Structure, Symmetry and ECC Theory

The structure of pristine PPy is not yet known since the material is amorphous, but can be guessed or extrapolated from the experimentally determined structures of the oligomers Pj and Py3 in the solid state. The structure of these molecules is trans and coplanar [106]. The results of semiempirical quantum chemical calculations are many and, being focussed at various physical aspects, are sometimes inconsistent. From MO calculations the structures of Py2 and Py3 in vacuo is not coplanar [107] but the barrier to coplanarity is relatively low such that it can be flattened when intermolecular forces are turned on in the crystal. Recent MO calculations in vacuo by the group of Kertesz send the important message that non coplanar structures should not be neglected [108]. In ref. 108 the possibility that PPy may also exist in a helical form is presented with serious theoretical and numerical arguments. A full discussion of this problem is presented in the case of Polythiophene in section 9 of this article. 

For the time being we work with assumption that, by extrapolation from the experimental results on Py2 and Py3/ the structure of PPy is trans (anti) and co-planar (in the literature in this field the terms trans-cis or anti-sin are freeley used). The symmetry of the polymer chain as 1-d perfect and infinite lattice is D2hand the qs=0 modes are classified as [109]  

The Raman spectrum should show predominantly the 8 Ag modes variously resonantly enhanced. If one Ag c-H stretching and one Ag N-H stretching are neglected we expect to find 6 dispersion branches in the f/F plot, in the frequency range 1700-100 cm. The 6 TS Ag frequencies must lie on these dispersion curves at a given value of the effective conjugation parameter F j, with relative intensity iL li. Indeed 6 Reunan lines are observed at =1569 (very strong). 

V2 1452 (vanishingly weak), V3= 1307 (medium), V4= 1051 (very week), Vs lOOO (very weak) and Vg=387 cm ( zero intensity). 

Upon doping 6 IR active modes, some of which very strong, must be found at Fg p p prist along the same dispersion branches as those used for the location of the Raman active Ag modes. 

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