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 t o 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 t w o R substituents are replaced by a bridging group (X = S, NH, or CH,) have similarly short S S distances. To account for this, another novel stereoelectronic interaction is proposed between specifically the equatorial orbitals o n 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 = CR, could reveal a discrimination between the axial and equatorial substituents.Both organic ' and inorganic 2 , 3 sulphur-nitrogen systems are

Does a Stabilising Interaction Favouring theZ,ZConfiguration of S‐NSN‐S Systems Exist?

“…The suggested interaction between the sulfur atoms has been studied by combining structural and energetic parameters but no definitive proof of its existence was found. Investigation of the bonding in the diselena derivative of 1, 1,5-diphenyl-2,4-diaza-1,5-diselena-3-thia-2,3-pentadiene, Ph-Se-N=S=N-Se-Ph (2) (see Figure 1), in combination with variable-temperature 77 Se NMR in solution strengthens the idea that the proposed [15,16] stabilising interaction might not exist and that packing forces are the main cause of the observed Z,Z configuration in the solid.…”

“…Based on the results of our calculations the equilibrium conformer composition at 293 K is 89 % C 2 -Z,Z-anti,anti, 10% C 1 -Z,E-anti,anti and 1 % C 1 -Z,E-anti,syn. Importantly Rzepas previous work [15,16] does not mention the possibility of other low-energy conformers besides the Z,Z conformer.…”

“…It is interesting to note that in a number of the more recent publications [7,[10][11][12] regarding the conformation of similar systems, the authors consistently refer to Rzepas work [15,16] for the explanation of the S···S interaction determining the conformational preference, even though this information is now over a decade old and the conclusions were based on the results of low-level quantum chemical methods. Additionally, Leitch suggested in his original paper on the crystal structure of 1,5-diphenyl-2,4-diaza-1,3,5-trithia-2,3-pentadiene from 1973, that "the outer sulfur atoms open out the systems rather than pull it together"; [13] his conclusion was based on a careful interpretation of the valence angles in the -S-N=S=N-S-fragment and almost suggests a repulsive effect.…”

“…[13,14] This configuration was later rationalised by Rzepa et al in terms of MO theory: according to the data of MNDO and HF/6-31G* calculations on model compounds such as H-S-N=S= N-S-H, the Z,Z configuration was said to be stabilised by the interaction of the n(N) orbitals with the n[S II ] orbitals. [15,16] This s,s interaction should logically lead to the presence of bonding electron density in the area between the two S II atoms due to which the S II ···S II distance is shortened to about 3.20 , [13,14] while the sum of the van der…”