oa South African Journal of Chemistry - Modelling sulphur clusters for an understanding of ultramarine
Ultramarine pigments are aluminosilicate-based and contain sulphur-based chromophores. Self-consistent-field Hartree-Fock and Moller-Plesset second order perturbation theory were applied to determine the relative stability of S2, S2-., S22-, and S3, S3-., S32-. The singly charged species was found to be the most stable in both sets. The transition from green to blue ultramarine is thought to be the transformation of the doubly charged species to the singly charged species and is known to be exothermic. Modelling studies supported this hypothesis. The open, C2v, isomer was found to be the most stable for the S3-. molecule, which is the blue chromophore in ultramarine. The closed, D3h, geometry represents a transition state. The S4 molecule is the most likely chromophore in ultramarine red; however the specific isomer is uncertain. Under the assumption that S4 was formed by a concerted reaction between S3-. and S+., a Woodward-Hoffmann analysis of the molecular orbitals of S3-. and S+. supported the formation of the puckered square S4, pyramidal S4, double triangle S4, and gauche S4 chain isomers. The gauche S4 chain is the most likely isomer in ultramarine red.
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