Infrared studies of some N-methylacetamide complexes

Abstract

Hydrogen bonding of N-methylacetamide (NMA) with dimethylsulfoxide (DMSO) and N,N-dimethylacetamide (DMA) in carbon tetrachloride and CCl4/CHCl3 mixed solvent was studied by a near-infrared method, which consists of measuring the effects of various concentrations of DMSO or DMA on the molar absorptivity of 0.06 M NMA at 6800 cm−1, corresponding to the first overtone of the uncomplexed NH stretching vibration. The association constants for the formation of 1:1 NMA–DMA and NMA–DMSO complexes in CCl4 are 10 and 121/mole, respectively, at 25°. In order to obtain an association constant for the formation of NMA–DMSO in CCl4/CHCl3 mixed solvent, it is necessary to have data on association constants of CHCl3–DMSO complexes in CCl4, and we have obtained these by determining the effects of various concentrations of DMSO on the proton magnetic resonance frequency of 0.05 M CHCl3 in CCl4. The association constants for the formation of NMA–DMSO and NMA–DMA in CCl4/CHCl3 mixed solvent are slightly higher than in CCl4, presumably due to hydrogen bonding between Cl3CH and C=O in NMA. NMA was chosen as the simplest molecule containing the peptide amide group and hence can be regarded as a structural unit of the polypeptide chain. Our results show that DMSO and DMA are effective in disrupting the interamide bonds, by acting as hydrogen acceptors to the NH group.