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Far-infrared spectra of solid ethanol at high pressures
19 July 1996
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters 257 (1996) 143-147
Far-infrared spectra of solid ethanol at high pressures A. Anderson, W. Smith Department of Physics, Universityof Waterloo, Waterloo Ontario Canada N2L 3G1
Received 3 April 1996; in final form 9 May 1996
Abstract
Far-infrared spectra of ethanol samples in diamond anvil cells at pressures up to 13 GPa are reported. The freezing pressure at 295 K is confirmed to be 1.8 + 0.1 GPa. No solid state phase transitions are observed in this pressure range. The evolution of six strongly absorbing lattice modes with increasing pressure is followed, and an approximate doubling of average hydrogen bond strength over this pressure range is deduced.
I. Introduction
Ethanol or ethyl alcohol, C2H 5 OH, forms a hydrogen-bonded molecular solid at low temperatures or high pressures. The freezing temperature at ambient pressure is 156 K [1] and the freezing pressure at room temperature is 1.78 GPa [2]. At low temperatures, ethanol is known to have only one crystalline phase, but two different glassy phases have also been observed [3]. The structure of the crystal phase has been determined by Jonsson [4]. The unit cell is monoclinic, space group Pc or C 2, and contains four molecules on general (C l) sites. At elevated pressures, ethanol is reported always to crystallize, in contrast to methanol, which usually forms a superpressed liquid or glass [2]. For this reason, methanol, either neat or in a 4:1 mixture with ethanol, is often used as a medium to ensure hydrostatic (isotropic) conditions for high pressure studies of solids [5]. There have been several previous spectroscopic studies of solid ethanol. These include an early mid-infrared work by Lake and Thompson [6] and two infrared investigations of single crystal ethanol
samples in diamond anvil cells by Jakobsen et al. [7] and Mikawa et al. [8]. In the latter, one set of observed lattice frequencies is listed, but no spectra are shown and the pressure is not given. In addition, a low temperature Raman and far infrared study of solid C2H 5 OH, C2H 5 0 D and C2D 5 0 D by Weng et al. [3] determined the frequencies of many lattice modes and several possible torsional modes and attempted to assign these on the basis of isotopic shifts. They observed a small increase in these frequencies as the crystal was cooled, which they attributed to a contraction of the lattice and subsequent strengthening of the hydrogen bonds. These effects should be enhanced by the application of external pressure to ethanol samples in diamond anvil cells, since it is known that larger changes in lattice constants occur under these conditions. In a Raman study of ethanol at high pressures [2], an observed shift to lower frequencies of the O - H stretching modes was interpreted as due to a strengthening of hydrogen bonding by charge transfers from the molecular bonds. However, the low frequency lattice modes, which directly involve these hydrogen bonds, could not be observed because of their weak inten-
0009-2614/96/$12.00 Copyright© 1996 Elsevier Science B.V. All rights reserved Pit S0009-261 4(96)00509-X
144
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 143-147
sity and proximity to the strong laser line. In contrast, because of the large dipole of the ethanol molecule, the far infrared absorption associated with many of these lattice modes should be quite strong. Hence, in principle this technique provides a method to measure changes in hydrogen bond strengths directly and quantitatively. In this Letter, we present results of a far-infrared room temperature study of ethanol at pressures up to about 13 GPa. The purposes are to confirm the melting pressure, to check for any evidence of solid state phase transitions, to correlate the results with low temperature Raman, infrared and X-ray data, and to obtain estimates of the changes in intermolecular forces over this pressure range.
gasket. The latter was indented by the diamonds to a thickness of about 0.025 mm, and a hole of 0.250 mm diameter was then drilled. The resulting sample volume was about 10 - 3 mm 3. Ethanol was obtained from BDH Chemicals, and had a stated purity of better than 99.7%. A few specks of ruby powder were added to the samples for in situ pressure measurements, using the calibrated shifts of two fluorescent lines [9]. A Fourier transform infrared spectrometer (Beckm a n - R I I C Model FS620) was used to record the far-infrared spectra. This evacuated instrument was fitted with a high pressure mercury lamp as source, Mylar beam dividers, crystal quartz and polyethylene filters, and a silicon bolometer detector, operating at 4.2K. An off-axis ellipsoidal mirror was used to focus radiation from the interferometer on to the diamond anvil cell which was placed directly in front of the entrance window of the bolometer cryostat. A resolution of about 2.5 cm-1 (corresponding to a mirror movement of 2 mm) was typically used, but several scans at higher resolution were obtained to check for any fine structure. Transmittance values
2. Experimental techniques Diamond anvil cells were obtained from Diacell Ltd., Leicester, U.K. They were the piston-cylinder type, with pressure adjustment through six Allen screws. Type 2a diamonds were used with an Inconel
~ ~
7.40GPa
2.53GPa
GPa
~
~ ~ G P a 80 240 400 I00
8 0 260
Wavenumber / cm" Fig. 1. Far-infraredspectraof crystalline ethanol at selected pressures.
GPa
GPa
420
145
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 1 4 3 - 1 4 7 Table 1 Peak wavenumbers of solid ethanol at various pressures Wavenumbers ( c m - t )
Pressure
(GPa)
1-'I
1)2
P3
~4
P5
~6
2.4 2.5 2.9 4.1 5.2 6.6 7.0 7.4 8.5 9.6 12.4 13.0
94 99 102 108 111 114 117 125 127 129 132 135
107 114 117 129 134 138 144 157 163 164 176 177
138 148 151 161 164 172 173 185 196 198 206 208
189 201 204 218 224 231 235 255 262 266 279 283
271 277 280 292 297 302 304 321 327 332 344 347
320 326 328 335 338 347 347 355 367 370 n.o. a 383
A (cm- l ) b B (cm- i GPa- i) b
91.4 3.64
99.1 6.51
131.6 6.36
179.5 8.48
259.2 7.12
309.2 6.00
78 90
115 120
137 143
190 193
279 282
294
Previous results ReL [8] c ReL [3] d a b c d
n.o. = not observed. Linear fit: v. = A + B P (see Fig. 2). At 295 K, pressure not known. At 20 K, vacuum measurement.
38o: i
~
1
#
.
I
!
!
!
376
V5 .......
; .....
: ................ i
m
•
..........
uW
: "~
280
...........
V4
; . . . .
...............
.........
; . . . . . .
;
i ......
: :
,, ."t ..........
;
•
. . . .
i
..........
......... +
180
.........
:
:
80
.
0.0.
i
......
!
+
. . . . .
: ....... o ~ :
i+
i,
..... *
~
V2
: . . . . . . . . . . . . . . . . . . . . . . .
;. . . . . . . . . .
•
;
:
i
,
4.0
-i.
x
x
,
8.0 Pressure I G P a
.
.
~
.
,..
i
12.0
Fig. 2. Plots of peak wavenumbers vs. pressure for crystalline ethanol. Lines represent fits to the equation v , = A + BP. See Table 1 for values of coefficients A and B.
146
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 143-147
were obtained by taking the ratio of sample to background spectra. Resolution and wave number accuracy were checked by recording the spectrum of water vapour. Ruby fluorescence spectra were measured with a double monochromator (Spex, model 1401) using an argon ion laser operating at 514.5 nm as source.
3. Results Far infrared spectra of ethanol at selected pressures between 2 and 13 GPa are displayed in Fig. 1. A single broad peak is observed for the liquid and several sharper features are apparent at higher pressures, characteristic of a crystalline solid. Wave numbers of principal absorption peaks at all recorded pressures are listed in Table 1 together with results from previous studies. Plots of these wave numbers versus pressure are displayed in Fig. 2. These plots have been fitted to linear functions, the coefficients of which are given at the foot of Table 1. All wave numbers listed are estimated to be accurate to + 1 cm-~ and pressures to + 0.1 GPa. From the sharpness of the fluorescent lines, and consistency of values from rubies in different parts of the sample, we believe that pressure gradients in the samples are small, even at the highest values. By observing the liquid to crystal transformation in a microscope fitted with polarizers and by detecting the onset of the lattice spectra, it was confirmed that the freezing pressure of C2H 5 OH, is 1.8 + 0.1 GPa at 295 K. There was no evidence of a glassy phase ever forming on application of pressure to the liquid sample.
4. Discussion The evolution of the spectra with pressure shows no sign of any major discontinuity. (A minor anomaly in the wavenumber versus pressure plots shown in Fig. 2 for some modes near 7.0 GPa is attributed to an inaccurate pressure reading resulting from relaxation or hysteresis effects). Observations with a microscope during pressure changes also indicated no major changes in appearance. We therefore deduce that there are no solid state phase transitions in ethanol over the pressure range studied. This con-
firms the conclusions reached from previous Raman [2] and infrared [7] investigations. Similarities in the low temperature and high pressure spectra indicate that they originate from the same structure, namely that determined by Jonsson [4] on samples at 87 K. The observed far-infrared spectra will therefore be analysed in terms of this monoclinic unit cell. Ethanol is an asymmetric top molecule, belonging to point group C s. With 9 atoms, there are 27 degrees of freedom, comprising 21 internal modes and 6 external modes, with no degeneracies. These external modes are the origin of the lattice modes in the molecular crystal, of which there are two types librations and translations. With 4 molecules in the unit cell, this leads to 24 lattice modes, 12 librational and 12 translational, of which 3 will be acoustic. Because of the low symmetry, all 21 optical lattice modes are nominally active in both Raman and infrared spectra, and most have been observed in a recent low temperature study [3], together with several peaks attributed to low frequency internal modes involving torsions about the C - C axis of the molecules. By noting frequency shifts of the deuterated species C2H 5 0 D and C2D 5 0 D , tentative assignments of all observed peaks were proposed. However, it was noted that the frequency ratios did not match those of the square roots of moments of inertia and molecular masses, indicating that strong mode mixing is prevalent, as permitted by the low site symmetry. In the far-infrared spectra, one would expect that the strongest peaks would result from modes primarily involving librations about principal axes which are almost perpendicular to the dipole axis. Modes originating from translations and torsions or librations about the principal axis almost parallel to the dipole axis would be weaker. From the observed spectra, 6 relatively strong peaks were selected on the basis that their shifts with pressure could be unambiguously tracked. Some weaker features were identified at lower pressures but were obscured at higher values. Most of these 6 peaks can be readily identified with ones recorded by previous workers at either low temperatures [3] or at an unknown pressure in a diamond anvil cell [8], as indicated in Table 1. Any differences can be attributed to the peaks being sharper at low temperatures or to different orientations of the single crystals in the diamond anvil cells. All six peaks increase in
A. Anderson, W. Smith/Chemical Physics Letters 257 (1996) 143-147
wave number as the pressure increases, corresponding to increases in intermolecular forces, in this case primarily hydrogen bonds. For the six modes, the ratios of frequencies at the highest pressure (13 GPa) to those at the melting pressure (1.8 GPa) range from 1.24 to 1.66 with a mean close to 1.4. This corresponds to an approximate doubling of average hydrogen bond strengths over this pressure range, if harmonic motions are assumed. The range of values for these ratios could result from anisotropic effects: under applied pressure, contraction of the lattice may vary for different directions in the unit cell, resulting in some bonds and hence some mode frequencies being affected more than others. A pronounced increase in hydrogen bond strengths was also found for methanol, from the results of a Brillouin scattering investigation which, of course, probes only the acoustic translational modes [10]. Future work is planned on the Raman and infrared spectra of the deuterated species of solid ethanol at high pressures in order to clarify the assignments of these and other modes and lead to a better understanding of the changes in hydrogen bond strengths. X-ray or neutron diffraction experiments on ethanol at high pressures would also be useful to correlate the observed changes in vibrational spectra with changes in unit cell size and shape.
147
Acknowledgements The assistance of K. Sivalingam and J.F. Wheeldon and helpful discussions with S.A. Lee and D.A. Pinnick are gratefully acknowledged. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the University of Waterloo.
References [1] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 55th ed. (Chemical Rubber Co., Cleveland, OH, 1974). [2] J.F. Mammone, S.K. Sharma and M. Nicol, J. Phys. Chem. 84 (1980) 3130. [3] S.X. Weng, J. Decker, B.H. Torrie and A. Anderson, Indian J. Pure and Appl. Phys. 26 (1988) 76. [4] P.G. JSnsson, Acta Crystallogr. B., 32 (1978) 232. [5] G.J. Piermarini, S. Block and J.D. Barnett, J. Appl. Phys. 44 (1973) 5377. [6] R.F. Lake and H.W. Thompson, Proc. Roy. Soc. A 191 (1966) 469. [7] R.J. Jakobsen, J.W. Brasch and Y. Mikawa, J. Mol. Struct. l (1967-8) 309. [8] Y. Mikawa, J.W. Brasch and R.J. Jakobsen, Spectrochim. Acta, 27A (1971) 529. [9] H.K. Mao, P.M. Bell, J.W. Shaner and D.J. Steinberg, J. Appl. Phys. 7, 49 (1978) 3276. [10] S.A. Lee, A. Anderson, S.M. Lindsay and R.C. Hanson, High Pressure Research, 3 (1990) 230.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical PhysicsLetters 257 (1996) 143-147
Far-infrared spectra of solid ethanol at high pressures A. Anderson, W. Smith Department of Physics, Universityof Waterloo, Waterloo Ontario Canada N2L 3G1
Received 3 April 1996; in final form 9 May 1996
Abstract
Far-infrared spectra of ethanol samples in diamond anvil cells at pressures up to 13 GPa are reported. The freezing pressure at 295 K is confirmed to be 1.8 + 0.1 GPa. No solid state phase transitions are observed in this pressure range. The evolution of six strongly absorbing lattice modes with increasing pressure is followed, and an approximate doubling of average hydrogen bond strength over this pressure range is deduced.
I. Introduction
Ethanol or ethyl alcohol, C2H 5 OH, forms a hydrogen-bonded molecular solid at low temperatures or high pressures. The freezing temperature at ambient pressure is 156 K [1] and the freezing pressure at room temperature is 1.78 GPa [2]. At low temperatures, ethanol is known to have only one crystalline phase, but two different glassy phases have also been observed [3]. The structure of the crystal phase has been determined by Jonsson [4]. The unit cell is monoclinic, space group Pc or C 2, and contains four molecules on general (C l) sites. At elevated pressures, ethanol is reported always to crystallize, in contrast to methanol, which usually forms a superpressed liquid or glass [2]. For this reason, methanol, either neat or in a 4:1 mixture with ethanol, is often used as a medium to ensure hydrostatic (isotropic) conditions for high pressure studies of solids [5]. There have been several previous spectroscopic studies of solid ethanol. These include an early mid-infrared work by Lake and Thompson [6] and two infrared investigations of single crystal ethanol
samples in diamond anvil cells by Jakobsen et al. [7] and Mikawa et al. [8]. In the latter, one set of observed lattice frequencies is listed, but no spectra are shown and the pressure is not given. In addition, a low temperature Raman and far infrared study of solid C2H 5 OH, C2H 5 0 D and C2D 5 0 D by Weng et al. [3] determined the frequencies of many lattice modes and several possible torsional modes and attempted to assign these on the basis of isotopic shifts. They observed a small increase in these frequencies as the crystal was cooled, which they attributed to a contraction of the lattice and subsequent strengthening of the hydrogen bonds. These effects should be enhanced by the application of external pressure to ethanol samples in diamond anvil cells, since it is known that larger changes in lattice constants occur under these conditions. In a Raman study of ethanol at high pressures [2], an observed shift to lower frequencies of the O - H stretching modes was interpreted as due to a strengthening of hydrogen bonding by charge transfers from the molecular bonds. However, the low frequency lattice modes, which directly involve these hydrogen bonds, could not be observed because of their weak inten-
0009-2614/96/$12.00 Copyright© 1996 Elsevier Science B.V. All rights reserved Pit S0009-261 4(96)00509-X
144
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 143-147
sity and proximity to the strong laser line. In contrast, because of the large dipole of the ethanol molecule, the far infrared absorption associated with many of these lattice modes should be quite strong. Hence, in principle this technique provides a method to measure changes in hydrogen bond strengths directly and quantitatively. In this Letter, we present results of a far-infrared room temperature study of ethanol at pressures up to about 13 GPa. The purposes are to confirm the melting pressure, to check for any evidence of solid state phase transitions, to correlate the results with low temperature Raman, infrared and X-ray data, and to obtain estimates of the changes in intermolecular forces over this pressure range.
gasket. The latter was indented by the diamonds to a thickness of about 0.025 mm, and a hole of 0.250 mm diameter was then drilled. The resulting sample volume was about 10 - 3 mm 3. Ethanol was obtained from BDH Chemicals, and had a stated purity of better than 99.7%. A few specks of ruby powder were added to the samples for in situ pressure measurements, using the calibrated shifts of two fluorescent lines [9]. A Fourier transform infrared spectrometer (Beckm a n - R I I C Model FS620) was used to record the far-infrared spectra. This evacuated instrument was fitted with a high pressure mercury lamp as source, Mylar beam dividers, crystal quartz and polyethylene filters, and a silicon bolometer detector, operating at 4.2K. An off-axis ellipsoidal mirror was used to focus radiation from the interferometer on to the diamond anvil cell which was placed directly in front of the entrance window of the bolometer cryostat. A resolution of about 2.5 cm-1 (corresponding to a mirror movement of 2 mm) was typically used, but several scans at higher resolution were obtained to check for any fine structure. Transmittance values
2. Experimental techniques Diamond anvil cells were obtained from Diacell Ltd., Leicester, U.K. They were the piston-cylinder type, with pressure adjustment through six Allen screws. Type 2a diamonds were used with an Inconel
~ ~
7.40GPa
2.53GPa
GPa
~
~ ~ G P a 80 240 400 I00
8 0 260
Wavenumber / cm" Fig. 1. Far-infraredspectraof crystalline ethanol at selected pressures.
GPa
GPa
420
145
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 1 4 3 - 1 4 7 Table 1 Peak wavenumbers of solid ethanol at various pressures Wavenumbers ( c m - t )
Pressure
(GPa)
1-'I
1)2
P3
~4
P5
~6
2.4 2.5 2.9 4.1 5.2 6.6 7.0 7.4 8.5 9.6 12.4 13.0
94 99 102 108 111 114 117 125 127 129 132 135
107 114 117 129 134 138 144 157 163 164 176 177
138 148 151 161 164 172 173 185 196 198 206 208
189 201 204 218 224 231 235 255 262 266 279 283
271 277 280 292 297 302 304 321 327 332 344 347
320 326 328 335 338 347 347 355 367 370 n.o. a 383
A (cm- l ) b B (cm- i GPa- i) b
91.4 3.64
99.1 6.51
131.6 6.36
179.5 8.48
259.2 7.12
309.2 6.00
78 90
115 120
137 143
190 193
279 282
294
Previous results ReL [8] c ReL [3] d a b c d
n.o. = not observed. Linear fit: v. = A + B P (see Fig. 2). At 295 K, pressure not known. At 20 K, vacuum measurement.
38o: i
~
1
#
.
I
!
!
!
376
V5 .......
; .....
: ................ i
m
•
..........
uW
: "~
280
...........
V4
; . . . .
...............
.........
; . . . . . .
;
i ......
: :
,, ."t ..........
;
•
. . . .
i
..........
......... +
180
.........
:
:
80
.
0.0.
i
......
!
+
. . . . .
: ....... o ~ :
i+
i,
..... *
~
V2
: . . . . . . . . . . . . . . . . . . . . . . .
;. . . . . . . . . .
•
;
:
i
,
4.0
-i.
x
x
,
8.0 Pressure I G P a
.
.
~
.
,..
i
12.0
Fig. 2. Plots of peak wavenumbers vs. pressure for crystalline ethanol. Lines represent fits to the equation v , = A + BP. See Table 1 for values of coefficients A and B.
146
A. Anderson, W. Smith / Chemical Physics Letters 257 (1996) 143-147
were obtained by taking the ratio of sample to background spectra. Resolution and wave number accuracy were checked by recording the spectrum of water vapour. Ruby fluorescence spectra were measured with a double monochromator (Spex, model 1401) using an argon ion laser operating at 514.5 nm as source.
3. Results Far infrared spectra of ethanol at selected pressures between 2 and 13 GPa are displayed in Fig. 1. A single broad peak is observed for the liquid and several sharper features are apparent at higher pressures, characteristic of a crystalline solid. Wave numbers of principal absorption peaks at all recorded pressures are listed in Table 1 together with results from previous studies. Plots of these wave numbers versus pressure are displayed in Fig. 2. These plots have been fitted to linear functions, the coefficients of which are given at the foot of Table 1. All wave numbers listed are estimated to be accurate to + 1 cm-~ and pressures to + 0.1 GPa. From the sharpness of the fluorescent lines, and consistency of values from rubies in different parts of the sample, we believe that pressure gradients in the samples are small, even at the highest values. By observing the liquid to crystal transformation in a microscope fitted with polarizers and by detecting the onset of the lattice spectra, it was confirmed that the freezing pressure of C2H 5 OH, is 1.8 + 0.1 GPa at 295 K. There was no evidence of a glassy phase ever forming on application of pressure to the liquid sample.
4. Discussion The evolution of the spectra with pressure shows no sign of any major discontinuity. (A minor anomaly in the wavenumber versus pressure plots shown in Fig. 2 for some modes near 7.0 GPa is attributed to an inaccurate pressure reading resulting from relaxation or hysteresis effects). Observations with a microscope during pressure changes also indicated no major changes in appearance. We therefore deduce that there are no solid state phase transitions in ethanol over the pressure range studied. This con-
firms the conclusions reached from previous Raman [2] and infrared [7] investigations. Similarities in the low temperature and high pressure spectra indicate that they originate from the same structure, namely that determined by Jonsson [4] on samples at 87 K. The observed far-infrared spectra will therefore be analysed in terms of this monoclinic unit cell. Ethanol is an asymmetric top molecule, belonging to point group C s. With 9 atoms, there are 27 degrees of freedom, comprising 21 internal modes and 6 external modes, with no degeneracies. These external modes are the origin of the lattice modes in the molecular crystal, of which there are two types librations and translations. With 4 molecules in the unit cell, this leads to 24 lattice modes, 12 librational and 12 translational, of which 3 will be acoustic. Because of the low symmetry, all 21 optical lattice modes are nominally active in both Raman and infrared spectra, and most have been observed in a recent low temperature study [3], together with several peaks attributed to low frequency internal modes involving torsions about the C - C axis of the molecules. By noting frequency shifts of the deuterated species C2H 5 0 D and C2D 5 0 D , tentative assignments of all observed peaks were proposed. However, it was noted that the frequency ratios did not match those of the square roots of moments of inertia and molecular masses, indicating that strong mode mixing is prevalent, as permitted by the low site symmetry. In the far-infrared spectra, one would expect that the strongest peaks would result from modes primarily involving librations about principal axes which are almost perpendicular to the dipole axis. Modes originating from translations and torsions or librations about the principal axis almost parallel to the dipole axis would be weaker. From the observed spectra, 6 relatively strong peaks were selected on the basis that their shifts with pressure could be unambiguously tracked. Some weaker features were identified at lower pressures but were obscured at higher values. Most of these 6 peaks can be readily identified with ones recorded by previous workers at either low temperatures [3] or at an unknown pressure in a diamond anvil cell [8], as indicated in Table 1. Any differences can be attributed to the peaks being sharper at low temperatures or to different orientations of the single crystals in the diamond anvil cells. All six peaks increase in
A. Anderson, W. Smith/Chemical Physics Letters 257 (1996) 143-147
wave number as the pressure increases, corresponding to increases in intermolecular forces, in this case primarily hydrogen bonds. For the six modes, the ratios of frequencies at the highest pressure (13 GPa) to those at the melting pressure (1.8 GPa) range from 1.24 to 1.66 with a mean close to 1.4. This corresponds to an approximate doubling of average hydrogen bond strengths over this pressure range, if harmonic motions are assumed. The range of values for these ratios could result from anisotropic effects: under applied pressure, contraction of the lattice may vary for different directions in the unit cell, resulting in some bonds and hence some mode frequencies being affected more than others. A pronounced increase in hydrogen bond strengths was also found for methanol, from the results of a Brillouin scattering investigation which, of course, probes only the acoustic translational modes [10]. Future work is planned on the Raman and infrared spectra of the deuterated species of solid ethanol at high pressures in order to clarify the assignments of these and other modes and lead to a better understanding of the changes in hydrogen bond strengths. X-ray or neutron diffraction experiments on ethanol at high pressures would also be useful to correlate the observed changes in vibrational spectra with changes in unit cell size and shape.
147
Acknowledgements The assistance of K. Sivalingam and J.F. Wheeldon and helpful discussions with S.A. Lee and D.A. Pinnick are gratefully acknowledged. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the University of Waterloo.
References [1] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 55th ed. (Chemical Rubber Co., Cleveland, OH, 1974). [2] J.F. Mammone, S.K. Sharma and M. Nicol, J. Phys. Chem. 84 (1980) 3130. [3] S.X. Weng, J. Decker, B.H. Torrie and A. Anderson, Indian J. Pure and Appl. Phys. 26 (1988) 76. [4] P.G. JSnsson, Acta Crystallogr. B., 32 (1978) 232. [5] G.J. Piermarini, S. Block and J.D. Barnett, J. Appl. Phys. 44 (1973) 5377. [6] R.F. Lake and H.W. Thompson, Proc. Roy. Soc. A 191 (1966) 469. [7] R.J. Jakobsen, J.W. Brasch and Y. Mikawa, J. Mol. Struct. l (1967-8) 309. [8] Y. Mikawa, J.W. Brasch and R.J. Jakobsen, Spectrochim. Acta, 27A (1971) 529. [9] H.K. Mao, P.M. Bell, J.W. Shaner and D.J. Steinberg, J. Appl. Phys. 7, 49 (1978) 3276. [10] S.A. Lee, A. Anderson, S.M. Lindsay and R.C. Hanson, High Pressure Research, 3 (1990) 230.