Molecular and vibrational structure of anthralin.
Infrared linear dichroism spectroscopy and quantum chemical calculations

(J. Mol. Struct., 475 (1999) 131-140)

Kristine B. Andersen, Morten Langgård, and Jens Spanget-Larsen

Dept. of Life Sciences and Chemistry, Roskilde University, Denmark

The IR spectrum of the antipsoriatic drug anthralin (Dithranol, Cignolin; 1,8-dihydroxy-9(10H)-anthracenone) is complicated, with compact regions of overlapping transitions. In this communication, anthralin is investigated by IR polarization spectroscopy on samples partially aligned in stretched polyethylene. The observed Linear Dichroic (LD) absorbance curves, corresponding to absorbance measured with the electric vector of the sample beam parallel (U) and perpendicular (V) to the stretching direction, are analyzed by the reduction procedure. As a result, otherwise hidden, differently polarized transitions are resolved. The normal vibrations of anthralin are predicted with density functional theory (B3LYP/6-31G*), leading to excellent agreement with observed transitions. A very complex IR spectrum is predicted: The vibrations of the intramolecularly H-bonded carbonyl and hydroxy groups couple strongly with CC and CH motions, and the significant IR intensity associated with these vibrations is distributed over a large number of modes.