CONFORMATIONAL STUDIES OF VIBRATIONAL PROPERTIES AND ELECTRONIC STATES OF LEUCOEMERALDINE BASE AND ITS OLIGOMERS

Ab initio geometry optimizations and normal mode analysis have been pe rformed on a series of oligomers of leucoemeraldine base (LB, the full y reduced amine form of polyaniline) over conformational variations wi th the HF/6-31G basis set. Aniline was used to obtain 12 force consta nt scaling factors with the best vibrational frequency root mean squar e error of 5.2 cm-l These scaling factors are used in the analysis of the vibrational spectra of diphenylamine, N,N'-diphenyl-1,4-phenylened iamine, aniline end-capped trimer, and LB. A polymeric normal mode ana lysis has been performed using the scaled quantum mechanical oligomer force field (SQMOFF) method where polymer force fields are constructed from the extrapolation of scaled ab initio oligomer force fields. CN stretching, NH rocking, and CH inplane bending modes are strongly affe cted by the molecular conformation. The peaks around 1220 and 1180 cm- l in the Raman spectrum turn out to be indicators of molecular and pol ymeric planarity. The differences between Raman spectra of Quillard et al. and Furukawa et al. of N,N'-diphenyl-1,4-phenylenediamine can als o be explained by differences of the molecular conformation of the dif ferent samples. The energetics of isolated chains favors a nonplanar c onformation by about 3-4 kcal/mol per phenyl ring, but intermolecular interactions seem to influence the favored conformations of various ol igomers. On the basis of IR and Raman spectroscopic evidence, most pro bable conformations for each oligomer and the LB polymer are proposed. A complete assignment of LB vibrational frequencies including symmetr ies of each normal mode has been achieved with excellent agreement wit h the published experimental values. We have found that, as the size o f the oligomer is increased, planarity increases. This preference of p lanarity in leucoemeraldine base was further supported by our band cal culation using MEHT (modified extended Huckel theory).