Mercè Deumal

Structure–Magnetism Relationships in α-Nitronyl Nitroxide Radicals

Statistical analysis of the packing of all known α-nitronyl nitroxide crystals (see diagram) with dominant ferro- and antiferromagnetic properties shows that there are no differences in the geometrical distribution of the NO⋅⋅⋅ON, C(sp3)−H⋅⋅⋅O−N and C(sp3)−H⋅⋅⋅O−N contacts in these two subsets. The magnetic properties are a result of the relative orientation of the molecules, that is, of their packing patterns (or synthons) and not of the geometry of an individual contact.

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin-canted antiferromagnet beta-p-NCC(6)F(4)CNSSN* at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first-principles bottom-up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=-33.8 cm(-1)), which generates a three-dimensional diamond-like magnetic topology within the crystal. The computed magnetic susceptibility, chi(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J(AB) radical-radical magnetic interactions change: the J(d1) radical-radical interaction becomes even more antiferromagnetic (-43.2 cm(-1)) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm(-1)). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The chi(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J(AB) contributions explains the change in the slope of the residual magnetic susceptibility in the low-temperature region.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.

Synthesis, Structure, Magnetic Behavior, and Theoretical Analysis of Diazine-Bridged Magnetic Ladders: Cu(quinoxoline)X2 and Cu(2,3-dimethylpyrazine)X2 (X = Cl, Br)

The synthesis, structure, and magnetic behavior of the complexes Cu(qnx)Br(2) (1), Cu(2,3-dmpz)Br(2) (2), Cu(qnx)Cl(2) (3), and Cu(2,3-dmpz)Cl(2) (4) (qnx = quinoxaline, dmpz = dimethylpyrazine) are described. Both X-ray structural data and fitting of the magnetic data suggest that the compounds are well-described as strong-rung, two-leg magnetic ladders with J(rung) ranging from -30 K to -37 K, and J(rail) ranging from -14 K to -24 K. An unexpected decrease in the exchange constant for J(rail) (through the diazine ligand) is observed when the halide ion is changed from bromide to chloride, along with a small decrease in the magnetic exchange through the halide ion. Theoretical calculations on 2 and 4 via a first-principles bottom-up approach confirmed the description of the complexes as two-leg magnetic ladders. Furthermore, the calculations provide an explanation for the experimentally observed change in the value of the magnetic exchange through the dmpz ligand when the halide ion is changed from bromide to chloride, and for the very small change observed in the exchange through the different halide ions themselves via a combination of changes in geometry, bond lengths, and anion volume.

The mechanism of the magnetic interaction in the β phase of the p-(nitro)phenyl nitronyl nitroxide (KAXHAS). A bottom-up study using only ab initio data

Abstract The magnetism of the β phase of p -(nitro)phenyl nitronyl nitroxide (KAXHAS) crystal has been studied using a recently developed theoretical approach ‘J. Phys. Chem. A 106 (2002) 1299’. This approach is a bottom-up study based on the evaluation of the magnetic interaction between all pairs of radicals ( J AB ), which allows the definition of the magnetic structure of the crystal. With only such knowledge, one solves an algebraic Heisenberg Hamiltonian on a properly chosen finite subset of the magnetic structure and then computes the magnetic susceptibility χ ( T ) and/or heat capacity C p ( T ) curves for the crystal. This method is applied here to the KAXHAS crystal. The theoretical χ ( T ) and C p ( T ) results are in very good agreement with the available experimental data. This theoretical methodology is first reviewed here on physical terms, and then used to rationalize the bulk ferromagnetic behavior of KAXHAS in terms of its corresponding microscopic J AB pair interactions.

The Mechanism of the Through-Space Magnetic Interactions in Purely Organic Molecular Magnets

The mechanism of the intermolecular magnetic (or through-space) interactions found in purely organic molecular crystals is reviewed, using results from the ab initio studies of model systems, and from the statistical analysis of the packing of nitronyl nitroxide crystals presenting dominant ferro or antiferromagnetic interactions. First of all, the foundations of the McConnell-I mechanism (more properly called a proposal) are reviewed from a rigorous theoretical point of view. It is shown that this proposal lacks a rigorous foundation and works in some prototype systems due to error compensations associated to the high symmetry of the model systems employed to evaluate such a mechanism. It will be shown how the McConnell-I mechanism fails in rationalizing the magnetic properties of well known nitronyl nitroxide crystals. We will also show the existence of pitfalls in many of the magneto-structural correlations employed today. One example of an erroneous correlation is that which associates the magnetic character of a nitronyl nitroxide dimer with the relative orientation of the ONCNO groups in these two molecules. Consequently, new magneto-structural relationships are needed based on unbiased assumptions. For such a purpose, we need to have solid data on the properties of the through-space magnetic interactions at the microscopic level. We will review in the last sections the current knowledge about these properties, obtained from ab initio computations and crystal packing studies.

Bulk ferromagnetism in nitronyl nitroxide crystals: a first principles bottom-up comparative study of four bulk nitronyl nitroxide ferromagnets (KAXHAS, YOMYII, LICMIT and YUJNEW)

Searching for general trends that could help to design new bulk (3D) ferromagnets, we have carried out a first-principles bottom-up theoretical study of four bulk ferromagnets of the nitronyl nitroxide (NN) family of crystals: β-p-(nitro)phenyl-NN (KAXHAS), α-o-(hydroxy) phenyl-NN (YOMYII), α-2,5-(dihydroxy)phenyl-NN (LICMIT) and p-(methylthio) phenyl-NN (YUJNEW). This first-principles bottom-up theoretical study connects, in a rigorous form, the microscopic magnetic pair interactions (J AB) with the macroscopic magnetic properties (e.g. magnetic susceptibility or heat capacity curves). The microscopic magnetic pair interactions are computed from the crystal structure using DFT methods. The network of non-negligible J AB magnetic pair interactions between AB radicals provides the magnetic topology of the crystal.  Using the room-temperature crystal structure (the only experimentally available) for KAXHAS, YOMYII and LICMIT, we found a 3D magnetic topology. However, although the computed magnetic susceptibility χ( T  ) curves reproduce, in all three cases, the ferromagnetic experimental χ( T  ) data, not all the J AB pair interactions are ferromagnetic. For YUJNEW, there are three sets of crystallographic coordinates (room temperature, 114 and 10K) and its magnetic topology depends on the temperature: at room temperature, there is a 2D magnetic topology but, using the 114 and 10K crystal structures, the magnetic topology becomes 3D. The computed magnetic susceptibility curve only reproduces the experimental data when using the magnetic topology computed at 10K (at 114K, there is only a qualitative agreement). The dependence of the J AB magnetic pair interactions on the geometry of the A–B radical–radical pair is also analysed. Our results show that small changes in the geometry induce changes in the sign of the interaction, a fact that can be only explained by considering that the nature of the interaction is a cooperative effect that depends on all atoms of the radical.

Theoretical analysis of the packing and polimorphism of molecular crystals using quantum mechanical methods : The packing of the 2-hydro nitronyl nitroxide

Abstract The crystal packing of molecular crystals can be rationalized using quantum mechanical ab initio methods. This work describes the principles in which this approach is based, applying the basic principles to the 2-hydro nitronyl nitroxides. Two forms of packing analysis using ab initio methods are described. The first one is based on the use of the molecular electrostatic maps for the molecules. The second one uses the information about the strength of the molecular interactions present in the crystal provided by accurate ab initio computations from model systems. Using this information it is possible to rationalize the packing in terms of primary, secondary and so on structures. For the 2-hydro nitronyl nitroxide radical the C(sp2)-H···O contacts should be the driving force behind the primary structure of the packing, while the C(sp3)-H···O contacts should be responsible of the secondary structure. The N-O···O-N contacts are found to be repulsive.

A general study of the spin population of α-nitronyl nitroxide radicals: radicals with crystals presenting dominant ferro or antiferromagnetic behavior

The atomic spin population of all the α-nitronyl nitroxide radicals whose crystals present dominant ferro or antiferromagnetic properties is computed at the B3LYP/6-31G(d) level, searching for changes and similarities in their spin distribution. The quality of the computed values is previously tested on the α-phenyl-nitronyl nitroxide radical using various basis sets. It is found that computed atomic spin population presents small changes with the basis set, when the computation is done using the B3LYP density functional. The B3LYP/6-31G(d) atomic spin population of the radicals presenting dominant ferromagnetic interactions is practically independent of the system. The same is true for the antiferromagnetic systems. The two sets present also similar values. This fact is of great help when analyzing the magnetic properties of the crystals of these radicals.

Through space magnetic exchange in tetrabromocuprates: theoretical considerations

The temperature dependence of the magnetic susceptibility of a two-dimensional Heisenberg antiferromagnet [bis(2-amino-5-chloropyridinium) tetrabromocuprate] is calculated via ab initio electronic structure methods. Individual pair-wise exchange values are used to determine the magnetic structure of the crystal and then to compute its magnetic susceptibility using a recently proposed method (J. Phys. Chem. A 106 (2002) 1299). Comparison of the calculated susceptibility to the experimental values shows excellent agreement.