a NICS(0) and NICS(1) represent NICS values at the center of and 1 Å above the four-membered rings.b NICS(0)zz and NICS(1)zzrepresent the out-of-plane component of shielding tensor at the center of and 1 Å above the four-membered rings.
We have further analyzed the strength of the π bonding ofS2N2 ring using the EDA–NOCV analysis. Frenking and co-workers have used the energy decomposition analysis (EDA) as a powerful tool for investigating the conjugation and aromaticity in carbocyclic and heterocyclic systems.[62,63] The EDA–NOCV analysis ofS2N2 is carried out using the fragmentation pattern shown the scheme S1, where the S–N bond was fragmented homolytically in two quartet SN fragments in the frozen geometry of the molecule S2N2for this purpose (Scheme S1). Since S–N single-bond has \(\sigma\)symmetry, the \(\pi\) contributions to the total orbital interactions (ΔEπ) can be considered as a measure for the\(\pi\)-electron delocalization as compared to the respective fragments.[18] The negative interaction energies ΔEint indicates stabilizing interaction between the fragments for the formation ofS2N2 (Table 3). The major contribution to the total interaction, ΔEint, comes from the orbital term ΔEorb (62.8%), indicating a strong covalent bonding. The breakdown of ΔEorb into pairwise orbital interactions shows that the bonding interaction between the fragments comes mainly from the σ-interaction (ΔEσ, 87.5%) between the respective quartet fragments. However, the contribution for the π-interaction is significant (-73.2 kcal/mol) and contributes 10.6% to the total ΔEorb. The EDA results indicate the nature of π-interaction in π electron-rich cyclic sulfur-nitrogen systems S2N2 and S3N3 are similar.[18]