title: Variable temperature and EPR frequency study of two aqueous Gd(III) complexes with unprecedented sharp lines.
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)(H(2)O)](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 ( approximately 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 ((155)Gd 14.8%, I = 3/2, gamma = -0.8273 x 10(7) s(-1) T(-1) and (157)Gd, 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 (157)Gd-enriched complex, and the value A((157)Gd)/gmu(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.
authors.affiliation: Illinois EPR Research Center and Department of Chemistry and Department of Veterinary Clinical Medicine, University of Illinois, Urbana, Illinois 61801, USA. alain.borel@ epfl.ch
authors.forename: Alain
authors.initials: A
authors.lastname: Borel
authors.forename: Hoon
authors.initials: H
authors.lastname: Kang
authors.forename: Christelle
authors.initials: C
authors.lastname: Gateau
authors.forename: Marinella
authors.initials: M
authors.lastname: Mazzanti
authors.forename: R B
authors.initials: RB
authors.lastname: Clarkson
authors.forename: R Linn
authors.initials: RL
authors.lastname: Belford
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005682:Gadolinium / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D009942:Organometallic Compounds; D014867:Water; D005682:Gadolinium