Molecular dynamics simulation study of ethylene glycol, ethylenediamine, and 2-aminoethanol. 1. The local structure in pure liquids

This paper describes a comparative molecular dynamics (MD) simulation study of the local structure of pure liquid ethylene glycol (EG), ethylenediamine (ED), and 2-aminoethanol (AE), which are three well-known representatives of 1, 2-disubstituted ethanes. As an essential component of this investigation, 12 molecular models were constructed and their gas-phase characteristics were determined. The results obtained for the molecular geometries were compared with the most reliable experimental estimates, to test different force fields and molecular representations. Liquid-phase simulations were then performed on the more successful (OPLS-based) models. The heats of vaporization and self-diffusion coefficients were used as criteria for the final selection of molecular models to be used in our subsequent detailed structural analysis. The dihedral angle distributions were calculated and relative populations of rotational isomers, with respect to the central dihedral angle, were determined. It was confirmed that, for pure liquid EG and AE, the gauche conformation accounts for the major population of isomers, whereas ED exhibits a significant population of trans conformers. The analysis of radial distribution functions (RDFs), in conjunction with calculated numbers of nearest neighbors around the O and N atoms of the main functional groups, provided some structural insights into the hydrogen-bonding pattern of the systems studied. The number of strongly hydrogen-bonded neighboring groups was determined, and their possible positions were located by means of spatial distribution functions (SDFs). The possibility of three- and four-membered nearest-neighbor arrangements (comprised of two strong and, at most, two weak hydrogen bonds) found around 0 and N atoms leads to the conclusion that, in the pure liquids of EG, ED, and AE, the generalized hydrogen-bonding pattern can be described as a three-dimensional, branched network.