A study of the 220 MHz proton resonance spectra for a series of biologically important acridine-type heterocycles

The high resolution proton magnetic resonance spectra of acridine, phenazine, phenoxazine and phenothiazine have been measured at 220 MHz in various solvents and at several temperatures. Spectra for all four molecules were analyzed iteratively and good agreement was obtained between observed and calculated spectra. Significant differences are noted between the coupling constants and chemical shifts of acridine and phenazine on the one hand and phenoxazine and phenothiazine on the other. These differences are attributed to a nonplanar heterocyclic ring conformation for phenoxazine and phenothiazine in solution. Moreover, it is concluded from the temperature measurements that the heterocyclic rings of phenoxazine and phenothiazine are conformationally flexible in the range covered. Possible mechanisms are suggested for the chemical shift and coupling constant changes.