Spin Relaxation Effects in Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of Nuclei with Strongly Anisotropic Hyperfine Couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation
processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale.
In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal ΔHFC−Δg
(hyperfine coupling anisotropy − g-tensor anisotropy) cross-correlation are shown to be negligibly slow.
The dominant relaxation process is transverse ΔHFC−Δg cross-correlation, which is shown to lead to an
inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently
large rotational correlation times. This inversion has recently been observed experimentally and used as
a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128,
10729−10737). The essential feature of the spin dynamics model employed here is the use of the complete
spin state space and the complete relaxation superoperator. On the basis of the results reported, we
recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are
important.