Ab initio study of vibrational anharmonic coupling effects in oligo(para-phenylenes)

In this study we present a theoretical approach to simulate vibrational anharmonic coupling effects seen in the Raman spectra of oligo(para-phenylenes). Quantum chemical ab inito methods are applied to determine anharmonic force constants and energy corrections on the harmonic vibrational frequencies of the isolated molecules. Sermempirical methods are applied to compute Raman intensities of fundamentals and combination bands. This methodology is then used to characterize a previously unassigned Fermi resonance around 1600 cm(-1). The evolution of this quantum mechanical resonance with oligomer length and planarity is compared to experimental data.