3130
J. Phys. Chem. 1980, 84, 3130-3134
Bureau of Standards through Grant No. GT-9020.
References and Notes (1) (a) C. J. Howard, J. Phys. Chem., 83, 3 (1979); (b) J. V. Michael and J. H. Lee, ibki., 83, 10 (1979); (c) F. Kaufman, Annu. Rev. Fhys. Chem., 30, 41 1 (1979), and references therein. (2) A. A. Westenberg and N. deHaas, J. Chem. Fhys., 58, 4061 (1973). (3) R. Zeliner and W. Steinert, Int. J. Chem. Kinet., 8, 397 (1976). (4) N. Cohen and K. Westberg, J. Phys. Chem., 83, 46 (1979). (5) R. Zellner, J. Phys. Chem., 83, 18 (1979). (6) (a) D. D. Davis, S. Fischer, and R. Schiff, J . Chem. Phys., 59, 628 (1974); (b) A. R. Ravishankara, P. H. Wine, and A. 0. Langford, ibkl., 70, 984 (1979), and references therein.
(7) J. 0. Cab& and J. N. P i , Jr., "Photochemistry", Wlley, New York, 1966. (8) P. H. Wine, N. M. Kreutter, and A. R. Ravishankara, J. Phys. Chem., 83, 3191 (1979). (9) The reactor was fabricated by R and D Optlcais, Inc., New Windsor, MD.
(IO) Ark Ravishankara, R. C. Shah, J. M. Nicovich, R. L. Thompson,
and F. P. Tullv. work in Droaress. (11) R. D.Hudson and E, I. Re&, %e Stratosphere: Present and Future”, NASA Ref. Publ., 1049, Dec 1979. (12) J. Peeters and G. Mahnen, Symp. (Int.) Combust., [Proc.], 14, 133 (1973). (13) J. Ernst, H. Gg. Wagner, and R. Zeliner, Ber. Bunsenges. Phys. Chem., 82, 409 (1978).
Raman Spectra of Methanol and Ethanol at Pressures up to 100 kbar J. F. Mammone,” S. K. Sharma, Geophysical Laboratory, Carnegie Institutlon of Washington, Washington, D.C. 20008
and M. Nlcol DepaHment of Chemistty, lJniversl& of California,Los Angeles, California 90024 (Received: Aprll 7, 1980)
The Raman spectra of methanol and ethanol were measured at pressures up to 100 kbar and room temperature. A t these pressures methanol either crystallizes or forms a glass (superpressed liquid), whereas ethanol always crystallizes. The pressure at which methanol crystallizes is estimated to be 35.1 f 1.0 kbar. Comparison of the Raman spectrum of high-pressure crystals with that of the low-temperature a-methanol indicates that the structure of the high-pressure phase resembles that of the a phase. The Raman spectra for ethanol, which crystallized at 17.8 f 1.0 kbar, were interpreted in terms of the monocliniic (Pc;2 = 4)low-temperaturestructure. In ethanol, the frequency of the 0-H stretching modes shows large negative shifts with pressure due to the strengthening of the hydrogen bonds.
Introduction At 1atm methanol freezes at 175 K and undergoes a X transition’ at 161 K; ethanol freezes2at 156 K. Vibrational spectra of the solid low-temperature forms of methanol have been extensively studied by and Raman5 spectroscopy. Polarized infrared spectra of single crystals of ethanol at room temperature have been measured in a diamond-window, high-pressure cella6 The nature of the intermolecular forces in these hydrogen-bonded systems not only is of fundamental theoretical interestlp’ but has become important in high-pressure experiments where a 4:l methanol-ethanol mixture is widely used as a pressure-transmitting medium.* In order to determine the nature of the 4:l mixture at high pressure, it is important to characterize the pure components. No Raman data at high pressure and room temperature have been reported previously for any solid phases of methanol or ethanol. The Raman spectra of methanol and ethanol at pressures up to 100 kbar have therefore been recorded, and the equilibrium pressures of crystallization have been measured. Experimental Section Spectroscopic-grade methanol, containing less than 0.08% water (Burdick and Jackson Laboratories, Inc., Muskegon, MI), and absolute ethanol (U.S. Industrial Chemicals Co., New York, NY) were used without further purification. The samples were compressed in a diamond-anvil cell designed by Mao and BelL9 The liquids were contained with a small ruby chip in a hardened steel gasket 0.25-0.4 0022-3654/80/2084-3130$0 1.OO/O
mm in diameter and approximately 0.15 mm thick, which had been preindented from an original thickness of 0.25 mm. Pressures were calculated from the shift of the ruby R1 fluorescence line.1° Diamond anvils cut from lowfluorescent, type-I1 diamonds, selected by the criteria of Adams and Sharma,l’ were used in the present work. A Spectra Physics Model 164 argon laser operating at 488.0 nm was used as the excitation source. The spectra were recorded at room temperature with a Jobin-Yvon double monochromator (HG-2s) with photon-countingdetection.12 Scattered radiation was collected in the forward direction by a 90’ off-axis ellipsoidal mirror.13 Polycrystalline samples could sometimes be formed by simply increasing the pressure on the liquid beyond the equilibrium crystallization pressure. Ethanol always crystallized in this way. Methanol could be crystallized by this method but only with difficulty; it usually formed a superpressed liquid. Sometimes crystallization of methanol could be induced by releasing pressure on the glass. Bridgman14also reported that ethanol would not superpress and that methanol was difficult to crystallize. Single birefringent crystals of each alcohol were grown by increasing the pressure on a single nucleation center. This nucleation center was obtained from a polycrystalline sample by releasing the pressure slowly until all but one grain melted, leaving a single grain. The crystallization pressures were measured with one or two crystals in equilibrium with the surrounding liquid and were found to be 35.1 f 1.0 kbar for methanol and 17.8 f 1.0 kbar for ethanol. Piermarini et a1.8 reported a value of 35.8 f 0.8 kbar for the freezing pressure of methanol. Bridgmanls 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 23, 1980 3131
Raman Spectra of Methanol and Ethanol
TABLE I: ldaman Frequencies (cm-') of Liquid and Crystdine Methanol at Different Pressure@ crystal liauid 1 iitm 12 kbar 39 kbar 63 kbar 101 kbar
-
-
-
1034 vs 1043 vs 1142 w 1179 w
1040 vs 1055 vs 1152 w 1187 w 1457 m 1475 m 1497 s 1507 sh 2881 m
1033 vs
1032 vs
1106 w 1149 w
1117 w 1164 w
1448 s
1467 s
1458 m 1473 m 1486 m
2832 vs
2844 vs
2865 s
2940 vs
2951 vs
3330 vw, br
vw, br
2986 vs 3019 s 3040 s 3193 vw, br 3250 vw, br
-
3003 vs 3047 s 3070 s
assignment
1052 vs 1068 vs 1160 w 1196 w 1433 w 1448 w 1465 m 1485 m 1510 s
C-0 stretch
2858 w 2907 w 3029 vs 3086 s 3102 s
C H sym str
CH, rock CH, bend
C-H antisym str
0-Hstretch
3217 vw, br vw, br Measurement accuracy: i 2 cm-' for sharp, strong bands; 10 cm-' for weak, broad bands. Intensity estimates: vs, very strong; s, strong; m, medium; w,weak; vw, very weak; br, broad; sh, shoulder. TABLE I1 : Raman Frequencies (cm -') for Superpressed Methanolu
-
1 atm 3 kbar 36 kbar 58 kbar - 92 kbar assignment 1031 s 1038 s 1047 s 1069 s C-0 stretch 1033 s 1112 w 1106 w 1127 w 1136 w CH, rock 1161 w 1170 w 1149 w 1181 w 1448 m 1462 m 1478 m 1487 m 1501 m CH, bend 2837 vs 283 2 vs 2862 s 2870 m 2884 w C-H sym str 2940 vs 2947 vs 2967 vs 2985 vs 3017 vs C-H antisym str 2992 sh 2996 sh 3019 sh 3028 sh 3076 sh 3330 vw, br vw, br vw, br 0-H stretch Measurement accuracy: f 2 cm-' for sharp, strong bands; i10 cm-' for weak, broad bands. Intensity estimates: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad. (I
could not measure the pressure at which methanol crystallized because of the sluggish nature of the transition. He found that at 20 OC ethanol froze at 19 kbar. It was not possible to study deuterated samples in this diamond-anvil cell because of interference with the second-order RaIman spectrum of diamond. In liquid CD30D the frequencies of the 0-D stretch, C--D symmetric stretch, and C-D asymmetric stretch are 2474,2082, and 2225 cm-', re~pectively,~ whereas the second-order Raman spectrum of diamond is very broad (-1800-2700 cm-l) with a maximum at 2458 cm-l.
Results and Discussion Methanol. Portions of the Raman spectra of crystalline, liquid, and vitreous methanol at several pressures are shown in Figures 1and 2, respectively. The positions and other characteristics of the bands are listed in Tables I and 11. The assignments of the internal modes of vibration presented in the tables are based on the characteristic group frequencies.16 The pressure dependences of the bands due to the symmetric and antisymmetric C-H stretching vibrations and the C-0 stretching vibration are plotted in Figure 3. No bands in the low-frequency region of the Raman spectra of crystalline methanol could be resolved. Such bands have very weak intensities and could have been obscured by background parasitic scattering. Because methanol was difficult to crystallize, small sample sizes were required, and the intensities of the Raman bands relative to baclcground scattering were therefore reduced. In the superpressed liquid methanol, the Raman bands remain broad and shift continuously with pressure. Upon crystallization, bands arising from internal modes of vibration in methanol become narrower, several bands split
r--;il
u 5
1000
I
I
I,
I
I
I
1500" 2700
I
3000
I
I
I
*
1
350C
Raman shift, cm.l Figure 1. Raman spectra of l i c p and crystalllne methanol at various pressures (laser 488.0 nm Ar ion, 350 mW, slits 6 cm-').
Raman shiff (cm -1)
Figure 2. Raman spectra of superpressed methanol at varlous pressures (laser 488.0 nm Ar' ion, 350 mW, SIRS 6 cm-I).
(Figure l),and the bands due to the C-H symmetric and antisymmetric stretches and the C-0 stretch shift dis-
3132
The Journal of Physical Chemistry, Vol. 84, No. 23, 1980
'' "'.
Mammone et ai.
'I
CH30H crystal CH30H superpressed llquid
500
1000 1500 3000 3300 Roman s h i f t (crn-')
Flgure 4. Raman spectra of liquid and crystalline ethanol at various pressures (laser 488.0 nm Ar' ion, 350 mW, slits 6 cm-l).
1
1
0
20
1
,
40 60 Pressure (kbar) -+
a0
100
Flgure 3. Pressure dependence of the C-0 stretch and the C-H symmetric and antisymmetric stretching mode frequencies (cm-') for methanol and ethanol.
continuously. The C-0 stretching model6 (at 1032 cm-' for the liquid at 12 kbar) splits by 15-16 cm-l in the crystal. (The weak bands at 1106 and 1149 cm-l are assigned to CH3 rocking modes.16) The CH3 bending mode (at 1448 cm-' for the liquid at 1 atm) and the antisymmetric CH3 stretching band (at 2904 cm-l for the liquid at 1 atm) also split upon crystallization. The effect of pressure on the C-H symmetric stretch is dramatic; its intensity decreases by almost an order of magnitude, and this band seems to split at 101 kbar (Figure 1). The 0-H stretching bands (at 3330 cm-l for the liquid at 1 atm) were very weak, broad, and difficult to resolve in the Raman spectra of methanol. The crystal structures of a- and P-methanol have been In the fl phase (space determined by X-ray diffra~ti0n.l~ group DiI-Cmem; 2 = 4)) the molecules are arranged in hydrogen-bonded zig-zag chains with two-molecule repeat units, resembling the structure of polyethylene. The carbon groups and the oxygen and hydroxyl hydrogen atoms lie in the plane of the chain; the hydroxyl hydrogen atoms are midway between two oxygen atoms. In a-methanol (space group Cih-P2Jm or Ci-P,,; Z = 2), the chain is puckered. The methyl groups on one side of the chain are located above the plane of the chain and on the other side are below the plane. The two space groups (Ciand cih) differ only by the position of the hydroxyl hydrogen atom, which is imprecisely determined by X-ray diffraction. Spectroscopy,however, suggests that
the appropriate symmetry for a-methanol is Cia6For e$, symmetry, the mutual exclusion principle reduces the number of allowed bands in the Raman spectrum from that permitted for the same number of molecules in a Ci structure and prohibits simultaneous infrared and Raman activity. Several vibrational bands, however, are active in both the Raman and the infrared spectra of a-methanol, including the C-0 stretching doublet; and the number of observed far-infrared bands exceeds those permitted for a two-molecule basis with CZh ~ymrnetry.~ The only changes observed in the Raman spectrum upon cooling methanol at 1 atm from the ,8 to the a phase are splittings of the C-O stretching and CH3bending modes.6J8 Similar splittings were observed in the Raman spectra above 35 kbar (Figure 1). Thus, the high-pressure phase can tentatively be identified with the low-temperature (a) structure. However, the Raman evidence is ambiguous concerning the correct space group for the high-pressure structure. Few bands that correspond to vibrations involving the hydroxyl hydrogen atoms were observed in the Raman spectrum of methanol at high pressure. Hydrogen bonding strengthens under pressure, and this strengthening of the bonding tends to move the hydroxyl hydrogen atoms toward the inversion center midway between the two oxygen atoms. In such a centrosymmetric structure, motions of atoms occupying the inversion center correspond to infrared-active and Raman-inactive vibrations. Raman intensities of these bands in the high-pressure spectra may mean that, even though the structure has C2 symmetry, the very weak distortion of the high-pressure structure from CZh symmetry is small. The large decrease in the intensity of the CHBsymmetric stretching mode with pressure is prominent in the Raman spectra of both crystalline and superpressed methanol (Figures 1 and 2). The observed intensity change is therefore not a result of crystallization. It appears to be due to the crossing of the CH3 symmetric stretch and the overtone of the CH3 bending mode with which it is in Fermi resonance. Similar effects have been reported in the vibrational spectrum of polyethylene under high pressure.20 Piermarini et a1.8 estimated the pressure of the glass transition in methanol at room temperature to be -80 kbar on the basis of line-broadening measurementa of the ruby R1fluorescence line. No discontinuities were observed in the Raman spectrum of the internal modes at this point, as expected, owing to the second-order nature of the glass transition. Ethanol. Raman spectra of ethanol at several pressures are presented in Figure 4. Band positions and other spectral characteristics are summarized in Table 111. Upon crystallization a narrowing and splitting of the Raman bands were observed. This result is illustrated by the spectrum of the liquid just before freezing at 17 kbar and
The Journal of Physical Chemistty, Vol. 84, No. 23, 1960 3133
Raman Spectra of Methanol and Ethanol
-
TABLE 111: Raman Frequencies (cm-' ) for Ethanola crystal liquid
-17 lrbar 893 vw
1060 I 1095 si
1458 s
2940 vs
vw, br
1 9 kbar 896 vs 1059 vs 1098 vs 1150 vw
37 kbar 900 vs 907 sh 1066 1103 vs 1156 vw
57 kbar 906 vs 915 s 1078 vs 1110 vs 1164 vw
1454 s
1455 s
1462 s
1468 s 1483 s
1471 s 1485 s 1495 sh 2739 w 2871 m 2889 sh 2906 s 2928 s 2963 vs 2991 s 3007 s 3023 s 3187 w, br 3286 w, br
1479 s 1490 s 1507 s 2749 w 2854 m 2899 sh 2924 sh 2944 s 2982 vs 3006 s 3026 s 3043 s 3149 w, br 3256 w, br
2734 w 2854 sh 2882 sh 2900 sh 2916 s 2946 vs 2981 s 2993 s 3004 s 3228 w, br 3317 w, br
82 kbar 914 vs 927 s 1083 vs 1122 vs 1406 sh 1465 s 1479 sh 1486 s 1497 s 1516 s 2755' w 2861 w 2910 sh
assignment
-
C-C stretch C - 0 stretch
CH, rock CH, bend CH, wag CH, bend CH, bend 2X 0-H in-dane bend C-H sym stE
2957 s
30021 vs 3030 s 3049 s 3069 s
C-H asym str
0-H stretch
3230 w, br
a Measurement accuracy: i 2 cm-' for sharp, strong bands; f 10 cm-I for weak, broad bands. Intensity estimates: vs, very strong; s, strong; m, medium; w, weak;vw, very weak; br, broad; sh, shoulder.
TABLE IV: l%amanFrequencies (cm'-') for Ethanol in the Low-Frequency Region 84 30 59 kbar kbar kbar assignment 90 124 198 436 465 5 _ 0_ 8_
a
a a
442 462
a a
449
lattice mode lattice mode lattice mode G C - 0 in-plane bend
530 530 539 562 684 7102 0-Hout-of-plane bend 850 8161 871 CH,roclring a Band could not be measured because of strong background.
the crystal spectrum at 19 kbar. The bands in the lowfrequency region (500-800 cm-l) are very weak. The positions of those low-frequency bands that could be measured reproducibly are listed in Table IV. Above 30 kbar the strong background in the region below 200 cm-' prevented resolution of any bands at these low frequencies. On the basis of infrared measurements, Jakobsen et aln2' detected only one polymorph for ethanol whether the crystal was grown by lowering the temperature or increasing the pressure. Thus, the Raman spectra of the high-pressure crystal will be interpreted in terms of the monoclinic crystal structure (Pc; 2 = 4) determined by X-ray diffraction2 at 87 K. If ethanol is considered as an infinite hydrogen-bonded chain mole~ule,'~ each repeat unit contains four molecules. There are 3n - 4 = 104 (n = 36 atoms) optic modes, all of which are expected to be Raman active in this low symmetry structure. However, only 30 bands are resolved in the spectra. When the unit cell is large, long-range forces within the unit cell are insufficient to separate all of the motions into discrete spectral bands. Thus, the main features of the spectrum are probably determined by a much smaller pseudocell. Consequently, band assignments have been based on characteristic group frequencies.ls The C-C-0 in-plane deformation appears at 425 cm-l in the high-pressure infrared spectrums but is split in the
Raman spectrum. In the low-temperature X-ray diffraction study: two crystallographicallyindependent molecules of ethanol were shown to form infinite hydrogen-bonded chains. The two different orientations could be responsible for the observed splitting of this skeletal vibration. Upon association, many alcohols exhibit a broad diffuse band in the infrared spectrum at about 650 cm-l, which has been assigned to the 0-H out-of-plane deformation.n Mikawa et ale6reported a value of 660 cm-' for this band, but the pressure at which the spectra were measured was not reported. With increasing pressure, the corresponding Raman band shifts to higher frequencies (702cm-l at 59 kbar), an effect that possibly reflecta the strengthening of the hydrogen bonding. The 0-H in-plane bending mode observed by Mikawa et a1.6 at 1358 cm-l in the infrared spectrum could not be observed in the present study because it coincides with the single, intense, fiist-order (Fa) diamond Raman line. With increasing pressure, the C-C stretching mode splits, with a shoulder becoming apparent at 37 kbar. The splitting increases tx~13 cm-l at higher pressures. The large difference in intensities (Figure 4) suggests that the doublet is not due to the presence of two different orientations of ethanol molecules. It could be due to adjacent molecules vibrating in and out of phase, an effect that would be sensitive to decreasing the distance between molecules. The C-0 stretching mode is coupled with the C-C stretching mode and thus can be considered as an asymmetric C-C-0 stretching mode. The variations of the relative intensitieu of the C-0 stretching modes with pressure are not systematic (Figure 4) and their origins are not understood. Assignments of the CH3 and CH2 bending modes and the CH2 wagging imode were based on those made by Mikawa et al.,6 who measured the dichroic ratios. In the C-H stretching region, nine Raman bands were observed, whereas only four bands were observed in the infrared spectrum.s The C-H stretching modes are arbitrarily assigned with the asymmetric stretches being assigned to higher frequencies than symmetric stretches. The band at 2734 cm-l in the spectrum at 19 kbar is too low in energy to be a C-H stretching mode and therefore is tentatively
3134
The Journal of Physical Chemistty, Vol. 84, No. 23, 1980
assigned to an overtone of the 0-H in-plane bending mode. The 0-H stretching vibration, which is very weak and broad in the liquid phase, splits into two well-resolved components in crystalline ethanol (Figure 4). With increasing pressure both bands shift to lower frequencies, with the lower frequency component exhibiting a greater shift. This negative shift is indicative of increased hydrogen bonding with pressure. The measured half-bandwidths were in the range 30-70 cm-I. These observations are consistent with those reported by Jakobsen et al.,2l who studied the infrared spectra of several solid alcohols. It has been proposedz1 that the splitting arises from the coupling of the 0-H stretching vibrations through nearest neighbors. The two bands represent in- and out-of-phase vibrations. In the lower frequency in-phase vibration, the hydroxyl protons move along the hydrogen bond in the same direction. This vibration favors tautomerization and is therefore expected to be more sensitive to the strengthening of the hydrogen bond with pressure.21 The bands that appear at 3228 and 3317 cm-l at 19 kbar are shifted by -79 and -41 cm-l, respectively, at 57 kbar (Table 111). The magnitude of the pressure-induced shift for the low-frequency component of the 0-H stretching band is larger than for any band in ethanol. In the spectrum recorded at 82 kbar the lower frequency component has shifted so much that it overlaps with the C-H stretching vibration (Figure 4). Comparisonof the Effectsof Pressure on Methanol and Ethanol. It was possible to superpress methanol without the formation of nuclei of the crystal, whereas ethanol would not superpress without freezing. Upon crystallization relatively large discontinuities in the vibrational frequencies were observed in methanol as compared with ethanol (Figure 3). These observations suggest that the structures of the liquid and crystal are more similar in ethanol than in methanol. The structure of a liquid such as ethanol is composed of polymer chains of hydrogenbonded alcohol molecules that were observed in dilute solutions to increase in average length with increasing concentration and decreasing temperaturemZ3 The band due to the 0-H stretching mode was difficult to resolve in methanol, in both the liquid and the solid states. Below 20 kbar in methanol, the rate of change of the frequency of the C-0 stretching vibration with pressure is near zero (Figure 3). Contrast this to the ethanol results. The dielectric constants of liquid metha.1101~~~ and ethanolz5have been measured, and the Kirkwood correlation factors, g, have been calculated at pressures up to 3 kbar. With increasing density, g decreases. This decrease has been explained by a partial breakdown in the hydrogen-bonded structure.z4 An alternative interpretation, based on the behavior of water,26is that the proportion of broken hydrogen bonds does not increase, but rather hydrogen bond angles are more greatly distorted. This may also account for the anomalous pressure dependence of the C-0 stretching frequency in methanol below 15 kbar. Destruction of hydrogen bonding might produce enough free 0-H polymer end groups to be observed in the Raman spectra. In liquids, the band due to the 0-H stretch is too weak and too greatly broadened by first-order coupling between hydroxyl groups2’ to be observed, The effect of pressure on methanol thus may be to increase the disorder in the structure and not necessarily to shift the equilibrium to longer or shorter polymer chains. If at high pressures such disorder is frozen in, a glass forms. In ethanol, the crystal structure has a lower symmetry and a more open chain structure that accommodates the seg-
Mammone et al.
ments present in the liquid, and therefore ethanol will not superpress. Summary Methanol crystallizes at 35.1 f 1.0 kbar at room temperature, and its Raman spectra were measured up to 100 kbar. The high-pressure crystal structure of methanol was assigned to the a phase by comparison to vibrational spectra of low-temperature crystals. Upon crystallization, discontinuities in the frequencies of the C-H and C-0 stretching vibrations were observed. Raman spectra of superpressed methanol also were measured to 92 kbar. Ethanol crystallizes at 17.8 f 1.0 kbar, and its Raman spectra were measured at pressures to 82 kbar. The high-pressure crystal structure of ethanol is interpreted in terms of the monoclinic low-temperature crystal structure. No additional phase changes were detected in either alcohol. It is proposed that the glass-forming tendency of methanol is a consequence of great differences between the structure of the liquid and that of the crystal. In ethanol, which will not form a glass, the structure of the liquid is closer to the structure of the crystal. The strengthening of hydrogen bonding with increasing pressure was dramatically demonstrated in ethanol by the large negative shifts of the frequencies of the 0-Hstretching modes. Acknowledgment. Critical reviews by Drs. J. W. Brasch, R. M. Hazen, T. C. Hoering, and D. Virgo are appreciated. We are grateful to Dr. H. S. Yoder, Jr., for his constructive criticism and suggestions. This work was carried out under a cooperative predoctoral program between the Geophysical Laboratory and the University of California, Los Angeles, and supported by NSF grant No. DMR-27428.
References and Notes (1) G. C. Plmentel and A. L. McClellan, “The Hydrogen Bond”, W. H. Freeman, San Francisco, CA, 1960, p 269. (2) P. G. Jonsson, Acta Cfystallogr., Sect. B , 32, 232 (1976). (3) M. Falk and E. Whalley, J. Chem. Phys., 34, 1554 (1961). (4) W. F. Passchier, E. R. Klompmaker, and M. Mandel, Chem. Phys. Lett., 4, 485 (1970). (5) J. R. Durig, C. B. Pate, Y. S. LI, and D. J. Antlon, J. Chem. Phys., 54, 4863 (1971). (6) Y. Mlkawa, J. W. Brasch, and R. J. Jakobsen, Spectrochim. Acta, Part A , 27, 529 (1971). (7) For reviews see ref 1 and P. A. Kollman and L. C. Allen, Chem. Rev., 72, 283 (1972). (8) G. J. Plermarlnl, S. Block, and J. D. Barnett, J. Appl. Phys., 44, 5377 (1973). (9) H. K. Mao and P. M. Bell, Carnegie Inst. Washington, Year Book, 77, 904 (1978). (10) H. K. Mao, P. M. Bell, J. W. Shaner, and D. J. Steinberg, J. Appl. Phys., 49, 3276 (1978). (11) D. M. Adams and S. K. Sharma, J. Sci. Instrum., 10, 680 (1977). (12) S. K. S h a m , Cam@ Imt. WasMngton, Year Bmk, 77,902 (1978). (13) D. M. Adams, S. K. Sharma, and R. Appleby, Appl. Opt., 16, 2572 (1977). (14) P. W. Bridgman, Proc. Am. Acad. Arts Sci., 77, 117 (1949). (15) P. W. Bridgman, Proc. Am. Acad. Arts Sci., 74, 399 (1942). (16) N. B. Colthup, L. H. Dab,and S. E. Wlberley, “Introduction to Infrared and Raman Spectroscopy”, 2nd ed.,Academlc Press, New York, 1975. (17) K. J. Tauer and W. N. Lipscomb, Acta Cfystaiiogr.,5, 606 (1952). (18) E. L. Pace, Specfrosc. Lett., 9, 411 (1976). (19) M. C. Tobin, J . Chem. Phys., 23, 891 (1953). (20) C.-K. Wu and M. Nicol, Chem. Phys. Lett., 18, 83 (1973). (21) R. J. Jakobsen, J. W. Brasch, and Y. Mlkawa, J. Mol. Struct., 1, 309 (1967). (22) A. V. Stuart and G. 6. B. M. Sutherland, J . Chem. Phys., 24, 559 (1956). (23) E. E. Tucker and S. D. Chrlstlan, J . Phys. Chem., 81, 1295 (1977). (24) E. U. Franck and R. Deul, Faraday Dlscuss. Chem. Soc., 66, 191 (1978). (25) K. R. Srlnlvasan and R. L. Kay, J. Solution Chem., 4, 299 (1975). (26) H. Kanno, R. J. Speedy, and C. A. Angell, Science, 189,880 (1975). (27) R. J. Jakobsen, Y. Mikawa, and J. W. Brasch, kture(London), 215, 1071 (1967).
