Experimental spin density in nitroxides

Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent

The study reveals heteronuclear-thermal mixing – a novel mechanism of dynamic nuclear polarization in a system with ¹H and ¹⁹F nuclei.

When combined X-ray and polarized neutron diffraction data challenge high-level calculations: spin-resolved electron density of an organic radical

Joint refinement of X-ray and polarized neutron diffraction data has been carried out in order to determine charge and spin density distributions simultaneously in the nitronyl nitroxide (NN) free radical Nit(SMe)Ph. For comparison purposes, density functional theory (DFT) and complete active-space self-consistent field (CASSCF) theoretical calculations were also performed. Experimentally derived charge and spin densities show significant differences between the two NO groups of the NN function that are not observed from DFT theoretical calculations. On the contrary, CASSCF calculations exhibit the same fine details as observed in spin-resolved joint refinement and a clear asymmetry between the two NO groups.

Determination of the relevant magnetic interactions in low-dimensional molecular materials: the fundamental role of single crystal high frequency EPR

A new one-dimensional copper(II) complex with formula [Cu(hfac)(2)(N(3)TEMPO)](n) (hfac = hexafluoroacetylacetonate and N(3)TEMPO = 4-azido-2,2,6,6-tetramethylpiperidine-1-oxyl) has been synthesized and investigated by X-ray crystallography, magnetometry and multifrequency single crystal EPR. The system crystallizes in the P1 space group with two non equivalent copper(II) ions in the unit cell, the two nitroxide radicals being coordinated to Cu(1) in axial positions. The copper(II) ions are bridged by N(3)TEMPO radicals resulting in a zig-zag chain structure. The magnetic susceptibility data were at first satisfactorily modeled assuming an alternating spin chain along the monodimensional covalent skeleton, with a ferromagnetic interaction between Cu(1) and the nitroxide moieties and a weaker antiferromagnetic interaction between these and Cu(2) (J(1) = -13.8 cm(-1), J(2) = +2.4 cm(-1)). However, single crystal EPR studies performed at the X- and W-band clearly demonstrate that the observed magnetic monodimensional character of the complex is actually due to the intermolecular contacts involving N(3)TEMPO ligands. This prompted us to fit the magnetic data using a consistent model, pointing out the fundamental role of single crystal EPR data in defining a correct model to describe the magnetic properties of molecular low dimensional systems.

Ferromagnetic Interaction in an Asymmetric End-to-End Azido Double-Bridged Copper(II) Dinuclear Complex: A Combined Structure, Magnetic, Polarized Neutron Diffraction and Theoretical Study

A new end-to-end azido double-bridged copper(II) complex [Cu(2)L(2)(N(3))2] (1) was synthesized and characterized (L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). Despite the rather long Cu-Cu distance (5.105(1) A), the magnetic interaction is ferromagnetic with J= +16 cm(-1) (H=-JS(1)S(2)), a value that has been confirmed by DFT and high-level correlated ab initio calculations. The spin distribution was studied by using the results from polarized neutron diffraction. This is the first such study on an end-to-end system. The experimental spin density was found to be localized mainly on the copper(II) ions, with a small degree of delocalization on the ligand (L) and terminal azido nitrogens. There was zero delocalization on the central nitrogen, in agreement with DFT calculations. Such a picture corresponds to an important contribution of the d(x2-y2) orbital and a small population of the d(z2) orbital, in agreement with our calculations. Based on a correlated wavefunction analysis, the ferromagnetic behavior results from a dominant double spin polarization contribution and vanishingly small ionic forms.

Structure and magnetic properties of nitroxide molecular crystals by density functional calculations employing periodic boundary conditions

The structure and magnetic properties of one-dimensional chains of representative nitroxides have been studied by a density functional model employing periodic boundary conditions. The optimized geometries are in better agreement with experiments than those obtained from optimizations of model dimeric systems. The spin populations and isotropic hyperfine couplings compare well with the values measured by polarized neutron and electron spin resonance experiments. Magnetic couplings computed by the broken symmetry approach reproduce the ferro- or antiferromagnetic behavior of different nitroxides derived from experiments. These results point out the reliability of the computational model and the significant tuning of all the magnetic properties by intermolecular hydrogen bridges.