Keto-enol tautomerism, spectral (infrared, Raman and NMR) studies and Normal coordinate analysis of 4-Methyl-2-hydroxyquinoline using quantum mechanical calculations

互变异构体 化学 拉曼光谱 红外线的 计算化学 甲基 化学位移 碳-13核磁共振 物理化学 结晶学 立体化学 群(周期表) 有机化学 光学 物理
作者
Ibrahim A. Shaaban,Mohammed A. Assiri,Tarik E. Ali,Tarek A. Mohamed
出处
期刊:Journal of Molecular Structure [Elsevier BV]
卷期号:1252: 132137-132137 被引量:3
标识
DOI:10.1016/j.molstruc.2021.132137
摘要

The Raman (3500−100 cm−1) and mid-infrared (4000−400 cm−1) spectra of 4-Methyl-2-hydroxyquinoline (MHQ) have been recorded. Owing to the keto-enol tautomerism and rotation of methyl group around C4–C13 single bond, six structures (S1–S6) were hypothetically proposed. Full geometry optimization and frequency calculations were performed for keto (quinolone) and enol (hydroxyquinoline) structures using the DFT methods including B3LYP, ωB97XD and mPW1PW91 with basis sets up to 6–311+G(2d,p). In addition, 1H/13C NMR chemical shifts (δ, ppm) were estimated utilizing GIAO approach and IEF-PCM solvation model. The computational outcomes favor keto tautomer, 4-Methylquinolin-2-one (MQO) while the methyl group is staggered to C4–C5 bond (S5). However, other structures were excluded owing to high energy differences besides having an imaginary wavenumber for the transition state structures S2, S4 and S6. These results agree with the observed/calculated vibrational spectral data along with 1H/13C NMR chemical shifts. Aided by the calculated wavenumbers and normal coordinate analysis, complete assignments for the observed infrared/Raman bands to their corresponding vibration modes were quantitatively reported herein for keto- MQO. Employing B3LYP/6–311+(2d,p) structural parameters, the thermo kinetic parameter F-number (5.3936707 cm−1) is estimated along with V3 barrier (1067±48/1219±62 cm−1, 3.05±0.14/3.48±0.18 kcal/mol) using the calculated/observed methyl torsion at 217/233 cm−1.
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