On the assignment of ¹³C resonances in unsymmetrical ortho‐disubstituted benzene rings. ¹³C N.m.r. spectrum of Piroxicam

Characteristic differences in multiplet hyperfine structure observed in coupled 13 C n.m.r. spectra of symmetrical o ‐disubstituted benzene rings are explained on general grounds. Contrary to earlier proposals, the observation of this effect in unsymmetrical systems is not a suitable basis for corresponding 13 C n.m.r. assignments unless the proton spectrum has been sufficiently analyzed. The 13 C spectrum of piroxicam, an antiinflammatory agent, is assigned and some interesting relaxation properties are noted.