Conformational information from vibrational spectra of polyaniline

Vibrational (IR and Raman) spectra contain information about local conformations in materials. Their extraction, however, is possible only through accurate modeling of the vibrational spectra and by comparing various predicted spectra with experiment. We have established a Scaled Quantum Mechanical Oligomer Force Field for oligomers of leucoemeraldine base and for one oligomer of the imine form of polyaniline (pemigraniline base). We conclude, that while the shorter oligomers of LB are far from planarity, the longer oligomers and LB itself are nearly planar. This result is confirmed by energy band structure calculations. There is no doubt, that an isolated LB polymer would energetically favor a non-planar structure. Planarity is probably a consequence of interchain interactions in the material. The vibrational analysis of the PNB form is not yet complete, but optical and energy band structure data suggest, that the samples contain the non-planar form.