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Jul 18, 2014 - Kinetics of Polymorphic Transitions of Cyclohexanol Investigated by Terahertz Absorption Spectroscopy. Hal Suzuki*, Hiromichi Hoshina, ...
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Kinetics of Polymorphic Transitions of Cyclohexanol Investigated by Terahertz Absorption Spectroscopy Hal Suzuki,* Hiromichi Hoshina, and Chiko Otani RIKEN, 519-1399 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-0845, Japan ABSTRACT: The phase behaviors and kinetics of the polymorphic transitions of cyclohexanol, C6H11OH, were investigated by terahertz absorption spectroscopy over the temperature range of 150−330 K. A new phase was found, labeled phase I′, which is more stable than the previously observed phase I but less stable than phase III′. The kinetics of the irreversible transitions from phase I to I′, phase I to III′, and phase III′ to III were analyzed using Avrami’s theory. The transition geometries of the first two transitions were found to be one-dimensional, while the latter was two-dimensional. The transitions from phase I to III and phase III to II were revealed to occur through two-step processes. The mechanism of these irreversible transitions is discussed in relation to the formation and dissociation of hydrogen bonds between the hydroxyl groups as well as the steric restriction effects of the cyclohexyl ring.

1. INTRODUCTION Predicting the structure and macroscopic properties of molecular crystals is very difficult for several reasons.1 First, there are many possible arrangements of molecules in space, and hence, identifying the actual arrangement is challenging.2 Second, organic molecules are often packed in different crystalline arrangements with similar thermodynamic stabilities, resulting in polymorphic behavior.3 Third, the crystal structures to be found are determined primarily by kinetics rather than thermodynamics, and the kinetics of nucleation and crystal growth are poorly understood.4 The first problem can be addressed by simplifying the molecular structure. For example, if the molecules are connected by covalent bonds, i.e., polymerized, the number of possible orientations of the molecular units will be significantly reduced. If the molecules have spherical structures, on the other hand, the effects of steric restriction will be reduced, so that the molecular orientation will be governed by specific intermolecular interactions such as dipole−dipole interactions, electrostatic forces, and hydrogen bonding. Among these interactions, hydrogen bonding is best suited for the control of molecular arrangement, because it is relatively strong (1−50 kJ mol−1)5 and extends over short distances with characteristic directionality. The role of hydrogen bonding in condensed molecular systems has been extensively studied by infrared (IR) and Raman spectroscopy.6−8 Via these techniques, the local stretching and bending modes in the donor or acceptor groups of the hydrogen bonds can be analyzed. Recently, interest in the low-frequency hydrogen bonding modes (50−300 cm−1) has also grown.9−13 These modes are related to the elongations that change the distance between the donor and acceptor groups and the relative orientations of the hydrogen-bonded groups. Such low-frequency modes are expected to provide direct insight into the structure of the hydrogen bonds as well © 2014 American Chemical Society

as the processes of bond formation and dissociation. Terahertz absorption spectroscopy is one of the spectroscopic techniques used for such low frequencies.14−17 [Below, the term terahertz absorption spectroscopy (spectrum) will be abbreviated as THz spectroscopy (spectrum).] In our previous work, we investigated the phase changes of hydrogen-bonded polymers poly(3-hydroxybutyrate) (PHB)18−21 and polycaprolactam (Nylon-6)22 by THz spectroscopy. The study of PHB suggested that, during isothermal crystallization from the supercooled liquid, CO···H−C hydrogen bonds are formed before the lamellar skeletal structure is established.21 In the study of Nylon-6, N−H··· OC hydrogen bonds were shown to form even in the supercooled liquid phase, and the structural transition in the crystalline phase, known as the Brill transition, gradually takes place over a wide temperature range.22 These results confirmed that the formation of hydrogen bonds is a leading factor in the aggregation for polymers. In the work presented here, we investigate the polymorphic phase transitions of cyclohexanol (C6H11OH) using THz spectroscopy. Cyclohexanol was chosen because it has a spherical body and the ability to form hydrogen bonds through its hydroxyl group. The purpose of this study was to reveal the extent to which hydrogen bonding contributes to the kinetics of phase transformation when steric effects are limited. Cyclohexanol shows several crystalline phases with different hydrogen bonding configurations.23 These phases exhibit thermodynamic stabilities that vary with temperature and result in complicated crystalline polymorphism. Some crystals can be supercooled to achieve metastable states, which are kinetically Received: May 14, 2014 Revised: July 9, 2014 Published: July 18, 2014 4087

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Figure 1. (a) Schematic diagram of cyclohexanol phase behaviors.27 The free energies of phases I−III and the liquid phase are plotted vs temperature using solid lines. The free energy of phase III′ is given by a dashed line because its thermodynamic stability has not been established. (b) Molecular arrangements in the four crystalline phases postulated in previous studies.23

that the transition from phase III to II is driven by relief of the strain from this close contact. Phase III can also be grown through an intermediate phase III′, which can be obtained by cooling phase I to 215 K. Phase III′ is kinetically unstable and can be sustained for only 15 min at this temperature before being transformed to phase III. The crystal structure of phase III′ is a monoclinic unit cell, and the molecules adopt equatorial conformations that are sequentially linked by hydrogen bonds to form 3-fold infinite helical chains (Figure 1b).23

stable. Depending on their thermal history, those metastable crystals irreversibly transform into more stable phases. We are interested in how hydrogen bonding contributes to those irreversible transformations. In this study, THz spectroscopy was chosen as a measurement technique, because not only the low frequency hydrogen bonding modes can be observed, but also because the optical phonon modes of the crystals can be detected, which may aid in the identification of polymorphic phases.24,25 In this work, details of the phase behaviors and transition kinetics of cyclohexanol revealed by THz spectroscopy are described. The phase behavior of cyclohexanol has been extensively studied for more than 80 years using the techniques of calorimetry,26−29 X-ray and neutron diffraction,23,30−32 infrared, THz (far-infrared), and Raman spectroscopies,33−35 thermal conductivity,36 dielectric permittivity,37,38 and observation of morphology.39 Figure 1 schematically shows (a) a diagram of the thermodynamic stability of cyclohexanol as a function of temperature and (b) the molecular arrangements in its four crystalline phases.23,27,29 The liquid transforms to a plastic crystal (phase I) at a Tfus of 298 K, which can be supercooled to vitrify below a Tg of 148 K. The molecules in phase I are arranged in a face-centered cubic (fcc) lattice. It has been proposed that both axial and equatorial configurations of the hydroxyl group are included, and the orientation of the cyclohexyl ring is disordered.23 Below 265 K, phase I is metastable, but the perfectly ordered crystalline phase II is the most stable.32 The structure of phase II is indexed on a pseudocubic tetragonal unit cell, in which the hydroxyl groups of the molecules adopt equatorial conformations.23 The molecules are linked by hydrogen bonds around a 4-fold axis into tetrameric rings (Figure 1b). Phase II is obtained via another metastable phase, III, which can be obtained by quenching phase I to 100 K and then heating it to ∼195 K. The transformation from phase I to III is irreversible and is known to be a two-step process.32 Phase III shows a transition to phase I at 245 K or irreversibly transforms into phase II after being heated to 220−240 K. Phase III is also perfectly ordered,27 and the crystal structure is indexed on a monoclinic unit cell in which the molecules adopt equatorial conformations and pack to form infinite hydrogen-bonded chains (Figure 1b).23 The hydrogen bond distance is short, 2.587 Å, and it is suggested

2. EXPERIMENTAL SECTION Cyclohexanol, C6H11OH (98%), was purchased from Wako Pure Chemical Industries and was used for the measurements without being further purified. The cyclohexanol sample in the liquid state was placed on a highly resistive Si plate that was horizontally attached on a temperature-controlled heating stage (Linkam 19113L). A transmission-type optical geometry was adopted, in which the THz waves passed through the hole in the stage and were transmitted through the Si plate and the sample. Highly resistive Si was used as the plate material because it is transparent in the THz frequency region and is highly thermally conductive. The sample thickness was estimated to be 100−200 μm by comparing its absorption intensity with that of a sample of a known thickness (0.5 mm) in a high-density polyethylene (HDPE) cell at room temperature. The sample temperature was indirectly measured using a platinum resistor sensor attached to the heating stage. The temperature difference between the stage and the Si surface was calibrated using a copper-constantan thermocouple. If the errors in calibration were included, the sample temperature was accurate within ±1 K. The THz spectra (1−11 THz) were obtained by Fourier transform far-infrared (FT-FIR) spectroscopy with a frequency resolution of 0.06 THz. The measurements were conducted using a JASCO FARIS instrument equipped with a high-pressure mercury lamp, a Mylar beam splitter, and a Si bolometer. The sample chamber was purged with dry nitrogen gas to reduce absorption of water vapor.

3. RESULTS AND DISCUSSION 3.1. Re-examination of Phase Behaviors and THz Spectra. The THz spectra of cyclohexanol in phases I−III have been obtained in previous studies.32,34,35 We re-examined the phase behaviors and THz spectra for all the crystalline phases and tried to obtain a spectrum of phase III′, which had not been obtained before. Phase I was obtained by cooling the liquid cyclohexanol below 298 K. Figure 2e shows the THz spectrum of phase I at 4088

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nucleation of phase III. The THz spectrum of phase III′, obtained for the first time in this study, is shown in Figure 2c. Peaks are observed at 3.3, 4.7, 5.4, 6.0, 6.4, 7.2, 10.2, and 10.6 THz, which are still broad but more intense than those in phases I and I′. The broad features of the THz peaks indicate that some orientational disorder remains. Phase III was obtained through several thermal paths. When supercooled phase I was annealed at 223 K, phase III was obtained by a direct irreversible transformation. Phase III was also obtained through phases I′ and III′ by annealing those phases above 223 K for ∼1 h. The THz spectrum of phase III at 223 K is shown in Figure 2b. The spectral shape coincides with that obtained in previous studies;34 i.e., there are eight large peaks at 3.0, 4.5, 5.1, 5.9, 7.5, 10.0, 10.3, and 10.7 THz, together with five small but sharp peaks below 3 THz. Phase II was obtained by annealing phase III at 239 K. This is an irreversible transformation, which is variable in occurrence depending on the thermal history and some minor conditions of measurement. Figure 2a shows the THz spectrum of phase II at 223 K. There are three intense peaks at 3.9, 5.7, and 10.2 THz. The peak maxima are not shown because of saturation. The spectral shape is consistent with that of previous studies.34 3.2. Kinetics of Polymorphic Transitions. On the basis of the phase relationships established in section 3.1, the transition kinetics among polymorphic phases was investigated by time-dependent THz spectroscopy. The results were analyzed using Avrami’s model and two-dimensional plots of first derivatives of absorbance with respect to time. The relationship between the geometry of the transitions and the motifs of the hydrogen bonding network is discussed. 3.2.1. Phase I to I′. Figure 3 shows the time evolution of the THz spectra for the transformation of phase I to I′ at 185 K.

Figure 2. THz spectra of cyclohexanol in (a) phase II (247 K), (b) phase III (223 K), (c) phase III′ (200 K), (d) phase I′ (185 K), and (e) phase I (274 K). The ordinate is for phase I. The results for the other four phases are successively shifted upward by 100 cm−1.

274 K, which resembles that previously reported at 223 K,32 in which broad peaks are observed at 3.0, 4.3, 5.4, 7.0, 9.3, 10.2, and 10.5 THz. These peaks also appear in the liquid phase, indicating that they are not related to the optical phonon modes specific to the fcc lattice of phase I. Instead, they are associated with low-frequency hydrogen bonding modes and/ or very soft intramolecular modes. The absence of the optical phonon mode suggests that the orientation of the molecular dipole originating from the hydroxyl group is disordered in phase I. Supercooled phase I was irreversibly transformed to another crystalline phase by annealing at 185 K for 30 min. This crystalline phase seemed, at first, to be that identified as phase III′ in previous works or a mixture of phases I and III′. However, it was later revealed to be a new phase, because some peaks characteristic of phase III′ were not observed in the spectrum. This phase (designated phase I′) was kinetically stable and sustainable for more than 1 h at 185 K. The spectrum of this phase at 185 K is shown in Figure 2d, which is similar to that of phase I, except that there are additional peaks at 3.8 and 6.7 THz for phase I′ while the peak at 9.3 THz is absent. In addition, the peak at 4.3 THz is sharper than that in phase I. The additional THz peaks can be explained by a partial ordering of the molecular orientation, which could give rise to an optical phonon response. In previous work, the peak at 9.3 THz in phase I was assigned to the bending mode of the CO− ring angle of the axial conformer of the unbonded molecule, which shifts ∼2.4 THz upon formation of hydrogen bonds.34 Therefore, the absence of the 9.3 THz peak in phase I′ suggests that the molecules adopt equatorial conformations to form hydrogen bonds. The larger absorbance at ∼11 THz in phase I′ may be the tail of the peak shifted up to 11.7 THz. When phase I′ was heated, it was irreversibly transformed to another state at 200 K, which was identified to be a mixture of phases III and III′. Pure phase III′ was successfully obtained by annealing supercooled phase I at 200 K for 15 min without experiencing the lower temperatures that would induce the

Figure 3. Time variations in the THz spectra of cyclohexanol during the transformation from phase I to I′ at 185 K. The inset shows the normalized peak intensity at 3.8 THz plotted vs time. The solid curve shows the results of fitting using Avrami’s model.

Clearly, the magnitudes of the peaks at 3.8, 4.3, and 6.7 THz increase, the peaks at 3.0 and 5.5 THz become sharper, and the magnitude of the peak at 9.3 THz decreases with time. The time-dependent peak intensities were analyzed by applying Avrami’s theory,40,41 wherein the volume fraction of the transformation can be described in terms of a simple time function: 4089

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x = 1 − exp( −kt n)

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(1)

where x is the volume fraction of the transformed phase, t is time, and k and n are time-independent constants. k depends both on the nucleation probability and on the rate of grain growth, and n depends on the geometry of the crystal growth configuration.42 If nucleation takes place only at the beginning of the transformation, it will be growth-dominated, and n will have a value of 1, 2, or 3 depending on the restrictive geometry of the growth to one, two, or three dimensions, respectively. If, on the other hand, nucleation continues at a constant rate during the transformation, the value of n will be equal to the dimension of growth increased by unity; i.e., n will have a value of 2, 3, or 4. In this study, the value of n at each transition was analyzed to discuss the transition geometry. The value of k was not analyzed because the kinetic rate of the transformation depends on the temperature and thermal history, which is beyond the scope of this work. Assuming that the THz peak intensity is proportional to the volume fraction of the phase, time-dependent normalized peak intensity X(t) at 3.8 THz can be directly fit using eq 1 (inset of Figure 3) to obtain an n value of 1.05 ± 0.08. The normalization is conducted using the formula X (t ) =

I(t ) − I(0) I(∞) − I(0)

Figure 4. Time variations in the THz spectra of cyclohexanol during the transformation from phase I to III′ at 200 K. The inset shows the fraction of phase III′ plotted vs time. The solid curve shows the results of fitting using Avrami’s model.

Avrami’s index (n ≈ 1) indicates that the transition from phase I to III′ is growth-dominated with one-dimensional geometry. Considering that the molecules in phase III′ are sequentially linked by hydrogen bonds forming 3-fold helical chains, the formation of the chains may be the leading factor in the growth of phase III′. 3.2.3. Phase I to III. The transition from phase I to III is known to be a two-step process.23,34 To reveal how the two processes take place, we analyzed the time-dependent THz spectra at the transition at 223 K (Figure 5a) by taking their first derivatives with respect to time. Figure 5b shows the first derivatives plotted two-dimensionally versus frequency and time. The red area represents positive values of the first derivative, i.e., the increase in spectral intensity with time, and the blue area shows the negative values, i.e., the decrease in intensity. It should be noted that such a two-dimensional plot resembles a synchronous perturbation-correlation movingwindow two-dimensional (PCMW2D) correlation spectrum, which shows a direct correlation between the observed spectral intensity changes with time.43 Figure 5b clearly shows that the transition occurs in a twostep manner, where the second process becomes dominant after 15 min. In the first step, the intensities at around 4.7, 5.1, and 5.9 THz significantly increase, whereas those from 3.5 to 4.2 THz and at 9.3 THz decrease. The increases in intensity at 4.7 and 5.9 THz indicate that the intermediate state is phase III′. The intensity increase at 5.1 THz indicates that some part of phase III has also been created during the first stage. In the second step, the intensities at 4.7, 5.4, and 6.4 THz decrease whereas those at 3.0, 4.5, and 7.5 THz increase. The decreasing peaks are characteristic of phase III′, indicating that the temporarily formed phase III′ is transformed to phase III. 3.2.4. Phase III′ to III. Figure 6 shows the time-dependent THz spectra of the transformation of phase III′′ to III at 223 K. It should be noted that the spectral shape at 0 min is different from that of pure phase III′ (Figure 2); for example, small peaks exist at 4.5, 5.1, and 7.5 THz. These peaks are characteristic of phase III, indicating that phase III begins to grow even before the temperature reaches 223 K. The peak at 7.5 THz was analyzed using a modified Avrami’s model

(2)

where I(t) is the absorption intensity at time t and I(0) and I(∞) are those before and after the transition, respectively. The same analysis was also conducted for the peak at 6.7 THz, yielding an n value of 1.08 ± 0.08. The peak at 9.3 THz was similarly analyzed, but eq 3 was used instead of eq 1, because the peak belongs to phase I, which disappears over time. x = exp( −kt n)

(3)

The n value for the peak at 9.3 THz was 1.18 ± 0.06. In terms of Avrami’s theory, these results indicate that the transformation from phase I to I′ is growth-dominated with one-dimensional geometry; i.e., the conformational change from axial to equatorial, accompanied by the formation of hydrogen bonds between hydroxyl groups, propagates onedimensionally. 3.2.2. Phase I to III′. Figure 4 shows the time evolution of the THz spectra for the transformation of phase I to III′ at 200 K. Significant peak growth at 3.3, 4.7, 5.4, 6.0, and 6.4 THz can be observed, whereas the intensities at around 3, 4, and 8 THz decrease with time. In addition, the peak at 9.3 THz vanishes, which similarly occurred in the transition from phase I to I′. This indicates that the unbonded axial conformers convert to the hydrogen-bonded equatorial conformers. These results were also analyzed on the basis of Avrami’s theory. Because it was difficult to isolate a single peak in the spectra, the degrees of transformation were not estimated from the peak intensities but were obtained from the ratios of spectral intensities for the two phases. The THz spectrum between 4 and 5 THz at each time point was represented by a combination of the spectrum at 0 min (phase I) and 15 min (phase III′), and the degree of transformation was estimated from the ratio of the combination. The fraction of phase III′ obtained in this way is plotted versus time in the inset of Figure 4, which was successfully fit by eq 1 (solid curve) to obtain an n value of 1.25 ± 0.10. The decreasing intensity of the peak at 9.3 THz was also analyzed using eq 3 and gave an n value of 1.16 ± 0.10. 4090

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Figure 5. (a) Time variations in the THz spectra of cyclohexanol during the transformation from phase I to III at 223 K. (b) First derivative of the THz spectra with respect to time, plotted two-dimensionally vs frequency and time. The red area represents an increase in spectral intensity and the blue area a decrease. The unit of the label number for the color bar is inverse minutes. The transition clearly occurs in a two-step manner, and the second process becomes dominant after 15 min.

upon whether the sample experiences low temperatures. Because the number of nuclei depends on the thermal history in general, the constant growth speed of phase III, independent of the thermal history, indicates that the nucleation rate of phase III at 223 K is much faster than the growth rate. The irreversible transition from phase III′ to III is, therefore, thought to be growth-dominated with a two-dimensional geometry. According to the prior structural study,23 molecules in phase III form infinite hydrogen-bonded chains. The two-dimensional growth of phase III from III′, therefore, suggests that not only hydrogen bonding but also the steric restriction of the cyclohexyl ring plays an important role in the transition kinetics. 3.2.5. Phase III to II. The transformation of phase III to II at 239 K was first analyzed by Avrami’s model. Figure 7a shows the time evolution of the THz spectra. The interconversion of the peaks characteristic of phases III and II is clearly observed. Interestingly, when Avrami’s model was applied to the increasing intensity of the peak at 2.0 THz and the decreasing intensity of the peak at 7.3 THz, different n values were obtained, 3.32 ± 0.46 and 1.04 ± 0.10, respectively, which suggests that the transformation is a multistep process. To confirm this, we analyzed the THz spectra by taking their first derivatives with respect to time, as in the case of the transformation from phase I to III. The two-dimensional plot of the first derivatives versus frequency and time (Figure 7b) clearly reveals the two-step transformation. In the first process (0−40 min), the intensities of all the peaks due to phase III decrease, whereas in the second stage (>40 min), the intensities of the peaks characteristic of phase II increase. This result suggests that phase II is formed after the collapse of phase III. The intermediate state does not show a characteristic structure in the THz spectrum, indicating some disordered state, such as a plastic crystal. Because the transition from phase III to I occurs at 245 K, it is not surprising that a significant entropic contribution to the thermodynamic stability makes some transient disordered state more stable than ordered phase III at 239 K.

Figure 6. Time variations in the THz spectra of cyclohexanol during the transformation from phase III′ to III at 223 K. The inset shows the normalized peak intensity at 7.5 THz plotted vs time. The solid curve shows the results of fitting using Avrami’s model.

x = 1 − exp[−k(t − t0)n ]

(4)

where t0 is the onset time of the transformation. The timedependent normalized peak intensity was well fit by eq 4 (inset of Figure 6) to give an n value of 2.09 ± 0.25. For the normalization in eq 2, the spectral intensity of phase III′ at 200 K was used as I(0). The fact that n ≈ 2 indicates that the transition from phase III′ to III is two-dimensional if nucleation takes place only at the beginning of the transformation or one-dimensional if nucleation continues at a constant rate during the transformation. To verify whether the transition is nucleationdominated or growth-dominated, another set of measurements was conducted in which phase III′ was cooled once to 156 K and then annealed at 223 K to transform it to phase III. The transition time scale from phase III′ to III was similar to that of the previous one; i.e., the transition time scale does not depend 4091

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Figure 7. (a) Time variations in the THz spectra of cyclohexanol during the transformation from phase III to II at 229 K. (b) First derivative of the THz spectra with respect to time, plotted two-dimensionally vs frequency and time. The transition occurs in a two-step manner, and the second process becomes dominant after 40 min.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Young Scientists (B) 25871138 from the Japan Society for the Promotion of Science (JSPS) and an Industry−Academia Collaborative R&D grant from the Japan Science and Technology Agency (JST).

The Avrami’s index n = 1.04 obtained from the decreasing intensity of the peak at 7.3 THz indicates that the collapse of phase III is one-dimensional, which may be dominated by the breaking of the infinite hydrogen-bonded chains. This is consistent with the suggestion from the structural study23 that the transition from phase III to II is driven by the relief of strain from the close contact due to hydrogen bonding.

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(1) Organised Molecular Assemblies in the Solid State, 1st ed.; Whitesell, J. K., Ed.; John Wiley & Sons: Chichester, U.K., 1999. (2) Price, S. L. Chem. Soc. Rev. 2014, 43, 2098. (3) Braga, D.; Grepioni, F.; Maini, L.; Polito, M. Molecular Networks 2009, 132, 25. (4) Dunitz, J. D.; Bernstein, J. Acc. Chem. Res. 1995, 28, 193. (5) Hydrogen Bond: Recent Developments in Theory and Experiment; Schuste, P., Ed.; Elsevier Science Publishing Co. Inc.: Amsterdam, 1976. (6) Egorochkin, A. N.; Skobeleva, S. E. Russ. Chem. Rev. 1979, 48, 1198. (7) Fujii, A.; Mizuse, K. Int. Rev. Phys. Chem. 2013, 32, 266. (8) Kim, Y. S.; Hochstrasser, R. M. J. Phys. Chem. B 2009, 113, 8231. (9) Vaz, P. D.; Nolasco, M. M.; Gil, F.; Ribeiro-Claro, P. J. A.; Tomikinson, J. Chem.Eur. J. 2010, 16, 9010. (10) Nibbering, E. T. J.; Elsaesser, T. Chem. Rev. 2004, 104, 1887. (11) Yamamoto, S.; Morisawa, Y.; Sato, H.; Hoshina, H.; Ozaki, Y. J. Phys. Chem. B 2013, 117, 2180. (12) Pal, S.; Bandyopadhyay, S. J. Phys. Chem. B 2013, 117, 5848. (13) Vaz, P. D.; Nolasco, M. M.; Ribeiro-Claro, P. J. A. Chem. Phys. 2013, 427, 117. (14) El Haddad, J.; Bousquet, B.; Canioni, L.; Mounaix, P. TrAC, Trends Anal. Chem. 2013, 44, 98. (15) Lee, Y.-S. Principles of Terahertz Science and Technology; Springer: New York, 2009. (16) Ueno, Y.; Ajito, K. Anal. Sci. 2008, 24, 185. (17) Yamaguchi, S.; Tominaga, K.; Saito, S. Phys. Chem. Chem. Phys. 2011, 13, 14742. (18) Hoshina, H.; Morisawa, Y.; Sato, H.; Minamide, H.; Noda, I.; Ozaki, Y.; Otani, C. Phys. Chem. Chem. Phys. 2011, 13, 9173. (19) Hoshina, H.; Morisawa, Y.; Sato, H.; Kamiya, A.; Noda, I.; Ozaki, Y.; Otani, C. Appl. Phys. Lett. 2010, 96, 101904. (20) Hoshina, H.; Ishii, S.; Yamamoto, S.; Morisawa, Y.; Sato, H.; Uchiyama, T.; Ozaki, Y.; Otani, C. IEEE Trans. Terahertz Sci. Technol. 2013, 3, 248.

4. CONCLUSIONS The phase behaviors and kinetics of the polymorphic transitions of cyclohexanol were investigated by THz spectroscopy. A new metastable phase I′ that is more stable than phase I but less stable than phase III′ was found. The basic structure of phase I′ seems similar to that of phase I, but the molecules adopt equatorial configurations to form hydrogen bonds. Phase I′ grows one-dimensionally. The THz spectrum of phase III′ was obtained for the first time. Phase III′ also grows onedimensionally, indicating that the formation of the hydrogenbonded chains is the leading factor in the transition. The transition from phase III′ to III progresses two-dimensionally, suggesting that not only the formation of hydrogen-bonded chains but also steric restriction effects contribute to the kinetics of the transformation for the growth of the ordered crystal. The transition from phase I to III at 223 K was verified to occur in two steps, where the intermediate phase was ascertained to be phase III′. The transition from phase III to II was also found to be a two-step process. The THz spectrum of the intermediate state did not show a characteristic structure, indicating some disorder. The collapse of phase III takes place one-dimensionally, which may be governed by the breaking of the hydrogen-bonded chains.

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REFERENCES

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Corresponding Author

*Phone: +81-22-228-2124. Fax: +81-22-228-2128. E-mail: h_ [email protected]. Notes

The authors declare no competing financial interest. 4092

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dx.doi.org/10.1021/cg500706f | Cryst. Growth Des. 2014, 14, 4087−4093