2H NMR and X-ray diffraction studies of methyl rotation in crystals of ortho-methyldibenzocycloalkanones

We have used 2H NMR lineshape analyses and single crystal X-ray diffraction (XRD) to investigate the effects of molecular structure and crystalline environment on the rotational dynamics of methyl groups in four aromatic cycloalkanones. These include two methyl-substituted anthrones, one anthraquinone and one dibenzosuberone, which are known to undergo excited state H-atom tunneling from the ortho-methyl group to the carbonyl oxygen. With experiments conducted between 100 and 300 K, samples 1,4-dimethylanthrone (DMAT) and 1,4-dimethylanthraquinone (DMAQ) were shown to enter the intermediate exchange regime (krot∼<107s-1) at ca. 120 K while samples of 1,4,10,10-tetramethylanthrone (TMAT) and 1,4-dimethyldibenzosuberone (DMDBS) remained in the fast exchange regime even at ca. 100 K. Single crystal XRD analyses suggest that high intramolecular hindrance is avoided by molecular distortions, and that intermolecular contacts play an important role.