Publications by authors named "Ulisses Z de Melo"
A conformational analysis of -methyl-2-pyrrolidinone 3-substituted by methoxyl, thiomethoxyl, and selenomethoxyl is reported by means of H nuclear magnetic resonance spectroscopy and electronic structure calculations. The five-membered ring has an envelope conformation with the α-carbonyl substituent being able to assume two positions: pseudo-axial and pseudo-equatorial. In vacuum, the calculations pointed to the pseudo-axial conformer as the most stable one, and this preference increases with the size of the substituent and a decrease in its electronegativity.
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- The study investigates the conformational behavior of -2-halocyclohexylamines, specifically focusing on compounds with fluorine, chlorine, bromine, and iodine substituents.
- Strong stereoelectronic interactions lead to an unexpected conformational preference, with over 90% of the molecules favoring the equatorial amine and axial halogen configuration.
- Both experimental and theoretical findings indicate that hyperconjugative interactions, particularly involving σ and σ* orbitals and halogen lone pairs, significantly influence the equilibrium, overshadowing factors like steric repulsion or solvent effects.
This study reports the results of ab initio and density functional theory (DFT) electronic structure calculations as well as (3)J(HH) experimental and calculated coupling constant data obtained in the investigation of the conformational equilibrium of 3-halo-derivatives of 1-methylpyrrolidin-2-one. The five-membered ring assumes an envelope conformation owing to the plane of formation of the O═C-N-R bond, with C4 forming the "envelope lid". When the conformation changes, the "lid" alternates between positions above and below the amide plane.
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