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The Raman spectrum of liquid dinitrogen tetroxide
Spact,rochimlcs
Acts, 1969, pp: 208 to 210. Pergamon
Press Ltd. Printed in Northern
Ireland
The Raman spectrum of liquid dinitrogen tetroxide* I. C. Department
and R. V. &rzsmmoNst
HISATSUNE of Chemistry,
Kansas
(Received
State
20 November
College,
Manhattan,‘Kansm
1958)
Ab&r&-The Raman spectsum of liquid N,O, at -10°C has been reinvestigated using the In addition to lines which had been reported earlier for the solid 4358 A Rg line for excitation. and liquid, one new line at 666 cm-l was observed. This line w&9 too weak to have its quahtative depol+zation ratio determined, but such ratios have been obtained for the remaining Raman lines for the &st time. Some conclusions on the assignment of the spectrum of N,O, are presented. Introdl.lction
the higher oxides of .mtrogen, N,O,, N,O, and N,O,, the only member for, which thermodynamic [l], spectroscopic [2-51 and structural [6,7] dataareavailable is N,O’,. The vibrational analysis of this molecule, therefore, will be of considerable interest in elucidating the properties of’the remaining two dinitrogen oxides. It was pointed out in an earlier paper [2], however, that an unequivocal assignment of the vibrational spectrum of N,O, was still not possible with the existing data. It was suggested then that the depolarization ratio studies of the Raman lines may afford a solution to this difficulty. Previous Raman studies on N,O, have been reported by SUTHERLAND [5] for Since the solid is colorthe solid and by GOULDEN and MILLEN [4] for the liquid. less, SUTHERLAND was able to obtain the spectrum using the 4358 A -Hg line for excitation. However, liquid N,O, is colored even at temperatures near its freezing point because of appreciable concentration of NO, which absorbs strongly in the blue region of the visible spectrum. GOTJLDEN and MILLEN therefore used the 5461 A Hg line for excitation, although the dispersion of their spectrograph in this region was poor compared to that in the 4358 A region. No depolarization ratio studies have been reported by either of these investigators. We have reinvestigated the Raman spectrum of liquid N,O, at -10°C and have succeeded in obtaining qualitative depolarization ratios of most of these observed lines. By controlling the temperature of the liquid so that it remained close to the freezing point, we have been able to use the 4358 d Hg line for excitation. Depolarization studies were made by using a single exposure method with a split Polaroid sheet at the entrance slit of the spectrograph. AMONG
* This investigation was supported by a research grant S-63 from the Division of Sanitary Engineering Services, U.S. Public Health Service. t Present address: Northern Utilization Research and Development Division, U.S. Department Agriculture, Peoria, Illinois. 11 GIAUQUE W. F. and KEMP J. D., J. Chem. Phya. 6, 40 (1938). :2] SNYDER R. G. and ECISATSUNE I. C., J. MoZ~rocrcopy Spec. 1, 139 (1957). ,3] WIENER R. N. and NIXON E. R., J. Chem. Phyys. 26, 906 (1957). :4] GOULDEN J. D. S. and MILLEN D. J., J. CJwn. Sot. 2620 (1960). :5] STJTHERLBND G. B. B. M., Proc. Roy. Sot. London A 141, 535 (1933). 61 SMITH D. W. and ECEDBER~ K., J. Chem. Phys. 25, 1282 (1956). '71 BROAJXEYJ.S. md RoBERTsoNJ.M.,N~~~T~ 164, 916 (1949).
206
of
The Ramtm spectrum of liquid dinitrogen
t&oxide
Experimental Samples of N,O, were prepared from the Matheson Company lecture bottle gas as follows. The gas was first dried by repeatedly passing it through a column containing glass beads coated with-phosphorus pentoxide. The sample was then allowed to react with excess purified oxygen until the condensed solid was colorless. Several distillations were made and Anally this sample was distilled directly into the Raman tube under vacuum. The sample remained a brown colored liquid in the sealed tube at room temperature. A Hilger E-612 Raman spectrograph with af/5.7 camera was used in this work. The spectra were recorded photographically using Eastman Kodak spectroscopic plates with ld3a emulsion
Fig. 1. A cross-section diagram of 8 low-temperature. Raman excitation SOUPX. The lower b8sse is that of 8 Hilger Raman Source Model FL-l. For description of the parts see text. which were in the sensitized Ckass E, 0 or J. These plates were developed according to the specification of the manufacturer and the spectrum read with a travelhng microscope. Raman shifts were calculsted by using the iron spectrum taken on the same plate. Wavelengths of the iron spectrum were obtained from the MIT Wavelength Tables [S]. A Hilger Raman Source Model FL-l was modi6ed 8s shown in Fig. 1 for the low temperature work [9]. The housing was enlarged so that 8 standard Hilger Raman sample tube could be placed inside an unsilvered Pyrex Dewar flask of size 2.7 cm i.d. by 45 cm o.d. by 25 cm length. The bottom of this Dewar ~8s m8de optically flat with a vacuum space of about 2 cm. A water cooling jacket was placed around this Dewar with about 5 mm space between the walls of the two glass pieces to allow for the insertion of plastic filter sheets. The 4359 A Hg line could be effectively isolated by using gelatin sheets containing rhodamine 5GDN-extra dye and Eastman Kodak Wratten 2B filter sheets. Both the Dewar and the cooling jacket were supported by a removable metal cup which was lined with asbestos and cork insulation. The outer housing was of metal construction and was supported rigidly by the source stand. The four low pressure mercury lamps are those from the Hilger source. Since these lamps dissipate considerable heat, the end plates and the walls were water cooled by means of copper [8] HARRISON [9]
FITZSIMMONS
G. R. (Editor) M.I.T R. N., M. S. Thesis,
Wavelength Tables. John Wiley, New York (1939). Kansas State College Library, Manhattan, Kamaa~(1958).
207
I. C. HISATSUNE
and
R. V. FITZSIMMOXS
Additional cooling was accomplished by having a stream tubings soldered directly on them. of air flowing through the hotiing and by an external electric fan. The lamp sockets at the two end plates provided good thermal contact between the lamp terminals and the cooled housing walls. The inner surfaces of the housing was coated with an ethanol suspension of TiO, baked This durable coating was found to be very effective in increasing the light intensity. on at 150°C. The Raman sample tube was centered in the Dewar flask by a metal ring at the bottom and The Dewar was centered in the optical path by a rubber coated metal ring at a cork at the top. the top and a bored cork which was cemented to the bottom and which fitted over the light tube on the Hilger Source stand. A fiber glass insulation and an aluminum ring held the water jacket vertically in the metal housing. The Dewar assembly or the sample tube could be removed simply by lifting upward. be cooled either by a tlow of cold gas or by a liquid The sample tube in the d ewar could refrigerant in the Dewar. For the former method, which was used in this work, a 6 mm o.d. Pyrex tubing which was sealed at the end and had several holes cut along the side was inserted Dried air passed through a dry-ice acetone bath or throuhh the cork at the top of the Dewar. nitrogen from boiling liquid nitrogen was connected to this tube. The temperature inside the work the temperature Dewar was controlled by the flow rate of the cooling gas. In the present in most of the runs were kept at about -10°C. Qualitative depolarization studies of thestrongRamanlinesweremade inthefollowingmanner. A light-shield was inserted into the Dewar vessel so that the sample was irradiated only in the plane which went through the axis of the sample tube and which made an angle of 45” with the optical axis at the entrance slit of the spectrogmph. Two Polaroid sheets were placed side-byside flatly against the entrance slit so that the adjoining edge of the two sheets made an angle of 45” with the spectrograph vertical axis. On one side of the adjoining edge, the Polaroid polarized incoming light so that its electric vector was parallel to the adjoining edge. The sheet on the remaining side polarized the same light so that the electrii: vector was perpendicular to the With The height of the slit was adjusted to about 3 mm with a V-shaped wedge. adjoining edge. this arrangement a single exposure gave simultaneously the two spectra resulting effectively from excitations by light polarized parallel or perpendicularly to the sample tube axis. These spectra which were exposed one over the other showed polarized lines with greatly different intensities in the two spectra, and depolarized lines with similar intensities in both (see-Fig. 3). This method was particularly suited for this work since long exposures were required to obtain the complete spectrum. The method of two exposures [lo] was also tried but the results were not too convincing for it was difficult to match the exposure times properly.
Results and discussion Frequencies, relative intensities and qualitative depolarization ratios of the Raman lines in liquid N,O, obtained in this work are listed in Table 1. The estimated uncertainties listed are based on average deviations over about twelve spectra. A densitometer trace of a typical plate is shown in Fig. 2. Outside of the new Raman line at 666 cm-l, which has not been reported before, the present results are in good agreement with the previous works which are also listed in the same table for comparison. In the depolarization measurements, the 265 cm-I line was usually overexposed in the time necessary to observe the remaining strong lines. The optimum exposure time for this line was too short for the others to be registered on the same plate. Thus a possibility could exist that the attenuation in intensity may be due to other factors beside polarization since there were no depolarized lines with which to compare. Several exposures were therefore made with N,O, in Ccl, solution. A densitometer trace of one of these plates is shown in Fig. 3. It should be noted [lo]
EDSALL
J. T. and WILSON
E. B., JR., J. C?MYJL Phys.
208
6, 124 (1938).
The
Raman
spectrum
of liquid
dinitrogen
Fig. 3. Spectra of N,O, m CCI, at -10°C showing the qualitative depolarization ratios of the 265 cm-l N,O, line and the 458 cm-l Ccl, line. Both spectra were obtained simultaneously using a single exposure.
Fig. 2. The Raman spectrum of liquid N,O, at - 10°C. The units of the Raman lines are in cm-’ and the Hg exciting line in A. The line labeled “BG” is the 4554 A Ba line produced by the mercury lamp.
Table
1. The Raman
spectrum
of N,O,
Liquid (-10%) This work 265 481 666 809 1325 1380 1712
&& & & 6 & f
p = polarized,
(cm-l) Solid (-180°C)
GOULDEN and MILLEN[~]
2 vs, p 7 w, d 7 VW, ? 3 9, p 5m,p 3 8, p 3 m, d
tetroxide
265 vs 500 m 8118 1325 mw 1382 s 1724 ms
SUTIDRLAND[~] 283 vs 500 m 812 6 1336 m 1382 s 1724 m
d = depolarized, v = very, s = strong, w = weak, m = medium
209
I. C. JTISATSUNE
and R. V. FITZSIKMONS:
The Raman
spectrum
of liquid
dinitrogen
t&oxide
that both spectra in this figure were taken simultaneously side by side. The 265 cm-l Raman line which appears between the two depolarized Ccl, lines appears deiinitely to be polarized. The depolarization ratio of the 666 cm-l line could not be determined since it was too weak. The results of the present investigation not only cotirm what had been expected all along, but they also clarify the assignment of a fundamental band which was expected to be around 680 cm-l and which gave rise to one of the Fermi doublet Raman lines observed in the 1360 cm-l region. As was predicted in the earlier paper [2], one of ,,these lines is due to the first overtone of a Raman active fundamental which is undoubtedly the newly observed line at 666 cm-l. The remaining line is due to the symmetric NO stretching mode. Prom intensity consideration, it seems reasonable to assign the stronger line at 1380 cm-r to the stretching mode and the weaker line at 1325 cm-l to the first overtone of 666 cm-l line. A weak band present in the infrared spectrum of gaseous N,O, at about 6S0 cm-l must therefore be explained as a combination band. As suggested previously [2], one possible combination is 430 + 265 = 695 cm-l. A pair of infrared bands at 652 and 667 cm-l, observed by WIENER. and NIXON [3] in the spectrum of solid N,O, enriched with 15N, can be adequately explained as bands arising from isotopic N,O, molecule of lower symmetry. The present depolarization ratio studies will now enable us to assign the A, symmetry modes unequivocally. A question still remains as to which of the two depolarized lines at 451 and 666 cm-l should be assigned to the B,, Glass, but a normal co-ordinate analysis will probably assist in settling this question.
210
Acts, 1969, pp: 208 to 210. Pergamon
Press Ltd. Printed in Northern
Ireland
The Raman spectrum of liquid dinitrogen tetroxide* I. C. Department
and R. V. &rzsmmoNst
HISATSUNE of Chemistry,
Kansas
(Received
State
20 November
College,
Manhattan,‘Kansm
1958)
Ab&r&-The Raman spectsum of liquid N,O, at -10°C has been reinvestigated using the In addition to lines which had been reported earlier for the solid 4358 A Rg line for excitation. and liquid, one new line at 666 cm-l was observed. This line w&9 too weak to have its quahtative depol+zation ratio determined, but such ratios have been obtained for the remaining Raman lines for the &st time. Some conclusions on the assignment of the spectrum of N,O, are presented. Introdl.lction
the higher oxides of .mtrogen, N,O,, N,O, and N,O,, the only member for, which thermodynamic [l], spectroscopic [2-51 and structural [6,7] dataareavailable is N,O’,. The vibrational analysis of this molecule, therefore, will be of considerable interest in elucidating the properties of’the remaining two dinitrogen oxides. It was pointed out in an earlier paper [2], however, that an unequivocal assignment of the vibrational spectrum of N,O, was still not possible with the existing data. It was suggested then that the depolarization ratio studies of the Raman lines may afford a solution to this difficulty. Previous Raman studies on N,O, have been reported by SUTHERLAND [5] for Since the solid is colorthe solid and by GOULDEN and MILLEN [4] for the liquid. less, SUTHERLAND was able to obtain the spectrum using the 4358 A -Hg line for excitation. However, liquid N,O, is colored even at temperatures near its freezing point because of appreciable concentration of NO, which absorbs strongly in the blue region of the visible spectrum. GOTJLDEN and MILLEN therefore used the 5461 A Hg line for excitation, although the dispersion of their spectrograph in this region was poor compared to that in the 4358 A region. No depolarization ratio studies have been reported by either of these investigators. We have reinvestigated the Raman spectrum of liquid N,O, at -10°C and have succeeded in obtaining qualitative depolarization ratios of most of these observed lines. By controlling the temperature of the liquid so that it remained close to the freezing point, we have been able to use the 4358 d Hg line for excitation. Depolarization studies were made by using a single exposure method with a split Polaroid sheet at the entrance slit of the spectrograph. AMONG
* This investigation was supported by a research grant S-63 from the Division of Sanitary Engineering Services, U.S. Public Health Service. t Present address: Northern Utilization Research and Development Division, U.S. Department Agriculture, Peoria, Illinois. 11 GIAUQUE W. F. and KEMP J. D., J. Chem. Phya. 6, 40 (1938). :2] SNYDER R. G. and ECISATSUNE I. C., J. MoZ~rocrcopy Spec. 1, 139 (1957). ,3] WIENER R. N. and NIXON E. R., J. Chem. Phyys. 26, 906 (1957). :4] GOULDEN J. D. S. and MILLEN D. J., J. CJwn. Sot. 2620 (1960). :5] STJTHERLBND G. B. B. M., Proc. Roy. Sot. London A 141, 535 (1933). 61 SMITH D. W. and ECEDBER~ K., J. Chem. Phys. 25, 1282 (1956). '71 BROAJXEYJ.S. md RoBERTsoNJ.M.,N~~~T~ 164, 916 (1949).
206
of
The Ramtm spectrum of liquid dinitrogen
t&oxide
Experimental Samples of N,O, were prepared from the Matheson Company lecture bottle gas as follows. The gas was first dried by repeatedly passing it through a column containing glass beads coated with-phosphorus pentoxide. The sample was then allowed to react with excess purified oxygen until the condensed solid was colorless. Several distillations were made and Anally this sample was distilled directly into the Raman tube under vacuum. The sample remained a brown colored liquid in the sealed tube at room temperature. A Hilger E-612 Raman spectrograph with af/5.7 camera was used in this work. The spectra were recorded photographically using Eastman Kodak spectroscopic plates with ld3a emulsion
Fig. 1. A cross-section diagram of 8 low-temperature. Raman excitation SOUPX. The lower b8sse is that of 8 Hilger Raman Source Model FL-l. For description of the parts see text. which were in the sensitized Ckass E, 0 or J. These plates were developed according to the specification of the manufacturer and the spectrum read with a travelhng microscope. Raman shifts were calculsted by using the iron spectrum taken on the same plate. Wavelengths of the iron spectrum were obtained from the MIT Wavelength Tables [S]. A Hilger Raman Source Model FL-l was modi6ed 8s shown in Fig. 1 for the low temperature work [9]. The housing was enlarged so that 8 standard Hilger Raman sample tube could be placed inside an unsilvered Pyrex Dewar flask of size 2.7 cm i.d. by 45 cm o.d. by 25 cm length. The bottom of this Dewar ~8s m8de optically flat with a vacuum space of about 2 cm. A water cooling jacket was placed around this Dewar with about 5 mm space between the walls of the two glass pieces to allow for the insertion of plastic filter sheets. The 4359 A Hg line could be effectively isolated by using gelatin sheets containing rhodamine 5GDN-extra dye and Eastman Kodak Wratten 2B filter sheets. Both the Dewar and the cooling jacket were supported by a removable metal cup which was lined with asbestos and cork insulation. The outer housing was of metal construction and was supported rigidly by the source stand. The four low pressure mercury lamps are those from the Hilger source. Since these lamps dissipate considerable heat, the end plates and the walls were water cooled by means of copper [8] HARRISON [9]
FITZSIMMONS
G. R. (Editor) M.I.T R. N., M. S. Thesis,
Wavelength Tables. John Wiley, New York (1939). Kansas State College Library, Manhattan, Kamaa~(1958).
207
I. C. HISATSUNE
and
R. V. FITZSIMMOXS
Additional cooling was accomplished by having a stream tubings soldered directly on them. of air flowing through the hotiing and by an external electric fan. The lamp sockets at the two end plates provided good thermal contact between the lamp terminals and the cooled housing walls. The inner surfaces of the housing was coated with an ethanol suspension of TiO, baked This durable coating was found to be very effective in increasing the light intensity. on at 150°C. The Raman sample tube was centered in the Dewar flask by a metal ring at the bottom and The Dewar was centered in the optical path by a rubber coated metal ring at a cork at the top. the top and a bored cork which was cemented to the bottom and which fitted over the light tube on the Hilger Source stand. A fiber glass insulation and an aluminum ring held the water jacket vertically in the metal housing. The Dewar assembly or the sample tube could be removed simply by lifting upward. be cooled either by a tlow of cold gas or by a liquid The sample tube in the d ewar could refrigerant in the Dewar. For the former method, which was used in this work, a 6 mm o.d. Pyrex tubing which was sealed at the end and had several holes cut along the side was inserted Dried air passed through a dry-ice acetone bath or throuhh the cork at the top of the Dewar. nitrogen from boiling liquid nitrogen was connected to this tube. The temperature inside the work the temperature Dewar was controlled by the flow rate of the cooling gas. In the present in most of the runs were kept at about -10°C. Qualitative depolarization studies of thestrongRamanlinesweremade inthefollowingmanner. A light-shield was inserted into the Dewar vessel so that the sample was irradiated only in the plane which went through the axis of the sample tube and which made an angle of 45” with the optical axis at the entrance slit of the spectrogmph. Two Polaroid sheets were placed side-byside flatly against the entrance slit so that the adjoining edge of the two sheets made an angle of 45” with the spectrograph vertical axis. On one side of the adjoining edge, the Polaroid polarized incoming light so that its electric vector was parallel to the adjoining edge. The sheet on the remaining side polarized the same light so that the electrii: vector was perpendicular to the With The height of the slit was adjusted to about 3 mm with a V-shaped wedge. adjoining edge. this arrangement a single exposure gave simultaneously the two spectra resulting effectively from excitations by light polarized parallel or perpendicularly to the sample tube axis. These spectra which were exposed one over the other showed polarized lines with greatly different intensities in the two spectra, and depolarized lines with similar intensities in both (see-Fig. 3). This method was particularly suited for this work since long exposures were required to obtain the complete spectrum. The method of two exposures [lo] was also tried but the results were not too convincing for it was difficult to match the exposure times properly.
Results and discussion Frequencies, relative intensities and qualitative depolarization ratios of the Raman lines in liquid N,O, obtained in this work are listed in Table 1. The estimated uncertainties listed are based on average deviations over about twelve spectra. A densitometer trace of a typical plate is shown in Fig. 2. Outside of the new Raman line at 666 cm-l, which has not been reported before, the present results are in good agreement with the previous works which are also listed in the same table for comparison. In the depolarization measurements, the 265 cm-I line was usually overexposed in the time necessary to observe the remaining strong lines. The optimum exposure time for this line was too short for the others to be registered on the same plate. Thus a possibility could exist that the attenuation in intensity may be due to other factors beside polarization since there were no depolarized lines with which to compare. Several exposures were therefore made with N,O, in Ccl, solution. A densitometer trace of one of these plates is shown in Fig. 3. It should be noted [lo]
EDSALL
J. T. and WILSON
E. B., JR., J. C?MYJL Phys.
208
6, 124 (1938).
The
Raman
spectrum
of liquid
dinitrogen
Fig. 3. Spectra of N,O, m CCI, at -10°C showing the qualitative depolarization ratios of the 265 cm-l N,O, line and the 458 cm-l Ccl, line. Both spectra were obtained simultaneously using a single exposure.
Fig. 2. The Raman spectrum of liquid N,O, at - 10°C. The units of the Raman lines are in cm-’ and the Hg exciting line in A. The line labeled “BG” is the 4554 A Ba line produced by the mercury lamp.
Table
1. The Raman
spectrum
of N,O,
Liquid (-10%) This work 265 481 666 809 1325 1380 1712
&& & & 6 & f
p = polarized,
(cm-l) Solid (-180°C)
GOULDEN and MILLEN[~]
2 vs, p 7 w, d 7 VW, ? 3 9, p 5m,p 3 8, p 3 m, d
tetroxide
265 vs 500 m 8118 1325 mw 1382 s 1724 ms
SUTIDRLAND[~] 283 vs 500 m 812 6 1336 m 1382 s 1724 m
d = depolarized, v = very, s = strong, w = weak, m = medium
209
I. C. JTISATSUNE
and R. V. FITZSIKMONS:
The Raman
spectrum
of liquid
dinitrogen
t&oxide
that both spectra in this figure were taken simultaneously side by side. The 265 cm-l Raman line which appears between the two depolarized Ccl, lines appears deiinitely to be polarized. The depolarization ratio of the 666 cm-l line could not be determined since it was too weak. The results of the present investigation not only cotirm what had been expected all along, but they also clarify the assignment of a fundamental band which was expected to be around 680 cm-l and which gave rise to one of the Fermi doublet Raman lines observed in the 1360 cm-l region. As was predicted in the earlier paper [2], one of ,,these lines is due to the first overtone of a Raman active fundamental which is undoubtedly the newly observed line at 666 cm-l. The remaining line is due to the symmetric NO stretching mode. Prom intensity consideration, it seems reasonable to assign the stronger line at 1380 cm-r to the stretching mode and the weaker line at 1325 cm-l to the first overtone of 666 cm-l line. A weak band present in the infrared spectrum of gaseous N,O, at about 6S0 cm-l must therefore be explained as a combination band. As suggested previously [2], one possible combination is 430 + 265 = 695 cm-l. A pair of infrared bands at 652 and 667 cm-l, observed by WIENER. and NIXON [3] in the spectrum of solid N,O, enriched with 15N, can be adequately explained as bands arising from isotopic N,O, molecule of lower symmetry. The present depolarization ratio studies will now enable us to assign the A, symmetry modes unequivocally. A question still remains as to which of the two depolarized lines at 451 and 666 cm-l should be assigned to the B,, Glass, but a normal co-ordinate analysis will probably assist in settling this question.
210