Orbital symmetry in sulphur pigmentary chromophores

Abstract

Pictorial molecular orbital theory, in a qualitative approach, is used in order to identify possible structures of the red chromophore in ultramarine red pigments. The known blue S₃^−⋅ molecule is assumed to be the starting reagent in a concerted reaction to form the S₄ molecule. Different proposed structures for the S₄ species are tested. Applying the frontier orbital symmetry rules of Woodward and Hoffmann eliminates the tetrahedral S₄ structure as a possibility.