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 Gd 3+ 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 T 1e, which much more closely agree with experimental values. Additionally, the LODEPR T 1e values for two standard chelates, [Gd(DOTA)(H 2 O)] - and [Gd(DTPA)(H 2 O)] 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.