The 13 C NMR chemical shifts of 3H-phenothiazin-3-one (1) and eleven of its methyl derivatives are reported. The effect of including sulfur 3d orbitals is discussed in relation to the 13 C chemical shifts. The chemical shifts of 1 can be explained in terms of both the successive polarization of the long π-conjugated framework promoted by the carbonyl group and the contribution of the sulfur d-orbitals. The 13 C chemical shifts gave a better ...