C„h point groups

Figure 6.19 Illustration of the three types of sets of identical nuclei that are possible in the C-h, point group...

Let us first consider the case of a long n-alkane molecule in the solid crystalline phase. From lattice dynamical calculations it is known that collective librational phonons (about the chain axis) exist in the lattice, some giving rise to scattering and/or absorption in infrared [70]. Let us take a n-alkane molecule with an even number of carbon atoms which belongs to the C h point group. The rigid rotation about its axis is of Bg species, the same as the Raman active d mode. When the... 

It is less easy to find examples for the C h point groups there are several C2h molecules, such as ( )- ,2-diphenylethene (common name stilbene), shown in Figure 3.18. This molecule is planar due to conjugation between the ethene double bond and phenyl substituents. Any planar molecule must have the plane of the molecule as a symmetry element, and in this case this is the only symmetry plane present. The principal axis is C2 and is perpendicular to the plane, which must, therefore, be labelled Since C2 and <7h are present we must have an S2 axis, but the only resulting operation is equivalent to an inversion (i = 2 ) and so only i is quoted. 

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