Abstract

We present an EPR study of two Gd(III) complexes in aqueous solution at multiple temperatures and EPR frequencies. These two complexes, [Gd(TPATCN)] and [Gd(DOTAM)(H2O)]3+, display remarkably sharp lines (i.e. slow transverse electron spin relaxation) in comparison with all complexes studied in the past, especially at X-band (~9.08 GHz). These unprecedented spectra even show, for the first time in solution, a distinct influence of hyperfine coupling to two magnetically active Gd isotopes (155Gd 14.8%, I = 3/2, gamma = -0.8273 × 10^7 s-1 T-1 and 157Gd, 15.65%, I = 3/2, -1.0792 × 10^7 s-1 T-1). The hyperfine coupling splitting in [Gd(TPATCN)] was determined accurately for a 157Gd-enriched complex, and the value A(157Gd)/gB = 5.67 G seems to be a good estimation for most chelates of interest. Consequently, we can safely assert that neglecting the Gd isotopes in line shape studies is not a significant source of error as long as the apparent peak-to-peak width is greater than 10-20 G. This is generally the case, except at very high EPR frequencies (>150 GHz). Analyzing the spectra within the physical model of Rast et al. we find that the slow electron spin relaxation is due to a nearly zero static ZFS. We discuss some structural features that might explain this interesting electron structure.

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