The interactions and structure of the -CONH- group in amides and polyamides

Abstract

1. The interactions of N-ethyl acetamide with a variety of other molecules have been studied in carbon tetrachloride solution by following changes in the infra-red frequencies of the CONH group. While the-C=O group readily forms hydrogen bonds to proton donating molecules the N-H group is comparitively inert, neither readily donating nor accepting protons by hydrogen bonding. With anhydrous hydrogen chloride a hydrochloride structure is formed. 2. A detailed comparison is made of the structures and bond polarities, of the amide and carboxyl groups based on published crystallographic data and a series of infra-red spectra of acetamide, N-ethyl acetamide, N,N-diethyl acetamide, formic acid, and acetic acid in solution in carbon tetrachloride, chloroform and CCl4/CHCl3 mixtures. 3. From a discussion of both series of experiments it is concluded that the electronic structure of the CONH group is quite unlike the carboxyl group. Peptide groups associate by dipole interaction andnot by hydrogen bonding as hitherto supposed. The implications of these conclusions in polypeptide and protein structures are briefly considered. 4. Intermolecular interactions are discussed and spectroscopic methods proposed for distinguishing non specific (e. g. dispersion forces) and specific (e. g. dipole-dipole forces, hydrogen bonds) interactions. The quantum mechanical nature of hydrogen bonding is considered and more exact criteria for defining hydrogen bonds are suggested.