Infrared and Nuclear Magnetic Resonance Spectroscopic Study of Secondary Amide Hydrogen Bonding in Benzoyl PABA Derivatives (Retinoids)

Abstract

Attenuated total reflection (ATR) Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) data are used to characterize the hydrogen bonding of the secondary amide N–H group of several structurally similar benzoyl derivatives of p-aminobenzoic acid esters (retinoids) in chloroform solution. The amide N–H can form intermolecular hydrogen bonds to several proton acceptors in these molecules or it can form an intramolecular hydrogen bond to a fluorine or oxygen atom in some of the molecules. The concentration dependence of the solution N–H infrared absorption bands is used to determine the formation of intramolecular and/or intermolecular H-bonds. Proton NMR spectra were obtained from deuterated chloroform solutions and the sec-amide N–H resonance was assigned for each compound. The downfield shift in the N–H resonance is correlated to intramolecular H-bond formation. Also, the NMR spectra of fluorine-containing compounds provide J_F–H through-space coupling values. Using infrared and NMR data, the relative intramolecular hydrogen bond strengths (N–H…F or N–H…O) of the compounds are approximately ranked.