Correlation of dihedral angles with 13C chemical shifts of quaternary carbon atoms of some phenothiazine derivatives

A direct relationship between the ¹³C nmr chemical shifts of the quaternary carbon atoms of the central ring of phenothiazine and the dihedral angle of the respective derivatives has been observed. This correlation allows for the useful and quick estimation of the dihedral angles of novel phenothiazines from readily obtainable ¹³C nmr solution measurements. In addition, the change in the dihedral angle appears to be directly related to the S-C_4a-C_10a bond angle; however, the C_4a-S-C_5a bond angle is not affected by changes in the dihedral angle. This indicates that the “flattening” of the phenothiazine tricyclic ring system is compensated for by the vertical displacement of the sulfur atom, by changes in hybridization of N₁₀, and by other angular distortions of the middle ring.