The electron-nucleus dipolar coupling in slow rotating systems. 2. The effect of g anisotropy and hyperfine coupling when and

Abstract

Abstract An equation has been derived for the field dependence of nuclear T1−1 enhancement due to dipolar coupling with an electron of spin 1 2 which takes into consideration g anisotropy under slow rotation conditions. In analogy with the fast rotating systems, g anisotropy effects are found to be relatively small. An equation has also been derived for the S = 1 2 system coupled with a nuclear spin vector I = 3 2 under isotropic coupling conditions. The effect of coupling on the nuclear T1−1 values is dramatic for coupling constant A values larger than ℏτs−1, where τs is the electronic relaxation time. We have derived an analytical solution also for the case A∥ ≠ 0, A⊥ = 0 and I = 3 2 , while for actual cases with A∥ ≠ 0, A⊥ ≠ 0 numerical solutions are given. They are shown to account for previously unexplained experimental data. As an example, the fitting of the field dependence of water proton T1−1 values in solutions of the copper(II)-containing enzyme superoxide dismutase is presented. The experimental curves, including their temperature dependence, are nicely reproduced by using the actual ESR parameters. It is also stressed that for magnetic field values > 5 MHz, i.e., gβeH ⪢ A∥, A⊥, the simple Solomon equation is adequate for fitting the data.