw 518~ 488~

514 w 489 m 467 VW 452m 345w

452 s 345 .wd

285m sh

283 md

278 s

274 sd 236 vwd 127 sd 118 m shd 97 sd

453m 347 VW 287m sh 280s 275s

Interpretation _-~-~ ee aa

crystamne 90 K

863 m

758~ 698vw 667 VW

556 s

Amorphous 90 K

*

862s 858s sb 828~~ 815~

300 Kb 859

*

s 11

~~__

“so bu

828 VR 815~ 11

763 w 755w * * * *

11

709

w

559

s 11

* 612~

* *

560 s 557 m sh 518 s 493 s * c

* * 292 s 279 w sh 277~ 246 m

* *

“51

bu “37%

518m 111 494s 111 472vw

292 s 11

246 m 111

“38%

139s 123s 92w

“39%

“39%

“54 bu lattice mode

“54

bu

aWeak bands outside the fundamental regions (4000-3000, 2800-2300, 2200-1500 cm-‘) pellet below 200 cm-‘. cs, strong; are omitted. bCsI pellet above 200 cm-* and polyethylene m, medium; w, weak; sh, shoulder; bd, broad; 111and 11 dichroic measurements; asterisks denote bands which are absent. dBands observed in C,H, solution.

High-pressure

spectra

It was reported previously [ 71 that for mono and tram-l ,4dihalocyclohexanes in solution the conformational equilibria are shifted towards the a (aa) conformer with pressure, revealing a smaller molar volume for the a (au) conformer. Moreover, isocyanatocyclohexane [ 141 and trans-1,2-bromochlorocyclohexane [ 161 crystallized in the a (aa) conformer under pressure, but in the e (ee) conformer at low temperature. Finally, for the six tram-1,4dihalocyclohexanes the “ee crystals” were converted to “au crystals” under pressure, and the ee conformers reversibly formed when the pressure was released [l--4] . For DCNC, pressurizing the solid to ca. 50 kbar in a diamond anvil cell led to no conformational changes as the ee bands were still present. The IR lines were generally displaced 3--15 cm -’ towards higher wavenumbers as commonly observed for solids under pressure. However, a phase transition probably occurred with increased pressure since the factor group splitting

124 TABLE

2

Raman

spectral

data for tram-l Solution

Melt

,4-dicyanocyclohexane Crystalline

C,H,

2962 s Pa 2940

sP

2905 s P

2872s

2237

1462

P

s P

2961

sb

2938

s P

2930

s

2910 2898

s sh P s P

2885

m sh

2867 2851 2297 2264 2241 2217 2190

s P sc m P w s P w P w

1455

1445 1375

m w

1442 m P? 1371mP 1357 w sh

1335

w

1332

w

1306

m

1302

m

1280

w

1283 w shb 1274m

1263

m

1259m

1062 1036

w m

869 w

796 w 772sP

Interpretation ee

m

1140m 1108 m P

(DCNC)

w

1216~ 1178 m shb 1136mb 1107 m P 1082 w 1058 w 1031 m P 1028 s shb 962 wb 945 vwb 865 wb 833 w shb 814w sh 790 m shb 771sP

2959 vs 2956 vs 2949 m sh 2936 2933 2911 2896 2890 2884 2865 2857 * *

w w1 vs w sh m sh s m w sh

vI

aa

ag

comb.

“17 bg “,ag 2 x “,,Ag

b,

“2, ” 17agv

2 x “,,Ag “,ag “18 bg

“3,”

dg,

b,

2 x *mAg

comb.

2240 vs 2223m sh * 1473w 1466 w 1 1448 m 1372m *

“,ag “19 + “z4Ag

“,ag “5 + “I2 Ag

“19 bg

“19 bg

vsag “,ag

“,ag “,ag “20 bg

“20 bg

“,ag

1336 1331 1308 1300 *

m sh 1 s s m 1

1268 1257 1223 1187 1138s 1111 *

s m 1 VW VW s

“10 + “27% “36 + “rsBg “22 bg “,ag

1062 1033 *

s m

“7.3 bg vlo ag

“?, vll ag, bg “21 bg “a ag

1016 w 961 m 948 vw sh * * 813m 794s 775vs

“,ag comb. “21 bg “,ag “23 b g

vloag 2 x “5zAg vllag “12 + “,6Ag

comb. “24 bg vlzag

vi1 ag 2 x “,z+Ag “24 b,

125 TABLE

2 (continued) Solution

Melt

Crystalline

C,H,

580~ 522sP 490w

764sP 655 wb 626 w shb 616 s Pb 601 w shb 580 wb 522mP 490 w

436 w 415w 384~ 365 m 315 m P 198 m sh 183s 142mP

433 w 418 vwb 382~ 360m P 312mP 196 mb 183mb 143 mb

765ssh

620m

P

P

Interpretation ee

* * * *

* *

*

655 w

578 521 492 471 437

“16 +

V26Bg

w s w VW w

U26

b,

u14ag 376 m

“14Qg

317 m 189m 177 m vlsag 98m 78m 55w 51 w 48m

aFor abbreviations, see Table 1; P, polarized. observed in (CH,),CO solution.

bBands

lattice

observed

modes

in CHCl,

solution.

CBands

changed significantly, e.g. the doublet at 1057/1042 cm-’ disappeared and a single band appeared at 1055 cm- ‘. Since no significant spectral changes took place under pressure, neither the actual spectrum nor the high-pressure data are given here. High-temperature

crystal

The crystalline powder of DCNC was heated between KBr plates to various temperatures before melting (50,70,95” C) and the spectra recorded. The melt spectrum was obtained at ca. 120” C, the sample again crystallized and the spectra recorded once more at 95,70 and 50” C. The aa bands at 1355 (vjl), 1350 (v&, 1331 (vS2), 1297 (combination), 1201 (vJ7), 1156 (Vet), 1120 (vS4), 980 (vS5), 829 (uSO)and 452 cm-’ (vS2) became apparent at 70” C and were enhanced at 95” C (although very weak compared to the melt spectrum). After melting and retooling, the intensities of the aa bands at 95 and 70” C were similar to the values obtained before melting. DCNC therefore behaved like trans-1,4dichlorocyclohexane [ 1,171 and the other trans-1,4dihalocyclohexanes [ 1,2] ;all of these contained increasing

126

amounts of the au conformer at higher temperatures before melting. Slight amounts of the au conformers were in many cases observed even at ambient temperature (ca. 100” C below melting); these were reduced to negligible amounts when cooled to liquid nitrogen temperature. When the melt was cooled under a temperature gradient, a partly oriented crystalline film of DCNC was formed. Dichroism of several IR bands was observed with polarized light (Fig. 4). Neither the X-ray structure of DCNC nor the orientation of the crystal axes in the oriented sample is known. However, the bands can be divided into two classes denoted I,, and I, in Table 1. The former (I,,) can be associated with the ee bands of species a,, the latter (II) with those of species b,, as previously adopted for the symmetric truns1,4dihalocyclohexanes [l--3] . Thioureu cluthrute In thiourea clathrates chloro- and bromocyclohexanes [ 18-201 are accommodated almost entirely as pure a conformer molecules in the channels. Clathrates with fluoro- [8] and iodocyclohexane [21], on the other hand, contain a significant amount of e conformer in addition. It has been observed that cyanocyclohexane is also preferred in the a conformation in thiourea clathrates [ 211 . Therefore, a DCNC-thiourea clathrate was prepared by dissolving thiourea in methanol, DCNC in chloroform, and mixing the solutions. What apparently was the clathrate crystallized from the mixed solvent after a few days. The IR spectrum of this sample in Nujol mull was recorded using the same technique as reported previously [21]. Bands at 918905,860, 556, 515,490 and 287 cm-‘, all assigned to the ee conformer, were observed while no au bands were detected. This is contrary to the results for the other substituted cyclohexanes studied [21] and thus it is tentatively concluded that the sample is not a clathrate, but a mixture of thiourea and DCNC crystals. Force constant

calculations

Based on the pioneering work of Snyder and Schachtschneider [ 221 and others [ 231 on cyclohexane and halocyclohexanes [ 24, 251 an extensive force field was developed for the halogenated cyclohexanes. From the well established fundamental frequencies for the ee and au conformers of the six truns-1,4dihalocyclohexanes a 49-parameter valence force field was fitted to ca. 600 observed frequencies by an overlay program [ 51. This force field was later transferred to the monohalocyclohexanes [8,9] and has now been adapted to some pseudohalogenated cyclohexanes C6H1,X in which X is CN, NC [15] or CCH [26] . The e and a conformer fundamentals for these molecules were treated together with the fundamentals of DCNC. Most of the force constants pertaining to the cyclohexane ring (30) were transferred from the truns-1,4dihalocyclohexanes [ 51 , while 28 parameters for the substituents (CN, NC, CCH) were subject to iteration. The calculated ee and

127 TABLE 3 Observeda and calculated fundamental frequencies for trans-1,4dicyanocyclohexane

_ ee

aa

PEDb

Obs.

Calc.

PED

Obs.

Calc.

97d(X)C 96d

2952

2957

99d(X)

2919 2856 2240 1436 1379 1313 1247 1113 1020 964 752 516 387 330 187

98d 98d 891

2938 2898

2958 2920

686 + 17y 30y + 269 + 21R 54y t 278 66y t 18R 597 t 13R 49R + 24-y + 11X 53X+ 31R 39R t 187 + 14X 31~~t 17~ t 127(X) + 118 4Ow(X) t 23~ t 207 44~ t 167 t 15X 420!t 232 + 157

2910 2867 2241 1445 1371 1304 1260 1108 1034 962 771 522 382 315 182

726 t 187 37,9t 277 t 19y(X) 507 t 18e t 19R 65y t 22R 437 + 19x 55R + 30y 32X t 29R t 27y 37R t 28X 23~ t 18X t 17R 52~ t 21s t 159 64~~+ 18~ 27~ t 2201t 217 t 12~

2867 2241 1442 1371 1332 1274 1108 1028 865 764 616 415 360 143

2856 2240 1426 1369 1334 1269 1114 1030 856 771 631 391 358 126

99d 99d 736 t 42y t 757 t 55R + 577 t 72y t 54x t 67~ t 56a t

2930

2913 2852 1442 1349 1319 1143 1069 798 523 468 197

99d

2898

2857 1455 1335 1304 1136 1058 800 490 471 196

99d 74s t 20y 78y + 17y(X) 42y(X) + 36R + 117 84~ 37y + 30R t !&y(x) 73y t 14R 441 t 23w t 2301 44w t 220! 53czt 41z

2867 1455 1357 1283 1178 1082 814 580 437 195

2913 2852 1441 1350 1288 1184 1061 800 567 425 201

2918 2855 1431 1380 1272 1217 1088 953 904 472 236 129

99d 98d 596 + 627 t 38y t 567 + 53R t 76y + 71R 452 t 37cYt 38~ +

2925

2957

aI3 VI

V¶ VS V4 V5 V6 V? V8 V9 VlO VI1 VI2 VI3

V14 VI6 V16

bg V17 VI8 V19

V20 V21 V*2 V23 VZ4 “21

V26 V27

98d 891

187 317(X) t 21R 217(X) 257 367(X) 13R 38a 117 38x

au VQ8 “29 VW3

u31 v32 v33

V34 “35 V36

V¶l ‘38 u39

99d

2940

98d

2862 1445 1370 1305 1217 1095 967

65s t 447 t 60y t 597 + 83R 53R t 737 t 53& t 46~ t 509 +

177 24y(X) t 24R 307(X) 197(X) t 12R 187 t 11~ 16y(X) 31x 42w 42or

490

246 127

2862 1425 1355 1331 1156 1120 980 862 522 272 127

2917 2855 1433 1358 1313 1144 1137 968 863 520 272 106

98d(X) 98d

2940 2895

2958 2916

99d

2862

2853

18y + 11R 18R t 137(X) 33-y(X) t 16R 28y(X) 28y 12R 36a

297 t 2ow 24ort 18~ t 137

ha u40 V4l

y4z

98d(X) 97d 98d

2950 2932

2862

2916 2852

128 TABLE

3 (continued)

ee

all -

PEDb

Obs.

Calc.

PED

891 696 + 22y 45y + 238 427 + 348

+ 13R + 15y(X)

8Or 35X 35R 26R 22X 27y 5601 397

+ 18y + 19X + 16X + 18~ + 1801 + 13y(X) + 13~ + 149 + 13or

2240 1460 1400 1282 1233 1050 918 860 556 515 287 118

2240 1454 1399 1293 1229 1052 907 869 557 500 307 82

891 686 320 61y 69y 42~ 36X 30R 37x 3201 40a 377

+ + + + + + +

25~ 34y 24y 2Oz 22~ 147 17w

+ 23y + 28y + 16e + 12e + 39y + 28R + 28y + 18y + 23~ + 20~ + 18~

+ 17R

+ + f + + +

20y 11X 11e 22~ + 12y 167 + 16~ 1401

Obs.

Calc.

2240 1438 1365 1350 1201 1011 931 829 667 452 280 97

2240 1453 1367 1346 1225 1018

aWhen possible frequency values taken from the liquid spectra are given. bThe potential energy distribution Xik = lOOFiiLi2k /.z iFiiLh. Cd(X) and y(X), C-H stretching and CCH bending in the C-CHX (X=C!=N) groups; d and y, CH stretchings and CCH bendings in the C-CH, groups; R, C-C stretchings in the ring; X, C-C stretching in the C-C-N groups; s , H-C-H bendings; 0, CCH bending in the CHC=N groups; W, C-C-C bendings in the ring; Z, C-C-C bending in the C-CHC=N groups; r, torsions around the C-C bonds in the ring; 2, C=N stretchings; 01, C--C=N bendings.

aa fundamentals for DCNC are listed in Table 3 together with the main contributors to the PED (potential energy distribution). The actual force constants and methods of calculation are not presented here, but will be given in a forthcoming paper on ethynylcyclohexane (C6H11C=C-H) [26] . Spectral interpretation The assigned fundamentals belonging to the ee and au conformers are listed in Table 3. With C2,, symmetry 54 fundamentals (16 ug, 12 b,, 11 a,, 15 b,) are expected for each conformer. The following criteria were followed: (1) bands present in the crystal belong to ee or are common to ee and au, while those present in the melt, solution and amorphous solid, vanishing in the crystal are uu bands; (2) the Raman-active bands belong to ug (polarized) or b, (depolarized); (3) the IR-active bands belong to a, or b, and for the ee conformer they could be partly assigned from the dichroic ratios. Since no pure aa spectra in IR or Raman could be obtained for DCNC by annealing or high-pressure techniques and the compound was less soluble than the corresponding tram-l ,4dihalocyclohexanes, the direct spectroscopic evidence was less conclusive for DCNC. Therefore, the results of the force constant calculations were relied on in our spectral assignments. ACKNOWLEDGEMENT

We are grateful to Jorunn E. Gustavsen who recorded some of the spectra.

946 831 668 422 302 82

129

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