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.

Detailed analysis of artefacts in the Cu(ii)–nitroxide RIDME experiments, related to orientation averaging, echo-crossing, ESEEM and background-correction is presented.

A fast, simple, and effective approach for postsynthetic modification (PSM) of amino-tagged metal− organic frameworks (MOFs) with carboxylic acids, acid anhydrides, and acid chlorides without additional solvent at elevated temperature was developed. In a comparative study optimal synthesis conditions for PSM were determined by systematic variation of the reactants in terms of reactivity and size for MIL-53(Al)-NH 2 , UiO-66(Zr)-NH 2 , and MIL-101(Al)-NH 2 . For this purpose, an acid-free digestion was prepared that allows accurate yield comparison of synthesized acid sensitive amides employing solution nuclear magnetic resonance (NMR). For MIL-53(Al)-NH 2 and UiO-66(Zr)-NH 2 best results were obtained using acid chlorides or anhydrides at 100°C. For MIL-101(Al)-NH 2 the application of acid anhydrides up to 100°C was a suitable pathway for an efficient PSM. Optimization of all reaction parameters led to a postfunctionalization yield with acetic anhydride at 100°C of 94.9 ± 0.5% for 2 h reaction time with MIL-53(Al)-NH 2 , 98 ± 3% for 10 min reaction time with UiO-66(Zr)-NH 2 , and 90 ± 1% yield for 10 min reaction time with MIL-101(Al)-NH 2 . Reactions with small carboxylic acid reactants or byproducts like acetic acid led to damage or decomposition of UiO-66(Zr)-NH 2 and MIL-101(Al)-NH 2 probably due to postsynthetic ligand exchange. However, this limitation can be overcome by using carboxylic acids with high steric demand that might decelerate or disable ligand exchange and hence prevent the MOF scaffold from damages.