11042
J. Phys. Chem. 1992,96, 11042-1 1047
(9) Mantsch, H. H.; Wong, R. T. T. Vibr. Spectrosc. 1990, 1, 151. (10) Kozulin, A. T. Opt. Spektrosk. 1972, 32, 709. (1 1) Kozulin, A. T. Opt. Spektrosk. 1973, 35, 163. (12) Menshutkin, V. N. Double systems of antimony trichloride and antimony tribromide with benzene and its substituted derivatives; St. Peterburg, 1912. (13) Lipka, A. Acta Crystallogr. 1979, 835, 3020. (14) Lipka, A,; Mootz, D. Z . Anorg. Allg. Chem. 1978, 440, 217. (15) Ishihara, H. J . Hiroshima Univ. 1981, A45 (2), 319. (16) Grechishkin, V. S. Nuclear Quardupole Interaction in Solids (in Russian); Nauka: Moscow, 1973. (17) Perkampus, H. H.; Schonberger, H. Chem. Phys. Lett. 1976,44348.
(18) Kondyurin, A. V.; Kozulin, A. T.; Mikov, S. N. J . Raman Spectrosc.
1991, 22, 249.
(19) Baumgarten, E.; Weigelt, G.; Perkampus, H. H. Spectrochim. Acta 1975, 31A, 1159. (20) Lipka, A.; Mootz,
D.Z . Anorg. Allg. Chem. 1978, 440, 224. (21) Mootz, D.; Handler, V. Z . Anorg. AIIg. Chem. 1986, 533, 23. (22) Hulme, R.; Szymanski, J. T. Acta Crystallogr. 1969,825, 753. (23) Kozulin, A. T.; Bogoslovski, N. B.; Karmanov, V. I. J. Appl. Spectrosc. (in Russian) 1975, 22 (6), 1052. (24) Pfann, W. G. Zone Melting, Wiley: New York, 1958. (25) Lunelli, 8.; Cazzoli, G.; Lattanzi, F. J. Mol. Spectrosc. 1983, 100, 174.
Stable and Metastable Solld Phases of Dlcyclopropylacetylene Vtasta MohaEek,* Krdimir FuriE, Ruder BoSkoviE Institute, POB 1016, 41001 Zagreb, Croatia
Marwan Dakkouri, and Martin Crosser Department of Chemistry, University of Ulm, Ulm, Germany (Received: May 21, 1992; In Final Form: August 10, 1992)
Different solid phases of dicyclopropylacetylene are studied for the first time using low-temperature Raman spectroscopy. Dicyclopropylacetylene(c8HIo)appears in three solid phases and one mesomorphic phase. Besides three independent welldefmed crystalline phases (II,12,11), on certain conditions a mixture of phases whose composition is not quite clear also exists. Between crystalline phases II and 12 a phase transition around 115 K is observed. The mesomorphic phase (I) which exists in the narrow temperature interval below the melting point (214 K) is assumed to be a glassy one.
1. Introduction Because of various inter- and intramolecular interactions, organic molecules display a wide repertory of possible structures, especially on transition from liquid to the crystalline state.' The nature of these mesomorphic phases (or mesophases) was the subject of many studies, particularly those concerning liquid and plastic crystals.*J Recently, attention is paid to glass transitions exhibited by different substances on cooling of either liquid, liquid crystal, or the crystal itself: Dicyclopropylacetylene(DCPA), CsHlo,represents such an example of an organic molecule with some of the mentioned properties. The number and nature of its phases below room temperature as far as the authors know have not been reported in the literature. No X-ray data and only the infrared spectrum of DCPA at the temperature of liquid nitrogen have been published so far.5 Vibrational spectroscopy and calorimetric methods are often used for the phase identification.68 Simpler experimental conditions and the possibility of measuring low-frequency bands make Raman spectroscopy preferable to an infrared technique in this type of investigation. We undertook therefore Raman measurements in the wide temperature interval-from room temperature down to -25 K. In the present paper all observed phase sequences together with the description of conditions in which phases were obtained will be presented. Frequencies of all observed Raman bands are listed in Table I. Observed and calculated vibrational frequencies of the DCPA molecule in liquid, which were the subject of our previous work: were a good basis for identification of bands in the solid. Our attention has been focused on two spectral regions: that below 160 cm-'which we shall call the low-frequency (LF) region and the interval between 350 and 550 cm-' which will be called the middle-frequency (MF) region. The main difficulty during assignment stemmed from the large bandwidth and asymmetry
of overlapping bands in the MF interval, where on the basis of the normal-coordinate calculation the appearance of skeletal modes was expected. The assignment in Table I is based on calculated values for the cis conformation and includes the Raman bands of liquid at 518, 460, and 390 cm-*. The apparent asymmetry of 518- and 390-cm-I bands could be caused by two additional which would indicate low-intensity bands at -480 and -420 cm-', that the number of conformers in liquid is at least two (see Figure 3 in ref 9). Excitations in M F and LF regions thus presented themselves as the most sensitive to changes in conformation and lattice structure, respectively. 2. Experimental Section DCPA was prepared according to the procedure of KBbrich et a1.I0 by the reaction of 1-chloro-2,2-dichloropropylethylene (CDCE) with n-butyllithium. CDCE was obtained from the chlorination of dicyclopropyl ketone. DCPA was purified by vacuum distillation, using a spinning band column,and the purity was checked by mass spectrometry and infrared spectroscopy. Low-temperature Raman spectra were recorded by a DILOR 2-24 Raman spectrometer with a triple monochromator using a CTI CRYOGENICS (HELIX, CRYODINE) Model 21 cryostat with closed cycle helium refrigerator (Lake Shore Cryotronics Model DRC-7OC controller). Attached to the cold finger was a specially constructed mount for capillary tube containing sample sealed under vacuum (see Figure 1). To provide better thermal contact, indium foil was used. On comparing the temperature of melting given by calorimetric measurements and that read on our cryostat display we observed a discrepancy. The difference between two temperature scales (Raman and calorimetric) is approximately 15 deg and originates from local heating in the focus of the laser beam. Namely, the temperature sensor within the cryostat head does not measure the true, higher temperature of the sample since it is attached to the
0022-3654/92/2096-11042$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 26,1992 11043
Solid Phases of Dicyclopropylacetylene vacuum ja k
"/
rnlhiiclgsr
Figure 1. Cryostat head with the mount for capillary tube.
n
11
WAVENUMBER(cm') Figure 3. Raman spectra of DCPA: (a) liquid at 297 K, (b) phase I at 210 K (7-250 cm-I).
d
c
WAVENUMBER (cm-') WAVENUMBER (cm-ll Figure 2. Raman spectra of different phases of DCPA in the middlefrequency region (350450 cm-'): (a) liquid at 297 K, (b) phase I at 210 K, (c) crystalline phase I1 at 25 K, (d) crystalline phase I, at 195 K, (e) crystalline phase I2 at 25 K, and (f) mixture of phases E at 180 K.
back side of the metal block. Therefore, all temperatures obtained from Raman experiments were adjusted on calorimetric ones and as such reported here. The highest cooling speed of the cryostat (-200 K/h) will be referred to as a "fast" cooling, while the speed of -10 K/h will be called "slow" cooling. The 514.5-nm line of a COHERENT INNOVA 100 argon laser served as excitation source. Most of the spectra were recorded in sequential mode with the step from 0.2 to 1 cm-'and wavenumber accuracy better than 1 cm-l. Laser power on the sample ranged from 100 to 200 mW, and the slit width was 2 cm-'. All spectra were almost perfectly reproducible (up to 10% of band intensity). 3. Results At room temperature dicyclopropylacetylene(DCPA) is a clear colorless liquid with perceptible smell. It remains transparent at all temperatures above the melting point (214 K), whereas below that point it becomes gelatin-like and grayish, apparently inhomogeneous. These properties remain unchanged only over the range of 16 K. This phase was denoted I. Corresponding changes in the Raman spectrum are most pronounced in the MF region, where the wide asymmetric band at 390 cm-I in liquid gains the intensity on the high-frequency side in phase I (as seen in Figure 2b). This could correspond to the increase in the intensity of the 42O-cm-l band in liquid. Figure 3 shows Raman spectra of liquid and phase I below 250 cm-I. Besides the shift of the 225-cm-l band toward higher wavenumbers, the shoulder at -200 cm-l is more pronounced. Furthermore, this phase decays over the course of time during an hour or so. Slight changes observed in Raman spectra in our opinion indicate I as a glassy phase, but
h
--
250
200
I
150
100
50
WAVENUMBER (cm")
F i i 4. Temperature dependence of low-frequency Raman spectra in phase I1 (7-250 c d ) . Laser plasma lines are denoted with asterisks.
the possibility of a liquid crystalline phase is not excluded. By slow cooling of DCPA in phase I, tiny white fibers emerge and multiply until the whole sample in the capillary turns white. Spectroscopically two new solid phases were detected. The first one is created through spontaneous decay of phase I at all temperatures in the 198-214 K interval. Since no phase transition from this new solid phase into another solid phase has been observed, it was denoted I1 according to ref 11. Phase I1 exists at all temperatures below 198 K. On heating above that temperature it does not transform back into phase I but melts. In Figure 4 the temperature dependence of low-frequency bands of phase I1 is shown. There are 24 bands in the LF region, and several group of four bands can be distinguished. The other phase transition at 198 K occurs by slow cooling of phase I. The same phase can be obtained by rapid cooling of DCPA from room temperature to 195 K. This second solid phase is denoted 11,and the corresponding spectrum is shown in Figure 5 . The difference between low-frequency spectra of phases I1 and I1 is obvious when the Figures 4 and 5 are compared. Only seven bands appear in the LF part of the spectrum of phase II. Upon further cooling,either slowly or rapidly, the sample gradually turns into a new phase I2 (seethe bottom of Figure 5 ) and remains unchanged until 25 K. In the MF spectral region the disap p r a n c e of the 371-cm-l band is clearly visible (see Figure 2d,e)
MohaZek et al.
11044 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 TABLE I: okcned RM.a Fmwenciw ( c d ) of Dicycbpmpyhcetykae h Different Phuer" ObSCNCd
E" (25 K) 3094 w 3084 m 3075 m, sh 3072 m 3064 vw, sh 3041 w, sh 3035 w, sh 3031 m 3026 m,sh 3012 m,sh 3006 s, sh 3003 s 2256 s, sh 2251 vs 2222 m 2210 m 2196 vw 2159 vw 1470 vw 1460 w, br 1455 w 1451 w, sh 1445 m 1438 vw, sh 1433 vw, sh 1430 w 1427 vw 1424 w, sh 1422 m 1415 vw 1384 vw 1381 vw 1376 vw, br 1351 w, sh 1346 m 1341 w, sh 1338 w, sh 1336 w, sh 1333 vw, sh 1325 vw, sh 1233 vw 1223 vw, sh 1219 vw 1215 vw, sh 1189 8, sh 1188 s 1182 vw 1177 w, sh 1169 w 1158 vw, sh 1099 w 1091 vw, br 1058 vw 1050 vw 1041 vw, sh 1036 w, sh 1032 m 1023 vw
I1 (25 K) 3105 vw 3095 m, br 3081 m 3078 m, sh 3075 m
I1 (25 K)
1, (190 K)
3094 w 3088 w 3084 m
3094 w, sh 3087 m
3076 m 3072 m
3075 m
liquid (297 K)
calculated cis-DCPA (ref 9)
3093 w
3098, CH2 stretching
3082 w
3090, CHI stretching 3066, CH stretching
3041 w 3033 m,sh
3030 m 3024 m 3018 m, sh 3012 s 3007 m, sh 3001 s 2249 vs 2242 s 2220 w 2211 w 2207 vw, sh 2202 w 2161 vw 1460 w 1457 w 1452 w 1447 vw, br
3029 m, sh 3025 m 3019 m, sh 3013 m 3007 m, sh 3003 s 2255 s, sh 2251 s
3012 m,sh 3005 s
3016 vs
3035, CH2 stretching
2249 vs
2248 vs
2277, triple bond stretching
2221 m,br 2211 m
2220 m 2209 m
2216 w 2206 w
2197 vw 2163 vw 1472 vw 1462 w
2195 w, sh 2160 w 1470 vw
2161 vw
3024 m
1460 w
1430 w
1381 vw, sh 1377 vw, sh 1375 vw
1425 m
1383 vw, sh 1381 vw
1380 vw
1468, CHI bending
1427 w
1434, CH2/CH bending 1419, CHI bending
1378 vw
1419, CH2 bending
1343 s
1347, CH/CH2 bending
1215 w
1203, ring breathing
1186 vs
1191, ring breathing
1174 w, sh
1192, CH2 twisting
1093 vw
1093, skeletal deformation/CH2 rocking 1087, CH2 rocking/CH twisting
1346 s, br
1346 s 1336 w, sh 1331 w, sh
1336 w, sh, br 1333 w, sh
1225 vw 1219 vw, sh 1213 w 1191 vw 1185 s 1183 s
1223 vw, sh 1219 vw
1218 vw
1047 w 1040 vw, sh 1037 vw 1033 w 1029 w 1024 vw, sh
1436 w, sh 1429 w
1428 w 1425 w 1423 w
1346 s 1343 m 1339 w, sh 1335 w, sh 1333 vw, sh
1175 vw, sh 1170 vw 1159 vw 1104 vw 1100 vw 1096 vw, sh 1093 vw 1089 vw, sh 1085 vw 1063 vw 1058 vw
1454, CHI bending
1448 m, br 1446 m 1438 vw 1432 w
1426 w 1423 vw, sh
1454 m 1453 vw, sh
1190 w, sh 1187 s 1182 vs 1180 m, sh 1177 w, sh 1170 vw
1187 s 1183 vs 1171 w
1098 vw
1095 vw, br
1091 vw 1079 vw
1087 vw, sh
1059 vw 1056 vw 1051 vw
1052 vw
1052 vw, sh
1036 m, br 1032 w 1029 w
1033 w 1029 w
1029 m
1057, CH2 wagging
1029, CH2 wagging 1022, CHI wagging
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 11045
Solid Phases of Dicyclopropylacetylene TABLE I (Continued) E” (25 K) 1005 vw 1002 vw 999 vw, sh 961 vw 921 w, sh 911 m 906 m 902 m, sh 896 m 891 w, sh 879 w, sh 873 m, sh 868 s 862 w, sh 834 w, br 830 w, sh 825 w 820 m 814 w, sh
I1 (25 K) 1020 w 1017 w 1004 w 1001 vw, sh 914 vw 911 vw, sh 907 m 903 s 897 m 878 m 876 m 873 s 867 vw, sh 834 w 831 w,sh 829 w 822 m 819 s 816 m, sh 814 vw 808 m
ObSeNed 12 (25 K)
I, (190 K)
1006 vw
1004 vw 1001 vw
998 vw 962 vw 918 vw, sh 910 m 905 s 893 m, br
874 m 868 s 861 vw, sh 835 w 831 vw, sh 824 w 820 m, sh 818 m
792 vw 789 vw
468 vw, sh 462 w, sh 459 m 457 w, sh 446 w, br
915, ring def/CH2 bending 907 s, sh 904 s 895 m, sh
903 m
9 12, ring deformation
875 w, sh
872 m
909 ring deformation
830 w
840, ring deformation/CH rocking
814 w
802, CH2 rocking/CH twisting
793 vw, sh
802, CHI rocking/CH twisting
779 vw, sh 750 vw
770, CHI twisting 745, CHI twisting
868 s 832 vw 823 w 817 m
797 vw 788 vw, br
789 vw
746 vw 662 vw 651 w, sh 648 w
746 vw
642 vw
642 w, sh 527 vw
642 w
626, skeletal deformation 528, linear bending
517 s 511 vw
517 s
518 w
506, linear bending
782 vw 775 vw 650 w
520 w, sh 516 s 511 vw 503 vw 493 w 480 w
934 CH2 rocking/CH bending
812 w, sh
798 vw 794 vw
648 w 646 vw, sh 640 vw, sh 528 vw
95 1, CH2 rocking/CH bending
800 vw
800 vw, br
777 vw 745 vw 660 vw, br
calculated cis-DCPA (ref 9)
liquid (297 K)
645 w 642 w 531 w 524 m 522 w, sh 517 m
647 w, br
504 w 497 w
469 vw 463 vw
481 w 472 vw 469 vw 462 w
?
476 w 464 w 460 vw
456 vw
457 vw, sh 448 w
437, skeletal deformation
446 vw 434 vw
430 w
431 w, br
?
417 w 413 w, br 401 w 394 vw 388 w
390 w
380, ring flapping
222 m
229, ring flapping
199 m
221, skeletal deformation
371 vw 229 w, sh 223 m, sh 220 s
205 w, sh 201 m, sh 195 s 192 m, sh
235 w 227 m, sh 226 m 223 m 220 w, sh 208 m 199 m 194 vw, sh
229 vw, sh 224 s 223 s 220 m
201 m 196 m 193 m, sh
196 e, br
‘Abbreviations: vs = very strong, s = strong, m = medium, w = weak, vw = very weak, and br = broad.
+ C-C stretch
MohaEek et al.
11046 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
E
2 4 I-
$ 0 Lu
I
7 fA W
K
a I
I
50
100
1
150
I
I
ZOO
250
TEMPERATURE (IO Figure 7. Presumed dependence of heat capacity C, on temperature.
WAVENUMBER (cm-7 Figure 5. Temperature dependence of low-frequency Raman spectra in phases I, at 195 K and I2 at 25 K (7-250 cm-I). Laser plasma lines are denoted with asterisks.
220
Figure 8. Raman spectra of the mixture E below 550 cm-l at 180 and at 25 K. Laser plasma lines are denoted with asterisks. Asterisks in brackets denote real Raman bands which coincide with plasma lines.
7 180
E V
I
IY
1LO
5 z
LU
2
100
3 60
20
’ *f 70 110 150 190 TEMPERATURE ( K ) Figure 6. Observed frequencies versus temperature for phases II and I*. On the right are indicated liquid bands at room temperature. (For the 55- and 100-cm-I bands see footnote b in Table 11.) 30
-
-
and strongly supports the existence of the I1 I2phase transition. (Observed frequencies versus temperature for the transition I1 I2 are shown in Figure 6.) Because of the increase of bandwidth with temperature, it was not possible to determine the phase transition temperature only on the basis of Figure 6. After careful measurements this temperature was determined to be -1 15 K, and it was concluded that the transition was reversible. In order to provide a better insight in the changes occurring in the DCPA by changing the temperature, the presumed behavior of heat capacity C, is qualitatively presented in Figure 7. The arrows indicate the direction of temperature change (heating, cooling). As seen in Figure 7, both I2 and I1 exist at the lowest
temperature. The stability of each phase was checked by leaving the sample at 25 K for 9 h. No changes in the LF region were observed for either of two phases, therefore leaving the question of the stable phase open. The unstable mixture of phases, denoted E, has proved to be particularly Micult to investigate. Its Raman spectra of the MF region and below at 180 and 25 K are shown in Figure 8. DCPA always solidifies in E when rapidly cooled from room temperature to 180 K or any lower temperature. In fact, it would be more correct to speak about E’ and E” because above and below n l 1 5 K the mixture E is not the same, as can be secn by inspection of Figure 8. When the sample is rapidly cooled from room temperature to any temperature between 180 and =115 K,one observes the phase we call E’, whereas when the sample is rapidly cooled from room temperature to any temperature below =115 K,the result is the phase E”. The changes E’ E” arc reversible. Above 115 K E’ decays in less than 2 h into phase 11. Analyzing its band positions above 115 K (Figure 8a), particularly in the MF interval where the band at 371 an-’appears, it is found that each band of E’ belongs either to phase II or to phase 11. The correspondence of intensities is also very good. Therefore, it was concluded that above 115 K the phase E’ is a mixture of phases I I and 11. Below 115 K the situation is more complex. In the MF region there is no 37l a - l band, but there arc two new bands at 394 and 413 cm-l which are not found in any other phase (12 or 11) at 25 K. (Sce Table I for comparison of observed band positions.) To see whether the mixture E” will decay in the cours~ of time, it was left at 25 K for 5 h, when the transformation into the phase Iz was observed. Differential scanning calorimetric measurements performed at Sektion Kalorimetrie, Universitgt Ulm, show crucial influence of cooling rate on the results obtained: the number of anomalies observed depends on the cooling rate used. Also, the specific heat
-
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 11047
Solid Phases of Dicyclopropylacetylene
~~
E" (25 K) 143 vw 121 vw 111 vw 94 w,sh 84m 74w,sh 63w 56m 47 m 44 m, sh 37 w 27 m 18 w, br
(25 K) 151 vw
(25 K)
liquid I, (190 K) (297 K)
144 vw, sh 134 vw 129 vw 121 vw 111 w 104 vw 94vw 88 vw, sh 83m 76 m 70 m 61m 57m 54 w, sh 50 w, sh 46 w 42 w 38 w 36 w, sh 32 vw 28 vw 23 m 19 w
144 vw 134vw
134vw
11
I2
calculated cis-DCPA (ref 9)
121 vw 112 vw 108 vw 97 vw
100 wb
109, linear bending
85 m 73 w,sh 7 4 s 71 w 63 w 58 w 54 s
55 wb
89 linear bending
52 vw
48 w, sh 45 m 40 s 38 w 28 vw 25vw 18 m 14w
23 s 15m,sh
'Abbreviations: vs = very strong, s = strong, m = medium, w = weak, vw = very weak, and br = broad. bThese bands were observed after correcting the spectrum with R(u) function.
of melting of the liquid-solid phase transition which is observed at 214 K is slightly cooling rate dependent. For identification of other phase transitions further measurements are needed. 4. Discussion The problem of phase identification manifests its complexity in the case of an organic molecule such as dicyclopropylacetylene. Judging on the basis of bands' positions in liquid, espccially in the MF region where skeletal deformations appear, one could determine the number of conformers present in liquid. There is no doubt about the existence of 5 18-, 460-, and 39a-Cm-1 Raman bands (see Figure 2a). The asymmetry of the bands at 5 18 and 390 cm-'could indicate the existence of two weak bands at -480 and 420 cm-I. The questionable existence of the 420-cm-l band becomes more apparent in phase I (see Figure 2b). If the 480and 420-cm-l bands do exist in liquid, the number of conformers in liquid could be two or more, as shown by the results of normalaordinatecalculations? On the other hand, if the bandwidth of 518- and 390-cm-I bands results from almost unhindered internal rotation of one cyclopropyl group with respect to the other, the increase in the intensity of the 39O-cm-I band could be caused by increased number of molecules with certain value of dihedral angle between two cyclopropyl groups. In both cases we think it is likely that the freezing of different conformers is responsible for the observed spectral changes in phase 1 (Figure 2b) and that phase I is a glassy state. In that case the measured C,( graph could exhibit a hysteresis like that of SnCI2.2H20 and other hydrates4 or show the continuous inflection effect like the one in 2-methylthi0phene.~In the case of DCPA this sort of hysteresis is indeed observed, because on heating phase I1 as well as phase I1 melts into liquid without passing through phase I. To conclude this discussion of phase I, we mention for comparison the case of dimethylacetylenewhose methyl groups rotate almost freely
in liquid and which has two crystalline phases which differ in the extent to which the methyl groups are mobile.I2 No mesomorphic phase has been reported for this molecule.2 On the other hand, bicyclic compounds are known to form plastic crystals (for example, bicycloheptane, -octane, and -nomne, see ref 3) and glassy phases, too (bicyclopropyl, ref 13). In Table I positions of all observed Raman bands in all crystalline phases and liquid are listed and compared to values calculated for C~S-DCPA.~ Very strong triplebond stretching which occurs in liquid at 2248 cm-l and at 2249 cm-' in phase II splits into two strong bands at 2255 and 2251 cm-l in phase 12. Reversible phase transition I1 I2 (Figure 5 ) transforms seven bands of phase I1 into 18 bands of phase I2 in the LF region. Bearing in mind the presence of several internal modes which exist in the same interval, doubling of the unit cell seems a plausible explanation for the spectral changes mentioned. The largest number of bands in the LF part of the spectrum (24) was observed for phase I1 (see Table 11). Four groups of four bands exist below 160 cm-':the first around 25 cm-I (weak intensity), the second group around 45 cm-I (weak intensity), the third one around 70 cm-I (medium intensity), and the fourth group around 140 cm-'(weak intensity). Comparing the bands at 199 and 222 cm-'in liquid and in phase I (Figure 3) with the observed bands in phase I1 (Figure 4), we observed the splitting of each band into four. There are thus firm reasons to believe there are four molecules in the unit cell of the phase 11. Observed frequencies of the mixture E" are also listed in Tables I and I1 for comparison with band positions of other phases. As already mentioned, bands at 41 3 and 394 cm-l do not have their counterparts in any other isolated phase. The number of bands in the LF region is 13 and is smaller than the number of bands of phase I2 (18) into which E" decays at 25 K. The origin of the mixture E" is still unclear and awaits further research. In conclusion, we briefly mention that DCPA appears in three crystalline phases and one mesomorphic phase which exists below the melting point. A reversible phase transition between crystalline phases II and I2 is observed at =115 K by Raman measurements and is accompanied by the doubling of the unit cell. The number of molecules per unit cell could be identified for phase I1 where it equals four.
-
Acknowledgment. We are thankhl to Dr.Z. MeiC for inspiring this study. This work was supported by Ministry of Science, Republic of Croatia, Grant No. 1-03-066. M. Dakkouri gratefully acknowledges financial support from Fond der Chemischen Industrie. Registry No. Dicyclopropylacetylene, 27998-49-8.
References md Notes (1) de Gennes, P. G. The Physics of Uquid Crysrals; Oxford University Press: London, 1974; Chapter 1. (2) Dunning, W. J. The Plarrically Crysralline Srare; John Wiley: ChiChester, 1979; Chapter 1. (3) Westrum Jr., E. F. Thermochemistry and Thermodynamics; University Park Prcss: Baltimore, MD, 1979; Chapter 9. (4) Suga, H.; Seki, S.Faraday Discuss. Chem. Soc. 1980,69,221. ( 5 ) Schrumpf, G.; Alshuth, T. J . Mol. Srrucr. 1983, 101, 47. (6) Figuite, P.;Szwarc, H.; Oguni, M.;Suga, H. J. Chem. Thermodyn. 1985, 17, 949. (7) Colombo, L.; Kirin, D.; FuriE, F.; Sullivan, J. F.; Durig, J. R.Croar. Chem. Acra 1988,61 (2). 301. (8) Novak, A. Croat. Chem. Acra 1988, 61 (2), 213. (9) MohaEck, V.;Furit, K. J. Mol. Srrucr. 1992, 266, 321. (IO) Kbbrich, G.; Merkel, D.; Thiem, K. W. Chem. Ber. 1980,354 1402. (11) Andre, D.; Dworkin, A.; Figuisre, P.; Fuchs, A. H.; Szwarc, H. J. Phys. Chem. Solids 1985,16 (4), 505. (12) Butler, I. S.: Newbury, M.L. Specrrochim. Acra 1980, 36A, 453. (13) Spiekermann, M. Dissertation, Universitat Dortmund, 1980.
