Formation of As 8 dimers in molecular solid-state arsenic

Abstract

Molecular yellow arsenic (y-As) consists of tetrahedral As 4 molecules that may be packed in various ways. All y-As modifications, both disordered and crystalline, are metastable and undergo irreversible transitions (polymerization) under action of heat and light, which cause a change in the nature of bonding in the molecules. Polymerization of y-As leads to the formation of amorphous arsenic (a-As) possessing a continuous random network structure. DTA studies show that polymerization is an activated exothermic process. The value of its enthalpy agrees satisfactorily with an estimate of the excess energy of strained ‘banana-shaped’ bonds in an As 4 molecule. Quantum chemical calculations, applying the semiempirical CNDO/BW approach, show that at the initial polymerization stage we have a formation of As 8 dimers due to cleavage of one bond in the tetrahedral As 4 molecule. Simulation of this process shows that formation of a stable As 8 cluster, possessing either D 2h or O h symmetry, may take place if the dimerization reaction path possesses D 2d symmetry. In this case a pair of approaching molecules is positioned in staggered ‘face-to-face’ configuration, which may be considered as a conformation with a six-membered chair-shaped ring dominating in the structure of polymerized a-As. The most favourable is found to be a molecular As 8 dimer with eclipsed ‘edge-to-edge’ configuration (D 2h symmetry).