Análise conformacional de 3-halo-derivados da 2-pirrolidinona por espectroscopia de ressonância magnética nuclear e cálculos de estrutura eletrônica

Abstract

This study deals with the conformational analysis of 2-pyrrolidone, a ?-butyrolactam, and its analogue N-methylated, both 3-halo-substituted. To achieve this purpose, we used nuclear magnetic resonance spectroscopy and electronic structure calculations. The compounds were synthesized by usual procedures, already described in the literature, and were properly characterized by its 1H and 13C NMR spectra. The study concerning the conformational equilibria was developed associating spin-spin coupling constants 3JHH with the data provided by the theoretical calculations. The five membered ring assumes the "envelope" conformation, due to the plane formed by the amide bond O=C?N?H(R), thus the 4C stands in the "envelope flap", moving above and down of the amide plane. Thereby, the ?-carbonilic hydrogen assumes two positions: pseudo-axial e pseudo-equatorial. In the gas phase, the theoretical calculations point out the pseudo-axial conformer as the most stable one. This preference becomes greater when we go down the halogens' family. Natural bond orbital analysis shows that the electronic delocalization is weighty only for the iodine-derivatives. To the others compounds, the oxigen? halogen electrostatic repulsion is driving the conformational equilibria. When the solvent effect was considered, the pseudo-equatorial conformer's population grows up with the increase on the solvent relative permittivity. This variation is large in the fluorine-derivatives, causing the inversion of the conformational preference. In the chlorine-derivatives, the pseudo-equatorial conformers population equals the pseudo-axial one in methanol and acetonitrile. For bromine and iodine derivatives, there is only a slight decrease in the pseudo-axial conformers population. This is a result of the conformation's dipole moment: the pseudo-equatorial one is more polar, so it's stabilized in high polarity solvents