Study on hydration enthalpy of N-methylacetamide in water.

In order to determine the hydration enthalpy of N-methylacetamide (NMA), the enthalpy change for mixing NMA with liquid water was measured calorimetrically. NMA molecules can self-associate both in the liquid state and in aqueous solutions. The population distribution of the self-associated polymeric species in liquid NMA was evaluated by using the results of ab initio MO calculations for the monomer, dimers, and trimers of trans-NMA, while that in aqueous NMA was obtained by NMR spectroscopy. The relationship between these distributions and the mixing enthalpy was formulated on the basis of the similar energy cycle as Born-Haber type, the schematic diagram of energy cycle is given, and the hydration enthalpy of the NMA monomer was determined accurately. The enthalpy thus obtained was found to be in good agreement with that calculated by ab initio MO theory.