[Rotational Isomers of Diphenhydramine].

[Rotational Isomers of Diphenhydramine]. Yakugaku Zasshi. 2017 Dec 27;: Authors: Ebitani M, Ebitani T Abstract   Diphenhydramine(DP), an antihistaminic agent, may become colored and daker or more fluorescent during storage. Herein, we spectroscopically examined the causes of this phenomenon under various DP storage conditions and durations. The infrared vibration-rotation spectrum shows multiple Gauche (G )-type conformers with different intramolecular n→π* interaction strengths. The splitting pattern of the dimethylamino group protons in the 1H-NMR spectrum indicates that DP is mainly in the G-type with a small portion in the Trans(T)-type. The correlation between the red-shifted peak intensity in the UV·VIS absorbance spectrum and the coloring progression indicates a decreased intramolecular n→π* interaction of the G-type under elevated temperature during storage. Enhanced fluorescence detected in the Excitation·Fluorescence spectrum demonstrates G-type(quenching) to T-type(fluorescent) conformation conversion, which is due to activated internal rotation of the dimethylamino group under elevated storage temperature and electronic excitation in the phenyl groups under light irradiation during storage. A signal detected in the ESR spectrum corresponds to the G-type charge transfer (CT) structure wherein part of the nonbonding electron pair on the N atom is intramolecularly redistributed to the phenyl groups. The CT structur...
Source: Yakugaku Zasshi : Journal of the Pharmaceutical Society of Japan - Category: Drugs & Pharmacology Authors: Tags: Yakugaku Zasshi Source Type: research