The local structure in liquid methylamine and methylamine-water mixtures

Molecular dynamics (MD) computer simulations are carried out on liquid meth ylamine and on 10 and 30 wt % aqueous solutions of methylamine. The local t hree-dimensional structure in these liquid systems is investigated using st andard one-dimensional radial distribution functions and the fully three-di mensional measure of the local structure around the molecule, spatial distr ibution functions. Time correlation functions for the linear and angular mo tion of methylamine in the molecular coordinate system are also explored. F rom this analysis, a detailed structural picture emerges, revealing local m olecular environments in these liquids that are both complex and varied. Hy drogen bond balance is found to play a key role in determining preferred ar rangements. Strong hydrogen bonds are formed around the amino group in the pure liquid and in aqueous solution. At the same time, there is a strong hy drophobic association of methyl groups. The hydration structure in aqueous solution is found to be particularly rich, where in addition to the usual H -bonding nearest neighbors in the first hydration shell of the amino group, there are bridging water molecules and a novel type of distinctly non-H-bo nding neighbors. The hydration structure in aqueous methylamine solution di ffers substantially from that found in methanol-water liquid mixtures. (C) 2000 American Institute of Physics. [S0021-9606(00)51142-8].