Proton NMR Investigation of Hindered Methyl Rotation in Paramagnetic Cr(II) Complexes

摘要

It is shown that in a paramagnetic complex, methyl rotation can act as an effective correlation time for spin relaxation through the Fermi contact interaction. For unhindered methyl groups, this mechanism can lead to methyl line narrowing. In the limit of slow methyl rotation resulting from steric hinderance, methyl rotation is capable of modulating the average hyperfine interaction (contact shift), which leads to methyl line broadening similar to that observed from chemical exchange effects. The proton linewidths of a series of trishyphen;Cr(II) chelates with methylhyphen;substitutedorthohyphen;phenanthrolines have been investigated. The linewidths of the aromatic protons in all complexes are determined by primarily the dipolar interaction. The linewidths of methyl groups which are expected to be sterically hindered are considerably broader than accountable by either dipolar or hyperfine exchange relaxation. We conclude that the anomalous methyl linewidths arise from modulation of the average hyperfine interaction by methyl rotation. The temperature dependence of the observed linewidths yields the barrier to methyl rotation.