Spectroscopic studies of the solvation of N,N-dimethyl amides in pure and mixed solvents

Abstract

The i.r. spectra of dilute solutions of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) have been studied in the >CO stretching region using a range of pure and binary mixed solvents in which the ‘probe’ amide concentration was kept as low as possible. Shifts of the ¹³C carbonyl resonance [Δv(¹³C)] were measured under identical conditions. A linear relationship between v_max(i.r.) and Δv(¹³C) exists for most pure solvents. For solvents in which two i.r. bands were observed, the correlation was obeyed when the weighted mean of these i.r. bands was used. For mixed protic–aprotic solvents the i.r. spectra displayed two discrete bands. By studying the complete mole-fraction range it was possible to assign specific bands to specific hydrogen-bonded amide units. Bulk water forms two hydrogen bonds to the carbonyl oxygen atom and addition of basic aprotic solvents such as dimethyl sulphoxide (DMSO) resulted in the sequential loss of these water molecules together with relatively minor shifts for the di- and mono-solvate bands. Using weighted averages of the mixed-solvent spectra together with the i.r.–n.m.r. correlation the ¹³C shifts for the mixed-solvent system water-cyanomethane over the whole mole-fraction range have been interpreted. For alcohol solvents two i.r. bands were detected, assigned to mono- and di-solvates on the basis of mixed-solvent studies. The frequencies for these bands are close to those for the mono- and di-hydrate bands, suggesting that the strengths of the corresponding hydrogen bonds are comparable. These results are compared with those from our previous study of the CO stretch and ¹³C(O) n.m.r. spectra for acetone. There is a linear correlation, but the shifts for both functions are greater for the amides. Bands assigned to individual solvates shift to low frequencies as the concentration of protic solvent increases. Limiting shifts, estimated for the units amide–––HOH–––B, where B is a basic solvent, are a function of the hydrogen-bond basicity of B, being at high frequencies for strong bases and low frequencies for weak bases. This is explained in terms of an ‘anti-cooperativity’ effect for water. Measurements of 2v(OH) and 3v(OH) for HOD in D₂O on the addition of these amides showed a fall in intensity in the band assigned to (OH)_free groups. By comparing this fall with that induced by triethylphosphine oxide (solvation number 3) we obtain a solvation number close to 2 for aqueous amides, in good agreement with the CO stretch and n.m.r. studies. These combined results strongly support the concept of dibonding to oxygen rather than bonding at oxygen and nitrogen, as has been recently suggested for matrix-isolated systems.