Distortion from planarity in arenes produced by internal rotation of one single hydroxyl hydrogen: The case of alternariol

Abstract

Alternariol (AOH) and alternariol methyl ether (AME) are compounds toxic to farm animals and humans produced by ubiquitous fungi of the Alternaria genus. Information on chemical and physical properties of these compounds is rather scarce although their X-ray structures are known. AOH and AME are composed of three aromatic rings with three hydroxyl groups. However, the crystal structure of AOH is nonplanar whereas that of AME which differs only in one methyl group substituting one hydroxyl hydrogen is planar. By means of quantum calculations we find that the internal rotation of that single hydrogen affects the structure of AOH producing deviations from planarity near 5°. We also show that although quantum calculations predict energy differences about 5kcal/mol between planar and perpendicular conformations of that hydrogen, its rotation has no noticeable effects on electron density properties that could indicate modifications of aromaticity features. Based upon these results, we suggest an explanation to the nonplanarity of the AOH crystal solely in terms of the spatial arrangement of molecules in the crystal interconnected through a network of hydrogen bonds involving rotatable hydroxyl hydrogens that produce distortions from planarity.