High-Pressure High-Resolution Nuclear Magnetic Resonance III: Concentration Dependence of A F * and A G * for the Rotation of the Dimethylaminogroup in Aqueous Solutions of Some N,N-Dimethylamides

Biophysikalische Chemie I Flüssigkeiten I Hohe Drücke I Magnetische Kernresonanz The concentration dependence of the activation volume AV* and the free activation enthalpy A G * for the inversion of the dimethylaminogroup has been determined in a series of dimethylamides in aqueous solution by high pressure H R N M R (p m a x = 150 MPa) . The compounds studied are the N,N-dimethylamides of formic-, acetic-, propionic-, isobutyric-, valerianicand benzoic acid. In all nonaqueous solvents AV* is only slightly concentration dependent, AV* being in the range of +10 + 2 c m 3 • m o l " 1 . For all compounds studied, AV* in the dilute aqueous solutions (x D 2 o > 0.9) decreases very rapidly to values around +2 c m 3 m o l 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 hydrophobic character of the amides.