Béatrice Gillon

Light-induced phase separation in the [Fe(ptz)6] (BF4)2 spin-crossover single crystal

We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [ Fe(ptz)6] (BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.

Interchain interactions and three-dimensional magnetic ordering in Mn(II)Cu(II) chain compounds; crystal structure and metamagnetic properties of MnCu(pbaOH)(H2O)3·2H2O, with pbaOH = 2-hydroxo-1,3-propylenebis (oxamato)

Abstract Single crystals of the novel phase MnCu(pbaOH)(H2O)3·2H2O (A) have been obtained; pbaOH stands for 2-hydroxy-1,3-propylenebis(oxamato). A crystallizes in the monoclinic system, space group P21/c,with a = 21.212(4) ,b = 5.033(1), c = 14.246(2) A , β = 100.72(2)° and Z = 4. The structure consits of oxamato-bridged Mn(II)Cu(II) bimetallic chains running along the a-axis direction. The shortest metal-metal separations between adjacent chains related by, respectively, the unit cell translation along the b-axis direction and the glide translation along the c-axis direction involve metal ions of the same type, in contrast with what was found in the previously described phase MnCu(pbaOH)(H2O)3 (B). According to our interpretation of the three-dimensional (3-D) magnetic effects in bimetallic chain compounds, the topology of the spin carriers in A should favor a 3-D antiferromagnetic ordering, while B exhibits a 3-D ferromagnetic ordering. The study of the temperature dependence of the magnetic susceptibility as well as the temperature and field dependences of the magnetization have confirmed this prediction. The antiferromagnetic ordering occurs at 2.4 K. A magnetic field of 0.9 kOe is sufficient to overcome the weak interchain interactions, so that the compound may be described as a metamagnet built from ferrimagnetic chains.

First spin-resolved electron distributions in crystals from combined polarized neutron and X-ray diffraction experiments

A method to map spin-resolved electron distribution from combined polarized neutron and X-ray diffraction is described and applied for the first time to a molecular magnet and it is shown that spin up density is 5% more contracted than spin down density.

Spin distribution in the diphenylpicrylhydrazyl (DPPH) radical measured by neutron diffraction

A complete experimental determination of the spin density has been performed on a DPPH : C6H6 single crystal using the polarized neutron diffraction technique. A parametric description of the spin density has been used, this being necessary for non-centrosymmetrical crystals. A large amount of the spin density (61 per cent) remains localized on the central hydrazyl group, in accordance with magnetic resonance data, with a slight excess of Nβ over Nα. The remaining part of the spin density (39 per cent) is delocalized on the three aromatic rings of DPPH, the amount of spin transferred on to a ring depending on the twist angle of the ring with respect to the hydrazyl backbone. On every ring the spin density changes its sign from carbon to carbon atom, in accordance with magnetic resonance and quantum theory. The spin populations measured for the carbons of the rings compare well with the hyperfine coupling constants of the adjacent protons, which permits a direct experimental verification of McConnells fir...

Structure, Magnetic Properties, Polarized Neutron Diffraction, and Theoretical Study of a Copper(II) Cubane

The paper reports the synthesis, X-ray and neutron diffraction crystal structures, magnetic properties, high field-high frequency EPR (HF-EPR), spin density and theoretical description of the tetranuclear CuII complex [Cu4L4] with cubane-like structure (LH2=1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one). The simulation of the magnetic behavior gives a predominant ferromagnetic interaction J1 (+30.5 cm(-1)) and a weak antiferromagnetic interaction J2 (-5.5 cm(-1)), which correspond to short and long Cu-Cu distances, respectively, as evidence from the crystal structure [see formulate in text]. It is in agreement with DFT calculations and with the saturation magnetization value of an S=2 ground spin state. HF-EPR measurements at low temperatures (5 to 30 K) provide evidence for a negative axial zero-field splitting parameter D (-0.25+/-0.01 cm(-1)) plus a small rhombic term E (0.025+/-0.001 cm(-1), E/D = 0.1). The experimental spin distribution from polarized neutron diffraction is mainly located in the basal plane of the CuII ion with a distortion of yz-type for one CuII ion. Delocalization on the ligand (L) is observed but to a smaller extent than expected from DFT calculations.

Experimental determination of spin-dependent electron density by joint refinement of X-ray and polarized neutron diffraction data

New crystallographic tools were developed to access a more precise description of the spin-dependent electron density of magnetic crystals. The method combines experimental information coming from high-resolution X-ray diffraction (XRD) and polarized neutron diffraction (PND) in a unified model. A new algorithm that allows for a simultaneous refinement of the charge- and spin-density parameters against XRD and PND data is described. The resulting software MOLLYNX is based on the well known Hansen-Coppens multipolar model, and makes it possible to differentiate the electron spins. This algorithm is validated and demonstrated with a molecular crystal formed by a bimetallic chain, MnCu(pba)(H(2)O)(3)·2H(2)O, for which XRD and PND data are available. The joint refinement provides a more detailed description of the spin density than the refinement from PND data alone.

Theoretical spin density in nitroxides: The effect of alkyl substitutions

The influence of alkyl substitution on the spin density distribution in nitroxide radicals is studied by performing ab initio UHF calculations on a series of radicals from H2NO to C5H10NO. Comparison of spin populations and spin density maps in the series shows a net spin migration from oxygen to nitrogen when hydrogens are replaced by methyl groups. This result does not depend on the size of the alkyl substituents. The substitution effect explains in part the discrepancy between the theoretical spin density in H2NO and the experimental result obtained by polarized neutron diffraction on tanol suberate biradical C8H12O4((CH3)4C5H5NO)2.

Local spin density theory of free radicals: Nitroxides

Local spin density theory is applied to the nitroxides H2NO, (CH3)2NO, (CH3)2NO⋅⋅⋅HOCH3. Spin density is localized on the NO group. In dimethylnitroxide the spin density is partitioned one to one between N and O. Substitution of the hydrogen atoms in dihydrogen nitroxide by methyl groups as well as addition of a hydrogen bridge result in an enhancement of the spin density on nitrogen. The hydrogen bridged compound models the spin density distribution in tanol (2,2,6,6‐tetrametyl‐4‐piperidinol‐1‐oxyl) which has been investigated by polarized neutron diffraction.

Spin density in a triazole-nitronyl-nitroxide radical presenting linear ferromagnetic interactions: role of hydrogen bonding

Abstract The compound 2-{3-[4-methyl-1,2,4-triazolyl]}-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide, abbreviated as Metrz-Nit, crystallizes in the non-centrosymmetric space group P2 1 2 1 2 1 . The investigation of the magnetic properties has revealed the occurrence of intermolecular ferromagnetic interactions. The crystal structure has been refined by neutron diffraction at 11 K. The spin density distribution has been determined from polarized neutron diffraction experiments carried out at 1.5 K under a magnetic field of 5 T. As expected, the main contributions of the spin distribution have been observed in the 2p π orbitals of the nitrogen and oxygen atoms of the two NO groups, and a significant negative spin population has been detected on the sp 2 carbon atom of the nitronyl nitroxide moiety. The spin distribution is slightly dissymmetrical, so that the sp 3 carbon atoms in α-position of the nitro nitrogen atoms carry spin populations of opposite signs. Concerning the triazole ring, the main spin population, of negative sign, has been found on the nitrogen atom occupying the 3-position. The carbon atom of the methyl group attached to the 4-position has been also found to carry a significant negative spin population. The spin populations on the hydrogen atoms have been determined. These experimental data have been compared to the results of LSD calculations performed on an isolated molecule. The role of intermolecular interactions in the spin distribution has been discussed.

Polarized Neutron Diffraction as a Tool for Mapping Molecular Magnetic Anisotropy: Local Susceptibility Tensors in CoII Complexes

Polarized neutron diffraction (PND) experiments were carried out at low temperature to characterize with high precision the local magnetic anisotropy in two paramagnetic high-spin cobalt(II) complexes, namely [Co(II) (dmf)6 ](BPh4 )2 (1) and [Co(II) 2 (sym-hmp)2 ](BPh4 )2 (2), in which dmf=N,N-dimethylformamide; sym-hmp=2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate, and BPh4 (-) =tetraphenylborate. This allowed a unique and direct determination of the local magnetic susceptibility tensor on each individual Co(II) site. In compound 1, this approach reveals the correlation between the single-ion easy magnetization direction and a trigonal elongation axis of the Co(II) coordination octahedron. In exchange-coupled dimer 2, the determination of the individual Co(II) magnetic susceptibility tensors provides a clear outlook of how the local magnetic properties on both Co(II) sites deviate from the single-ion behavior because of antiferromagnetic exchange coupling.