The infrared spectra of the C-H stretching vibrations of pyridine, its dimer and trimer, and the N-H stretching vibrations of (pyridine)m-(NH3)n complexes were investigated with infrared (IR) - vacuum ultraviolet...

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.

The structures of a pyrazine dimer (pyrazine) 2 and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C−H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine) 2 and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π−π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm −1 , with intervals of 8 cm −1 in (pyrazine) 2 and 11 cm −1 in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene-d 6 ), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine) 2 and (pyrazine)(benzene) in a supersonic jet. For (pyrazine) 2 , the two isomers which were previously assigned to the H-bonded planar and the π−π stacked structures respectively were reassigned to the cross-displaced π−π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine) 2 , especially the split at ∼3065 cm −1 . However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.