High‐Pressure High‐Resolution Nuclear Magnetic Resonance III: Concentration Dependence of ΔV≠ and ΔG≠ for the Rotation of the Dimethylaminogroup in Aqueous Solutions of Some N, N‐Dimethylamides

The concentration dependence of the activation volume Δ V ≠ and the free activation enthalpy Δ G ≠ for the inversion of the dimethylamino‐group has been determined in a series of dimethylamides in aqueous solution by high pressure HRNMR ( p max = 150 M Pa). The compounds studied are the N, N‐dimethylamides of formic‐, acetic‐, propionic‐, isobutyric‐, valerianic‐ and benzoic acid. In all nonaqueous solvents Δ V ≠ is only slightly concentration dependent, Δ V ≠ being in the range of +10 ± 2 cm 3 ·mol −1 . For all compounds studied, Δ V ≠ in the dilute aqueous solutions (x ≥ 0.9) decreases very rapidly to values around + 2 cm 3 ·mol −1 . It is attempted to explain this decrease by the formation of an open hydration shell around the dimethylaminogroup and to correlate the concentration effects with the hydro‐phobic character of the amides.