An ab initio self-consistent field molecular-orbital study of novel stereoelectronic effects in linear R2S3N2 and cyclic XS3N2 systems

Abstract

Ab initio self-consistent field molecular-orbital calculations confirm that compounds of the type RSNSNSR (1; R = H) exist preferentially as the syn,syn isomer in which the S ⋯ S distance is significantly shorter than the non-bonded van der Waals value, an effect attributed to stereoelectronic interactions involving the terminal sulphur lone-pair orbitals. A five-membered ring valence isomer (2) is calculated to be much higher in energy and not a true minimum. A series of related compounds (4) in which the two R substituents are replaced by a bridging group (X = S, NH, or CH₂) have similarly short S ⋯ S distances. To account for this, another novel stereoelectronic interaction is proposed between specifically the equatorial orbitals on X and the adjacent sulphur lone-pair orbitals, which results in bonding electron density in the transannular S ⋯ S region, and has the greatest effect for X = S. On this basis, it is predicted that the bridging group X = NR will show an axial preference for the R substituent, and that groups such as X = CR₂ could reveal a discrimination between the axial and equatorial substituents.