Physicochemical Properties and Structural Study of 1,2-Dithiolylium-4-Olates and Isothiazolium-4-Olates

The physicochemical properties of two series of mesoionic compounds, 1,2-dithiolylium-4-olates 3 and isothiazolium-4-olates 4 have been examined. The negative solvatochromy shown by 3 and 4 in u.v.-vis. is discussed. The unusual i.r. band of 3g (1488 cm−1 in KBr) and 4 (1530 cm−1 in KBr) is identified by its large solvent shift. The experimental dipole moments of a few compounds of series 3 and 4 are compared with the theoretical values corresponding to the canonical structures A, B and C. Carbon-13 NMR chemical shift data are given for a number of 3 compounds and compared with data from related 1,2-dithiolylium salts. In connection with the geometry of compound 3a, determined by X-ray diffraction, the CO group is found to be intermediate between a carbonyl and a single bond C[sbnd]O. The discussion deals with the comparison of the CO group in compounds 3 and in tropone 10 or in (4H) thiopyran-4-one 11 and with the comparison of the experimental data with a theoretical electronic structure published by another author.