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Langmuir 1997, 13, 4474-4479
NMR Investigation of Molecular Motion in the Neat Solid and Plastic-Crystalline Phases of Cyclohexane1 Kenneth J. McGrath* Code 6122, Naval Research Laboratory, Washington, D.C. 20375-5342
Richard G. Weiss Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received November 14, 1996X The molecular dynamics of neat solid and plastic phase cyclohexane are investigated using solid state carbon, deuteron, and proton nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on deciphering the dynamical origin of a line width/line shape transformation in the static proton NMR spectrum of plastic phase cyclohexane near 199 K, and it is examined in terms of rotational, translational, and conformational contributions. It is shown that translational diffusion within lattice defect sites is the probable dynamical mode associated with the static proton NMR line shape modulation. The rate of conformational isomerism by ring inversion in the plastic phase is measured for the first time at two temperatures, and the time scale of this process suggests a mechanistic correlation with translational diffusion within single crystals in the plastic phase. Finally, the time scale and symmetry associated with rotational diffusion in solid-phase cyclohexane are found (respectively) to be longer and more complex than previously reported.
Introduction In spite of extensive investigations of the neat phases of cyclohexane, several aspects of the dynamics within them remain enigmatic. For instance, the neat plasticcrystalline phase is known to be thermodynamically stable between 186 and 279 K.2,3 Nevertheless, we have observed that the static proton nuclear magnetic resonance (NMR) spectrum of cyclohexane exhibits a gradual line width/ line shape transformation beginning near 199 K. Its origin is not apparent, in part because of inconsistencies among existing dynamical studies of rotational and translational self-diffusion in neat and solid phase cyclohexane; also, the rate of chair-chair conformational inversion (a possible cause of the change at 199 K) had not been previously characterized in the plastic phase. On the basis of evidence from proton, carbon-13, and deuteron solid state NMR spectra, we report a detailed analysis of the line width/line shape transformation. Experimental Section Nuclear magnetic resonance spectra were recorded on a Bruker MSL 300 spectrometer with a static field of 7.04 T. Solid state deuteron quadrupole echo and static Bloch decay NMR spectra were acquired using a probehead equipped with a 5 mm transverse coil, π/2 pulse widths of 3 µs, and a 20 µs refocusing delay (quadrupole echo experiment). Deuteron and carbon-13 magic angle spinning (MAS) experiments were performed using a double tuned, broadband probe equipped with 7mm o.d. zirconia rotors, MAS spin rates of 3000 ( 5 Hz, and π/2 pulse widths of 6 (2H) and 5 µs (13C). The proton decoupling field used in the 13C MAS experiments was 50 kHz. Static proton NMR spectra were also acquired using the MAS probe, with π/2 pulse widths of 5 µs. Temperature was maintained with a precision of (1 deg using a Bruker variable temperature controller. Calibration is based on the solid-plastic (186 K) and plastic-liquid (279 K) phase X
Abstract published in Advance ACS Abstracts, July 15, 1997.
(1) Taken in part from the Ph.D. Thesis of K. J. McGrath, Georgetown University, Washington, DC, 1994. (2) Dunning, W. J. The Plastically Crystalline State; Wiley: New York, 1979. (3) Kahn, P.; Fourme, R.; Andre, D.; Renaud, M. Acta Crystallogr. 1975, B29, 131.
S0743-7463(96)02002-1 CCC: $14.00
transitions of neat cyclohexane-h12. Samples were maintained at each temperature for at least 5 min before recording spectra; equivalent results were obtained upon heating or cooling to a target temperature. High-power radiofrequency amplifiers (maximum power output ≈ 1000 W) were used in solid state experiments requiring short pulse durations and/or high-intensity proton decoupling fields. No apodization of the free induction decays was used in any of the experiments. Cyclohexane-h12 (99.9%, Aldrich Chemical Co.) and cyclohexane-d12 (99.7%, 99.5% D, Cambridge Isotope Laboratories) were used without further purification.
Results and Discussion Static Proton NMR Analyses. Figure 1 shows the static proton NMR spectra of neat (plastic phase) cyclohexane-h12 between 245 and 184 K. An apparent Lorentzian line shape is observed at 245 K, with a line width at half-height (δν1/2) of ca. 2 kHz. At lower temperatures, where the correlation time associated with molecular reorientation is longer, less efficient averaging of proton dipolar and chemical shift anisotropic interactions results in broader resonances: the proton NMR line width is approximately 4.5 kHz at 234 K and 10 kHz at 222 K. Below 234 K, the static proton spectrum undergoes a distinct transformation (that is complete at 199 K; δν1/2 ≈ 16 kHz) to an apparent Gaussian line shape. Virtually no variation in the line shape or line width is observed between 199 and 189 K. Below 189 K, a distinctly broader (δν1/2 ≈ 37 kHz), apparently Gaussian component, superpositioned with the existing narrower peak (δν1/2 ≈ 16 kHz) is discernible. At 184 K, only the broader line shape, attributed to the plastic crystalline-solid phase change at 186 K, remains. From best fits to a mixed Lorentzian/ Gaussian function, the spectra below 199 K exhibit only Gaussian character (χ2 ≈ 28), and those above 210 K are only Lorentzian (χ2 e 2). Attempts were also made to fit the proton spectra using combinations of up to two Lorentzian and two Gaussian functions, with no limitation on offset or intensity. A reasonably precise fit to the experimental data at 199 K was obtained only when using a single, on-resonance Lorentzian (δν1/2 ) 8560 Hz, normalized intensity ) 1.8) combined with two Gaussian functions, one 4830 Hz © 1997 American Chemical Society
NMR Investigation of Molecular Motion
Figure 1. Static proton NMR spectra (64 scans) of neat cyclohexane-h12 at 184-245 K. Note the transformation from an apparent Lorentzian to an apparent Gaussian line shape between 199 and 234 K.
downfield (δν1/2 ) 7020 Hz, normalized intensity ) 1.0) and the other 4870 Hz upfield (δν1/2 ) 8114 Hz, normalized intensity ) 1.2). However, this result cannot be rationalized from the physical nature of the problem, particularly with respect to the large offsets of the Gaussian peaks. The lack of a physically reasonable good fit is not surprising given the complex nature of cyclohexane dynamics in the plastic phase: cyclohexane is unique among low entropyof-fusion plastic solids with regard to its extremely high concentration of “pipe defects”; self-diffusion within these defects is known to dominate over the bulk lattice process at low temperatures in the plastic phase and is believed to proceed by a complex jump mechanism involving, on average, 12-18 molecules.6,8 The broadening of the NMR spectra (Figure 1) as temperature is lowered can be attributed generally to the increase in proton magnetic dipole-dipole and chemical shift anisotropic interactions as molecular correlation times increase. However, the transformation in line shape between 199 and 210 K and the absence of line width and line shape variation below 199 K were of more specific interest from a molecular dynamics perspective. Pure Lorentzian NMR line shapes are generally observed in systems characterized by rapid (on the NMR time scale), isotropic molecular reorientations while pure Gaussian line shapes are characteristic of restricted molecular (4) Egelstaff, P. A. J. Chem. Phys. 1970, 53, 2590. (5) Andrew, E. R.; Eades, R. G. Proc. R. Soc. London 1953, A215, 398. (6) Hampton, E. M.; Sherwood, J. N. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2398. (7) McGrath, K. J.; Weiss, R. G. J. Phys. Chem. 1993, 97, 2497. (8) Chadwick, A. V.; Sherwood, J. N. J. Chem. Soc., Faraday Trans. 1 1972, 68, 47.
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motions. Thus, the onset of line width (and line shape) variation in Figure 1 at 199 K suggests a molecular reorientational averaging process which is occurring at a rate approximately equal to the inverse static proton NMR line width (e.g., 16 kHz, the relevant NMR time scale at 199 K). On the basis of published data for plastic-crystalline cyclohexane, rotational reorientation ought to be too rapid (>106 s-1 4,5) and translational self-diffusion should be too slow (
