Electrostatic effects on 19F NMR chemical shifts in N-phenyl γ-lactam derivatives

Abstract

Covalently identical ortho-F nuclei display dual 19F NMR signals in di-ortho-F substituted N-phenyl γ-lactams. Structural variations at C9-substituent on the γ-lactam lead to larger chemical shift changes at downfield 19F signals (δdown) than those at upfield 19F signals (δup). A “reverse” correlation between downfield 19F chemical shifts and electron-withdrawing abilities of C9-phenylamides is observed. Electrostatic effects are the major forces between C9-substituents and ortho-F nuclei, and Hammett parameters can be used to describe such through-space interactions. Structural factors influencing ortho-19F shielding among diverse C9-substituted N-phenyl γ-lactams are discussed.