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