JOURNAL OF SOLID STATE CHEMISTRY ARTICLE NO.
136, 16—20 (1998)
Spectroscopic and Differential Scanning Calorimetric Studies of the Order–Disorder Phase Transition in Bicyclononanone Ralph M. Paroli, Denis F. R. Gilson,1 and Ian S. Butler Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada Received June 6, 1997; in revised form September 15, 1997; accepted September 22, 1997
temperature (9). If conformational interconversions occur in phase I, this could provide an additional disordering mechanism and thus contribute to the large entropy change associated with the transition. No information on the crystal structures of the two phases of bicyclononanone is available.
The phase transition in bicyclononanone, C9 H14O, has been examined using differential scanning calorimetry, variabletemperature vibrational spectroscopy, and proton spin–lattice relaxation time measurements. The high-temperature phase is disordered and the low-temperature phase either has an orthorhombic crystal with D2 or C2 symmetry or is tetragonal with D2d symmetry. The barrier to rotation in the low-temperature phase between 250 and 300 K is 45.1 kJ mol21. In the hightemperature phase, two relaxation processes were observed, with activation energies of 11.7 (300–330 K) and 7.7 kJ mol21 (>330 K). ( 1998 Academic Press
Bicyclononanone was obtained from Aldrich Chemical Co. and was sublimed immediately before use. Differential scanning calorimetric measurements were performed on a Perkin-Elmer DSC-7 calorimeter with temperature and enthalpy calibrations based on the phase and melting transitions of cyclohexane. The samples were scanned at 5 K min~1 for both the cooling and heating directions. Proton spin—lattice relaxation times were measured by the inversion—recovery method, using a spin-lock CPS spectrometer operating at 33 MHz. Raman spectra were obtained using an Instruments S.A. spectrometer with a JobinYvon U-1000 1.0-m double monochromator. The 514.5-nm line of an argon-ion laser was used for excitation with a laser power of 100 mW at the sample. The samples were contained in sealed glass capillary tubes mounted, using indium foil, on the cold finger of a Cryodyne Model 21 cryocooler. Infrared spectra were recorded on an Analect AQS-18 FT spectrometer. The samples were examined as KBr pellets, which were allowed to relax for 1 month before use.
Structures based on the bicyclo[3.3.1]nonane skeleton have received attention as a result of an unusually short intramolecular contact distance between hydrogens at C3 and C7, estimated to be less than 2.0 As (1—4). In spite of this very close approach, the chair—chair conformations are of lowest energy but interconversion between the chair and boat forms is possible. The keto derivative, bicyclo[3.3.1] nonan-9-one, C H O, has interesting solid-state thermal 9 14 properties. A recent adiabatic calorimetry study (5) reported a solid—solid phase transition at 300.5 K, with enthalpy and entropy changes of 14.11 kJ mol~1 and 46.99 J K~1 mol~1, respectively. The entropy change is one of the largest observed for organic molecular crystals and indicates that considerable disorder must occur in the high-temperature phase (phase I). This confirmed the results of an NMR study, in which whole molecule motion was assumed to be responsible for the 13C spin—lattice relaxation (6) in the high-temperature phase. Although the chair—chair form of bicyclononanone predominates in the vapor (7), interconversion between conformers in solution is fast on the time scale of 13C NMR measurements (8), and lanthanideinduced-shift measurements indicated that about 22% of the boat—chair conformation exists in solution at room
RESULTS AND DISCUSSION
Only one solid—solid phase transition was observed by DSC, at 282 K on cooling and at 299 K on heating. No endotherms were observed on heating and so no glassycrystalline state was produced. The transition enthalpy and entropy changes were 13.9 kJ mol~1 and 46.5 J K~1 mol~1, respectively. These results are in excellent agreement with those obtained by White and Perrott using adiabatic calorimetry (5). In addition, the hysteresis in the transition temperature corresponds to the minimum observed at 285 K in the cooling rate in the adiabatic calorimetric study;
1 To whom correspondence should be addressed. 16 0022-4596/98 $25.00 Copyright ( 1998 by Academic Press All rights of reproduction in any form reserved.
PHASE TRANSITION IN BICYCLONONANONE
thus bicyclononanone can exist in both orientationally ordered and orientationally disordered states at room temperature. In the absence of X-ray diffraction data on the crystal structure, useful information can be obtained from the splitting of peaks in the vibrational spectra which depend upon the site and factor group symmetry in the crystal. A total of 66 Raman and 53 infrared bands are expected for the C H O molecule with C symmetry: !"20a #13a # 9 14 2v 1 2 17b #16b . The a , b , and b modes are both IR and 1 2 1 1 2 Raman active, whereas the a species are solely Raman 2 active. Only 40 Raman and 36 IR bands were actually observed (Table 1). At ambient temperature both the Raman and infrared spectra contained broad vibrational peaks and the Raman spectrum of the lattice region showed TABLE 1 Raman and Infrared Vibrational Frequencies (cm21) of Bicyclo[3.3.1]nonan-9-one Phase I (300 K) Raman
2989 m 2958 w 2941 vs 2932 s
2995 sh 2990 vs 2965 sh 2960 s 2940 sh 2932 vs
2990 sh 2998 s 2960 sh
2926 vs 2917 vs 2915 sh 2910 vs 2886 m 2880 sh 2865 sh 2855 s 1719 w 1709 w
2910 vs 2900 vs
2879 vs 2860 vs 2853 vs 1733 w 1720 vs 1710 vs 1700 sh
1680 vw 1490 vw 1458 w 1455 w
2582 vs 1730 sh 1720 w
1490 vw 1483 m 1457 m
1365 sh 1363 vw
1496 vw 1456 m
1455 m 1447 m
1440 vw 1436 w 1430 w
2854 vs 1730 sh 1722 vs
1448 m 1438 m
1408 vw 1368 w 1358 vw
1358 w 1356 vw
1349 vw 1335 vw
1349 vw 1328 m 1316 m
1348 w 1328 vw
1328 vw 1313 vw
Phase I (300 K) Raman
1280 vw 1271 vw
CH def 2
1245 vw 1240 s
1275 vw 1254 vw 1247 m 1236 w
1165 w 1125 w 1123 w 1085 s
807 s 785 vw 754 w 750 w 735 vw 714 vw 661 s 605 vw 470 vw
1004 w 941 s 929 vw 920 vw 907 vw 834 w
Phase II (150 K)
1068 m 1029 w
Phase II (150 K) IR
372 w 337 vw 305 vw 293 vw 228 vw 121 m 80 vw 70 vw 55 vw
1165 vw 1136 w
1088 vw 1078 m 1068 w 1026 vw 1020 sh 1001 vw 938 w 927 vw 915 m 904 s 830 sh 829 vw 804 vw 784 vw 753 w 748 vw 739 vw 655 vw 603 vw 474 w 466 m 460 w
1235 sh 1231 m 1228 sh
1257 w 1244 w 1240 sh
1218 w 1186 sh 1162 vw
1121 vw 1081 s 1075 m 1033 w
1076 m 1070 w 1025 vw
1001 w 937 m
1001 vw 936 w
904 vw 830 vw 828 vw 805 s
750 w 729 w 714 vw 661 m 616 vw 605 vw
917 w 904 m 830 sh 829 vw 803 vw 783 vw 752 w
655 vw 601 vw 466 vw
456 w 430 vw 384 sh 370 vw 340 vw 300 vw 288 vw 228 vw 221 vw 120 m
H H H
CC str CH def 2 CH twist 2 CCH def
CH wag 2 CCH def
only the broadened Rayleigh line and no discrete peaks. These observations are characteristic of an orientationally disordered solid. Upon cooling, the bands in the Raman spectrum became sharper and narrower and, by 150 K,
PAROLI ET AL.
many peaks had split into two components and the lattice region now showed three peaks. Similarly, the IR bands narrowed or split into two peaks at low temperature (Figs. 1 and 2). Possible site symmetries are C , C , C , or C , 2v 2 s 1 since the site symmetry must be equal to or lower than the molecular point group. The C and C factor groups with 1 i a C site would not lead to splitting of the vibrational peaks 1 and the triclinic groups can be ruled out. Thus the most likely possibilities are (i) a D crystal with a C site or a C 2 2 2v crystal with a C site, (ii) a D crystal with a C site, or (iii) S s 2d s 4 or C crystals with a C site. For the first case, all Raman 4 1 peaks and some infrared peaks should be doubled. In case ii all infrared peaks should be split into doublets and the Raman peaks split into two or into three peaks, and in case iii all Raman peaks split into three peaks. Since no splitting into three peaks was observed, the low-temperature phase is either orthorhombic, D or C , with two molecules per unit 2 2v cell, or tetragonal, D , with Z"4. 2d Spin—lattice relaxation time measurements showed a sharp break in ¹ at the phase transition (Fig. 3) and also 1 the hysteresis in the transition. In phase II, below 299 K, ¹ 1 increased with decreasing temperature but at about 225 K, where values exceeded 20 s, the signal-to-noise ratio deteriorated to the extent that the errors in ¹ were too great to 1 give reliable values. It appeared, however, that ¹ may 1 decrease again below 200 K. The plot of relaxation times versus reciprocal temperature gave an activation energy of 45.1 kJ mol~1 for the temperature region 250—300 K. In phase I there was a change of slope at about 330 K, with
FIG. 1. Infrared spectra of bicyclononanone: Phase I at 300 K; phase II at 100 K.
activation energies of 11.7 (between 300 and 330 K) and 7.7 kJ mol~1 (above 330 K), which are relatively low barriers. Since no ¹ minima were observed, no second1 moment values could be obtained and it was not possible to determine which motions are responsible for relaxation. Since the changes in molecular conformation are possibly involved in the disordering process, the energetics have been calculated by molecular mechanics methods. Similar calculations have been reported previously (3), but using a modified force field (2). The heat of formation calculated by the MM2 (10) method was !249.9 kJ mol~1 compared with !232.6 kJ mol~1 obtained by MM3 (11) and !219.5 kJ mol~1 from PCMODEL (Serena Software). The experimental value13 is 239.9 kJ mol~1. Using the dihedral driver option in MM2, we calculated the energy as a function of the two dihedral angles, C C C C and 1 2 3 4 C C C C , on driving the structure from the chair—chair to 1 8 7 6 the chair—boat and then to the boat—boat conformation. The latter structure is not symmetrical, as the rings adopt twist-boat conformations with dihedral angles of 36.7° and 58.5°. A contour plot of the MM2 energy is given in Fig. 4. The chair—boat form lies 4.8 kJ mol~1 above the chair— chair structure, with a barrier to interconversion of 22.7 kJ mol~1. The boat—boat form is 18.5 kJ mol~1 above the chair—boat form, with a barrier of 15.9 kJ mol~1. In the boat—boat form, there is a very small barrier of 4.0 kJ mol~1 between the two twisted structures; this value, however, is barely larger than the error in the calculation. Osawa and co-workers (3), using a harder torsional potential, obtained a difference of 10.9 kJ mol~1 and a barrier of 29.7 kJ mol~1 for the chair—chair to chair—boat forms. PCMODEL gave much smaller energy differences of 4.0 and 16.6 kJ mol~1 between the chair—chair, chair—boat, and boat—boat structures, respectively. The variations in calculated energies probably extend to the dihedral driver calculations of the barrier heights and are too large to permit assignment of the activation energies measured by spin—lattice relaxation to a given motion. In solution, the interconversion of conformers is fast on the NMR time scale (8), suggesting that the barrier is low. In phase I, isotropic rotation is the most probable cause of relaxation, plus, with increasing temperature, conformational motions of the cyclohexyl rings involving the chair—boat interconversions causing the additional relaxation. The maximum change in molecular dimensions is not large, as the distance between those hydrogen atoms which are the furthest apart increases from 5.31 to 5.85 As (from chair—chair to boat—boat). In the expanded disordered phase this should not be a problem and the interconversions of the rings might well be responsible for the additional spin—lattice relaxation in phase I, since, in this phase, intermolecular forces are not strong. In the calorimetric study (5), a significant contribution to the entropy of transition was attributed to the volume change, since the analogous compound 2-adamantanone (12), which
PHASE TRANSITION IN BICYCLONONANONE
FIG. 2. Raman spectra of bicyclononanone: Phase I at 300 K; phase II at 150 K.
Spin—lattice relaxation times versus reciprocal temperature.
FIG. 4. Contour plot from dihedral driver calculations. Energies are in kJ mol~1.
PAROLI ET AL.
is rigid, had an entropy of transition that was much lower (the actual value depending on the thermal history of the sample). The upper limit of the temperature range covered by the adiabatic calorimetry study (5) was 306 K, and so any increased heat capacity from the conformational change was not observed, but this might contribute to the high value of the entropy of transition. ACKNOWLEDGMENTS This work was supported by grants from NSERC (Canada) and FCAR (Quebec).
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