Mode assignment for linear phenyl acetylene sequence: Phenylacetylene, di-phenylacetylene and 1,4-di(phenylethynyl)benzene

Normal mode derivation for a large molecule at the harmonic approximation level, is still, a demanding task for ab initio large-size basis or functional densities methods. Build in-process additive errors, affect the accuracy and often limit the applicability of such. It is argued here, that in cases, where point group symmetry of a large molecule is not lower then the symmetry of the related structural "building-block", normal mode unfolding scheme could be effectively predicted from group theory considerations and the vibrational structure of the building block. A simple semiempirical Hamiltonian could be then utilized in order to obtain a mode assignment for the large molecule. In the present study, vibrational mode assignment for members of a poly-phenylacetylene linear chain: 1,4-di(phenylethynyl) benzene (DPB), diphenylacetylene (DPA) and phenyl acetylene (PA) is obtained. Their structure was calculated by means of the computationally efficient semiemprical RHF PM3 Hamiltonian. Raman and FTIR spectra were taken and assign with mutual correspondence for the three compounds. The remaining weak peaks observed in the spectra were successfully described as overtones and combinations. Few medium sized modes were identified as Fermi resonances. The overall, mode-unfolding scheme of the DPB, which was originated in the DPA, which in turn was originated in the PA, was established. The successive elements of the linear poly-phenylacetylene chain are all members of a same D_2h point group, except the phenyl acetylene which is of lower C_2v point group. It is argued here, that the same principle allows us to predict the unfolding scheme further for linear DPB...+ PA, but not for the lower symmetry phenyl acetylene macro cycles.