Variable temperature and EPR frequency study of two aqueous Gd(III) complexes with unprecedented sharp lines

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 (similar to 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 (Gd-155 14.8%, I = 3/2, gamma = -0.8273 x 10(7) s(-1) T-1 and Gd-157, 15.65%, I = 3/2, -1.0792 x 10(7) s(-1) T-1). The hyperfine coupling splitting in [Gd(TPATCN)] was determined accurately for a Gd-157-enriched complex, and the value A(Gd-157)/g mu(B) = 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.