Nuclear magnetic resonance studies of the hindered internal rotation of NN-disubstituted amides. Part VII. Solvent effects on the N-methyl resonance shifts and activation parameters

Abstract

Activation energies E_a for the hindered internal rotation of NN-dimethylformamide and NN-dimethylacetamide in a variety of aprotic, proton-accepting, and proton-donating solvents have been systematically analysed to check the discrepancies found by several authors. The high-resolution line-shape method was used and the consistency of the determinations much improved by evaluating individual dω values for each line-shape. Temperature- and solvent-dependence of dω for both amides have thus been systematically investigated in the same variety of solvents and over a large temperature interval.

The results for dω show marked solvent- and temperature-dependence and their variations have been correlated with variations in the activation parameters.

ΔG^‡ Values are not significantly affected by solvents and experimental conditions. Significant E_a decreases with dilution are instead given for both amides in aprotic solvents, whilst significant E_a increases are detected only for dimethylformamide in formamide, dimethylacetamide showing in this solvent a singular behaviour also revealed in the temperature-dependence of dω. The E_a and ΔS^‡ results are discussed in terms of intermolecular associations.