Ab initio and NBO studies of methyl internal rotation in 1-methyl-2(1H)-quinolinone: effect of aromatic substitution to 1-methyl-2(1H)-pyridone

Sinha, Rajeev K.

doi:10.1007/s00894-020-04358-9

Cited by 3 publications

(3 citation statements)

References 23 publications

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“… Specific hyperconjugative contributions from the methyl groups to the eight conformers in Figure 2 were obtained by deletion of all donor→acceptor interactions involving the methyl groups. Hyperconjugation effects on molecular geometries: following a strategy suggested elsewhere [ 29 , 78 , 79 , 80 , 81 , 82 ], geometry optimizations for the rotations of the methyl groups within the lowest energy structure were carried out by deleting all hyperconjugative interactions involving each methyl group separately and then involving all methyl groups simultaneously. …”

Section: Methodsmentioning

confidence: 99%

“…Hyperconjugation effects on molecular geometries: following a strategy suggested elsewhere [ 29 , 78 , 79 , 80 , 81 , 82 ], geometry optimizations for the rotations of the methyl groups within the lowest energy structure were carried out by deleting all hyperconjugative interactions involving each methyl group separately and then involving all methyl groups simultaneously.…”

Section: Methodsmentioning

confidence: 99%

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Analysis of Conformational Preferences in Caffeine

2022

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High level DLPNO–CCSD(T) electronic structure calculations with extended basis sets over B3LYP–D3 optimized geometries indicate that the three methyl groups in caffeine overcome steric hindrance to adopt uncommon conformations, each one placing a C–H bond on the same plane of the aromatic system, leading to the C–H bonds eclipsing one carbonyl group, one heavily delocalized C–N bond constituent of the fused double ring aromatic system, and one C–H bond from the imidazole ring. Deletion of indiscriminate and selective non-Lewis orbitals unequivocally show that hyperconjugation in the form of a bidirectional –CH3 ⇆ aromatic system charge transfer is responsible for these puzzling conformations. The structural preferences in caffeine are exclusively determined by orbital interactions, ruling out electrostatics, induction, bond critical points, and density redistribution because the steric effect, the allylic effect, the Quantum Theory of Atoms in Molecules (QTAIM), and the non-covalent interactions (NCI), all predict wrong energetic orderings. Tiny rotational barriers, not exceeding 1.3 kcal/mol suggest that at room conditions, each methyl group either acts as a free rotor or adopts fluxional behavior, thus preventing accurate determination of their conformations. In this context, our results supersede current experimental ambiguity in the assignation of methyl conformation in caffeine and, more generally, in methylated xanthines and their derivatives.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Analysis of Conformational Preferences in Caffeine

2022

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“…In this context, several studies have been reported in the literature related to the rotational barrier of substituents in aromatic compounds [ 16 , 17 , 18 , 19 , 20 ]. The works already reported bring innumerable importance and allow the understanding of the stereoelectronic effects of the most different substituents, such as the work of Cimiraglia and Hofmann (1994) [ 17 ], who investigated the relationship between the rotation and inversion mechanisms in the E/Z isomerization of the N-phenyldiazene and azobenzene compounds.…”

Section: Introductionmentioning

confidence: 99%

Theoretical study of internal rotational barriers of electrons donating and electrons withdrawing groups in aromatic compounds

Lima

Filho

et al. 2020

Heliyon

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The presence of internal rotation in sigma bonds is essential for conformational analysis of organic molecules and its understanding is of great relevance in chemistry, as well as in several other areas. However, for aromatic compounds that have substituent groups, withdrawers or donors of electron, there are no data in the literature to explain their rotational barriers. In this context, the work studied the internal rotational barriers of electron donating and withdrawing groups in aromatic compounds using the MP3, MP4, and CCSD(T) methods and the influence of substituents' nature on barrier heights was investigated through calculations based on the theory of Natural Bond Orbitals (NBO) and Quantum Theory of Atoms in Molecules (QTAIM). The results obtained showed that the CCSD(T) method is the one that best describes the internal rotational barriers, followed by MP4 and MP3 and the electron donating groups decrease the barrier, whereas electron withdrawing groups increase. Through the NBO analysis it was possible to observe that for withdrawing groups the interaction of the molecular orbitals is more accentuated promoting the increase of the rotational barrier of these compounds. Through the QTAIM analysis it was possible to show that, for electron donating groups, the internal rotation is influenced by the loss of electronic density when the substituents is perpendicular to the ring plane, however, for withdrawing groups the density is little influenced, regardless of the two conformations (minimum and maximum energy). Two molecules showed free rotation, trichloromethylbenzene and methylbenzene, and the theoretical calculations NBO and QTAIM showed that for these species there is no difference in the properties studied when there is rotation of the dihedral angle.

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