Quantitative analysis of zero-field splitting parameter distributions in Gd(iii) complexes

Abstract

The magnetic properties of paramagnetic species with spin S > 1/2 are parameterized by the familiar g tensor as well as "zero-field splitting" (ZFS) terms that break the degeneracy between spin states even in the absence of a magnetic field. In this work, we determine the mean values and distributions of the ZFS parameters D and E for six Gd(iii) complexes (S = 7/2) and critically discuss the accuracy of such determination. EPR spectra of the Gd(iii) complexes were recorded in glassy frozen solutions at 10 K or below at Q-band (∼34 GHz), W-band (∼94 GHz) and G-band (240 GHz) frequencies, and simulated with two widely used models for the form of the distributions of the ZFS parameters D and E. We find that the form of the distribution of the ZFS parameter D is bimodal, consisting roughly of two Gaussians centered at D and -D with unequal amplitudes. The extracted values of D (σD) for the six complexes are, in MHz: Gd-NO3Pic, 485 ± 20 (155 ± 37); Gd-DOTA/Gd-maleimide-DOTA, -714 ± 43 (328 ± 99); iodo-(Gd-PyMTA)/MOMethynyl-(Gd-PyMTA), 1213 ± 60 (418 ± 141); Gd-TAHA, 1361 ± 69 (457 ± 178); iodo-Gd-PCTA-[12], 1861 ± 135 (467 ± 292); and Gd-PyDTTA, 1830 ± 105 (390 ± 242). The sign of D was adjusted based on the Gaussian component with larger amplitude. We relate the extracted P(D) distributions to the structure of the individual Gd(iii) complexes by fitting them to a model that superposes the contribution to the D tensor from each coordinating atom of the ligand. Using this model, we predict D, σD, and E values for several additional Gd(iii) complexes that were not measured in this work. The results of this paper may be useful as benchmarks for the verification of quantum chemical calculations of ZFS parameters, and point the way to designing Gd(iii) complexes for particular applications and estimating their magnetic properties a priori.

Improving the accuracy of Cu(ii)–nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)–nitroxide rulers

“…A further design criterion was that the ligand, which is needed for anchoring the metal ion, forms stable complexes with several metal ions of importance for metal ion-nitroxide EPR spectroscopy. [17][18][19][20][21][22]43,46,49,[84][85][86][87][88][89][90][91] To reach these goals the rod-like spacer was furnished with highly hydrophilic, branched oligo(ethyleneglycol) (PEG) chains and the ligands PyMTA 66,92,93 and TAHA 83,92,94 were selected (Fig. 1).…”

“…1). These ligands form readily 1 : 1 complexes with Cu(II) ions as well as with other paramagnetic ions, such as Mn(II) 92 and Gd(III) 94 and are of small size which becomes relevant when being used as spin labels. Cu(II)-nitroxide rulers and, more generally, compounds labeled with Cu(II) and nitroxide, reported in the literature, contain terpyridine (terpy), 17,28,29 porphyrine, 18 peptide-based binding loops, [95][96][97] as the Cu(II) binding unit.…”

SMM-ET: An SMM Evaluation Tool for the Quantitative Treatment of Ac Susceptibility and Magnetic Hysteresis Data

“…Ueff offering a choice between Gaussian, Lorentzian or Voigtian probability density functions. Indeed, it was shown by Weihe and coworkers [37], and subsequently by this author [38] and others [39] Argand or Cole-Cole plots). We can also plot them as isofrequency/isofield χʹ,χʺ(T) plots.…”

Chromium(III)-pyrazole complexes. X-Ray crystal structures, 1H NMR investigation of ligand fluxional behavior and EPR studies

“…Due to the relevance of zero-field splitting (ZFS) in the appearance of the satellite features, and due to the minor influence of ligand-field effects on the g-tensor components and in inducing gstrains (see below), only a D-strain model was considered in order to account for parameter distributions, assuming that the g-tensor components are monodisperse. The D parameter was considered as following a normal distribution according to the methodology of recent studies considering distributed spin Hamiltonian parameters [36,37]. Fitting yielded central D values of 0.15-0.16 cm -1 for values for 1 and 3 and about half that for 4 ( Table 5).…”