EPR on aqueous Gd3+ complexes and a new analysis method considering both line widths and shifts

We performed variable temperature (0–100°C), concentration and frequency (9.425, 75, 150 and 225 GHz) continuous wave electron paramagnetic resonance (EPR) measurements on three different Gd(III) compounds: [Gd(H2O)8]3+, [Gd(DOTA)(H2O)]− (DOTA: 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane) and [Gd(DTPA-BMA)(H2O)] (DTPA-BMA: 1,5-[bis(N-methylcarbamoyl)methyl]-1,3,5-tris(carboxymethyl)-1,5-diamino-3-azapentane) in aqueous solution. A simultaneous analysis of peak-to-peak widths and dynamic frequency shifts provides access to the transverse electronic relaxation, which is described using a transient zero field splitting (ZFS) mechanism with a spin rotation contribution. Our simultaneous analysis procedure involves numerical calculations using the full relaxation matrix and yields results in acceptable agreement with experimental data for reasonable values of the ZFS parameters (trace of the square of the ZFS Hamiltonian Δ2=1019–1020 s−2 depending on the complex, correlation time of the fluctuations τv298=10−11–10−10 s). We also discuss the relationship between our approach and recent developments found in the literature.