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

• Alternariol is a three-ring arene with three rotatable hydroxyl hydrogens.
• The crystal structure of alternariol is nonplanar.
• The arene planar structure is distorted by internal rotation of one single hydrogen.
• Energy varies noticeably along the rotation but aromaticity features do not change.
• Nonplanar crystal structure arises from distortion caused by intermolecular H-bonds.

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 5 kcal/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.

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