J. Phys. Chem. 1993,97, 1998-2001
1998
Vibrational and Deuterium N M R Spectroscopic Studies of the Phase Transition in Solid Cyclohexanone Yining Huaag, Denis F. R. G i n , ' and Ian S. Butler' Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received: September 30, 1992; In Final Form: November 30, 1992
Variable-temperature Raman and infrared spectroscopic methods have been used to examine the order-disorder phase transition in cyclohexanone. A factor group analysis predicts that the low-temperature phase has an orthorhombic unit cell with Cb symmetry with two molecules per unit cell. Deuterium N M R line-shape studies show that, on heating, motional narrowing commences well below the phase-transition temperature of 235 K, and isotropic rotation occurs in the high-temperature phase.
Introduction
Order-disorder transitions are common in cage hydrocarbons, such as adamantyl and bicycloheptyl compounds and their derivatives, and also occur in monocyclic alkanes.' Compared with the rigid cage compounds, the latter have an additional mode of disordering since out-of-plane vibrations of the ring structure can lead to ring inversion or pseudorotation. Modification of the ring structure by introducing double bonds or the inclusion of a keto substituent, for example, reduces the flexibility of the ring, but disordering transitions are still quite common. The information available on the solid-state properties of these compounds is often limited to the calorimetric measurements of the phase-transitiontemperature and enthalpy and to molecular dynamics studies by NMR methods. Structural information on the low-temperature phase is seldom available. Phase transitions occur in medium ring cyclic ketones,* and the transition in cyclohexanone has been known for many years from dielectric constant measurements34and calorimetre' and NMR studitss2 At ambient pressure, cyclohexanone melts at 245.21 Kand undergocsonesolid+olidphasetransitionat 224.75 K (phase I phase 111). The entropies of transition and fusion are 39.22 and 5.41 J K-l mol-I, respecti~ely.~ Such a low entropy of fusion and a high entropyof transition are typical of the behavior of disordered organic molecular crystals. An additional phase (phase 11) exists under high pressure: but the triple point for all three phases is very close to 1 atm of pressure. Nakamura and co-workers7have reported that the differential thermal analysis curves of cyclohexanoneare quite different in shape upon heating and cooling, and they attributed the difference to a transition involving two steps, but this observation might be related to the closeness of the transition to the triplet point. These DTA results showed a large hysteresisin transition temperature. Proton NMR studiesas a function of temperaturehave shown that, upon heating, there is a sharp decrease in the line width and second moment at the transition point, suggesting that the molecules form a rigid lattice in phase I11 and reorient isotropically in phase I, with molecular diffusion also occurring.2 These NMR results support the conclusions from the dielectric constant measurements.34 A very thorough study of the vibrational spectra of liquid cyclohexanone has been reported, including a normal coordinate analysis,* but the spectra of the solid phases have received little attention except for the infrared spectra, which were reported: but no analysiswas given. The crystal structure of the disordered phase, like many materials of this type, is face centered cubiclo with four molecules per unit cell, but the structure of the ordered phase is not known. Since the vibrational spectra of thedifferent phases can offer useful information on the structures of each phase and on the nature of molecular motions, low-temperature
-
0022-3654193f 2097- 1998tM.00f 0
A
1;
-
3000
2800
1800
I
1800
1400
B
1200
1000
800
800
400
Wavenumber (cm-')
Figure 1. Infrared spectra of cyclohexanone: (A) phase I at 230 K; (B) phase I1 at 57 K.
infrared and Raman spectroscopic studies have been performed. As these led to the probability that the low-temperature phase is not completely rigid, some *HNMR line-shape measurements were also undertaken.
Experimental Section Cyclohexanone (99.8%) was obtained from Aldrich Chemical Co. Raman spectra were recorded using an Instruments S.A. spectrometer with Jobin-Yvon Ramanor U- 1000 double monochromator and Spectra-Physics5-W argon ion laser. The 5 14.5nm line was used for excitation, with about 300 mW at the sample. Infrared spectra were obtained on a Nicolet 6000 FT-IR spectrometer with an MCT(B) detector. A closed-cycle helium cryostat and temperature controller were used for the variabletemperature studies. Deuterium NMR spectra were measured at 46 MHz using the 90°x-T-900y quadrupole echo sequence" on a Chemagnetics CMX-300spectrometer. The 90° pulse length was 2.5 ps, and a T value of 25 ps was used. Results a d Discussion The infrared and Raman spectra of cyclohexanone for phases I and I11 are shown in Figures 1 and 2, respectively. The vibrational frequencies and corresponding assignmentsare given in Table I. The assignments are based on those given by Jones and co-workers.8 The IR and Raman spectra are characteristic of each phase and showed marked changesat the phase transition. All the bands in the spectra of phase I are very broad and featureless, implying that this phase is highly disordered. Upon the transformation to phase 111, the vibrationalbands underwent Q 1993 American Chemical Society
Phase Transition in Solid Cyclohexanone
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1999
TABLE I: Vibrational Data (cm-I) for Cyclohexanone phase I11 IR (57K) 2951 vs 2934vs 2928~s 2910s 2893 m 2869s 2851 s 2a35w 2a1avw 1765 vw 1758 vw 1751 vw 1738 vw 1706sh i702Vs i67aw 1664 w 1639 vw 1497 1472sh 1461 m 1454m 1444m 1431 m 1413m 1350m i338m 1324w 1313s 1266m 1255vw 1230m 1219m 1145vw
phase I
Raman (73K) 2953 vs 2937s 2929m 2906sh
IR (230K) 2952 sh 2936 vs
phase I11
Raman (235 K)
assignment”
2911 m 2903 m 2861 vs
2a76w 2870 VS 2a52w 2896 vs 2a37w 2a1avw
1062vw 1054w 1047 w 1019w 1014m 995w
2895 S, sh 2828 w 1769 vw
1713 sh 1708 vs 1679 vw
1717sh 1705 s
) u7*a’
1
1636 1478vw 1463w 1451 w 1447m 1434w 1432 w 1419m 1414 m 1356w 1353 w 1341vw 1326m 1317w 11276w 1266 w 1251vw 1235 w 1227m 1221 w
1
overtones + combinations
1749 vw 1720 vw 1705m 1698s wavw
IR (57K) 1123m llllw 1074w
overtones
+ combinations
1462 w 1449 m 1429 m 1422 m 1347 m 1339 m 1312 s 1266 w
1223 s 1144 vw
phase I
Raman (73 K) 1124vw 1122vw 1076m 1073 w 1063vw 1053w
IR (230K) 1119m
Raman (235K)
assinnmento
1072w 1052w
1022vs 1019w 1017w 992w 997 w 990 vw 904m 907w 909m 8 9 5 ~ 896Vw ~ a95vw 8 6 2 ~ 8 6 4 ~ a65w ~ 8 5 6 ~ a5avw 8 4 5 ~ 846s ~ 841 VW 841 sh 750m 752vs 752m 749 w 666 vw 663 w 651 w 650m 654w 645 w 6 4 8 ~ ~ 492s 494w 491s 475vw 478vw 478 vw 414m 415vw 406w 407w 311 vw 308 vw 209 vw 206 vw 145 vw 120 w 107 vw 92 w 81 w 69 w 64 m 56 m 52 w 41 w 32 m
1
intermolecular vibrations
I
From ref 8.
A
I
s 10
E
Ik
3000
2850
I
1
-
B
LOO
.
le00
, 1400
, , I200 1000
I
800
I
800
,
-
400
-
200
Wavenumber (cm-‘)
Figure 2. Raman spectra of cyclohexanone: (A) phase I at 235 K;(B) phase I1 at 73 K.
substantial narrowing, and many of them were split into doublets in both the IR and the Raman spectra: Y14, U l 5 r U16, and u21, for example. The effects of band splitting and narrowing became more apparent with decreasing temperature. The phase-transition behavior of cyclohexanoneis also readily
150
200 250 T (K) Figure 3. Temperature dependence of the full width at half-height (FWHH)of YZO upon cooling and heating.
followedby the changes in the line width of the vibrational modes, and Figure 3 illustrates the sudden decrease and increase in the width of u20, an IR-active C-C stretching mode, upon cooling and heating, respectively. A 30 K hysteresis in transition temperatureisclearlyevident,inagreement withthe DTAresults.7 The vibrational spectra showed no evidence of a metastable state resembling that found for the larger ring ketone cyclooctanone.12 The energetically favored geometry for cyclohexanone is
Huang et al.
2000 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 258
K
183 K
A
203 K
183 K (after I 1R hours)
L m
100
0
-100
400
kHx ~
I
100
'
80
I
60
~
40
I
~
20
Wavenumber (em-') Figure 4. Low-frquency Raman spectra of cyclohexanone: (A) phase I at 235 K;(B)phase I1 at 73 K.
160
140
120
100
80
Wavenumber (cm-I) Figure 5. Temperature dependence of the ring deformation modes of cyclohexanone (labeled by *).
believed to be the chair conformation.13 The point group symmetry of the free molecule, therefore, is C,. Consequently, the possible lattice sites that a molecule of C, symmetry can occupy are either CIor C,. Since all the vibrational modes under C, symmetry are nondegenerate, cyclohexanone cannot exhibit any site splitting and the observed peak doublings are due solely to the correlation coupling. The majority of the observed vibrational modes in phase 111of cyclohexanonewere coincident in both the IR and Raman spectra, and therefore, the unit cell is probably noncentrosymmetric. The correlationsfor the possible crystal symmetries lead to two types of site and factor group combinations: CzUcrystal (orthorhombic) with a C, site; and C, crystal (monoclinic) with a CIsite. In the first case, where the point group and site symmetriesare the same, each a' mode should theoretically split into two componentsin both theIR and Raman spectra under the CZ,factor group symmetry, while each a" mode
Figure 6. Deuterium NMR powder line shapes of cyclohexanone on I
cooling.
~
I
should double in the Raman spectrum but remain unsplit in the IR spectrum. For the second case, the CIsite should lead to a doubling of all the modes under the C, unit cell symmetry. A survey of those bands which split in the IR spectrum of phase 111 indicates that most of the splitting did happen to the a'-related modes, and therefore, the most likely symmetry of the unit cell for phase I11 is CZ,. The low-frequency Raman spectrum of cyclohexanone also underwent significant changes, Figure 4. In phase I, no discrete external modes were observed, indicating that the overall molecular motion is a rapid isotropic reorientation on the time scale of the experiment, which is consistent with the results of the proton second moment measurements.2 In the spectrum of phase I11 at 73 K, the presence of nine external modes implies that the crystal is ordered and that there are probably two molecules per primitive unit cell. Two low-frequency skeletal vibrations, at 120 ( ~ 1 5 and ) 145 cm-I in the Raman spectrum of phase I11 (at 73 K), deserve further comment. The 120-cm-l band is a highly delocalized symmetric ring distortion involving changes in the CCC angles, CC torsions, and the C - 0 out-of-plane wag. This band, therefore, would be very sensitive to any conformational changes. Since the behavior of the 145-cm-l band is typical of that found for the ring inversion modes in several other cyclic molecules,'3-'5 where large changes in the frequencies of the outof-plane vibrations occur at the phase transitions, this mode may be alternatively assigned as the ring inversion vibration. The band at 196 cm-l ( ~ 4 5 ) has been partially related to a chair -., boat conformation inver~ion.~ Figure 5 shows that, upon warming, both bands quickly broadened and shifted to lower energies, indicating that the molecules are totally rigid only at very low temperatures, i.e., below 73 K, and start gaining motional freedom with increasing temperature. At temperatures close to the transition, the 120-cm-l peak became extremely broad and the 145-cm-' mode completely disappeared. The broadening of the two ring deformation modes may be due to the conformational change between the chair and boat forms. However, the line widths of these modes may also be sensitive to other types of motion possibly existing in this phase, such as uniaxial rotation. Some 2H NMR line-shape measurements were undertaken in an attempt to resolve this situation. The liquid spectrum at 258 K, Figure 6, showed a sharp singlet, which persisted down to 203 K as the liquid phase supercooled. The spectrum of phase I also contained a single peak, but the line width of this peak was 10 kHz. These results confirm that phase I is highly disordered and the molecules in this phase rotate isotropically, but at a rate much slower than in the liquid. Phase I also underwent supercooling, and a Pake powder pattern only started to emerge at 183 K, corresponding to the beginning of the
Phase Transition in Solid Cyclohexanone formation of an ordered phase. The intensity of the powder pattern increased with time until after l'/z h when the spectrum became constant, except for the central singlet which did not totally disappear until 143 K, implying that therearestillsome molecules which rotate spherically. At 143 K, the width of the *H powder line width was 127 Wz and the Pake powder pattern line shape was typical of static C-2H bonds. The splitting in the spectrum at this temperature corresponds to a quadrupole coupling constant (e2qQ/h) of 170 Wz, which is typical for deuterium bound to aliphatic carbons.I7 Molecular motion must be slower than the value of e2qQ/h, and the molecules are considered to be rigid. However, in the lowfrequency Raman spectrum at 150 K, Figure 5, the two skeletal vibrational modes have already shown some broadening, suggesting that the motion has already begun but at a rate between the NMR and Raman scattering time scales. Upon heating the sample to 173 K, the sharp central line started to appear again in the spcctrum and increased in intensity with increasing temperature, while the Pake doublet decreased in intensity. Upon passing through the 111- I transition at 235 K, the temperature immediately prior to the transition point, the powder pattern finally vanished and the spectrum of phase I contained only the central line. Interestingly, although the intensity of the powder pattern decreased with increasing temperature, the shape of the Pake doublet remained axially symmetric with a constant width of about 127 Wz, and no significant line narrowing was observed throughout the temperature range examined. The 2H spectra indicate that some molecules commence spherical rotation far below the transition temperature, even at 170 K, and the number of such molecules increases with increasing temperature. This process was not detected by the proton second moment although the second moment and line width began to decrease several degrees before the actual transition. Other than the isotropic rotation, there are no other types of motion in phase 111. For instance, a reorientation of the molecule about an axis perpendicular to the molecular plane would reduce the splitting
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 2001 of the Pake doublet, and a rotation about the axes lying in the molecular plane would result in a different line shape from the Pake doublet. The large hysteresis in transition temperature observed in the vibrational spectra and deuterium NMR studies is consistent with the behavior of other order-disorder materials where a large volume change (estimated by Wurflinger and KreutzenbeclP to be 7.0 cm3 mol-') and the hysterwis are associated with a high transition entropy.'* Acknowledgment. This research was supported by operating grants to D.F.R.G. and I.S.B.from NSERC (Canada), EMR (Canada), and FCAR (Quebec). Y.H. acknowledges the award of a Fellowship from McGill University.
References and Notes (1) Parsonage, N. G.; Staveley, L. A. K. Disorder in Crystals;Clarendon Press: Oxford, U.K., 1978. (2) Fried, F. Mol. Cryst. Liq. Cryst. 1973, 20, 1. (3) White, A. H.; Bishop, W. S.J. Am. Chem. Soc. 1940, 62, 8. (4) Crowe, R. W.; Smyth, C. P. J . Am. Chem. Soc. 1951, 73, 5406. (5) Corfield, G.;Davies, M. Trans. Faraday Soc. 1964, 60, 10. (6) (a) Wurflinger, A.; Kreutzenbeck, J. J . Phys. Chem. Solids 1978, 39, 193. (b) Wurflinger, A. Faraday Soc. Discuss. 1980.69, 146. (7) Nakamura, N.; Suga, H.; Seki, S . Bull. Chem. Soc. Jpn. 1980,53,
2755. (8) Fuhrer, H.; Kartha, V. B.; Krueger, P. J.; Mantsch, H. H.; Jones, R. N. Chem. Rev. 1972,72,439. (9) Burer, T.; Gunthard, Hs. H. Helv. Chim. Acra 1957, 40, 2054. (IO) Hassel, 0.;Sommefeldt, A. M.2.Phys. Chem. 1938, IOB, 391. (11) Davis, J. H.; Jeffery, K. R.;Bloom, M.;Valic, M.I.; Higgs, T. P. Chem. Phys. Lett. 1976.42, 309. (12) Huang, Y.; Butler, I. S.;Gilson, D. F. R.J . Phys. Chem. 1992, 96, 1151. (13) Tai, J. C.; Allinger, N. L. J . Am. Chem. Soc. 1966, 88, 2179. (14) Haines, J.; Gilson, D. F. R. Can. J . Chem. 1990, 68, 604. (15) Haints, J.; Gilson, D. F. R. Can. J . Chem. 1989, 67, 941. (16) Hainw, J.; Gilson, D. F. R.J . Phys. Chem. 1989, 93, 6237. (17) Sparks,S. W.;Budhu,N.;Young,P. E.;Torchia,D.A.J. Am. Chem. Soc. 1988,110,3359. (18) Kawai, N. T.; Gilson, D. F. R.;Butler, I. S.J . Phys. Chem. 1992, 96, 8556.
