INFRARED SPECIAL STUDIES ON LACTAMS AS CIS-TRANS MODELS FOR PEPTIDE BOND

The infrared spectra of a homologous series of lactams have been studied as a function of temperature and concentration in various solvents. There are at least three spectral regions which show distinguishing features for the cis- and trans-peptide configurations in carbon tetrachloride; the free N-H and the associated N-H stretching absorptions, the amide I band, and the amide II band. The bands around 1550 cm-1 and 1510 cm-1 are assigned to the amide II band of the associated N-H group and the free N-H group in trans amides, respectively. The n = 9 lactam is the dividing size for the cis and the trans configuration in carbon tetrachloride. It exists in both the cis and the trans configurations but associates to form mainly cis cyclic dimers. This cis-trans equilibrium seems to be affected by the physical state and solvent. The doubling of the free N-H stretching bands, which was observed for larger lactams (n ≥ 8), is thought to be due to the different conformers of the lactams. The interactions between solvents, such as carbon tetrachloride, chloroform, dioxane, dimethyl sulfoxide, and water, and the amide group of the lactams are dominated by the relative hydrogen-bonding abilities and the dielectric properties of the solvents.