Journal
JOURNAL OF THE CHINESE CHEMICAL SOCIETY
Volume 69, Issue 1, Pages 160-172Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/jccs.202100366
Keywords
aromatic azine; C-H stretching vibration; Fermi-resonance; IR-VUV spectroscopy; supersonic jet
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Funding
- Academia Sinica
- Ministry of Science and Technology of Taiwan [MOST 108-2811-M-009-508, MOST109-2113-M-001-040, MOST110-2639-M-A49-001-ASP, MOST107-2923-M-001-008-MY2, MOST107-2628-M-001-002-MY4]
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The infrared-vacuum ultraviolet spectroscopy was used to measure the C-H stretching vibrations of pyrimidine and pyrazine, and an anharmonic analysis with evaluation of QP at DFT level was carried out to interpret the F-R mode patterns.
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.
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