The effect of solvent polarity upon rotational barriers in nikethamide

Solvent dependency of rotational barriers in ethamivan and comparison to nikethamide

Carbon-13 nuclear magnetic resonance (NMR) techniques were employed to examine the effects of solvent environment on rotational barriers in two drugs known to cause widespread stimulation in the mammalian central nervous system: ethamivan and nikethamide. Total NMR bandshape analysis was performed for the exchanging alkyl carbon resonances of these compounds as a function of temperature in six solvent systems: D2O, CH3OD, CH3CH2OD, CDCl3, C6D6 and CF3CH2OH. The rate constants for rotation about the amide bond obtained in this way were used to calculate free energy (delta G++), enthalpy (delta H++) and entropy (delta S++) of activation parameters for this process. Our results indicate that the magnitude of rotational barriers is affected markedly by (1) the size and polarity of the solvent molecules, and (2) the nature of the aromatic ring system attached to the amide grouping. Comparative interpretation of the thermodynamic parameters in light of the structures of nikethamide and ethamivan (in the various solvent systems examined) has further clarified the manner in which hydrogen bonding interactions between solvent molecules and the carbonyl oxygen of these analogues stabilize transition state conformers.

Solvent interactions with N, N-dialkylnicotinamides and their effects on rotational barriers

Carbon-13 nuclear magnetic resonance techniques were employed to examine the effects of solvent environment on rotational barriers in a series of molecules structurally-related to the analeptic, nikethamide: N,N-dimethylnicotinamide, N,N-di-n-propylnicotinamide, and 1-nicotinoyl piperidine. Total bandshape analysis was performed for the exchanging alkyl carbon resonances of these compounds as a function of temperature in four solvent systems: D2O, CH3OD, CH3CH2OD and CDCl3. The rate constants for rotation about the amide bond obtained in this way were used to calculate free energy (delta G), enthalpy (delta H) and entropy (delta S) of activation parameters for this process. Our results indicate that rotational barriers are less affected by the nature of the alkyl chain attached to the amide nitrogen than by the size and polarity of the solvent molecules. Interpretation of the thermodynamic parameters in light of both nikethamide analogue structure and solvent type has further clarified the manner in which hydrogen bonding interactions between solvent molecules and the carbonyl oxygen of these analogues stabilize transition state conformers.