Strong-coupling case, optical centers

Translational and unit cell symmetries greatly simplify the vibrational spectra of infinite polymers. In the case of /-PA, whose Cyi unit cell is shown in Fig. 6.3, we have just six in-plane, optically active q = 0 modes. The four g Raman-active modes are even under inversion at bond centers, whereas the two IR-active hu modes are odd. Only C—C stretches couple to it electrons in Hiickel theory, and they remain the strongly coupled modes of PPP models. Since C—C stretches do not appear in the subspace, the IR modes are not coupled efficiently to tt electrons. Formally, the symmetry of the polymer is exploited by defining a set of symmetry coordinates S related to the internal coordinates by the... 

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