Ab initio geometry optimizations and normal mode analysis have been performed on a series of oligomers of leucoemeraldine base (LB, the fully reduced amine form of polyaniline) over conformational variations with the HF/6-31G* basis set. Aniline was used to obtain 12 force constant scaling factors with the best vibrational frequency root mean square 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-phenylenediamine, aniline end-capped trimer, and LB. A polymeric normal mode analysis 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 affected by the molecular conformation. The peaks around 1220 and 1180 cm-l in the Raman spectrum turn out to be indicators of molecular and polymeric planarity. The differences between Raman spectra of Quillard et al. and Furukawa et al. of N,N’-diphenyl-1,4-phenylenediamine can also be explained by differences of the molecular conformation of the different samples. The energetics of isolated chains favors a nonplanar conformation by about 3-4 kcal/mol per phenyl ring, but intermolecular interactions seem to influence the favored conformations of various oligomers. On the basis of IR and Raman spectroscopic evidence, most probable conformations for each oligomer and the LB polymer are proposed. A complete assignment of LB vibrational frequencies including symmetries of each normal mode has been achieved with excellent agreement with the published experimental values. We have found that, as the size of the oligomer is increased, planarity increases. This preference of planarity in leucoemeraldine base was further supported by our band calculation using MEHT (modified extended Huckel theory).