DENSITY-FUNCTIONAL CALCULATIONS OF ELECTRONIC-STRUCTURE, GEOMETRIC STRUCTURE AND STABILITY FOR MOLECULAR MANGANESE SULFIDE CLUSTERS

Questions about the geometrical structures, electronic structures and stabilities of [MnxSy](-) clusters observed in the gas phase have been approached using density functional calculations of electronic struct ure and energy, optimised geometrical structure, and electron affinity Results are presented for 61 postulated isomers for 23 different comp ositions ranging in size from [MnS2] to [Mn15S15], and general structu ral types and principles are identified. Structures based on stacks of Mn3S3 triangles are thermodynamically favourable, as are regular Mn-x polyhedra with (mu(3)-S) caps. Structures with doubly-bridging S are frequently favourable, but there is no evidence that (mu(4)-S) capping confers extra stability. Trigonal MnS3 local co-ordination is desirab le. Bonding between Mn atoms occurs throughout, at distances from cn. 2 Angstrom in the smallest underco-ordinated structures up to ca. 2.5 Angstrom. Electronically these clusters are characterised by a band of closely-spaced bonding molecular orbitals, composed mainly of Mn 3d a tomic orbitals, at and below the Fermi level. The HOMO-LUMO gaps are g enerally very small, and low-lying electronic states with associated m agnetic and optical properties are expected. The electron affinities g enerally increase with cluster size, up to ca. 35 eV.