- Email: [email protected]
Raman spectra of ordered crystalline ethane
~ochimicnAetaVol.Uhpp.715to716 Pergamon Press Ltd., 1979.Printed in Great Britain
Raman spe-ctm of ordered RICHARD
Department
E.
crys@line
ethane
WILDE
of Chemistry, Texas Tech University, Lubbock, TX 79409, U.S.A. (Receiued 23 Muy 1978)
Abstract-The Raman spectrum of ethane in phase II at 74 K has been obtained. The spectrum lends support to the crystal structure previously deducted from i.r. spectra.
INTRODUCTION
RRSULTS
Recently. TEIADA and EGGESU[l] reported the i.r. spectra of ethane and ethane& in the ordered crystalline phase (phase II). Tbey concluded that, except for a shoulder kt 824.6 cm-‘, the i.r. spectral evidence indicates at least two molecules per unit cell at sites of C2 or C, symmetry. A proton magnetic resonance study [2] of solid ethane indicates that between 75 K and the I-II transition (89.72 K) the molecules are executing rotational diffusion about the three-fold axis and a libration about the axis perpendicular to the C, axis. Below 75 K the librational motion is lost. As part of our program of spectroscopic studies of small hydrogen-containing molecules in condensed phases, we have obtained Raman spectra of ethane in phase II between 74 and 89 K. The spectIa we have obtained provide new evidence in support of T’~JADA and EGGEZU[ 11.
EXPERIMENTAL
The Raman spectra were obtained with a Jane&Ash 0.5 m Ebert double monochromator . Wavenumber accuracy was +1 cm-‘. The slit width was varied between 2.2 and 2.8 cm-i. The source was a Coherent Radiation Model 54 argon-ion laser; the 488 nm Ar line was used with 150 mW on .the ‘sample. Right-angle scattering was detected with an S-5 response PMT. The signal was analyzed with a Spex DPC-2 photon counter. The C2Hb sample was condensed into an 8 mm o.d. Pyrex tube mounted on the tail section of an Air Products~ Cryo-Tip refrigerator. Temperature control to within fO.l K was &ected with a Scientific Instruments digital &nperature controller. The temperature was accurate to f 1 K. The ethane was purchased from Ma&son Gas Products and was stated to be 99% pure; no further purification was done. Because phase I can be supercooled and exist simultaneously with phase II [2, 31, it is important that the procedure for sample preparation be discussed. We prepared the solid sample by two different processes and obtained nearly identical spectra both times For one sample, the temperature was iowered from 93 to 88 K over a 10 min neriod and then held at 88 K for 45 min. The second sample-was prepared by rapidly cooling to 84 K, holding the temperature at 84 K for 1 h, and then rapidly lowering the temperature to 74 K. No spectral changes were detected after 1 h at 74 K. No attempt was made to get spectra of phase I, because it exists over only a 0.65 K temperature range. Local laser heating is more than this and would make observation of phase I extremely difficult.
The wavenumbers of the observed Raman bands, along with the gas-phase values, are listed in Table _. The observed bandwidth (FWHH) of the A,, bands is about 2.7 cm-‘, while that of the EI band (vrr) is about 4 cm-r. The actual bandwidths may be less than this because of the wide spectral slit widths necessary to observe the weak scattering from the solid. However, we do not expect exceedingly narrow bands because of the diffusional motion around the Ca axis. This motion should give, rise to a certain amount of vibrational dephasmg [4], which will broaden the bands. A very broad band was observed with an apparent maximum at 2941 cm’ ’ ; this is asslgned to V-10. The non-degenerate vs band is a single band in the liquid [5j. However, in phase II a doublet is observed in the intensity ratio of 10:7, the 993 cm-’ band being the more intense. The overtone 2v11 band at 2880 cm-’ is a singlet within the resolution of the experiment. This band has been shifted from its liquid-phase value of 2885 cm-‘. On the other hand, the nondegenerate vr band is shifted up from its liquid-phase value of 2943 cm- ‘. Not only is it shifted up in frequency, but it is split into a doublet, also in the ratio of about 10:7, the 2960 cm-’ band being the stronger. Finally, the EN species vlt band, which is a broad band (34 cm-’ FWHH) in the liquid at 1466 cm-‘, is a doublet in the solid in the intensity ratio of about 3 : 1, the 1463 cm-’ band being the more intense. We find nothing in the I&man spectrum to refute the analysis of TBJALMand EGGERS[l]. The two A ip fundamentals are both doublets suggesting at least two molecules in the unit cell. The observation of a doublet for vi 1 and a singlet for 2v 11 suggests only one set of sites in the unit cell. Because the doublet observed for vi I can arise either as a result of correlation-field splitting or as a result of the lifting of the double degeneracy, it is not possible to speculate as to whether or not the site group contains a three-fold axis.
715
support of the Robert A. Welch Foundation is gratefully acknowledged.
Acknowledgement-The
716
RICHARDE. WILDE Table 1. Wavenumbers of Raman spectral bands of CtH, at 74 K
Band
Solid (cm-')'
"1
"3
%O
*lS E8
“11 %
ca,akm-l)
2955 2960
2953.7
993 996
994.8
2941 (br)
2968.69
1448 1463 2880
2092.1
* D. W. LBPARD,D. E. Sruw and H. L. WELSH, Can. J. Phys. 44.2353
(1966). REFERENCES [l] S. B. TRIADA and D. F. Ecxxw, JR, Specrmchim. Acra 32A, 1557 (1976). [2j F. L. GMNS and W. D. MCCOIU&C, J. Chem. Phys. 67,115O (1977).
[3] D. F. EGGERS,JR,J. Phys. Chem. 79, 2116 (1975). [4] S. F. F-and A. LA UBEREAU, Chem. Phys. Leo. 35, 6 (1975).
f5] R. E. WILDE,J. Cbem. Phys. (to be published).
Raman spe-ctm of ordered RICHARD
Department
E.
crys@line
ethane
WILDE
of Chemistry, Texas Tech University, Lubbock, TX 79409, U.S.A. (Receiued 23 Muy 1978)
Abstract-The Raman spectrum of ethane in phase II at 74 K has been obtained. The spectrum lends support to the crystal structure previously deducted from i.r. spectra.
INTRODUCTION
RRSULTS
Recently. TEIADA and EGGESU[l] reported the i.r. spectra of ethane and ethane& in the ordered crystalline phase (phase II). Tbey concluded that, except for a shoulder kt 824.6 cm-‘, the i.r. spectral evidence indicates at least two molecules per unit cell at sites of C2 or C, symmetry. A proton magnetic resonance study [2] of solid ethane indicates that between 75 K and the I-II transition (89.72 K) the molecules are executing rotational diffusion about the three-fold axis and a libration about the axis perpendicular to the C, axis. Below 75 K the librational motion is lost. As part of our program of spectroscopic studies of small hydrogen-containing molecules in condensed phases, we have obtained Raman spectra of ethane in phase II between 74 and 89 K. The spectIa we have obtained provide new evidence in support of T’~JADA and EGGEZU[ 11.
EXPERIMENTAL
The Raman spectra were obtained with a Jane&Ash 0.5 m Ebert double monochromator . Wavenumber accuracy was +1 cm-‘. The slit width was varied between 2.2 and 2.8 cm-i. The source was a Coherent Radiation Model 54 argon-ion laser; the 488 nm Ar line was used with 150 mW on .the ‘sample. Right-angle scattering was detected with an S-5 response PMT. The signal was analyzed with a Spex DPC-2 photon counter. The C2Hb sample was condensed into an 8 mm o.d. Pyrex tube mounted on the tail section of an Air Products~ Cryo-Tip refrigerator. Temperature control to within fO.l K was &ected with a Scientific Instruments digital &nperature controller. The temperature was accurate to f 1 K. The ethane was purchased from Ma&son Gas Products and was stated to be 99% pure; no further purification was done. Because phase I can be supercooled and exist simultaneously with phase II [2, 31, it is important that the procedure for sample preparation be discussed. We prepared the solid sample by two different processes and obtained nearly identical spectra both times For one sample, the temperature was iowered from 93 to 88 K over a 10 min neriod and then held at 88 K for 45 min. The second sample-was prepared by rapidly cooling to 84 K, holding the temperature at 84 K for 1 h, and then rapidly lowering the temperature to 74 K. No spectral changes were detected after 1 h at 74 K. No attempt was made to get spectra of phase I, because it exists over only a 0.65 K temperature range. Local laser heating is more than this and would make observation of phase I extremely difficult.
The wavenumbers of the observed Raman bands, along with the gas-phase values, are listed in Table _. The observed bandwidth (FWHH) of the A,, bands is about 2.7 cm-‘, while that of the EI band (vrr) is about 4 cm-r. The actual bandwidths may be less than this because of the wide spectral slit widths necessary to observe the weak scattering from the solid. However, we do not expect exceedingly narrow bands because of the diffusional motion around the Ca axis. This motion should give, rise to a certain amount of vibrational dephasmg [4], which will broaden the bands. A very broad band was observed with an apparent maximum at 2941 cm’ ’ ; this is asslgned to V-10. The non-degenerate vs band is a single band in the liquid [5j. However, in phase II a doublet is observed in the intensity ratio of 10:7, the 993 cm-’ band being the more intense. The overtone 2v11 band at 2880 cm-’ is a singlet within the resolution of the experiment. This band has been shifted from its liquid-phase value of 2885 cm-‘. On the other hand, the nondegenerate vr band is shifted up from its liquid-phase value of 2943 cm- ‘. Not only is it shifted up in frequency, but it is split into a doublet, also in the ratio of about 10:7, the 2960 cm-’ band being the stronger. Finally, the EN species vlt band, which is a broad band (34 cm-’ FWHH) in the liquid at 1466 cm-‘, is a doublet in the solid in the intensity ratio of about 3 : 1, the 1463 cm-’ band being the more intense. We find nothing in the I&man spectrum to refute the analysis of TBJALMand EGGERS[l]. The two A ip fundamentals are both doublets suggesting at least two molecules in the unit cell. The observation of a doublet for vi 1 and a singlet for 2v 11 suggests only one set of sites in the unit cell. Because the doublet observed for vi I can arise either as a result of correlation-field splitting or as a result of the lifting of the double degeneracy, it is not possible to speculate as to whether or not the site group contains a three-fold axis.
715
support of the Robert A. Welch Foundation is gratefully acknowledged.
Acknowledgement-The
716
RICHARDE. WILDE Table 1. Wavenumbers of Raman spectral bands of CtH, at 74 K
Band
Solid (cm-')'
"1
"3
%O
*lS E8
“11 %
ca,akm-l)
2955 2960
2953.7
993 996
994.8
2941 (br)
2968.69
1448 1463 2880
2092.1
* D. W. LBPARD,D. E. Sruw and H. L. WELSH, Can. J. Phys. 44.2353
(1966). REFERENCES [l] S. B. TRIADA and D. F. Ecxxw, JR, Specrmchim. Acra 32A, 1557 (1976). [2j F. L. GMNS and W. D. MCCOIU&C, J. Chem. Phys. 67,115O (1977).
[3] D. F. EGGERS,JR,J. Phys. Chem. 79, 2116 (1975). [4] S. F. F-and A. LA UBEREAU, Chem. Phys. Leo. 35, 6 (1975).
f5] R. E. WILDE,J. Cbem. Phys. (to be published).