Theoretical studies on the conformational preferences of 1,3-diazabuta-1,3-dienes

Abstract

Ab initio MO and density functional calculations have been performed to study the conformational preferences for 1,3-diazabuta-1,3-dienes. Complete optimisations using B3LYP/6-31G* level on HNCH–NCH2 indicate that the conformational preferences are different for the E and Z isomers, E preferring s-cisoid arrangement and Z preferring s-trans arrangement. Natural Bond Order analysis has been carried out on the various conformations of 1,3-diazabuta-1,3-dienes to understand the electron delocalisations and the origin of conformational preferences in these systems. To understand the effect of substituents on conformations, complete optimisations have been carried out with Me, SH, and NH2 substituents at C-2 and C-4 positions. The results indicate that the s-cisoid structure is more stable in most cases and in some cases the s-trans even does not exist. A systematic change in the N1–C2–N3–C4 torsional angle is observed as a function of the electron delocalisation in these systems. The s-cisoid structure tends to be closer to s-cis in the E isomers and such an effect has not been observed in the Z isomers. The preference for the s-trans arrangement and the ϕ values in E-s-cisoid structure are larger in solvent media.