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Torsional vibrations in CO2 and N2O crystals
SpectrochimieaActa, 1959,Vol. 13, pp. 308 to 319. Perramon Press Ltd.
Printed in Northern Ireland
Torsional vibrations in CO, and N,O crystals* D. A. Dows Department of Chemistry, University of Southern California, Los Angeles 7, California (Received 13 October1958) Abstract-Infix-red spectra of thick films (several hundred microns) of crystalline CO, and N,O show bands in the vicinity of the bending vibrations which are attributed to combinations with several torsional frequencies. Information present.ly available on torsional vibrations in these crystals is summarized.
INFRA-RED spectra of crystalline
CO, and N,O have been reported respectively by and HORNIG[‘] and by Dows[~’ and GIGUERE and HARVEY.[~] Certain absorption bands in the spectrum of each substance are identified with transitions involving torsional motions of the molecules in the lattice. We have recently had the occasion to study the infra-red spectrum of a thick film of solid CO,, and have found additional combinations involving torsional levels; subsequent re-examination of solid N,O using films of much greater thickness than previously has revealed combinations analogous to the new ones found for CO,. OSBERG
1 10 cm-’
Fig. 1. Infra-red spectra of thick films of crystalline CO, and N,O in the vicinity of the bending vibration. The samples were taken from Matheson lecture bottles, distilled in V~CUO,and condensed on a silver chloride plate cooled to 82°K in a cell similar to that used before.‘21 Spectra in the vicinity of the bending fundamentals are shown in Fig.1. NaCl optics were used for CO, and KBr for N,O (the use of KBr optics with * This work was supported by the Office of Ordnance Research, U.S. Army.
30s
Torsional vibrations in CO, and N,O crystals
showed only small differences attributable to lower resolution). Thin films of CO, gave spectra in agreement to those of OSBERCJ and HORNIG. Peak frequencies listed in Table i are believed accurate to two wavenumbers.
CO,
Table 1. Observed frequencies (cm-l) in crystalline CO, and N,O
Assignment
“ohs
Aasigmnent
h2
-_ 692 (sh)
v2 + 2 x 63
778 (ah) 767 (ah)
v2 + 2 x 51
675
758 731 718 670 660 653 635
reflection
Ii
v2 +
655 642 590 574
6 6
7
v2
v2(14N15NO)
-I
-i sb = shoulder
The band seen at 667 cm-l in the spectrum of CO, is assigned by OSBERG and maximum. The unusual changes in intensity of this band with sample thickness which formed the basis for their interpretation were reproduced in these experiments, using the NaCl prism, with a different sample substrate (AgCl instead of KBr). In spectra obtained with the KBr prism, no distinct peak was seen at 667 cm- l, but the band broadened asymmetrically to higher frequencies as the sample was thickened. The crystal structures of the two substances being the same except for the centre of symmetry in CO,, the previous theoretical analysisul applies to both cases. Torsional oscillation falls in the class E, of the site symmetry (drop subscripts “g” and “u” for N,O), and consultation with the correlation diagramul results in a prediction of three torsional fundamentals of the unit cell (E, + 2F,). In combination with the bending vibration the torsions are symmetry allowed in the infra-red spectrum. It is realized that there are no strict selection rules for combination bands, but the lack of evidence that the usual rules for limiting modes are not obeyed makes it prudent to establish the allowedness of these transitions. Table 2 lists the frequencies observed in combination with the bending vibrations. These are differences between the observed combination band frequencies and 590 cm-l (N,O) or 653 cm-l (CO,). The lower of the two CO, frequencies was arbitrarily chosen as discussed below. The Raman spectrum of crystalline CO, has been observed by GAIZAUSKAS and WELSH,[~] who also studied the region of the symmetric stretching vibration in very thick films in the infrared. They obtain values for torsional frequencies which are included in Table 2. HORNIG as a reflection
4
309
D. A. Dows:
Torsional vibrations in COP and NaO crystals
Also listed are the values given in references 1 and 2. All entries under “other occurrence” give the band with which the lattice frequency is observed in combination and the frequency difference found (or the Raman shift). The agreement between different observations seems quite good, and it is simply for this reason that the lower COB frequency (653) was chosen as a basis for the differences in compiling Table 2; the frequencies resulting are then in close Table 2. Torsional frequency summary
Other occurrence
Difference from vs
Other occurrence
Dif%rence from ~a
I
I
I
125
101
114 105 78 65 * Raman, ref. [4].
R*(112),
Ysf(ll0)
R*(77),
rrS(36)
R*(64),
QS(57)
t Ref. [l].
85 65 62
$ Ref. [4].
%§(3%
r&(92)
5 Ref. [2].
agreement with the other data. The close accord suggests that limiting modes of the librational and vibrational branches are being observed, as is suggested by usual selection rules for fundamentals. The most serious numerical disagreement is in the case of combinations with the symmetric stretching fundamental of CO,, where combinations with the limiting modes are symmetry forbidden. If the highest combination frequencies in each case are assigned as the bending frequency plus twice the lowest observed torsional frequency, then there remain three torsional frequencies for N,O and four for CO,. No more specific assignment, nor any reason for the appearance of an extra frequency for CO, will be given here. The curious preference for combining with the 653 wavenumber level of CO, must also go unexplained. Acknowledgement-It is a pleasure to acknowledge an interesting discussion with Professor H. L. WELSH, and to thank him for permission to refer to the work of GAIZAUSKAS md WELSH before its public&ion.
References [l] [2] [3] [4]
OSBERQ W. Dows D. A. GICXJEREP. G~IZAUSK_~
E. J. A. V.
and HORNIU D. F. J. Chem. Phys. 1952 20, 1345. Chem. Whys. 1957 26, 745. and HARVEY K. B. Spectrochim. Actcc 1957 9, 204. Ph.D. Thesis, Toronto 1955; WELSH H. L. Private communication.
310
Printed in Northern Ireland
Torsional vibrations in CO, and N,O crystals* D. A. Dows Department of Chemistry, University of Southern California, Los Angeles 7, California (Received 13 October1958) Abstract-Infix-red spectra of thick films (several hundred microns) of crystalline CO, and N,O show bands in the vicinity of the bending vibrations which are attributed to combinations with several torsional frequencies. Information present.ly available on torsional vibrations in these crystals is summarized.
INFRA-RED spectra of crystalline
CO, and N,O have been reported respectively by and HORNIG[‘] and by Dows[~’ and GIGUERE and HARVEY.[~] Certain absorption bands in the spectrum of each substance are identified with transitions involving torsional motions of the molecules in the lattice. We have recently had the occasion to study the infra-red spectrum of a thick film of solid CO,, and have found additional combinations involving torsional levels; subsequent re-examination of solid N,O using films of much greater thickness than previously has revealed combinations analogous to the new ones found for CO,. OSBERG
1 10 cm-’
Fig. 1. Infra-red spectra of thick films of crystalline CO, and N,O in the vicinity of the bending vibration. The samples were taken from Matheson lecture bottles, distilled in V~CUO,and condensed on a silver chloride plate cooled to 82°K in a cell similar to that used before.‘21 Spectra in the vicinity of the bending fundamentals are shown in Fig.1. NaCl optics were used for CO, and KBr for N,O (the use of KBr optics with * This work was supported by the Office of Ordnance Research, U.S. Army.
30s
Torsional vibrations in CO, and N,O crystals
showed only small differences attributable to lower resolution). Thin films of CO, gave spectra in agreement to those of OSBERCJ and HORNIG. Peak frequencies listed in Table i are believed accurate to two wavenumbers.
CO,
Table 1. Observed frequencies (cm-l) in crystalline CO, and N,O
Assignment
“ohs
Aasigmnent
h2
-_ 692 (sh)
v2 + 2 x 63
778 (ah) 767 (ah)
v2 + 2 x 51
675
758 731 718 670 660 653 635
reflection
Ii
v2 +
655 642 590 574
6 6
7
v2
v2(14N15NO)
-I
-i sb = shoulder
The band seen at 667 cm-l in the spectrum of CO, is assigned by OSBERG and maximum. The unusual changes in intensity of this band with sample thickness which formed the basis for their interpretation were reproduced in these experiments, using the NaCl prism, with a different sample substrate (AgCl instead of KBr). In spectra obtained with the KBr prism, no distinct peak was seen at 667 cm- l, but the band broadened asymmetrically to higher frequencies as the sample was thickened. The crystal structures of the two substances being the same except for the centre of symmetry in CO,, the previous theoretical analysisul applies to both cases. Torsional oscillation falls in the class E, of the site symmetry (drop subscripts “g” and “u” for N,O), and consultation with the correlation diagramul results in a prediction of three torsional fundamentals of the unit cell (E, + 2F,). In combination with the bending vibration the torsions are symmetry allowed in the infra-red spectrum. It is realized that there are no strict selection rules for combination bands, but the lack of evidence that the usual rules for limiting modes are not obeyed makes it prudent to establish the allowedness of these transitions. Table 2 lists the frequencies observed in combination with the bending vibrations. These are differences between the observed combination band frequencies and 590 cm-l (N,O) or 653 cm-l (CO,). The lower of the two CO, frequencies was arbitrarily chosen as discussed below. The Raman spectrum of crystalline CO, has been observed by GAIZAUSKAS and WELSH,[~] who also studied the region of the symmetric stretching vibration in very thick films in the infrared. They obtain values for torsional frequencies which are included in Table 2. HORNIG as a reflection
4
309
D. A. Dows:
Torsional vibrations in COP and NaO crystals
Also listed are the values given in references 1 and 2. All entries under “other occurrence” give the band with which the lattice frequency is observed in combination and the frequency difference found (or the Raman shift). The agreement between different observations seems quite good, and it is simply for this reason that the lower COB frequency (653) was chosen as a basis for the differences in compiling Table 2; the frequencies resulting are then in close Table 2. Torsional frequency summary
Other occurrence
Difference from vs
Other occurrence
Dif%rence from ~a
I
I
I
125
101
114 105 78 65 * Raman, ref. [4].
R*(112),
Ysf(ll0)
R*(77),
rrS(36)
R*(64),
QS(57)
t Ref. [l].
85 65 62
$ Ref. [4].
%§(3%
r&(92)
5 Ref. [2].
agreement with the other data. The close accord suggests that limiting modes of the librational and vibrational branches are being observed, as is suggested by usual selection rules for fundamentals. The most serious numerical disagreement is in the case of combinations with the symmetric stretching fundamental of CO,, where combinations with the limiting modes are symmetry forbidden. If the highest combination frequencies in each case are assigned as the bending frequency plus twice the lowest observed torsional frequency, then there remain three torsional frequencies for N,O and four for CO,. No more specific assignment, nor any reason for the appearance of an extra frequency for CO, will be given here. The curious preference for combining with the 653 wavenumber level of CO, must also go unexplained. Acknowledgement-It is a pleasure to acknowledge an interesting discussion with Professor H. L. WELSH, and to thank him for permission to refer to the work of GAIZAUSKAS md WELSH before its public&ion.
References [l] [2] [3] [4]
OSBERQ W. Dows D. A. GICXJEREP. G~IZAUSK_~
E. J. A. V.
and HORNIU D. F. J. Chem. Phys. 1952 20, 1345. Chem. Whys. 1957 26, 745. and HARVEY K. B. Spectrochim. Actcc 1957 9, 204. Ph.D. Thesis, Toronto 1955; WELSH H. L. Private communication.
310