THE INTRAMOLECULAR VIBRATIONS OF PROTOTYPICAL POLYTHIOPHENES

Inelastic neutron scattering experiments are combined with infrared an d Raman data to obtain a uniquely defined description of the intramole cular vibrations of three oligomers of polythiophene, Through refineme nt of ab initio force fields, the three vibrational spectra of each ol igomer are simulated with remarkable accuracy. Two different basis set s of atomic orbitals are used: the first, is 6-31G and is used to opt imize the geometries,and calculate the relevant force fields of alpha- 2T and alpha-4T, the second is 3-21G and is used for the same purpose for alpha-4T and alpha-6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameter s is generated for the 6-31G basis and another for the 3-21G* basis. The parameters are common to both molecules calculated with either bas is sets and are believed to be transferable to higher isomers, The fit ting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimental infrared and Raman activity, then a fitting of the Inelastic Neutron Scattering pr ofile is performed, finally, the infrared and Raman spectra are calcul ated with the new normal modes and the ab initio derivatives of the di pole moment and the polarizability. The procedure is iterated until th e three spectra of each oligomer are satisfactorily reproduced. For al pha-4T, two scaled force fields are obtained-one for each basis set-an d are shown to yield very similar normal modes. It is important to emp hasize that not only the vibrational frequencies but also the spectral intensities are well reproduced by the simulations. Implicitly, this means that the dipole moment and the polarization tensor surfaces calc ulated nb initio at the potential energy surface minimum are of good q uality. The procedure is absolutely general and can be applied to any molecular system. In the present case, it leads to well defined force fields that give us a stringent picture of the vibrations of these mol ecules. (C) 1996 American Institute of Physics.