Infrared-vacuum ultraviolet spectroscopy of the C-H stretching vibrations of jet-cooled aromatic azine molecules and the anharmonic analysis

Vibrational spectra of the C-H stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)-vacuum ultraviolet (VUV) spectroscopy under the jet-cooled gas-phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi-resonance (F-R). The anharmonic analysis of the F-R pattern was performed with second-order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6-311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F-R between the C-H stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in-plane C-H bending vibrations and the contribution of the higher-order anharmonic coupling to the observed spectra seems to lead to red-shift of the F-R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug-cc-pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level.