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Infrared and Raman spectra of the condensed phases of hydrogen bromide
Spectmchhica Acta,Vol.27A,gp.401 to 499.Pergamon Press 1971.Prht8dInNorthemIrelsnd
Inhued and Ramm ~pectraft~~ondensed
phases of hydrogen
E. L. PACE Department of Chemistry Case Western Reserve University Cleveland, Ohio (Received 16 June 1970)
Al&act--A temperature-dependent study of the infrared and Ramau speatra of the liquid and solid ,9, y and 6 phases over the range from 100 to 190’K hae been carried out. Of particular interest is the fact that band maxima in the infrared aud Ramau spectra have identical frequency values iu the liquid phase but attain a separation of 12 cm-r in the 6 or plaetioally crystalhue phase at 120’K. A qualitative explauation of the behavior of the 6 crystalline phase is presented. INTRODUCTION
work, hydrogen bromide is known to have three solidsolid transitions [l] and therefore to exist in four crystalline forms, (1) the a form which is stable below 89.2”K, (2) the /l form stable between 89.2 and 113.4’K, (3) the y form stable between 113.4 and 116.8’K, and (4) the 8 form which ia stable between 116.8’K and the melting point at 1862°K. The transitions at 89.2, 113.4 and 116.8”K involve respectively 63.4, 78.8 and 85.7 calories per mole. As a solid, hydrogen bromide belongs to a type which has been characterized by TIMXERMANS [Z] 8s “plastic”. Generally, in such solids, the molecules in the crystalline phase below the melting point crystallize in a lattice of cubic symmetry and, in their motion, tend to local disorder at the lattice sites. In the case of deuterium chloride, SANDOR and FARROW have found that the unit cell is face centered cubic from X-ray powder diffraction data [3] and that a disordered model is necessary to account for the alternating intensity of the lines observed in the neutron diffraction pattern [4]. It is quite likely that analogous behavior pertains to the high temperature 6 form of hydrogen bromide. The bulk of the infrared and spectroscopic research work has been carried out on the a phase which shows pronounced splittings at temperatures around 77°K [5-7-J. FROM precise calorimetric
These investigations show the presence of four componentfl at 77’K with frequencies of 2406, ~2417, 2440, -2449 cm- l. Some uncertainty exists in the frequencies at
2417 and 2449 in that the bands appear as shoulders in the spectra. Corresponding to the peaks in the Raman spectrum, there have definitely been observed two asymmetric bands with maxima at 2406 and 2440 cm-l in the infrared spectrum. A high frequency component around 2450 cm -l which has been attributed to the formation of linear chains [5] has also been noted. In genersl, the spectroscopic results for the [I] W. F. GIAUQUEand R. WIEBE, J. Am. Churn. Sot. SO,2193 (1928). [2] J. !thMERMANS, J. Phya. Chern. SoZic?s18,1 (1961). [3] E. S-OR and R. F. C. FURROW,Nature 215,171 (1967). [4] E. S-OR and R. F. C. Fmow, Nature fU5,1265(1907). [5] D. F. HORNIQ and W. E. OSBERU, J. Chem. Phys. 23, 1662 (1955). [6] R. SAVOIE and A. ANDERSON, J. Chews. Phys. 44, 648 (1966). [7] M. ITO, M. Snzmrr and T. YOKOYA~, J. Chem. Bye. 50, 2949 (1969). 491
492
E. L. PACE
a phase are consis~nt with a factor group symmet~ of C,, (space group C,,*) with four molecules per unit cell located at sites of C, symmetry. The spectroscopic results, however, are in contrast to the neutron diffraction results for hydrogen and deuterium chloride [3] on the basis of which a unit cell containing two molecules (space group C,ta) has been proposed. A temperature dependent study of the infrared spectrum of hydrogen bromide for the high temperature forms has been carried out by BRUNELand PEYRON[S]. They observed a shift in the absorption maximum from 2477 to 2468 cm-l as the liquid was cooled from 190’K through the fusion to 165°K. The &r transition was marked by a discontinuous change in intensity of absorption. Ito et al. report a Raman spectrum of the 8 form of HBr at 168°K. Otherwise no temperature dependent study of the Raman spectrum of the high ~rn~rat~e forms of hydrogen bromide exists. It was the purpose of the present ~vestigatiou to obtain both the infrared and Raman spectra of the high temperature forms of hydrogen bromide at a variety of temperatures in order to help establish the symmetry of the 6 phase in particular, as well as to provide some information on the structural changes involved in the transitions. EXPERIMENTAL The hydrogen bromide was J. T. Baker Co. brand and had a stated purity of 99.8 %. Air was removed from the sample by a number of cycles involving the melting and freezing of the sample followed by evacuation with a high vacuum pumping system. The Raman spectrum was taken with a Cary M[odel 81 s~ot~rne~r in which coaxial excitation with a helium-neon laser source was provided. The sample contamer was a portion of a 3 mm Pyrex glass tube with a flattened end which served as a window for the exciting and scattering light. Refrigeration was provided by liquid nitrogen used in conjunction with a dewar and cold tiger set-up which could be inserted into the path of the light from a location outside the sample compartment. Evacuation of the refrigerant to produce temperatures below 77°K was possible. Nitrogen gas cooled in liquid nitrogen was also used to obtain temperatures above 77’K. In general, temperature control of the order of A2 could be obtained with the sample during scans of the spectrum. The temperature of the sample was determined by a copper-constantan thermocouple attached to a copper coil which was tightly wrapped around the sample tube at a point near the window. Thermal contact of the coil with the tube was also assured by the use of stopcock grease. Calibration of the thermocouple was achieved by comparing the above thermocouple with an identical one which was inserted directly into the sample itself. A correction was also applied for the warming of the sampIe produced by the absorption of laser light, an effect which was particularly noticeable at the very lowest temperatures. Calibration points checked to within &2’ for all but the very lowest temperatures where the uncertainty may have been double the preceding value. Slit widths used for the spectra wereapproximately 3 cm -l. A wave length calibration of the spectrometer was performed against a neon arc so that the frequencies recorded during scans of the spectrum corrected with the calibration are accurate to within 1 cm-l.
Infrared and Raman spectra of the condensedphases of hydrogen bromide
493
Because of the broadness of the bands, the assignment of frequency values to the band maxima may be in error by as much as f2 cm-l. The Raman spectra for a number of temperatures involving the 6 and liquid phases are presented in Fig. 1 and for a range of temperatures involving the a, t9, y, and 8 solid phases are presented in Fig. 2. I
I
I
2500
2400
Fig. 1. Raman spectrum of liquid and 6 phases.
The infrared spectra were taken with a Beckman IR-12 spectrometer employing a low temperature apparatus which has previously been described [9]. Spectra were found to be reproducible when either silver bromide or silver chloride windows were used in the sample cell. The temperature was recorded with a copper-constantan thermocouple soldered to a silver screw which was in direct contact with one of the windows of the cell. Temperature control during scans of the spectrum was f2’. Since it was not possible to obtain a temperature of the window at a point adjacent to the sample and not directly of the thin layer of the sample, errors of the order of 15% might well exist in the recorded temperatures. A frequency calibration of the spectrum against the spectrum for water vapor showed the instrument to be correct within 1 cm-l for observations in the region around 2400 cm-l. Slit widths of approximately 2 cm-l were used to take the spectra. Tracings of the actual spectra as recorded are shown in Figs. 3 and 4. The high frequency end of the traces show the effect of decreased transmission on the silver chloride windows. A calibration of this [9] E. L. PACE and L. J. NOE, J. Chtm. Phys. a@,5317 (1968).
404
E. L. PACE
2400
2500
Fig. 2. Raman spectrumof solid phasesbetween83 and 120%. effect for the empty cell showed a 5% decrease in transmission for every 100 cm-1 increase in frequency for the frequency range around 2400 cm-l. The effect was essentially temperature independent throughout the entire temperature range of the observations. In Fig. 3, the traces obtained for a temperature range involving the 6 and liquid phases are shown. In Fig. 4, a series of traces in the temperature range involving the /3 and y phases are shown. It was possible to attain temperatures only of the order of 100°K because of a vacuum in the cell of the order of 10-s torr instead of one of the order of 1O-5 torr which is necessary. The values of the band maxima which are marked in Figs. l-4 are contained in Table 1. Corrections of the order of 1 cm-l were made for the asymmetry in the infrared bands resulting from the effect on the band shapes produced by the changing transmission of the silver chloride windows. Also, because of the broadness of the bands, the quoted values cannot be considered more accurate than &-2 cm-l. DISCUSSION
In the liquid phase around 192°K both the infrared and Raman spectra in Figs. 1 and 3 have a band maximum at the same frequency within the experimental error of f2 cm-‘. The value of 2477 cm-l observed in the infrared spectrum is in exact agreement with that noted by BRUNELand PEYRON[S]. Asymmetry in the shape of both the infrared and Raman band results from the increased absorption on the low
Infrared and Raman spectra of the condensedphases of hydrogen bromide
496
E. L.
496
PACE
Table 1. Maxima in absorption bands Temperature CW
Band maximum (cm--f) Infrared Raman 2408, 2442,2464” 2415, 2463 2461
70
83 97 102 114 115 119 120 123 126 146 148 176 177 192 194
2462 2454 2460 2467 2466 2454 2466 2467 2467 2466 2472 2476 2477
* Not shown in figuree.
frequency side of the band maximum. Since the 0 and P rotationrtl branches of diatomic molecules in general have lower intensities than the other branches [lo], it is possible that the envelope of the rotational branches of the vibration-rot&en band is involved in determining the shape of the bands in the liquid phase.
The S-phase is the so-called “plastic” phase. No direct experimental evidence is available but its moleoulctr arrangement is probably oubic. During the change from liquid to the 6 phase, several effects are noticeable in the Raman spectrum. First, there is an immediate shift tow&rd lower frequency of the order of 10 cm-l in the bsnd rn~~rn~ and a narrowing of band width and a decrectsein the asymmetry of the band. As the temperature is lowered, the asymmetry disltppears and the band becomes reasonably symmetric in contour. Also, there is a continuous shift in the band maximum toward lower frequency whmh eventually amounts to more than 20 cm-l at the lowest temper&urea of the 8 phese. The effects in the infrared spectrum contrast strongly with those in the Ram&n spectrum, The shift in fkequenoy of the rn&~rnurnwith ~m~r&ture is toward lower frequency as in the Raman spectrum but the magnitude is less than half of that in the Raman spectrum. The magnitude of the shift in the infrared spectrum is in agreement with that reported by Brunei and Peyron who recorded a value of 2468 cm-l at 166’K. There is also a much more obvious narrowing of the band width in the infrared bsnd but the a,symmetry is retained even at the lowest temperatures of the Qphtlse. The behavior [lo) G. HERZBJERCJ, Molmdur
~S’pectru and Molm&zr p. 133. Prentice-Hall (X939).
S~rwture,
Vol. I, DkztornicMdexdea,
Infrared and
Raman spectra of the condensed phases of hydrogen bromide
497
exhibited by the 6 phase appears to indicate the presence of two distinct bands separated by approximately 12 om-i at temperatures around 12O”K, one of which is associated with activity in the Raman spectrum and the other in the infrared spectrum. The results for the cubic phase which have been discussed by IT0 et al. [7] appear to show one important difference when oompared to hydrogen chloride. In hydrogen chloride, the Raman spectrum has a peak at 2755 cm-l and a shoulder at 2785 cm-i which appear to match in frequency a peak at 2785 cm-l and a lowfrequency shoulder observed by HIEBERTand HORNIG[l l] in the infrared spectrum.
Our temperature control and temperature readings were probably not accurate enough to speak with certainty of the behavior of the y-phase in view of the 4’ range of existence of the phase. As the hydrogen bromide goes through the e-8 transitions there is a noticeable shift in the frequency of the maximum toward longer wave lengths in infrared spectrum which is followed by a broadening of the band without any splitting. Other investigators [8, 121have observed other effects such as a discontinuous change of the intensity of absorption and optical anisotropy at temperatures around those of the Sy transition. The Raman spectrum of the /3 phase shows the appearance of a small asymmetry in the band without an appreciable change in band width. Both the infrared spectrum [8] and the Raman spectrum confirm the appearance of a double band at temperatures around WK which splits into several components as the temperature is lowered further. COMMENTS ON THE ST~TJCTTJRE OF THE 6 PHBSE The most complete study of the crystal structure of the 8 phase of any of the deuterium or hydrogen halides is that carried out by SANDORand FARROW[3, 41 on deuterium chloride by means of powder X-ray dilfraction and neutron diffraction. The space group was established as Fnz37n (Ohs)with four molecules per unit cell. The four chlorine atoms were located at equivalent crystallographic positions of the lattice (000). However, to account for the alternating intensity observed in the neutron difl?action pattern, a twelve-fold disordered model in which the deuterium atom of each molecule had twelve equally probable equilibrium positions situated on sites equivalent to zz0 (Ozz, 2Oz, zz0, OZZ,POZ,220, OzZ, fOz, zZ0, OzZ, sOZ, ZrO) in which x is the fractional position parameter of the unit cell. The position of the hydrogen atom was thus assumed to be in a line joining nearest neighbor chlorine atoms and in the direction of the primitive lattice vectors. Each of the equilibrium positions has C2c symmetry providing all others are simultaneously occupied. A completely random model was found unable to account for the alternation of observed intensities. Also, a six-fold disordered model in which the deuterium atoms were located along crystallographic axes of the face-centered lattice was found to be more unacceptable than the random model. The results and model derived from neutron [Ill G. L. HIEBERT and D. F. HORNIO, J. Own. Phya. 1121A. ETJOKEN,2. Electrochem. 45, 126 (1939).
27, 1216 (1967).
498
E. L. PACE
diffraction data led S~WDORand FURROW[4] to suggest that deuterium chloride molecules carry out librations of large angular amplitudes about their equilibrium positions accompanied by a random “flipping over” from one equivalent orientation to another. From the fact that a rotational lattice band at 200 cm-1 is observed while a translational lattice band expected in the frequency range below 100 cm-l is absent, Ito et al. estimate the lower limit of the average lifetime of a hydrogen bond in hydrogen chloride as 2 x lo--l3 sec. Therefore the greatest frequency of the “flipping over” process is probably of the order of 6 x lOl* set-l in the hydrogen halides whereas the frequency of the internal modes of vibration is of the order of 1014se+. The experimental infrared and Raman results indicate that the band maximum in the infrared spectrum differs from that in the Raman by approximately 10 cm-l in hydrogen bromide and by approximately 30 cm-l in hydrogen chloride. The band contours in the infrared spectrum differ from those in the Raman. In the case of hydrogen bromide, the effects are shown here to be associated with the transition from the liquid to the 6 phase since the maxima for both the infrared and Reman spectrum coincide exactly in the liquid phase. No direct structural information is available for the J-phase of hydrogen bromide. However, it would appear reasonable to assume that the general features of the model used in interpreting the X-ray and neutron diffraction results for deuterium chloride pertain in the case of hydrogen bromide. For deuterium chloride, the space symmetry which has been found to be 0,,5 is accounted for in the proposed model by assuming a site symmetry of C,, for any one of the 12-fold positionsialong the primitive lattice vectors surrounding each chlorine atom. However, the interpretation of the neutron diffraction results is based on the integration of the scattering intensity over intervals of time (of the order of one week) which are much longer than the time required to break a hydrogen bond. Thus, the density of the proton at each of the Wfold positions averages to the same value. On the other hand, since a molecular vibration occurs in an interval of time at least two orders of magnitude less than required for the “flipping over” of a hydrogen bond, the nature of the Raman and infrared spectrum will essentially depend on the instantaneous and not the average symmetry of the lattice. Consequently, the infrared and Raman activity will be determined by temporary configurations in which the site symmetry will be lower than C,, on the average and will correspond to that of the correlational subgroups of C,, (C,, C, and 0,). As a result, the symmetry of the factor group and the space group would be that of a correlational subgroup of 0,6 (B’m3m). The spectroscopic results may then be interpreted in the following manner. At temperatures near the melting point, the site symmetry is probably C, and the space and factor group symmetry is so low that the symmetry of the site essentially determines the nature of the spectrum. Under these conditions the results would be comparable to that for the liquid and a convergence of the maximum in the Raman spectrum and that in the infrared would be expected. As the temperature of the d-phase is lowered, it is reasonable to assume that in localized regions of the crystal, configurations of higher symmetry than C, would exist for periods of time which are long relative to the time of a vibration. The presence of low symmetry at the site with some symmetry (C,, C,, D, etc.) in the crystal would render adjacent sites
Infrared and Raman spectra of the condensedphases of hydrogen bromide
499
non-translationally equivalent and set up the necessary conditions for resonance coupling between molecules. Qualitatively one can see that symmetry vibrational modes resulting from resonance coupling of aggregates of two, three and more molecules can exist. A majority of such symmetry modes are active in both the infrared and the Raman spectrum. The net effects of the superposition of the large number of such modes would be to give bands of considerable width and probably different maxima since the parameters which determine the intensity in the infrared differ from those in the Raman spectrum.
10
Inhued and Ramm ~pectraft~~ondensed
phases of hydrogen
E. L. PACE Department of Chemistry Case Western Reserve University Cleveland, Ohio (Received 16 June 1970)
Al&act--A temperature-dependent study of the infrared and Ramau speatra of the liquid and solid ,9, y and 6 phases over the range from 100 to 190’K hae been carried out. Of particular interest is the fact that band maxima in the infrared aud Ramau spectra have identical frequency values iu the liquid phase but attain a separation of 12 cm-r in the 6 or plaetioally crystalhue phase at 120’K. A qualitative explauation of the behavior of the 6 crystalline phase is presented. INTRODUCTION
work, hydrogen bromide is known to have three solidsolid transitions [l] and therefore to exist in four crystalline forms, (1) the a form which is stable below 89.2”K, (2) the /l form stable between 89.2 and 113.4’K, (3) the y form stable between 113.4 and 116.8’K, and (4) the 8 form which ia stable between 116.8’K and the melting point at 1862°K. The transitions at 89.2, 113.4 and 116.8”K involve respectively 63.4, 78.8 and 85.7 calories per mole. As a solid, hydrogen bromide belongs to a type which has been characterized by TIMXERMANS [Z] 8s “plastic”. Generally, in such solids, the molecules in the crystalline phase below the melting point crystallize in a lattice of cubic symmetry and, in their motion, tend to local disorder at the lattice sites. In the case of deuterium chloride, SANDOR and FARROW have found that the unit cell is face centered cubic from X-ray powder diffraction data [3] and that a disordered model is necessary to account for the alternating intensity of the lines observed in the neutron diffraction pattern [4]. It is quite likely that analogous behavior pertains to the high temperature 6 form of hydrogen bromide. The bulk of the infrared and spectroscopic research work has been carried out on the a phase which shows pronounced splittings at temperatures around 77°K [5-7-J. FROM precise calorimetric
These investigations show the presence of four componentfl at 77’K with frequencies of 2406, ~2417, 2440, -2449 cm- l. Some uncertainty exists in the frequencies at
2417 and 2449 in that the bands appear as shoulders in the spectra. Corresponding to the peaks in the Raman spectrum, there have definitely been observed two asymmetric bands with maxima at 2406 and 2440 cm-l in the infrared spectrum. A high frequency component around 2450 cm -l which has been attributed to the formation of linear chains [5] has also been noted. In genersl, the spectroscopic results for the [I] W. F. GIAUQUEand R. WIEBE, J. Am. Churn. Sot. SO,2193 (1928). [2] J. !thMERMANS, J. Phya. Chern. SoZic?s18,1 (1961). [3] E. S-OR and R. F. C. FURROW,Nature 215,171 (1967). [4] E. S-OR and R. F. C. Fmow, Nature fU5,1265(1907). [5] D. F. HORNIQ and W. E. OSBERU, J. Chem. Phys. 23, 1662 (1955). [6] R. SAVOIE and A. ANDERSON, J. Chews. Phys. 44, 648 (1966). [7] M. ITO, M. Snzmrr and T. YOKOYA~, J. Chem. Bye. 50, 2949 (1969). 491
492
E. L. PACE
a phase are consis~nt with a factor group symmet~ of C,, (space group C,,*) with four molecules per unit cell located at sites of C, symmetry. The spectroscopic results, however, are in contrast to the neutron diffraction results for hydrogen and deuterium chloride [3] on the basis of which a unit cell containing two molecules (space group C,ta) has been proposed. A temperature dependent study of the infrared spectrum of hydrogen bromide for the high temperature forms has been carried out by BRUNELand PEYRON[S]. They observed a shift in the absorption maximum from 2477 to 2468 cm-l as the liquid was cooled from 190’K through the fusion to 165°K. The &r transition was marked by a discontinuous change in intensity of absorption. Ito et al. report a Raman spectrum of the 8 form of HBr at 168°K. Otherwise no temperature dependent study of the Raman spectrum of the high ~rn~rat~e forms of hydrogen bromide exists. It was the purpose of the present ~vestigatiou to obtain both the infrared and Raman spectra of the high temperature forms of hydrogen bromide at a variety of temperatures in order to help establish the symmetry of the 6 phase in particular, as well as to provide some information on the structural changes involved in the transitions. EXPERIMENTAL The hydrogen bromide was J. T. Baker Co. brand and had a stated purity of 99.8 %. Air was removed from the sample by a number of cycles involving the melting and freezing of the sample followed by evacuation with a high vacuum pumping system. The Raman spectrum was taken with a Cary M[odel 81 s~ot~rne~r in which coaxial excitation with a helium-neon laser source was provided. The sample contamer was a portion of a 3 mm Pyrex glass tube with a flattened end which served as a window for the exciting and scattering light. Refrigeration was provided by liquid nitrogen used in conjunction with a dewar and cold tiger set-up which could be inserted into the path of the light from a location outside the sample compartment. Evacuation of the refrigerant to produce temperatures below 77°K was possible. Nitrogen gas cooled in liquid nitrogen was also used to obtain temperatures above 77’K. In general, temperature control of the order of A2 could be obtained with the sample during scans of the spectrum. The temperature of the sample was determined by a copper-constantan thermocouple attached to a copper coil which was tightly wrapped around the sample tube at a point near the window. Thermal contact of the coil with the tube was also assured by the use of stopcock grease. Calibration of the thermocouple was achieved by comparing the above thermocouple with an identical one which was inserted directly into the sample itself. A correction was also applied for the warming of the sampIe produced by the absorption of laser light, an effect which was particularly noticeable at the very lowest temperatures. Calibration points checked to within &2’ for all but the very lowest temperatures where the uncertainty may have been double the preceding value. Slit widths used for the spectra wereapproximately 3 cm -l. A wave length calibration of the spectrometer was performed against a neon arc so that the frequencies recorded during scans of the spectrum corrected with the calibration are accurate to within 1 cm-l.
Infrared and Raman spectra of the condensedphases of hydrogen bromide
493
Because of the broadness of the bands, the assignment of frequency values to the band maxima may be in error by as much as f2 cm-l. The Raman spectra for a number of temperatures involving the 6 and liquid phases are presented in Fig. 1 and for a range of temperatures involving the a, t9, y, and 8 solid phases are presented in Fig. 2. I
I
I
2500
2400
Fig. 1. Raman spectrum of liquid and 6 phases.
The infrared spectra were taken with a Beckman IR-12 spectrometer employing a low temperature apparatus which has previously been described [9]. Spectra were found to be reproducible when either silver bromide or silver chloride windows were used in the sample cell. The temperature was recorded with a copper-constantan thermocouple soldered to a silver screw which was in direct contact with one of the windows of the cell. Temperature control during scans of the spectrum was f2’. Since it was not possible to obtain a temperature of the window at a point adjacent to the sample and not directly of the thin layer of the sample, errors of the order of 15% might well exist in the recorded temperatures. A frequency calibration of the spectrum against the spectrum for water vapor showed the instrument to be correct within 1 cm-l for observations in the region around 2400 cm-l. Slit widths of approximately 2 cm-l were used to take the spectra. Tracings of the actual spectra as recorded are shown in Figs. 3 and 4. The high frequency end of the traces show the effect of decreased transmission on the silver chloride windows. A calibration of this [9] E. L. PACE and L. J. NOE, J. Chtm. Phys. a@,5317 (1968).
404
E. L. PACE
2400
2500
Fig. 2. Raman spectrumof solid phasesbetween83 and 120%. effect for the empty cell showed a 5% decrease in transmission for every 100 cm-1 increase in frequency for the frequency range around 2400 cm-l. The effect was essentially temperature independent throughout the entire temperature range of the observations. In Fig. 3, the traces obtained for a temperature range involving the 6 and liquid phases are shown. In Fig. 4, a series of traces in the temperature range involving the /3 and y phases are shown. It was possible to attain temperatures only of the order of 100°K because of a vacuum in the cell of the order of 10-s torr instead of one of the order of 1O-5 torr which is necessary. The values of the band maxima which are marked in Figs. l-4 are contained in Table 1. Corrections of the order of 1 cm-l were made for the asymmetry in the infrared bands resulting from the effect on the band shapes produced by the changing transmission of the silver chloride windows. Also, because of the broadness of the bands, the quoted values cannot be considered more accurate than &-2 cm-l. DISCUSSION
In the liquid phase around 192°K both the infrared and Raman spectra in Figs. 1 and 3 have a band maximum at the same frequency within the experimental error of f2 cm-‘. The value of 2477 cm-l observed in the infrared spectrum is in exact agreement with that noted by BRUNELand PEYRON[S]. Asymmetry in the shape of both the infrared and Raman band results from the increased absorption on the low
Infrared and Raman spectra of the condensedphases of hydrogen bromide
496
E. L.
496
PACE
Table 1. Maxima in absorption bands Temperature CW
Band maximum (cm--f) Infrared Raman 2408, 2442,2464” 2415, 2463 2461
70
83 97 102 114 115 119 120 123 126 146 148 176 177 192 194
2462 2454 2460 2467 2466 2454 2466 2467 2467 2466 2472 2476 2477
* Not shown in figuree.
frequency side of the band maximum. Since the 0 and P rotationrtl branches of diatomic molecules in general have lower intensities than the other branches [lo], it is possible that the envelope of the rotational branches of the vibration-rot&en band is involved in determining the shape of the bands in the liquid phase.
The S-phase is the so-called “plastic” phase. No direct experimental evidence is available but its moleoulctr arrangement is probably oubic. During the change from liquid to the 6 phase, several effects are noticeable in the Raman spectrum. First, there is an immediate shift tow&rd lower frequency of the order of 10 cm-l in the bsnd rn~~rn~ and a narrowing of band width and a decrectsein the asymmetry of the band. As the temperature is lowered, the asymmetry disltppears and the band becomes reasonably symmetric in contour. Also, there is a continuous shift in the band maximum toward lower frequency whmh eventually amounts to more than 20 cm-l at the lowest temper&urea of the 8 phese. The effects in the infrared spectrum contrast strongly with those in the Ram&n spectrum, The shift in fkequenoy of the rn&~rnurnwith ~m~r&ture is toward lower frequency as in the Raman spectrum but the magnitude is less than half of that in the Raman spectrum. The magnitude of the shift in the infrared spectrum is in agreement with that reported by Brunei and Peyron who recorded a value of 2468 cm-l at 166’K. There is also a much more obvious narrowing of the band width in the infrared bsnd but the a,symmetry is retained even at the lowest temperatures of the Qphtlse. The behavior [lo) G. HERZBJERCJ, Molmdur
~S’pectru and Molm&zr p. 133. Prentice-Hall (X939).
S~rwture,
Vol. I, DkztornicMdexdea,
Infrared and
Raman spectra of the condensed phases of hydrogen bromide
497
exhibited by the 6 phase appears to indicate the presence of two distinct bands separated by approximately 12 om-i at temperatures around 12O”K, one of which is associated with activity in the Raman spectrum and the other in the infrared spectrum. The results for the cubic phase which have been discussed by IT0 et al. [7] appear to show one important difference when oompared to hydrogen chloride. In hydrogen chloride, the Raman spectrum has a peak at 2755 cm-l and a shoulder at 2785 cm-i which appear to match in frequency a peak at 2785 cm-l and a lowfrequency shoulder observed by HIEBERTand HORNIG[l l] in the infrared spectrum.
Our temperature control and temperature readings were probably not accurate enough to speak with certainty of the behavior of the y-phase in view of the 4’ range of existence of the phase. As the hydrogen bromide goes through the e-8 transitions there is a noticeable shift in the frequency of the maximum toward longer wave lengths in infrared spectrum which is followed by a broadening of the band without any splitting. Other investigators [8, 121have observed other effects such as a discontinuous change of the intensity of absorption and optical anisotropy at temperatures around those of the Sy transition. The Raman spectrum of the /3 phase shows the appearance of a small asymmetry in the band without an appreciable change in band width. Both the infrared spectrum [8] and the Raman spectrum confirm the appearance of a double band at temperatures around WK which splits into several components as the temperature is lowered further. COMMENTS ON THE ST~TJCTTJRE OF THE 6 PHBSE The most complete study of the crystal structure of the 8 phase of any of the deuterium or hydrogen halides is that carried out by SANDORand FARROW[3, 41 on deuterium chloride by means of powder X-ray dilfraction and neutron diffraction. The space group was established as Fnz37n (Ohs)with four molecules per unit cell. The four chlorine atoms were located at equivalent crystallographic positions of the lattice (000). However, to account for the alternating intensity observed in the neutron difl?action pattern, a twelve-fold disordered model in which the deuterium atom of each molecule had twelve equally probable equilibrium positions situated on sites equivalent to zz0 (Ozz, 2Oz, zz0, OZZ,POZ,220, OzZ, fOz, zZ0, OzZ, sOZ, ZrO) in which x is the fractional position parameter of the unit cell. The position of the hydrogen atom was thus assumed to be in a line joining nearest neighbor chlorine atoms and in the direction of the primitive lattice vectors. Each of the equilibrium positions has C2c symmetry providing all others are simultaneously occupied. A completely random model was found unable to account for the alternation of observed intensities. Also, a six-fold disordered model in which the deuterium atoms were located along crystallographic axes of the face-centered lattice was found to be more unacceptable than the random model. The results and model derived from neutron [Ill G. L. HIEBERT and D. F. HORNIO, J. Own. Phya. 1121A. ETJOKEN,2. Electrochem. 45, 126 (1939).
27, 1216 (1967).
498
E. L. PACE
diffraction data led S~WDORand FURROW[4] to suggest that deuterium chloride molecules carry out librations of large angular amplitudes about their equilibrium positions accompanied by a random “flipping over” from one equivalent orientation to another. From the fact that a rotational lattice band at 200 cm-1 is observed while a translational lattice band expected in the frequency range below 100 cm-l is absent, Ito et al. estimate the lower limit of the average lifetime of a hydrogen bond in hydrogen chloride as 2 x lo--l3 sec. Therefore the greatest frequency of the “flipping over” process is probably of the order of 6 x lOl* set-l in the hydrogen halides whereas the frequency of the internal modes of vibration is of the order of 1014se+. The experimental infrared and Raman results indicate that the band maximum in the infrared spectrum differs from that in the Raman by approximately 10 cm-l in hydrogen bromide and by approximately 30 cm-l in hydrogen chloride. The band contours in the infrared spectrum differ from those in the Raman. In the case of hydrogen bromide, the effects are shown here to be associated with the transition from the liquid to the 6 phase since the maxima for both the infrared and Reman spectrum coincide exactly in the liquid phase. No direct structural information is available for the J-phase of hydrogen bromide. However, it would appear reasonable to assume that the general features of the model used in interpreting the X-ray and neutron diffraction results for deuterium chloride pertain in the case of hydrogen bromide. For deuterium chloride, the space symmetry which has been found to be 0,,5 is accounted for in the proposed model by assuming a site symmetry of C,, for any one of the 12-fold positionsialong the primitive lattice vectors surrounding each chlorine atom. However, the interpretation of the neutron diffraction results is based on the integration of the scattering intensity over intervals of time (of the order of one week) which are much longer than the time required to break a hydrogen bond. Thus, the density of the proton at each of the Wfold positions averages to the same value. On the other hand, since a molecular vibration occurs in an interval of time at least two orders of magnitude less than required for the “flipping over” of a hydrogen bond, the nature of the Raman and infrared spectrum will essentially depend on the instantaneous and not the average symmetry of the lattice. Consequently, the infrared and Raman activity will be determined by temporary configurations in which the site symmetry will be lower than C,, on the average and will correspond to that of the correlational subgroups of C,, (C,, C, and 0,). As a result, the symmetry of the factor group and the space group would be that of a correlational subgroup of 0,6 (B’m3m). The spectroscopic results may then be interpreted in the following manner. At temperatures near the melting point, the site symmetry is probably C, and the space and factor group symmetry is so low that the symmetry of the site essentially determines the nature of the spectrum. Under these conditions the results would be comparable to that for the liquid and a convergence of the maximum in the Raman spectrum and that in the infrared would be expected. As the temperature of the d-phase is lowered, it is reasonable to assume that in localized regions of the crystal, configurations of higher symmetry than C, would exist for periods of time which are long relative to the time of a vibration. The presence of low symmetry at the site with some symmetry (C,, C,, D, etc.) in the crystal would render adjacent sites
Infrared and Raman spectra of the condensedphases of hydrogen bromide
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non-translationally equivalent and set up the necessary conditions for resonance coupling between molecules. Qualitatively one can see that symmetry vibrational modes resulting from resonance coupling of aggregates of two, three and more molecules can exist. A majority of such symmetry modes are active in both the infrared and the Raman spectrum. The net effects of the superposition of the large number of such modes would be to give bands of considerable width and probably different maxima since the parameters which determine the intensity in the infrared differ from those in the Raman spectrum.
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