PS2Br and AsS2X (X = Br, I): similar molecules with completely different shapes – a combined experimental and theoretical study

Abstract

By the reaction of solid As₄S₄ with gaseous Br₂ at a temperature of 410 K gaseous AsSBr and AsS₂Br are formed; the reaction with gaseous I₂ at 468 K leads to the formation of AsSI, AsS₂I, As₂S₂I₂ and As₂S₃I₂. Thermodynamic data on these novel species are obtained by mass spectrometry. The experimental findings are extended and confirmed by ab initio quantum chemical calculations. Undoubtedly the molecules AsSX (X = Br, I) and As₂S₂I₂ contain As-atoms in the formal oxidation state III. Unexpectedly, in AsS₂X arsenic is not of formal oxidation state +V but +III: due to the absence of As–S π-bonding, the molecular structure of AsS₂X is arranged as a dithia-arsirane with an AsS₂ three-membered ring. For comparison with AsS₂X, PS₂Br is investigated for the first time. This high temperature species is planar with symmetry C_2v and phosphorus of the formal oxidation state +V in which the two PS π-bonds are realized due to their strength over one S–S σ-bond. As₂S₃I₂ contains a five-membered As₂S₃-ring with one S–S-bond. The ionization energies are determined and compared with those obtained from calculations; they serve as a further indication for the structures realized in these molecules.