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

Abstract

Infrared shifts of the CO stretch (ν_co) 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 ν_co 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 (ν_co) 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 ν_co 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.

In all cases, as well as the gain and loss of i.r. bands, those assigned to the hydrogen-bonded units shifted considerably as [B] increased. For aqueous systems these shifts are assigned to changes in secondary solvation.

We have looked for specific differences between the results for the dimethyl derivatives and the present compounds, in the expectation that N—H solvation would have a discernible affect on CO solvation. There are small differences, but these are not systematic on going from RCONMe₂via RCONHMe to RCONH₂, and it is concluded that cooperativity effects involving CO and N—H solvation are small compared with those for water and alcohols. The ¹³C resonance (¹³CO) shifted systematically for mixed protic–aprotic solvent systems. Using the i.r.–n.m.r. correlation and the intensity changes for the ν_CO bands, reasonable predictions of these n.m.r. shifts were obtained.

Marked changes in the amide II band were also observed, but these are less readily interpreted. Studies of the overtone infrared spectra for aqueous solutions reveal the presence of (OH)_free and (NH)_free bands, showing that the N—H groups are not fully hydrogen-bonded in water despite the effect of two bonds to the carbonyl group.