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 13C NMR relaxation and 1H-13C 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 gauge-the "methyl interaction volume"-is introduced. Implications for correlated rotational diffusion of methyl groups are discussed. Copyright 1997 Academic Press. Copyright 1997Academic Press
Wimmer
R
SINTEF UNIMED MR-Center, Trondheim, N-7034, Norway.
Muller
N