Molecular Simulation Study of Tetraalkylammonium Halides. 1. Solvation Structure and Hydrogen Bonding in Aqueous Solutions

The hydration structure in aqueous solutions of three tetraalkylammonium halides (TAAX), tetramethyl-ammonium chloride (TMACl), tetrapropylammonium bromide (TPABr), and tetrabutylammonium bromide (TBABr) at concentrations of 2.2, 1.4, and 1.0 m, respectively, is studied via molecular dynamics (MD) simulation at ambient conditions. The results are compared directly with neutron diffraction with isotopic substitution (NDIS) experimental results for the same systems. In agreement with the NDIS results, we find evidence of a small enhancement of water structure around the apolar solutes, based on water H-H radial distribution functions. Decomposition of the water-water O-H radial distribution function into hydrogen-bonded and non-hydrogen-bonded components, using the geometric criterion for hydrogen bonding of Mezei and Beveridge (1981), allowed us to calculate the hydrogen-bonded (HB) contribution to the total O-H radial distribution function, both in the bulk solution and in the first solvation shell of the cations. For the TPA and TBA cations, there is a small enhancement in the probability of forming hydrogen bonds between solvation shell water molecules, while the reverse is true for TMA. We find that the HB contribution to the total O-H coordination number remains approximately constant in going from pure water to the TAAX solutions. An analysis of the distributions of the parameters of the hydrogen-bonding criterion between pure water and water in the first solvation shells of the cations reveals that there is a slight tendency for the water in the first solvation shell to form more “ideal ” hydrogen bonds compared to pure water, particularly for the the larger cations. However, the differences in the water-water structure between TPA and TBA are negligibly small. 1