Solvation of a macrocyclic compound in a water–ammonia mixture

Monte Carlo simulations

Abstract

The structural properties of a system consisting of one 1,4,7,10-tetraazacyclododecane (cyclen) ligand in 18.45 mol% aqueous ammonia have been studied, based on the Monte Carlo method. Pair potential functions have been developed using ab initio MO calculations. The simulations were carried out for 35 million configurations, in order to examine the dynamic characteristics of the solution. A clear picture of the cyclen solvation sphere, consisting of two layers, has been observed. The inner solvation shell contains six water molecules, two of them (the ratio for having one and two water molecules is 1:2.3, leading to an average number of 1.7) located close to the cyclen molecule with one N–H bond pointed towards the cyclen cavity, and the other four water molecules lie near NH functional groups of the cyclen and bond to the two innermost water molecules via hydrogen bonds, but are not bound directly to the NH groups. Apart from numerous water molecules, an inner part of the outer solvation shell accumulates three ammonia molecules, held in place by hydrogen bonds with the six molecules of the inner shell, while the outer part of this shell shows ideal characteristics of the bulk water–ammonia solvent.