Proton n.m.r. spectra of furo[3,2-c] and furo[2,3-c]thiapyrylium cations,compared with their thieno homologues. Correlations between chemical shifts and charge densities from CNDO/2 and PPP calculations

Significant variations of thiapyrylium chemical shifts, by comparison with their thienohomologues, are shown by ¹H n.m.r. spectra of furo[3,2-c]thiapyrylium and furo[2,3-c]thiapyrylium perchlorates. The observed changes can be ascribed to a different distribution of the electron charge (mainly caused by participation of the 3d orbitals of sulphur in the thienohomologues) and to a different contribution of the ring current; this is shown by MO calculations performed by the CNDO/2 and the coupled Hartree–Fock methods, respectively.