C-13 NMR AND CRYSTALLOGRAPHIC STUDY OF THYMOLSULFONEPHTHALEIN

Up to now the correct structure of the undissociated thymolsulfonephth alein has not been determined unequivocally and three possible resonan ce structures have been proposed so far: a zwitterionic one, a quinoid one, and a lactonoid one. The present C-13 NMR and X-ray crystallogra phic study resolves this problem and it can be explicitly shown that t he correct structure is zwitterionic with some delocalization of posit ive charge into the phenol rings. The correct assignment of the comple tely dissociated blue basic form is also included and permits the conc lusion that a fourfold-charged resonance structure is as probable as a quinoid one.