Spectroscopic studies of the solvation of amides with N—H groups. Part 1.—The carbonyl group

Abstract

Infrared shifts of the CO stretch (νco) band, and n.m.r. shifts for the 13CO carbon have been studied for formamide, acetamide, N-methyl formamide and N-methyl acetamide for dilute solutions in a range of pure and mixed solvents. The results are compared with those previously reported for N,N-dimethylamides in the same systems. There are good linear relationships between Δν(13C) and νco for the pure solvent systems, provided allowance is made for the presence of two types of solvate for methanol. For mixed methanol-aprotic solvents (B) the low-frequency (νco) component for pure methanol was lost as the concentration of B was increased. The high-frequency band initially gained intensity, but this was ultimately replaced by a third band characteristic of the amide in pure B. These results suggest that the CO group forms both one and two hydrogen bonds in methanol. Aqueous solutions have a single νco band close to that for the disolvate in methanol. As [B] was increased, this gave way to a band close to that for the mono-solvate, which again was steadily replaced by the non-hydrogen-bonded form. Hence it is concluded that for all the amides, the di-hydrogen-bonded species dominates in water. Reasons for the different behaviour in methanol and water are discussed.