H.G.M. Edwards/Journal of Molecular Structure 415 (1997) 37–44
43
−
1
(
n Ϸ 715 cm ), but this is hardly tenable as some of
of liquid cyanogen and this is also consistent with its
2
these features occur in the gas, liquid and solid phases,
and in the infrared and Raman spectra. In Table 4, the
assignment of the strongest features near 740 cm has
been made in accord with combination bands rather
than an alternative assignment which invokes the
adoption of P- and R- branches for the 766 and
41 cm components. A suggestion has also been
made [15] that a new assignment of n be considered
to the strongest feature near 741 cm ; however, no
explanation can be given for the intensity of this
infrared-forbidden band or for the combination
bands involving a n Ϸ 503 cm which are generally
compatible with this latter assignment, particularly in
the gas phase spectra. The observation of a weak
feature at ϳ1028 cm in the Raman spectrum of
the liquid [9] has not been confirmed in the current
work; Langseth and Møller invoked the occurrence of
assignment as n . Since the n fundamental involves a
5
5
lateral displacement of the nitrogen atoms relative to
the carbon atoms out of the molecular axis (Fig. 1), it
is tempting to suggest that in the solid (and, perhaps,
also in the liquid phase) cyanogen molecules are sub-
ject to molecular interactions at right angles to the
molecular axis. This would facilitate the dynamic
creation of ‘bent’ C N molecules and cause activa-
−1
−1
7
2
2
2
−1
tion of the n mode in the Raman spectrum and would
5
−1
explain the band observed at 234.5 cm in the present
work. This conclusion conflicts with the X-ray dif-
fraction results [7] on solid cyanogen, but a possible
contributory factor to the discrepancy could be the
temperature at which the solid state Raman spectrum
was obtained viz. 240 K, just below its melting point.
At this temperature, the population of the lowest
−1
4
−1
vibrational state, n , is still about 50% that of the
5
a Fermi resonance between 2n Њ and n2 (whose
ground state and this would favour the observation
of a non-linear structure. The temperature of the X-
ray diffraction experiment was not defined, but that of
the solid-state infrared spectra was 80 K; at this tem-
4
−1
imperturbed levels are separated by 150 cm ) to
explain the appearance of this band in their Raman
spectrum. Support for the Fermi resonance idea is
provided by the infrared work of Maki [6], who has
derived some band constants.
perature, the population of the n state is negligibly
5
small and therefore the linear structure would be in
preponderance.
The Raman spectrum of solid cyanogen at 240 K
reported here is again much more simple than the
corresponding infrared spectrum; the most character-
−1
istic feature is the strong band at 2327 cm which is
−1
References
the n fundamental, displaced some 6 cm from its
1
gas-phase value. The n and n fundamentals are also
2
4
[1] C.K. Møller, B.P. Stoicheff, Can. J. Phys. 32 (1954) 635.
[2] I.-Y. Wang, A. Weber, J. Chem. Phys. 67 (1977) 3084.
[3] H.G.M. Edwards, H.R. Mansour, J. Mol. Struct. 160 (1987)
209.
−1
clearly seen as strong bands at 847 and 505 cm ,
respectively. However, a medium intensity band at
−1
234.5 cm
observed in the solid-state Raman
[
[
[
[
4] A. Bersellini, C. Meyer, C.R., Part B, 270 (1970) 1672.
5] A. Picard, Spectrochim. Acta Part A: 30 (1974) 691.
6] A.G. Maki, J. Chem. Phys. 43 (1965) 3193.
7] A.S. Parkes, R.E. Hughes, Acta Crystallogr. 16 (1963) 734.
spectrum in the current work is clearly a new feature,
which does not appear in our gas-phase or liquid-
phase studies. The assignment of this band to the
infrared-active n fundamental, which has not been
[8] L. Pauling, H. Springall, K.J. Palmer, J. Am. Chem. Soc. 61
(1939) 927.
5
recorded directly but whose wave number value is
well known from combination band measurements,
is quite realistic. However, the interesting conclusion
which can then be made is the breakdown of Dϱh
molecular symmetry in cyanogen in the solid state
which gives rise to this feature. The strongest
infrared-active band in D symmetry is the n mode
[9] A. Langseth, C.K. Møller, Acta Chem. Scand. 4 (1950) 725.
[
[
[
10] A. Petrikaln, J. Hochberg, J. Phys. Chem. 3 (1929) 217.
11] A.W. Reitz, R. Sabathy, Monatsh. Chemie 71 (1938) 131.
12] G.J. Cartwright, D. O’Hare, A.D. Walsh, P.A. Warsop, J. Mol.
Spectrosc. 39 (1971) 393.
[13] G.B. Fish, G.J. Cartwright, A.D. Walsh, P.A. Warsop, J. Mol.
Spectrosc. 41 (1972) 20.
ϱh
3
−1
[14] J.W. Schultz, D.F. Eggers, J. Mol. Spectrosc. 2 (1958) 113.
15] B.H. Thomas, W.J. Orville-Thomas, J. Mol. Struct. 3 (1969)
near 2160 cm ; no evidence for this mode is observed
in the Raman spectrum of the solid state here.
However, Langseth and Møller [9] have reported a
[
191.
[16] F.D. Verderame, J.W. Negben, E.R. Nixon, J. Chem. Phys. 39
(1963) 2274.
−1
weak feature at ϳ240 cm in their Raman spectrum