Ab initio and DFT anharmonic spectroscopic investigation of 1,3-cyclopentadiene

Abstract

Anharmonic infrared spectra of 1,3-cyclopentadiene and its hexadeuterated isotopomer, 1,3-cyclopentadiene- d 6, have been calculated by means of second-order perturbation theory using Møller–Plesset second-order perturbation and density functional theory (B3LYP and B97-1) methods with correlation consistent Dunning’s basis sets (cc-pVDZ and cc-pVTZ). Computed anharmonic frequencies of fundamental, overtone and combination transitions are in satisfactory agreement with available experimental data. With exception of low-frequency ring torsion vibrations, introduction of the anharmonic corrections reduces the harmonic wavenumbers. The largest anharmonic effects are found for C H and CH 2 stretches, which decrease the harmonic frequencies by 100–130 cm −1 (3–4%) and 140–160 cm −1 (5–6%), respectively, reproducing the observed values within ca. 1%. For the antisymmetric CH 2 stretching mode, the most significant anharmonic corrections are given by the diagonal and coupling with the symmetric CH 2 vibration terms, together contributing to about 80% of the total anharmonic adjustment. Anharmonic calculations are important for a reliable prediction of the H/D isotopic frequency shift involving the C H and CH 2 stretching vibrations.