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

Anharmonic infrared spectra of 1,3-cyclopentadiene and its hexadeuterated isotopomer, 1,3-cyclopentadiene-d6, 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 CH and CH2 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 CH2 stretching mode, the most significant anharmonic corrections are given by the diagonal and coupling with the symmetric CH2 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 CH and CH2 stretching vibrations.