Nuclear magnetic resonance spectra of heterocycles. Analysis of carbon-13 spectra of thioxanthenones using calculated chemical shifts, substituent constants, and PRDDO molecular-orbital calculations

Abstract

Changes in ¹³C chemical shifts of a series of 2-substituted thioxanthenones indicate that the 2-substituent causes electronic changes in the unsubstituted ring. Analysis of the ¹³C n.m.r. data using the dual-substituent parameter (d.s.p.) method and PRDDO molecular-orbital calculations shows that the 2-substituent alters, in a regular fashion, the electron density at C-6 and C-7, and at carbons adjacent to sulphur. The analysis also suggests that transmission of substituent effects from the 2-substituent to the unsubstituted ring takes place via a π-polarization mechanism operating through the heterocyclic sulphur.