ORCID Profile
0000-0002-1459-137X
Current Organisations
KU Leuven
,
University of Melbourne
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Publisher: Wiley
Date: 21-09-2021
Abstract: Vibrational spectra of the CH 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 CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH 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.
Publisher: AIP Publishing
Date: 16-10-2022
DOI: 10.1063/5.0025778
Abstract: The appearance of multiple bands in the N–H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend–stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (νsymD), asymmetric stretchin (νasymD), and bending overtone (2νbendD) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (νbendD + νbendA) and with the combination of the intermolecular translational mode over the donor bending overtone (νtrans + 2νbendD). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm−1 for the N–H bend–stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend–stretch Fermi resonance coupling.
Publisher: American Chemical Society (ACS)
Date: 26-02-2021
No related grants have been discovered for Ha-Quyen Nguyen.