J. Phys. Chem. B 1998, 102, 6493-6498
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Orientational Diffusion of Methyl Groups in Crystalline CH3F: An Infrared Study A. A. Stolov,† W. A. Herrebout,‡ B. J. van der Veken,*,‡ and A. B. Remizov§ Department of Chemistry, Kazan State UniVersity, KremleVskaya Street 18, Kazan 420008, Russia; Department of Chemistry, UniVersitair Centrum Antwerpen, Groenenborgerlaan 171, B2020 Antwerp, Belgium; and Kazan State Technological UniVersity, Karl Marx Street 68, Kazan 420015, Russia ReceiVed: May 5, 1998; In Final Form: June 22, 1998
The mid-infrared (4000-400 cm-1) spectra of polycrystalline methyl fluoride have been investigated at temperatures between 8 and 85 K. Least-squares band fitting was performed for all fundamental bands belonging to symmetric and asymmetric vibrations of the methyl group, and the contributions of inhomogeneous broadening and vibrational and orientational relaxation to the bandwidths have been evaluated. From the temperature dependence of the bandwidths, using the modified Rakov approach, the activation enthalpy, ∆H*, and activation entropy, ∆S*, for orientational diffusion of the methyl groups have been determined to be ∆H* ) 0.85(7) kJ mol-1 and ∆S* ) -7(1) J mol-1 K-1.
1. Introduction Methyl halides, CH3X, are very useful as model compounds for testing both theoretical and experimental approaches to various problems of molecular relaxation phenomena.1-3 These molecules are symmetric tops with noticeably different moments of inertia for spinning (i.e., rotation about the symmetry axis, ||) and tumbling (i.e., rotation of the symmetry axis, ⊥) motions. While for an isolated CH3X molecule the spinning rotation is free, in the crystalline phase the rotation is hindered by a barrier, whose origin is completely intermolecular. The orientational dynamics of CH3 and NH3 groups in crystals has been of interest over many years.4-22 There are two main aspects that attracted major attention. First, it is believed that data on orientational diffusion are useful in understanding the origin of intermolecular interactions in crystals.11,15 Second, the small masses of atoms involved in methyl reorientations and the relatively low barriers to internal rotation provide a strong possibility for quantum tunneling. Thus, the lowtemperature solid methyl halides represent a limited number of molecular systems for which the tunneling phenomenon can be investigated experimentally.12-19,22 The crystal structure of CH3F has been found to be monoclinic, with four molecules in the unit cell.23 The methyl quantum rotation in crystalline CH3F has been studied previously by NMR16 and inelastic neutron scattering spectroscopy.18 It was shown, as was later confirmed by the crystal structure determination,23 that all methyl groups are equivalent, and their rotational potential can be well described by a 3-fold cosine function with a barrier equal to 2.27 kJ mol-1.18 Unfortunately, no attempts were made to determine the spinning diffusion constant from the obtained data. Complementary and independent information on the orientational dynamics of methyl groups can be obtained using vibrational spectroscopy.1,21,24-26 Although the vibrational spectra of crystalline methyl fluoride have been studied several * Corresponding author:
[email protected]. † Kazan State University. ‡ Universitair Centrum Antwerpen. § Kazan State Technological University.
times,27-31 to our knowledge the infrared and Raman spectra have not been used to obtain information on orientational diffusion. The present work is devoted to a study of the shapes and widths of the absorption bands in the mid-infrared (IR) spectra of solid methyl fluoride. On the basis of the spectra obtained at different temperatures (8-85 K), the activation parameters of spinning diffusion and the magnitude of the diffusion constant are determined. 2. Experimental Section Methyl fluoride was purchased from PCR, Inc., and was used without further purification. In the vapor-phase IR spectrum of this compound no impurities could be detected. The sample was grown by vapor deposition on a CsI substrate mounted in Leybold Heraeus ROK 10-300 cryostat. The temperature of the substrate was measured by a Si diode with an accuracy of 0.1 K. It is assumed that the sample had the same temperature as the substrate. A slow deposition, over a time period of approximately 1 h, of CH3F vapors on the CsI plate, maintained at 8 K, yielded an amorphous solid film of the substance. This film was subsequently crystallized into a polycrystalline film by annealing at 45 K for 10 min. The investigations were carried out at temperatures between 8 and 85 K. Use of temperatures higher than 85 K was prevented by the fast evaporation of the sample. The IR spectra were recorded using a Bruker IFS 113v Fourier transform infrared spectrometer, equipped with a Globar source, Ge/KBr beam splitter, and a broad-band MCT detector. The interferograms, averaged over 200 scans, were boxcar apodized and Fourier transformed using a zero filling factor of 2. To investigate possible effects of instrumental resolution on the obtained bandwidths, the measurements were performed at 0.17, 0.08, and 0.04 cm-1 resolution. Even for the narrowest band registered at the lowest available temperature (fwhm ) 0.53 cm-1), no effects of the instrumental resolution were detected. Therefore, no deconvolution of the obtained spectra was performed.
S1089-5647(98)02128-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/01/1998
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Figure 1. IR spectra in the region of ν1 and ν4, obtained at the resolution of 0.17 cm-1, and the result of the fitting using Voigt components. The top and the bottom spectra were registered at 8 and 85 K, respectively. In each case the top trace is the superposition of experimental and simulated spectra, while the lower traces give the individual components.
Stolov et al.
Figure 2. IR spectra in the region of ν2 and ν5 and the result of the fitting using Voigt components. The top and the bottom spectra were registered at 8 and 85 K, respectively. In each case the top trace is the superposition of experimental and simulated spectra, while the lower traces give the individual components.
3. Results and Discussion IR spectra obtained at different polarization of the incident beam showed a weak dichroism, indicating that the sample was an oriented polycrystalline film. The procedure of the crystal growth was repeated several times, reproducing the axis of orientation and the dichroic ratios for various absorption bands. Similar phenomena of nonrandom orientation with respect to the substrate have been noticed earlier for a wide number of molecular crystals.21 The isolated molecule CH3F belongs to the point group C3V and exhibits six normal modes. Three of the normal vibrations (ν1, ν2, ν3) are of symmetry A1, and the other three (ν4, ν5, ν6) are doubly degenerate (E symmetry). The crystal belongs to the C52h space group, and each molecule occupies a general site in the primitive cell, with local C1 symmetry.18 The reduction of the symmetry of the molecule to the site symmetry leads to removal of the degeneracy of the E modes. This and coupling of the molecules in the primitive unit cell give rise to splitting of the vibrational bands into components, which are also called vibrons.32 Internal modes of A1 species give rise to four components of Ag, Bg, Au, and Bu symmetry, the Au and Bu species being IR-active. Internal modes of the E species yield eight components, of which two have Au and two have Bu symmetry, and are IR-active. In Figures 1-3 the CH3 stretching and deformational regions of the spectra are shown at the lowest (a, 8 K) and at the highest temperature (b, 85 K). Because the band positions are temperature-dependent, for simplicity we will describe each component by its peak wavenumber in the spectra recorded at 8 K. In the CH3 stretching region separate bands due to the ν1 and ν4 modes are observed. The former is represented by only one peak at 2972.0 cm-1, while the latter splits into at least two components, at 3034.2 and 3028.8 cm-1. Also, a weak shoulder is observed near 3021.7 cm-1, which has been assigned as a component of ν4.27 The fundamentals ν2 and ν5 fall in the 1480-1440 cm-1 region shown in Figure 2. Strong overlap of ν2 and ν5 in the spectrum of the liquid CH3F makes it difficult to assign the vibrons observed in the solid-phase spectrum.27-29 In the most recent study,30 the components at 1454.3 and 1446.9 cm-1 were
Figure 3. IR spectra in the region of ν6 and the result of the fitting using Voigt components. The top and the bottom spectra were registered at 8 and 85 K, respectively. In each case the top trace is the superposition of experimental and simulated spectra, while the lower traces give the individual components.
ascribed to the ν2 mode and those at 1474.3, 1463.4, and 1459.4 cm-1 to the ν5 mode. The temperature broadening of the components can be used to verify this assignment, and it will be shown below that such an analysis proves that the vibron at 1446.9 cm-1 must be assigned to ν2, while all the others must be due to ν5. The small perturbations in the spectra in the 1480-1440 cm-1 region are due to incomplete compensation of the atmospheric water absorption in our experiments. The region of ν6 is shown in Figure 3. In the crystalline phase this band splits into three components, observed at 1184.8, 1182.2, and 1175.4 cm-1. In addition, weak bands are present at 1173.2 and at 1170.9 cm-1, and a shoulder can be seen on the low-frequency side of the 1175.4 cm-1 band. These weak features probably are due to the 13CH3F isotopomer present in the sample. It was mentioned above that no effects of the instrumental resolution on the obtained spectra have been detected in the
Methyl Groups in Crystalline CH3F spectra. This was inferred from spectra recorded at the same temperature, 8 K, using 0.08 and 0.17 cm-1 resolution. The numerical subtraction of one of these spectra from the other shows that there is no difference between them, confirming that the use of resolutions higher than 0.17 cm-1 was unnecessary. From the above discussion follows that for each internal mode of CH3F less vibrons are observed than expected on the basis of the symmetry of the crystal. This means that either some vibrons are very weak and escape detection or they occur nearly degenerate with another vibron. Thus, for the A1 vibrations ν1 and ν2 two components are predicted, but even at the lowest temperature and highest resolution no signs of splitting into doublet of the single observed bands were detected. Also, if two components with different intensities and slightly different band centers would be present, the resulting band profiles should be asymmetric. No such asymmetry was observed, and it is, therefore, concluded that if a second component is present with measurable intensity, the two components have very nearly the same position and they have very close shapes. Then, little error is made by treating the envelopes of the observed A1 transitions as single vibrons. For the E-type transitions ν4 and ν6 triplets, instead of the predicted quadruplets, are observed. This means that at most one of the observed bands is composed of two vibrons. From the analysis below it will be seen that different components of the same internal mode lead to very similar results on the rotational diffusion of CH3F. Hence, the fact that one of them is a doublet does not appear to influence the observed contour. Therefore, also for the E-type modes the observed bands are treated as single vibrons. Widths and shapes of vibrational bands in crystals are being studied extensively.32-42 It is well-known that the vibrational band shape is a result of both inhomogeneous and homogeneous broadening.32,35,40,42 There are several factors that may contribute to the inhomogeneous broadening. One of the factors is the above-mentioned accidental degeneracy of two or more vibrons. The inhomogeneous broadening can be also due to crystal symmetry distortions, which may be caused by the presence of impurities, including isotopomers. For the polycrystalline CH3F sample the main contribution to the inhomogeneous broadening is, probably, due to symmetry distortions, which occur at the boundaries of single-crystal domains. Vibrational and orientational relaxation processes are responsible for the homogeneous broadening.1-3,43,44 Usually both vibrational and orientational relaxation produce nearly Lorentzian band profiles in the IR spectra of crystals.32-42 If one assumes a Gaussian distribution of the inhomogeneities in the sample, the resulting bands should have Voigt profiles, in which the Lorentzian, δL, and Gaussian, δG, widths are approximately equal to the contributions of homogeneous, δhom, and inhomogeneous, δinhom, broadening, respectively.40 To evaluate the characteristics of the observed components, least-squares band fitting has been performed, using Voigt functions. The best fits for the lowest and the highest temperatures studied are also shown in Figures 1-3. The components of the 1184.8/1182.2 cm-1 doublet are slightly asymmetric. The spectra in this region could be satisfactorily reproduced by introducing a third Voigt component near 1184 cm-1. In analogy with the weak bands found in the ν6 region, this component presumably is due to the 13CH3F isotopomer. At higher temperatures the profile of the 1446.9 cm-1 band becomes noticeably asymmetric (Figure 2). Therefore, also in this region a supplementary component, centered at approximately 1445 cm-1, was introduced.
J. Phys. Chem. B, Vol. 102, No. 34, 1998 6495 TABLE 1: Orientational Diffusion and Spectroscopic Parameters of the Crystalline CH3F mode ν1 ν2 ν4 ν5 ν6
component/ cm-1
δGa/ cm-1
2972.0 1446.9 3034.2/3028.8b 1463.4 1454.3 1184.8 1182.2 1175.4
1.4(1) 0.14(3) 2.1(1) 1.61(6) 1.25(6) 0.38(2) 0.14(4) 0.18(1)
δLa/ δ0inhom+vib/ ∆H*/ ∆S*/ cm-1 cm-1 kJ mol-1 J mol-1 K-1 4.3(1) 0.56(3) 3.2(1) 0.95(6) 0.82(6) 0.40(2) 0.58(2) 0.43(1)
4.88(2) 0.60(1) 5.67(3) 2.40(1) 1.89(2) 0.68(1) 0.62(2) 0.53(1)
0.97(37) 0.86(6) 0.77(6) 0.84(3) 0.77(6) 0.90(5)
-6.6(28) -7.2(17) -9.6(23) -6.9(20) -6.0(18) -6.6(19)
a At 8 K; the data given are full widths at half-height. b The average values for two components.
It should be noted that some of the vibrons are substantially overlapped by their neighbors. In those cases the accuracy of the band characteristics obtained from the fitting is poor. For this reason we will not be considering the results obtained for the 1459.4 cm-1 component of ν5, which at higher temperatures becomes completely hidden by the wings of its neighbors (Figure 2). In addition, because of strong overlap, the widths of the 3034.2 and 3028.8 cm-1 components of ν4 cannot be determined accurately. However, the average value of their widths exhibits little scatter, and it seems reasonable to use this average value in the discussion below. The full widths at half-maximum of the Lorentzian and Gaussian contributions, further noted as δL and δG, determined at 8 K are given in Table 1. It can be seen that at this temperature the inhomogeneous and homogeneous broadening give comparable contributions. The homogeneous width of a vibron (δhom ≈ δL) can be represented as a sum of contributions from vibrational δvib, and orientational, δor, relaxation:1,2,43
δhom ) δvib + δor
(1)
The orientational relaxation part is a function of the diffusion constants for spinning, D|, and tumbling, D⊥, motions. For A1type bands only tumbling diffusion contributes to their broadening.26 As a consequence,
δor(A1) ) D⊥/πc
(2)
where c is the speed of light. In contrast, the E-type bands are broadened by both spinning and tumbling diffusion processes. For the orientational diffusion of various CH3X and CD3X molecules in liquids it has been shown that45-47
δor(E) ) (1/πc)(D⊥ + (1 - ζ)D|)
(3)
where ζ is the first-order Coriolis coupling constant. For CH3F these constants for ν4, ν5, and ν6 are equal to 0.089, -0.280, and 0.284, respectively.48 At low temperatures the classical rotation of the molecules in the crystal becomes impossible, leaving only quantum tunneling as possible rotational diffusion. The barrier hindering tumbling of molecules is very high, and it may be safely assumed that D⊥ at low temperatures becomes effectively zero. However, the barrier hindering spinning is much lower, and the contribution of tunneling to D| must be considered. The tunneling diffusion constant can be calculated by the approximate relationship49
D| )
{
∫ss [2µ(V(s) - Evib]1/2 ds}
ντ 2π exp π h
2
1
(4)
6496 J. Phys. Chem. B, Vol. 102, No. 34, 1998
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Figure 4. Temperature dependence of the total bandwidth δ for the component of ν1 at 2972.0 cm-1 (a) and the average widths for the components of ν4 at 3034.2 and 3028.8 cm-1 (b). The solid lines were obtained by fitting an Eyring-type function, eq 8.
Figure 5. Temperature dependence of the total bandwidth δ for the components of ν5 at 1463.4 cm-1 (a), 1454.3 cm-1 (b), and the component of ν2 at 1446.9 cm-1 (c). The solid lines were obtained by fitting an Eyring-type function, eq 8.
where ντ is the torsional frequency, µ is the reduced mass for the spinning motion, Evib is the total vibrational energy, V(s) is the rotational potential, s is the vibrational coordinate, and h is Planck’s constant. The integration interval (s1, s2) is defined as the region where V(s) - Evib g 0. The value of 81 cm-1 for ντ has been determined from inelastic neutron scattering spectra.18 It is readily shown that at T ) 8 K the occupation numbers for excited torsional states are negligible, and consequently, Evib equals hcντ/2. Describing, as in ref 18, the rotational potential by a 3-fold cosine function with a barrier of 2.27 kJ mol-1 and using the geometry of the isolated CH3F molecule,50 we obtained the spinning diffusion constant of 5.0 × 109 s-1. It follows from eq 3 that the contributions to the vibrational bandwidths due to tunneling are of the order of 0.03-0.07 cm-1. Comparing these with δL values from Table 1, it can be concluded that tunneling represents a minor part of the observed homogeneous bandwidths. According to eqs 1-3 the homogeneous part of the bandwidths can be used for the determination of the orientational diffusion constants, D⊥ and D|, at different temperatures. However, at temperatures above 50 K the overlap between the vibrons becomes severe, which results in a poor reproducibility of the δL and δG values. In contrast, the total bandwidths, δ, were obtained with fairly high reproducibility. Therefore, in the further analysis the total bandwidths were used. The justification for this is given below. In Figures 4-6 the values of δ for the components of the A1- and E-type bands, δ(A1) and δ(E), are plotted as functions of the temperature. The results in Figures 4 and 5 show that the widths of the E-type band components increase with the temperature more rapidly than those of the A1-type bands. As we will show, this may be rationalized on the basis of eqs 1-3 in the assumption that the main difference in the temperature behavior of δ(A1) and δ(E) is due to the presence of the (1 ζ)D| term in (3). The plots shown in Figure 5 make it possible to confirm the tentative assignment of the components observed in the 14801440 cm-1 region. The vibron at 1454.3 cm-1 exhibits the same temperature behavior as that at 1463.4 cm-1. Therefore, in contrast with previous assignments,30 these vibrons must both be assigned to the ν5 mode. When analyzing temperature dependencies of the bandwidths, it is often assumed32,35,42,51 that
where δinhom is the width of the inhomogeneous envelope of the band. Observed band profiles are convolutions of the inhomogeneous and homogeneous contributions. However, if δinhom does not significantly vary with the temperature and if only the temperature effect on δhom is investigated, the above assumption is reasonable. Literature data32,33,35,42,51 indeed show that in the 0-100 K range the temperature derivatives of δinhom do not exceed 3 × 10-3 cm-1 K-1. This value is much smaller than the temperature derivative of δ in our experiments, 5 × 10-2-10 × 10-2 cm-1 K-1. To obtain information on the spinning diffusion of CH3F, it is necessary to separate the contribution due to δor from the δ(E) values. This can be done by comparing the temperature effects on δ(A1) and δ(E). The contributions of vibrational relaxation and inhomogeneous broadening can be separated into temperature-independent and temperature-dependent parts. Incorporating eqs 1, 2, 3, and 5, one obtains
δ ) δinhom + δhom
(5)
0 δ(A1) ) δinhom+vib (A1) + δinhom+vib(A1,T) + (D⊥/πc) (6) 0 δ(E) ) δinhom+vib (E) + δinhom+vib(E,T) + (1/πc)(D⊥ +
(1 - ζ)D|) (7) The first two terms on the right-hand side of these expressions include contributions due to both vibrational relaxation and inhomogeneous broadening. It is believed that, as in refs 32, 33, 35, 42, and 51, the temperature-dependent term is mainly due to the contribution of vibrational relaxation. The temperature-independent terms of (6) and (7) can be obtained by extrapolating the experimental curves to 0 K. Following the modified Rakov approach,21,52 we fitted the experimental widths with an Eyring-type function: 0 + aT exp(-b/T) δ ) δinhom+vib
(8)
where δ0inhom+vib, a, and b were adjustable parameters. The results of this are given in Figures 4-6 by continuous lines, and the obtained δ0inhom+vib values are collected in Table 1. Next, from the difference in the moments of inertia for spinning and tumbling rotation of CH3F (I| ) 5.489 and I⊥ ) 32.95 × 10-40 g cm2, ref 50) and the larger free volume requirements for the tumbling motion, it follows that D⊥ , D|. Hence, the temperature dependence of δ(E) should be mainly due to vibrational and spinning orientational relaxation processes.
Methyl Groups in Crystalline CH3F
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The temperature effect on the vibrational relaxation part of bandwidths in crystals has been extensively studied both in theory53,54 and in experiment.32-42,51 The experimental data are analyzed in terms of the anharmonic coupling between intramolecular modes, the vibrons, and the lattice phonons. The anharmonic coupling results in both depopulation of the excited vibrational states and pure vibrational dephasing. The rates of the depopulation and dephasing depend on the frequency of the vibron under study relative to those of the other vibrons and phonons that take part in the coupling. The temperature dependence of δvib is due to the fact that the occupation numbers of phonons vary with temperature.53 Thus, for all vibrons a positive temperature derivative of δvib is observed.32-42,51,55 If this derivative is known from experiment, then the contributions to δvib due to the different processes can be fitted using a phenomenological model.32-35,41,42,51,55 Unfortunately, this model does not allow the prediction of the temperature derivative of δvib without empirical data. However, it may be noted that if two vibrons have close frequencies, then, as a rule, their vibrational relaxation occurs through interactions with the same, or close, inter- and intramolecular modes of the crystal. It follows that vibrons having close frequencies should exhibit close temperature derivatives of δvib, and that indeed was observed experimentally.32-35,41,42 Thus, it seems reasonable to assume that the ν1 and ν4 modes of CH3F will be characterized by similar values of (dδvib/dT), and the same should be true for the (ν2,ν5) pair. Summarizing the above assumptions, relations 6 and 7 can be rewritten as 0 0 δ(E) ) δinhom+vib (E) + β[δ(A1) - δinhom+vib (A1)] + [(1 - ζ)D|/πc] (9)
This equation enables one to determine values of D| from the experimental bandwidths δ(E) and δ(A1). The parameter β was introduced to account for the uncertainties arising from the various assumptions. Since there is no direct way to test the validity of the latter, we have estimated the uncertainties on D| in a somewhat arbitrary way. To this end, D| was calculated for β ) 0, 1, and 2. The calculation at β ) 0 corresponds to neglecting the temperature dependence of δinhom+vib(E,T), while at β ) 2 the temperature dependence of δinhom+vib(E,T) is judged to be overestimated. The results obtained for β ) 1 were taken as the average D| values, while the results for β ) 0 and 2 were used to estimate the uncertainties. When calculating the values of D|, the δ(A1) for ν1 was used in pair with δ(E) for ν4 and δ(A1) for ν2 in pair with δ(E) for ν5. Unfortunately, there are no A1-type bands in the vicinity of ν6. In this case, following the approach described in refs 26, 45, and 56, we modeled the δinhom+vib(E,T) of ν6 using the δinhom+vib(A1,T) data for ν2, multiplied by (ν6/ν2)2. The D| values obtained at different temperatures were fitted by the Eyring function:
D| ) (kT/h) exp((-∆H*/RT) + (∆S*/R))
(10)
where ∆H* and ∆S* are the activation enthalpy and entropy for spinning diffusion, respectively. The resulting parameters are given in Table 1. From the data in Table 1 it can be seen that there is good agreement between the values obtained from different vibrons. This suggests that the assumptions that were used do not introduce large systematic errors for ∆H* or ∆S*. Moreover, it is clear that variation of β between 0 and 2 does not significantly alter the value of the activation enthalpy. Thus,
Figure 6. Temperature dependence of the total bandwidth δ for the components of ν6 at 1182.2 (a), 1184.8 (b), and 1175.4 cm-1 (c). The solid lines were obtained by fitting an Eyring-type function, eq 8.
Figure 7. Temperature dependence of the spinning diffusion constant. The circles with error bars correspond to the average values of D|; the solid line is obtained by fitting the data with the Eyring function, eq 10.
the procedure of accounting for the temperature dependence of δinhom+vib(E,T) has only a minor effect on ∆H*. In contrast, the absolute value of D| is noticeably affected by the variation of β, what results in rather large uncertainties for ∆S*. In Figure 7 the average values of D| are plotted as a function of temperature. Within the temperature range studied, the uncertainties do not exceed 15% of the absolute D| values. It can be seen that the temperature dependence of D| is accurately described by the Eyring equation (10), and the values obtained for the activation parameters are ∆H* ) 0.85(7) kJ mol-1 and ∆S* ) -7(1) J mol-1 K-1. The activation enthalpy is relatively small and has the same order of magnitude as those obtained for CH3NO2, (CH3)2SnCl2, (CH3)2SnBr2, and (CH3)2SnClBr.13,21 Since orientational diffusion includes not only classical transitions between the potential wells but also tunneling transitions and small-angle reorientations inside potential wells, it is obvious that ∆H* should be smaller than the barrier hindering the rotation of CH3 groups. The documented18 value for the barrier, 2.27 kJ mol-1, is indeed larger than the obtained ∆H*. At temperatures higher than ∆H*/R ≈ 100 K the reorientations of CH3 groups in solid CH3F must be expected to become quasi-free. Nearly free rotation has been reported for various CH3X molecules in the liquid phase and solutions.26,46,47,57,58 Thus, it is of interest to compare the spinning diffusion constant for crystalline CH3F with that for the liquid phase of similar compounds. Extrapolating relation 10 to higher temperatures, we obtained the D| value of 180(30) × 1010 s-1 for the hypothetical “solid” CH3F at 295 K. This, as anticipated, agrees well with the D|(295 K) values determined for liquid CH3Br,
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