A combined ab initio and semi-empirical study on the theoretical vibrational spectra and physical properties of polypyrrole

This study concentrates on the important conducting polymer, polypyrrole. Detailed atomistic molecular models have been developed with the help of ab initio and semi-empirical quantum mechanical calculations. The vibrational spectra of isolated pyrrole monomers and oligomers from n = 1 and 2, where n is the number of structural repeat units used, have been computed using the ab initio 3-21G basis set. The results obtained are compared with data for the case of oligomers with n = 2-5 for both neutral benzenoid and quinonoid oligopyrroles, from semi-empirical predictions obtained by AM1 and PM3. The trends in the computed harmonic force fields, vibrational frequencies and intensities are monitored as a function of the chain length. The data are analysed in conjunction with the trends in computed equilibrium geometries. Also the examination of the heat of formation of these two degenerate forms (quinonoid and benzenoid) has been conducted with respect to increases in the number of rings and the change of methods from AM1 to PM3. (C) 2000 Elsevier Science Ltd. All rights reserved.