J. Phys. Chem. 1993,97, 11092-1 1095
11092
The Solid Phases of Cyclooctanone Studied by Thermal, Nuclear Magnetic Resonance, and Dielectric Techniques J. Mark Adams, David M. Snyderman, and Mark S. Conradi’ Department of Physics-1 105, Washington University, One Brookings Drive, St. Louis, Missouri 631 30-4899 Received: June 4, 1993; In Final Form: August 10, 1993’
Differential thermal analysis has been used to reexamine the stable and metastable solid phases of cyclooctanone and the phase transitions. N M R and dielectric measurements were used to characterize the extent of molecular motions in each phase. Rotor phase I can be supercooled into a glassy crystal, I,. A single orientationally ordered phase, 11, is stable at low temperatures. Another phase, 11’, is always metastable with respect to phase 11. The activation energies of self-diffusion and molecular reorientation in I are reported.
Introduction The cyclic hydrocarbons and their derivatives have very rich phase diagrams.14 Because of the globular molecular shapes, rotor solid phases (also known as orientationally disordered and plastic solids) are found at high temperature^.^.^ Supercooling of the rotor phases is common, often resulting in glassy crystalline Additional metastable phases occur in some of these systems,including cyclooctane10Jl andcycloheptane.12 It is likely that the conformational flexibility of the large rings” is partly responsible for the rich phase behavior. The goal of the present work is to understand the stable and metastable solid phases of cyclooctanone. A previous thermal study14 arrived at an incorrect picture of the phases and their transitions, which has been accepted in the l i t e r a t ~ r e . ~ JThus, ~J~ we report herenew, thorough thermal measurements. From these data, a free energy diagram of the phases is constructed. Nuclear magnetic resonance (NMR) and dielectric measurements are used to confirm the thermal data and to characterize the extent of molecular motions in each phase.
The cooling coils provided a means of halting and reversing the warmup. Occasionally, more warming data were recorded after recooling. However, the homebuilt DTA did not perform well during rapid cooling, so a Du Pont 9 10 DSC was used for a few cooling measurements. NMR. Proton NMR measurements at 85.03 MHz (2T) were performed. Samples previously studied by DTA were used in the same tubes. This allowed cyclooctanone to be prepared into a specific phase by DTA and then transported in liquid N2 to the NMR apparatus. Temperature control was provided by feedbackcontrolled on-off heating of a N2 gas stream, obtained by boiling liquid Nz. The temperature was determined with a type-T thermocouple external to the sample. Longitudinal relaxation times TI were measured with the saturate-wait-inspect method. Inspection was commonly performed with a 90° pulse ( - 6 ps). Saturation was accomplished with a comb of six 90° pulses. The transverse relaxation time T2 was determined from the free induction decay (FID) g(t),
Experimental Section
Samples. Cyclooctanone was purchased from Aldrich, with stated purity of 98%. Several samples were purified by fractional crystallization and fractional distillation. One small, especially pure sample for DTA was obtained by preparative gas chromatography. Benzene with stated purity 99.9%was used as received from EM Sciences for the DTA reference. The samples were sealed into glass tubes for the measurements. DTA. A homebuilt differential thermal analyzer (DTA) was constructed with a massive copper block, 23-cm high and 8-cm diameter.1° Sealed glass tubes containing cyclooctanone(sample) and benzene (reference) were located in snug-fitting holes in the copper block. In each tube, a type-T thermocouplewas immersed in the material; the wires were sealed with epoxy where they exited the glass at room temperature. Most of the measurements used 5-mm diameter tubes, but the early work was with 12-mm tubes. Typically, the sample and reference tubes were cooled to 77 K by gradual immersion (- 30 s) into liquid N2. The copper block was cooled by flowing liquid N2 through attached cooling coils. The sample and reference tubes were then inserted into the block. As the assembly warmed (-2 h to 270K), the temperature difference AT between the two thermocouples was recorded as a function of the sample temperature T using an x-y recorder. The rateof warmingdecreasedcontinuouslyas Tincreased toward room temperature, decreasing the relative amplitude of the hightemperature peaks. *Abstract published in Advance ACS Absrrucfs, October 1, 1993.
0022-3654/93/2097-11092$04.00/0
This definition of T2 and its practical implementation have been discussed.1° In particular, the time zero is taken as the earliest time for which the receiver has fully recovered from the transmitter pulse. Dielectric. An AC bridgeoperating at 5000 Hzwas constructed around a Gertsch ratio transformer. A cyclooctanone sample (99%) was melted and allowed to fill the region between the plates of a small air-variable capacitor (18 pF nominal value). The capacitor and sample were held in a glass vessel with a greased taper seal and epoxy-sealed wires passing through the glass. The temperature of the sample was measured with a thermocouple immersed in the sample. Temperature control was performed as for the NMR measurements.
Results and Discussion DTA. The DTA thermogram for a cyclooctanone sample during warmup is presented in Figure la. This sample had been cooled from room temperature (phase I) to 77 K in about 10 s. The succession of exothermic and endothermic peaks is quite similar to a previously reported re~u1t.l~ The heat capacity step at 131 K indicates a glass transition. The exothermic peak at 150 K indicates an irreversible transition to a more stable phase. In a separate run, the sample was cooled from 300 to 77 K, was allowed to traverse the 150 K exotherm, and was then recooled to 77 K. Subsequent warming of this sample resulted in the 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11093
Solid Phases of Cyclooctanone
I I
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Figure 2. Sketch of Gibbs free energy as a function of temperature for the solid phases of cyclooctanone. The path taken by the sample during the warming run duplayed in Figure la is represented by arrows. The dashed lines indicate exothermic transformations. Phase 11' is always metastable with respect to phase 11.
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200 240 TEMPERATURE (K)
Figure 1. (a) DTA warming thermogram for a cyclooctanone sample that had been cooled from room temperature (phase I) to 77 K in about 10 s. (b) DTA warming thermogram for a sample that had been quickly rccooled to 77 K after passing through the exotherm near 150 K in (a). (c) DTA warming data for cyclooctanonethat had been quickly rccooled to 77 K after passing through the 181 K exotherm in (a).
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DTA curve of Figure 1b. There, the phase obtained after the 150
K exotherm shows no transitions below about 170 K. In Figure la,b, a small endotherm is evident at 171 K this is followed immediately by a large exotherm. A sample allowed to traverse the 181 K exotherm and then recooled to 77 K showed no transitions below 222 K upon subsequentwarming, as presented in Figure IC. The most straightforward explanation of all of the DTA results is shown in Figure 2, a sketch of the Gibbs free energy vs temperature of the several phases. The naming of the phases can be a source of confusion. Rudman and Post,14 Gilson," and the present work agree on phase I. Phase I1 in this work and in Rudman and Post is called I11 in Gilson's paper. Phase 11' here is called I11 by Rudman and Post and I1by Gilson. We note that phase 11' is always higher in free energy than 11, so that 11' is never the thermodynamically preferred state. This is in accord with the finding of the vibrational spectroscopy study" but in disagreement with the previous thermal work.14 In Figure 2, arrows are used to show the path of the sample in the DTA warming run of Figure la. We note that the I, I glass transition is not represented in Figure 2, because a glass transition is essentially a rate effect. The complex behavior near 171 K can be understood as follows. The endotherm at 171 K is a transition from 11' to I. Presumably, strains produced by the transition cause the newly formed I to fall rapidly into the more stable phase 11. The small exotherm just below 171 K suggests that phase II'also begins to fall directly to I1at these temperatures. We note that similar behavior is found in cyclooctane, where an endothermic transition into a rotor phase is followed promptly by an exotherm.1° With slower cooling of phase I from room temperature, phase I converts to phase 11', but only below approximately 170 K. Still slower cooling of I from room temperature results in I transforming to I1 at somewhat less than 200 K. These behaviors are also in accord with Figure 2. In all cases, the phase of the product was determined by subsequent DTA measurements. To confirm that phase I underwent no transitions during rapid cooling from 300 to 77 K,a commercial scanning calorimeter
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Figure 3. Proton NMRrelaxationtimes ?'I and T2for solidcyclooctanone. The data are labeled as phase I (O), 11' (A),or I1 (0)based on the DTA results. In rotor phase I, translational self-diffusion narrows the line and increases T2 for temperatures above 250 K. Below 145 K, the molecular reorientations in phase I become too slow to narrow the line.
was employed. No transitions were observed upon rapid cooling (300 to 77 K in about 4 min). In Figure la, a small endothermic peak is evident at 222 K, just below the large I1 I endotherm at 225 K. A DTA measurement of cyclooctanone purified by preparative gas chromatography (99.97%) did not have the 222 K peak. A very impure sample, taken from the liquid material after fractional crystallization, showed a substantially enhanced peak at 222 K. Thus, this peak (also evident in Figure 3 of ref 14) is either from a phase transition of the impurity or it involves the formation of a cyclooctanone phase which is only stable with the impurity present. NMR. Proton NMR measurements of TIand T2 are reported in Figure 3. The data are labelled by phases, with the phases determined from the picture in Figure 2. Between 160 and 245 K, phase I has a nearly constant T2 of about 30 ps. This is much
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11094 The Journal of Physical Chemistry, Vol. 97, No.42, 1993
longer than in phases I1 and 11’, demonstrating that I is a rotor phase. Comparison with other rotor crystals indicates that the 30ps plateau correspondsto fast molecular reorientation without translational self-diffusion.18 The X-ray determined structures of the rotor phases of ~yclooctanone~~ and cyclooctane19are the same, Pm3n. This is also the structure of the rotor phases of y-OzZoand /3-F2.z0,21In this structure there are eight molecules per cell: two spherically disordered and six with planar or disklike disorder. The plateau Tz value of phase I cyclooctane is similar, approximately 40 ps.10 The FID time Tz is quite short in phases 11, 11’, and I (below 140 K), about 6 ps. With such short T2 values, finite rf pulse lengths and receiver deadtime affect the value of this decay time. Nevertheless, these effects will be equal in the three phases. The short and nearly equal Tzvalues demonstrate that these phases have relatively little molecular motion faster than the 10-5 s time scale. The shorter T1 of phase 11’ compared to T1 of phase I1 suggests that 11’ has more disorder and motion than 11. This suggestionis in accord with vibrational ~pectroscopy.~~ We cannot determine the nature of the motions driving spin-lattice relaxation in phases I and 11’ at low temperatures. However, the short, nearly equal, and temperature-independent TZvalues in phases I and 11’ at low temperatures demonstrate that the motions are of small amplitude and/or affect only a small fraction of the molecules. The T1 in phase I1 is long but varied from run to run, suggesting that dissolved 0 2 may have limited T1 in this rigid phase. At temperatures above 250 K in phase I, Tz increases with temperature. This is due to translational self-diffusion. The T2 data of Figure 3 yield an activation energy of self-diffusion of 4100 f 400 K, using graphical analysis (34 f 3.4 kJ/mol). The motion driving the longitudinal relaxation ( T I )is most likely molecular reorientation by comparison with previous work.18 The activation energy for reorientation over the 200-300 K interval is found to be 1500f 150K (12.5 f 1.3kT/mol). For comparison, the activation energies of rotation and diffusion in cyclooctane phase I are 1650 K (13.7 kJ/mol) and 4350 K (36 kJ/mol), respectively,lO remarkably similar to the present cyclooctanone results. In phase I, the overall reorientational motion is described by a correlation time T~ with W,T, < 1 above 200 K, as evidenced by the temperature dependenceof T Iin Figure 3 (w, is the resonance frequency, 2n.85 X lo6 s-l& Near 145 K, 7, is near l e 5 s, as demonstrated by the decrease in T2 toward its rigid-lattice limit. Presumably, the glass transition at 131 K evident in Figure l a indicates 7, E lo2 s. Together, these values show a strong deviation from Arrhenius behavior, as is common for glass transitions. Furthermore, the revalues so deduced are much too large to account for T I in phase I below 140 K thus a motion other than overall reorientation is responsible for T I there. One aspect of Figure 2 that cannot be ascertained by thermal measurementsis the identity of the phase into which 11’transforms at 171 K. It could be phase I or some other phase. To settle this issue, a successionof FIDs was recorded as a sample of 11’ warmed through 171 K. The short decay of phase 11’ was temporarily replaced by a longer FID, which upon examination was indistinguishable from the decay of phase I (obtained upon cooling through the same temperature). This long FID persisted for only about 60 s, being replaced by the short FID of phase 11. Of course, we cannot rule out a different rotor phase, but the straightforward explanation is that 11’ transforms to rotor phase I at 171 K, which subsequentlyfalls exothermically to 11. Similar observations of the long FID of phase I during the 11’ I I1 transformation aided the identification of the phases in Figure 3. Dielectric. Dielectric measurementson the polar cyclooctanone molecule are sensitiveto the extent of molecular reorientations.22.23 The real part e’ of the dielectric constant and the dielectric loss
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Adams et al.
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F i p e 4 . Real part e’ofthedielectricconstant (upper)andloss (arbitrary units, lower) of solid cyclooctanone at 5000 Hz. (a) Data obtained on rapid cooling of phase I. (b) Data obtained on subsequentwarming; note the expanded scales for e’ and for temperature. The peak in d near 177 K is due to the presenceof phase I during the 11’- I I1 transformation.
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at 5000 Hz are plotted in Figure 4a for a phase I sample cooled rapidly from room temperature. The dielectric constant of I is large and has a Curie-like temperature variation above 160 K. This is as expected for a solid with rapid reorientation~.~2~23 The rapid decrease in e’ and the loss peak near 150 K demonstrate that the molecular reorientation rate passes through -5000 Hz at this temperature. We note that this is in accord with the decrease in TZof phase I near 150 K (Figure 3). For motions to be effective in line-narrowing, the rate of motions must exceed the rigid-lattice line width of -40 kHz, just slightly larger than the 5 KHz of the dielectric measurement. In separate measurements (not shown), the dielectric constants of I1 and 11’ were found to be small (near 3.0), independent of temperature, and loss-free. Thus, molecular reorientation is essentially absent in these phases. We note that dielectric measurements alone can only exclude motions that reorient the molecular electric dipole moment. The large dielectric loss in phase I above 260 K is due to ionic conductivity; the loss is approximately described by an activation energy of 5000 f 1000 K. It is not known whether the ions are intrinsic or impurities. The dielectric response upon warming of an initially phase I, sample is presented in Figure 4b. Near 145 K, the dielectric constant e’ and the loss both increase. These warming data below 150 K are essentially equal to the cooling data in Figure 4a, indicating that both represent phase I. Upon further warming, however, the sample transforms to the nonrotor phase 11’, so the dielectric constant and loss decrease. As the sample of 11’ is heated further (near 170 K) in Figure 4b, it transforms to I which then falls exothermically to phase 11. Because phase I has a large dielectric constant, the presence of phase I causes the sample-averaged dielectric constant to rise. Comparing the e’ data of Figure 4b,a, no more than 25% of the sample was in phase I at any time. Because phase I is essentially free of dielectric loss near 177 K (see Figure 4a), the peak in e’ near 177 K in Figure 4b at the 11’ I I1 transition is not accompanied by appreciable dielectric loss.
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Conclusions The DTA results are summarized in the sketch of Gibbs free energy vs temperature, Figure 2. With rapid cooling, rotor phase I can be supercooled to 77 K. Upon warming, a steplike increase in the heat capacity indicates a glass transition, 1,- I. Depending on the cooling/warming rate and the temperature, phase I falls exothermically into the metastable phase 11’ or the stable phase 11. The phase 11’ converts to 11 only near, during, or after the
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The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
Solid Phases of Cyclooctanone 11’ I phase transition. NMR and dielectric measurements both confirm the temporary presence of a rotor phase in this process; this is most likely to be phase I. We suggest that strains created during the 11’ I transition assist the subsequent I I1 exothermic conversion. A small endotherm at 222 K just prior to the I1 I transition is associated with impurities. The above picture of the phases and phase transitions in solid cyclooctanone is in essential agreement with results from a vibrational spectroscopy study.” Specifically,phase II’is always higher in free energy than phase 11. We note that the phases are named differently in the present and past w0rks.1~9~~ In the IR and Raman work, phase I could not be supercooled to a glassy crystal, presumably because of a smaller available cooling rate.17 The proton NMR results indicate that translational selfdiffusion occurs in rotor phase I with an activation energy of 4100 K (34kJ/mol). Reorientation in this phase is described by a smaller activation energy, 1500 K (12.5 kJ/mol). Phases I1 and 11’have nearly equal and short Tzvalues, indicating structures with little molecular motion. The longitudinal relaxation time T1 is shorter in phase 11’ than in 11, suggesting that 11’ may be less ordered and have more motion than I1 (but a small enough amplitude of motion that substantial line narrowing is not observed). Phase I below 145 K has a short Tz,indicating that rmrientational molecular motions have been frozen out. The dielectric measurements confirm that phases I1 and 11’have little molecular reorientation. The dielectric constant of I shows a Curie-like increase upon cooling, from 300 to 160 K. The dielectric constant of I remains low below 140 K,indicating that molecular motions are indeed frozen out on the time scale of 1V
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Gilson were quite valuable. The ratio transformer was generously loaned by H. Meyer. The funding of this work by NSF through Grant DMR-9024502is acknowledged.
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Acknowledgment. The skillful assistance of T. Kowalewski with the commercial scanning calorimeter is appreciated. We thank S. Gilbertson for his guidance and assistance in the purification of cyclooctanone. Communications with D.F. R.
References and Notes (1) Finke, H. L.; Scott,D. W.; Gross, M. E.; Messerly, J. F.; Waddington, G. J. Am. Chem. Soc. 1956,78, 5469. (2) Parsonage, N. G.; Staveley, L. A. K. Disorder in Crystals; Clarendon: Oxford, 1978. (3) Nakamura, N.; Suga, H.; Seki, S.Bull. Chem. Soc. Jpn. 1980,53, 2155. (4) Haines, J.; Gilson, D. F. R. J . Phys. Chem. 1990, 94, 4712. (5) Timmermans, J. J . Phys. Chem. Solids 1961, 18, 1. (6) Dunning, W. J. In The Plastically Crystalline State; Sherwood, J. N., Ed.; Wiley: New York, 1979; Chapter 1. (7) Adachi, K.; Suga, H.; Seki, S.Bull. Chem. SOC.Jpn. 1968,41,1073. (8) Suga, H.; Seki, S.J. Non-Cryst. Solids 1974, 16. 171. (9) Haines, J.; Gilson, D. F. R. J. Phys. Chem. 1990, 94, 3156. (10) Keller, R. C.; Coffey, M. S.;Lizak, M. J.; Conradi, M. S.J. Phys. Chem. 1989, 93, 3832. (1 1) Huang, Y.; Gilson, D. F. R.; Butler, I. S.J. Phys. Chem. 1991, 95, 5051. (12) Snyderman, D. M.; Adams, J. M.; Conradi, M. S.,manuscript in
preparation. (13) Strauss, H. L. Ann. Rev. Phys. Chem. 1983, 34, 301. (14) Rudman, R.; Post, B. Mol. Cryst. 1968, 3, 325. (15) Fried, F. Mol. Cryst. Liq. Cryst. 1973, 20, 1. (16) Walker, W. W.; Meritt, W. G.;Cole,G. D. J . Chem. Phys. 1972,56, 3729. (17) Huang, Y.; Butler, I. S.;Gilson, D. F. R. J . Phys. Chem. 1992,96, 1151. (18) Boden, N. in The Plastically CrystallineState; Sherwood, J. N., Ed.; Wiley: New York, 1979; Chapter 5. (19) Sands, D. E.; Day, V. W. Acta. Crystallogr. 1965, 19, 278. (20) Jordan,T.;Streib, W.;Smith,H.; Lipscomb,W. N.Acta. Crystallogr. 1964, 17, I l l . (21) Jordan, T.; Streib, W.; Lipscomb, W. N. J. Chem. Phys. 1964, 41, 760. (22) Smyth, C. P. Dielectric Behavior and Structures. McGraw-Hill: New York, 1955. (23) Davies, M. Some Electrical and Optical Aspects of Molecular Behavior; Pergamon: New York, 1965
