J. Schweizer

Experimental Spin Density in a Purely Organic Free Radical: Visualisation of the Ferromagnetic Exchange Pathway in p‐(Methylthio)phenyl Nitronyl Nitroxide, Nit(SMe)Ph

Polarisedneutrondiffractionstudies of the purely organic 2-(4-methylthiophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, Nit(SMe)Ph, ferromagnet (Tc = 0.2 K) have enabled the distribution of the global spin density to be determined (see picture), and a pathway for the magnetic interaction as well as a possible mechanism are proposed. The experimental results are compared with those from DFT calculations.

An imino nitroxide free radical: Experimental and theoretical spin density and electronic structure

Abstract An imino nitroxide, the 2-(3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (m-NPIM), a potential building block for molecular-based magnets, was investigated by conventional and polarized neutron single crystal diffraction. Several methods, including the recently developed Default Model Maximum of Entropy (DMME) technique, were used to reconstruct the spin density distribution in the radical. Quantitative (atomic spin populations) and qualitative (shape of the spin density around the nuclei) results were obtained. Our data provide the basis for the discussion of the electronic configuration of imino nitroxides. The nodes of the singly occupied molecular orbital (SOMO) were directly observed. The spin polarization effect, which gives rise to negative spin densities of several atomic sites in the radical and is liable for the spin density transfer to the nitrophenyl group, is discussed. The spin distribution is compared to that previously observed experimentally in nitronyl nitroxides and with theoretical predictions obtained by density functional first principle calculations.

Neutron diffraction observation of a ferromagnetic phase transition in a purely organic crystal

Abstract The ferromagnetic phase transition was observed by zero field neutron diffraction for the first time in a purely organic material, the β form of the para- nitrophenyl nitronyl nitroxide p-NPNN. The direction of spontaneous magnetization along the 19A crystallographic axis proposed by previous zero field muon spin rotation experiments is consistent with our results. The observed increase of Bragg intensity below TC is in agreement with the value expected from previous polarized neutron diffraction studies.

Spin Densities in Nitronyl Nitroxide Free Radicals

Abstract The spin density distributions of three nitronyl nitroxide (NIT) free radicals have been determined by polarized neutron scattering. For such a determination the neutron is an ideal tool as it is a magnetic probe which feels the spin density at the microscopic level, and also as it is an itinerant probe which sees the spin density everywhere throughout the molecule.

Spin density in the free radical NitPy(CCH)

The nitronyl nitroxide compound NitPy(CCH) (space group P21/2) crystallizes in chains with molecules connected via an hydrogen bond. Each molecule carries a delocalized, S=12, unpaired electron. The intrachain magnetic interactions are positive. We have investigated the spin density distribution of this radical by polarized neutron diffraction. Both the spin density population on the hydrogen, and the depletion of spin density on the related oxygen, give evidence that the hydrogen bond is involved in the path of the ferromagnetic interactions

Spin density in an organic biradical ferromagnetic crystal

Abstract The Dupeyredioxyl nitroxide biradical is one of the few purely organic ferromagnets known ( T C = 1.48 K). The concern of this paper is a polarized neutron diffraction determination of the spin density distribution in this compound. The results are discussed in comparison to those obtained by first principle theoretical calculations and to previous experimental studies of other nitroxide radicals.

The spin density in an imino nitroxide free radical: A polarized-neutron study

Abstract Imino nitroxide free radicals are potential building blocks for molecular-based magnets. They are purely organic and carry a spin ½ which is delocalized over several atoms of the molecule. We have determined the spin density of one of them, the 2-(3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl(m-NPIM),by polarized-neutron diffraction on a single crystal. The delocalization of the singly occupied molecular orbital (SOMO) and the spin polarization effects, which give rise to negative spin densities, are observed directly.

The Experimental Spin Density of Two Nitrophenyl Nitroxides: A Nitronyl Nitroxide and an Imino Nitroxide

Abstract We have determined, by polarized neutron diffraction, the spin density of two nitrophenyl nitroxides: the para-nitrophenyl nitronyl nitroxide (p-NPNN), which is a ferromagnet at low temperature, and the meta-nitrophenyl imino nitroxide (m-NPIN), which exhibits antiferromagnetic coupling. Several methods were used to reconstruct the spin density distribution, including the maximum entropy method. Our data provide the basis for a better knowledge of the electronic configuration. The nodes of the SOMO are directly observed and the spin polarization effects which give rise to a negative spin density on several atomic sites is discussed.

Neutron diffraction studies of the first purely organic ferromagnetic crystal

Abstract The first purely organic ferromagnetic material, the β form of the para-nitrophenyl nitronyl nitroxide p-NPNN, was studied by single crystal neutron diffraction. A zero field unpolarized neutron experiment performed in the temperature range 50–800 mK provides the first direct evidence of spontaneous ferromagnetic bulk ordering below T C = 0.67 K. The data obtained in the polarized neutron experiment was used to reconstruct the spin density distribution and explain the mechanisms of magnetic interaction.

Experimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex

Abstract The association of phenylboronic acid (no unpaired electron) with the free radical phenyl nitronyl nitroxide (S = 1/2) constitutes an inter-heteromolecular hydrogen bonding system presenting ferromagnetic intermolecular interactions. We have investigated its spin density distribution in order to visualize the pathway of these magnetic interactions. The spin density of this complex was measured by polarized neutron diffraction. The data were treated using both direct and indirect methods. As in the isolated PNN, the main part of the spin density is located on the O-N-C-N-O fragment of the PNN radical. But, with the PNNB, the global spin density distribution give evidences that the phenylboronic acid constitutes a spin transmission path between PNN radicals via hydrogen bonds. The experimental results are compared to those obtained by density functional theory calculations.