The effect of electron-withdrawing groups on 15N and 13C chemical shifts: a density functional study on a series of pyrroles

Abstract

Electron-withdrawing groups (EWGs) on the nitrogen atom of pyrroles have significant effects on the properties of the pyrrole. It has been suggested that the experimental 13C chemical shifts show a general increase in deshielding effect with the increase of N-EWGs strength [A. Thompson, S. Gao, G. Modzelewska, D.S. Hughes, B. Patrick, D. Dolphin, Org. Lett., 2, 3587 (2000)]. However, recently observed 15N chemical shifts of pyrroles do not correlate with the N-EWG strength. To elucidate the relationship between the electronic structures of pyrroles and their nitrogen and carbon chemical shifts, density functional theory calculations were performed on pyrroles with various substituents. A correlation between the paramagnetic shift and the 15N chemical shift was observed for the pyrroles, indicating that the nitrogen chemical shift trend for the pyrroles arises entirely from variations of the paramagnetic shift contribution. However, a general correlation between the 15N chemical shifts and the EWG strength does not exist. Natural chemical shielding (NCS) analysis shows that the changes in the σ(N5–R)–π* transitions and changes in the sum of the σ(C1–N5)–π* and σ(C4–N5)–π* transitions account for the nitrogen chemical shift trend observed in the pyrroles.