title: Solvation structure and dynamics of ammonium (NH4+) in liquid ammonia studied by HF/MM and B3LYP/MM molecular dynamics simulations.
abstract: The characteristics of NH4+ solvated in liquid ammonia have been investigated by means of combined HF/MM and B3LYP/MM molecular dynamics simulations, in which the ion and its surrounding ammonia molecules were treated by HF and B3LYP methods, respectively, using the D95* basis set. For both HF/MM and B3LYP/MM simulations, it is observed that four nearest-neighbor ammonia molecules directly hydrogen-bonded to each of the ammonium hydrogen atoms, forming a well-defined tetrahedral cage structure of the NH4+ solvate. Nevertheless, the solvation shell of NH4+ is rather flexible, in which several possible species of solvated NH4+ exist, ranging from 4- to 7-fold and from 3- to 6-fold coordinated complexes for the HF/MM and B3LYP/MM simulations, respectively. In terms of the dynamical details, i.e., the self-diffusion coefficients and the mean residence times of ammonia molecules surrounding the ion, the B3LYP/MM simulation shows slower dynamics of the solvated NH4+ when compared with the HF/MM results. With regard to the reported tendency of density functional theory (DFT) methods to predict overly rigid ion solvations as well as hydrogen bonds that are too short, the ab initio HF formalism has been demonstrated to be more reliable for providing a detailed description of this solvated ion.
authors.affiliation: School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand. anan_tongraar@yahoo.com
authors.forename: Anan
authors.initials: A
authors.lastname: Tongraar
authors.forename: Supot
authors.initials: S
authors.lastname: Hannongbua