The NMR spectra of the carbanions of xanthene and thioxanthene. Evidence for paratropism

The proton NMR spectra of the carbanions of xanthene [XH] − and thioxanthene [TxH] − have been recorded and interpreted. Paratropism in the central rings of [XH] − and [TxH] − is inferred from a comparison of the chemical shifts with those of the carbanion of 9,10‐dihydroanthracene [AH] − . The contributions to the chemical shifts arising from n‐electron excess charges, local dipoles and magnetic anisotropies are discussed. Numerical values for the various ring currents have been estimated by a least squares analysis of the observed chemical shifts after applying corrections for the excess charge effect. The results point to a strongly increasing paramagnetic ring current in the central ring in the order [AH] − , [TxH] − , [XH] − .