Molecular models of brown coal containing inorganic species

Molecular models of inorganic solution species, brown coal and brown coal containing inorganic species have been optimised using the computer-aided molecular package for Windows, CAChe, and the general-purpose semi-empirical molecular package MOPAC. The coal models were based on elemental composition and the experimentally measured distribution of aliphatic carbon, aromatic carbons, and oxygen functional groups. The calculated partial charges and bond polarities of the model are consistent with the hydrophilic properties of brown coal. Water molecules within the model exert the major stabilisation through H-bonds and electrostatic interactions. Calculated values of bond lengths and angles of the inorganic models were in good agreement with reported values, but variations occur because they are modelled as isolated structures. Brown coal models with NaCl, Na+, Mg2+ and Ca2+ are stabilised when the respective cations have been located in spaces that reduce steric hindrance and are surrounded by oxygen functional groups, carboxyl anions and water molecules. Coal models with octahedral Fe(III) or Ni(II) complexes may form stable structures bonded to carboxyl groups, but mono-nuclear complexes with bi-dentate carboxyl ligands usually form distorted structures. Polynuclear complexes form energetically favoured structures in coal, especially when carboxyl ligands form mono-dentate bonds to the metal centres. Reported experimental data on iron and nickel complexes, and for polymeric iron species in brown coal, are consistent with the modelling results. (c) 2005 Elsevier Ltd. All rights reserved.