Hydration behavior of alkyl amines and their corresponding protonated forms. 1. Ammonia and methylamine

The structure and dynamics of hydration of ammonia/ammonium and methylamine/methylammonium systems have been studied by Car-Parrinello molecular dynamics simulation. While methylamine interacts weakly with the aqueous environment, the interaction of ammonia is found to be much stronger than expected. Both protonated species show a highly structured first solvation sphere. The solvent exchange mechanisms for all species were also investigated, along with the geometry of the hydration spheres. Comparison of these exchange mechanisms with that published for the ammonium ion shows only a minor difference. Analysis of the respective distribution functions has allowed insight into the thermodynamics of solvation for both systems. The calculated pK(a) values (9.23/10.65) correspond very closely with the published experimental values of 9.25 and 10.65.