T1e in Four Gd3+ Chelates: LODEPR Measurements and Models for Electron Spin Relaxation
n a previous paper, we reported the first longitudinally detected electron paramagnetic resonance (LODEPR) measurements of fast T1e values in aqueous solutions of two Gd3+ chelates, and we included predicted values for these relaxation times, based on zero field splitting (ZFS) parameters derived from multifrequency EPR data on the two systems [Atsarkin, V. A.; Demidov, V. V.; Vasneva, G. A.; Odintsov, B. M.; Belford, R. L.; Radüchel, B.; Clarkson, R. B. J. Phys. Chem. 2001, 105, 9323-9327]. The model used in that analysis was derived from the original work of Hudson and Lewis and did not explicitly consider the static and dynamic parts of the ZFS. A more comprehensive model for relaxation in these S = 7/2 systems has been recently published. Here, we reexamine the multifrequency data in light of this new model, recalculate the ZFS parameters, and calculate new predictions for T1e, which much more closely agree with experimental values. Additionally, the LODEPR T1e values for two standard chelates, [Gd(DOTA)(H2O)]- and [Gd(DTPA)(H2O)]2- are reported, together with predicted relaxation times. Both the importance of the new model and the validity of the LODEPR values are strengthened by these data.