Isomers of 3-methylthio-1,5-diarylformazans and their interconversion in solution

Abstract

¹ H and ¹³C n.m.r. and i.r. spectroscopic studies, including ¹⁵N-labelling, on solutions of ArN:N·C(SR):N·NHAr (Ar = Ph, p^–, or o-tolyl; R = CH₃ or CD₃) taken in conjunction with established X-ray crystal structures of pure isomers have made it possible to establish the structural causes responsible for temporal changes in the visible spectra. Relative to the formal CN double and C–N single bonds the yellow isomer is shown to have a single anti,s-trans structure whereas the pink isomer comprises syn,s-trans species in rapid tautomeric equilibrium.