Density functional calculations of electronic structure, geometric structure and stability for molecular manganese sulfide clusters

Abstract

Questions about the geometrical structures, electronic structures and stabilities of [Mn _x S _y ] ^- clusters observed in the gas phase have been approached using density functional calculations of electronic structure and energy, optimised geometrical structure, and electron affinity. Results are presented for 61 postulated isomers for 23 different compositions ranging in size from [MnS ₂ ] to [Mn ₁₅ S ₁₅ ], and general structural types and principles are identified. Structures based on stacks of Mn ₃ S ₃ triangles are thermodynamically favourable, as are regular Mn _x polyhedra with (µ ₃ -S) caps. Structures with doubly-bridging S are frequently favourable, but there is no evidence that (µ ₄ -S) capping confers extra stability. Trigonal MnS ₃ local co-ordination is desirable. Bonding between Mn atoms occurs throughout, at distances from ca. 2 Å in the smallest underco-ordinated structures up to ca. 2.5 Å. Electronically these clusters are characterised by a band of closely-spaced bonding molecular orbitals, composed mainly of Mn 3d atomic orbitals, at and below the Fermi level. The HOMO–LUMO gaps are generally very small, and low-lying electronic states with associated magnetic and optical properties are expected. The electron affinities generally increase with cluster size, up to ca. 3.5 eV.