Conformational Isomerism of 3-Chalcogenomethyl-N-Methyl-2-Pyrrolidinones: Insights from NMR Spectroscopy and Molecular Modeling.

A conformational analysis of N-methyl-2-pyrrolidinone 3-substituted by methoxyl, thiomethoxyl, and selenomethoxyl is reported by means of 1H 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. Natural bond orbital analysis evidenced the importance of electron delocalization on the stability, and a principal component of analysis (PCA) plot of the hyperconjugative interactions revealed the main ones. Steric and electrostatic effects were also investigated by energy decomposition analysis.