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 (<i>retinoids</i>) 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<sub>F–H</sub> 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.

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