T1e in four Gd3+ chelates:: LODEPR measurements and models for electron spin relaxation

In a previous paper, we reported the first longitudinally detected electron paramagnetic resonance (LODEPR) measurements of fast T-1e 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.; Raduchel, 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 T-1e which much more closely agree with experimental values. Additionally, the LODEPR T-1e 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.