HYDROGEN-BONDING OF N-METHYLACETAMIDE IN CDCL3 SOLUTION STUDIED BY NMR AND IR-SPECTRA

The amide proton NMR chemical shifts have been widely used for the determination of the population fraction of the non-hydrogen-bonded states in studies of hydrogen bonding in peptides and proteins. However, in such works the determination of the limiting chemical shifts for the fully hydrogen-bonded state and for the non-hydrogen-bonded state is quite problematic, because they cannot be measured directly. In the present study, we carried out variable-temperature H-1 NMR and IR measurements for N-methylacetamide in CDCl3 solution. We derived the expression for the limiting chemical shift for the fully hydrogen-bonded state as a function of IR band intensity and chemical shift. According to the equation, we determined the values of the limiting shifts at various temperatures. The present method may be applicable to other hydrogen-bonding systems.