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 l evel, is still, a demanding task for ab initio large-size basis or function al 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 mol ecule is not lower then the symmetry of the related structural "building-bl ock", normal mode unfolding scheme could be effectively predicted from grou p theory considerations and the vibrational structure of the building block . A simple semiempirical Hamiltonian could be then utilized in order to obt ain a mode assignment for the large molecule, In the present study, vibrati onal mode assignment for members of a poly-phenylacetylene linear chain: 1, 3-di(phenylethynyl) benzene (DPB), diphenylacetylene (DPA) and phenyl acety lene (PA) is obtained. Their structure was calculated by means of the compu tationally efficient semiemprical RHF PM3 Hamiltonian. Raman and FTIR spect ra were taken and assign with mutual correspondence for the three compounds . The remaining weak peaks observed in the spectra were successfully descri bed as overtones and combinations. Few medium sized modes were identified a s Fermi resonances. The overall, mode-unfolding scheme of the DPB, which wa s originated in the DPA, which in turn was originated in the PA, was establ ished. The successive elements of the linear poly-phenylacetylene chain are all members of a same Dill pr,int group, except the phenyl acetylene which is of lower C-2v point group. It is argued here, that the same principle a llows us to predict the unfolding scheme further for linear DPB... + PA, bu t not for the lower symmetry phenyl acetylene macro cycles. (C) 2001 Elsevi er Science B.V. All rights reserved.