Internal rotation of mutually interacting methyl groups:: A 13C NMR study

The overall and intramolecular rotational diffusion behavior of 1,3,7,10-tetramethylbenzo[c]cinnoline was determined from longitudinal C-13 NMR relaxation and {H-1}-C-13 NOE measurements in dilute chloroform solution, The four methyl groups in this compound represent three different situations of sterical hindrance, One pair of methyl groups is in close mutual sterical contact, forming the ends of an open six-membered ring, Assuming completely anisotropic overall molecular tumbling combined with a 120 degrees jump model for the internal methyl rotations the jump rates of methyl groups were evaluated and compared to earlier results on different sterically hindered compounds, in particular with respect to a potential cogwheel-like intermethyl interaction, To characterize intermethyl interactions in different sterical situations, a new guage-the "methyl interaction volume"-is introduced. Implications for correlated rotational diffusion of methyl groups are discussed. (C) 1997 Academic Press.