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

Abstract

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.

Structures of (Pyrazine)2 and (Pyrazine)(Benzene) Dimers Investigated with Infrared–Vacuum Ultraviolet Spectroscopy and Quantum-Chemical Calculations: Competition among π–π, CH···π, and CH···N Interactions

“…The IR-VUV spectrum of (Pyra) in He in the C–H stretching region and its analysis has been reported in our previous paper; the spectrum is reproduced in Figures c and S1a. The band widths of (Pyra) are larger than those of (Pyra) 2 .…”

“…Thin solid curves in (a), (b), and (f) are the spectrum of (f) convoluted with a Lorentzian function with an FWHM of 6, 4, and 5 cm –1 , respectively; see text. The IR-VUV spectrum of (Pyra) (c) was adapted with the permission from ref . Copyright 2022 of Wiley-VCH.…”

“…In our previous study, we used the B2PLYP/6−311 + + G(d,p) method for the anharmonic calculation of (Pyd) and diazine monomers. 23 Although this calculation reproduced well the positions of the observed IR spectra, the agreement of the relative intensities was unsatisfactory. In the Supporting Information (Figure S1), we compared the observed spectra with the calculated anharmonic ones by using these three methods for (Pyra).…”

“…For the vibrational analysis, we calculated the anharmonic spectra by the vibrational second-order perturbation theory (VPT2) method at the B3PW91 + D3/6–311 + + G­(2d,2p) and B3LYP + D3/6–311 + + G­(2d,2p) levels. In our previous study, we used the B2PLYP/6–311 + + G­(d,p) method for the anharmonic calculation of (Pyd) and diazine monomers . Although this calculation reproduced well the positions of the observed IR spectra, the agreement of the relative intensities was unsatisfactory.…”

High‐pressure infrared spectroscopy of ionic liquids mixed with Tween 80

“…[4] Two dominant imidazolium C H absorptions at 3163 (C 4,5 H) and 3,122 cm À1 (C 2 H or Fermi resonance between C 2 H and in-plane ring modes) of pure [BMIM]BF 4 are revealed in Figure 1b. [16][17][18][19][20][21][22][23][24] Several aliphatic C H bands of cation locate at 2966, 2940 and 2,877 cm À1 for neat [BMIM]BF 4 in Figure 1b. IR spectrum of [BMIM]PF 6 (Figure 1c) exhibits similar spectral features in comparison to those of pure [BMIM]BF 4 (Figure 1b), and C H stretching absorptions appear at 3169, 3125, 2,965, 2,940, and 2,877 cm À1 for [BMIM]PF 6 (see Figure 1c).…”