Effects of fluorine bonding and nonbonding interactions on 19F chemical shifts

Abstract

¹⁹ F-NMR signals are sensitive to local electrostatic fields and are useful in probing protein structures and dynamics. Here, we used chemically identical ortho-F nuclei in N-phenyl γ-lactams to investigate the relationship between ¹⁹F NMR chemical shifts and local environments. By varying the structures at the C5- and C7-substituents, we demonstrated that ¹⁹F shifts and Hammett coefficients in Hammett plots follow typical relationships in bonding interactions, while manifesting reverse correlations in nonbonding contacts. Quantum mechanics calculations revealed that one of the ortho-F nuclei engages in n → π* orbital delocalization between F lone pair electrons (n) and a CO/ArN antibonding orbital (π*), and the other ortho-F nucleus exhibits n ↔ σ orbital polarization between the n electrons and the C–H σ bonding orbital. As ¹⁹F NMR spectroscopy find increasing use in molecular sensors and biological sciences, our findings are valuable for designing sensitive probes, elucidating molecular structures, and quantifying analytes.