High pressure Raman studies on the structural conformation of oligophenyls

The goal of this combined experimental and computational study is to invest igate the structural conformation of oligo(para-phenylenes) in the crystall ine phase, in particular the planarity of this type of molecules. To this e nd we have performed Raman experiments on paraterphenyl and para-quaterphen yl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C-C interring stretch mode at 1280 cm(-1) have been tracked. We find that and the C-H in-plane b end mode at 1220 cm(-1) upon increasing temperature at ambient pressure the intensity ratio I-1280/I-1220 drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increa sing pressure up to about 15 kbar. In the computational part, the Raman fre quencies and activities of isolated 3P and 3P molecules were calculated wit hin restricted Hartree-Fock formalism with the interring tilt angles varyin g from 0 to 90 degrees. These calculations confirm that the I-1280/I-1220 i ntensity ratio can be related to the planarity of the molecules. Three-dime nsional bandstructure calculations within density functional theory were ap plied to determine phonon frequencies and estimate Raman activities for the polymer poly(paraphenylene). These simulations show that the same conclusi ons hold for crystalline environment. (C) 2001 Published by Elsevier Scienc e B.V.