A general study of the spin population of α-nitronyl nitroxide radicals: radicals with crystals presenting dominant ferro or antiferromagnetic behavior (original) (raw)
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Structure - Magnetism relationships in α-nitronyl nitroxide radicals
The crystal packing of α-nitronyl nitroxide radicals that have dominant ferromagnetic or antiferromagnetic interactions is analyzed in order to test if there are characteristic orientations of their functional groups that can be associated with these magnetic interactions. From a large crystalline structural database of compounds containing α-nitronyl nitroxide radical units (143 structures), 23 representative cases with dominant intermolecular ferromagnetic interactions, and 24 cases exhibiting dominant antiferromagnetic interactions were selected. The spatial distribution of the N-O ⋯ O-N, C(sp3)-H ⋯ ON, and C(sp2)-H ⋯ ON contacts whose distance is smaller than 10 Å was analyzed, with special emphasis on the 0-5 À region for the N-O ⋯ O-N contacts and 0-3.8 Å for the C-H ⋯ O-N contacts. No correspondence is found between the presence of intermolecular ferro- or antiferromagnetic interactions and the geometry of any of the previous isolated contacts. Therefore, there is a need to c...
Polyhedron, 2009
The effects of acceptor-donor interactions in thienyl substituted benzimidazole-nitronyl nitroxides (TBNN) on the absorption spectroscopy, spin density distribution, magnetic behavior, and crystallographic packing were explored through spectroscopy, computation, and characterization of structure and magnetic properties in the crystalline phase. The electronic spectra of the radicals exhibit a strong broad absorption in the NIR (k max $ 1000 nm) that exhibits solvatochromism consistent with charge transfer between the thienyl (donor) and benzonitronyl nitroxide (acceptor) dyads. Computational analysis allowed assignment of the transition as a HOMO-SOMO transition (TD-DFT UB3LYP/6-31G**). The TBNN radicals form highly disordered slipped p-stacks in the solid state that give rise to antiferromagnetic interactions consistent with 1D chain interactions. The magnetic behavior was well-fit to a Bonner-Fisher model to give exchange parameters of J = À2 to À10 cm À1 depending on substitution. The weak exchange parameters are attributed to the degree of solid-state disorder, and the observed properties can be rationalized by the effects of substitution on the electronic structure and topology of the radicals.
Structure-Magnetism Relationships inα-Nitronyl Nitroxide Radicals
Chemistry-a European Journal, 1999
oxidized species of the compounds gave rise to only one sub-gap electronic transition near 1.25 (pentamer) and 1.15 eV (hexamer). More detailed experimental conditions of electronic spectral measurements can be found elsewhere [9b,14].
1992
The spin density distribution in the indolinonic nitroxide radical 1,2-dihydro-2-methyl-2-phenyl-3H-indole-3-oxo-l-oxyl has been determined by polarized neutron diffraction measurements. An analytical description of the spin density is obtained from the experimental data using the multipole expansion method. The results show that the unpaired electron is not confined to the N-O group but is delocalised over the indolinic moiety.
2007
Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (4) were synthesized and crystallized. Single crystal X-ray studies of 3 and 4 show the formation of supramolecular head-to-tail one-dimensional H-bonded (Ns O‚‚‚HsCtCs type) chain structures with O‚‚‚C distances of 3.181 and 3.155 Å, respectively. High-resolution isotropic liquid state (c e 10 -4 M) electron spin resonance (ESR) spectroscopy studies of the well-isolated molecules confirmed the intramolecular spin polarization from the nitronylnitroxide radical group (acceptor, NsO) to the acetylenic proton (donor, HsCtCs), which is mediated by the π-conjugated backbone. The influence of the heteroatom (pyridine nitrogen-14 N) in the ESR hyperfine splitting pattern was clearly seen in radical 4, with an additional number of lines appearing in the M I ) 0 line of the total five-line spectrum. The solution state paramagnetic 1 H NMR investigation of radicals 2-(4-trimethylsilylethynyl-1-phenyl)-4,4,5,5tetramethylimidazoline-1-oxyl (1) and 3 clearly support the intramolecular spin density propagation from the acceptor to the donor groups as well as the proton hyperfine coupling (hfc) values of the conjugated backbone determined by ESR studies. Bulk magnetic investigations of the polycrystalline chain compounds (3 and 4) in the temperature range 300 down to 4.5 K display antiferromagnetic exchange interactions at very low temperature. The experimental bulk magnetic data were found to be fit by using the dimer model with exchange coupling 2J/K B values of -3.10 ( 1.16 and -8.00 ( 3.83 K for 3 and 4, respectively, as well as by adopting the Heisenberg-chain model with 2J/K B values of -0.62 ( 0.02 and -2.21 ( 0.13 K for 3 and 4, respectively.
Experimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex
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.
Chemical Physics Letters, 1998
The zero-field muon-spin rotationrrelaxation technique has been used in the direct observation of spontaneous magnetic Ž. order below the Curie temperature of T ; 500 mK in 2-hydroxyphenyl-a-nitronyl nitroxide 2-HOPNN and two successive C Ž. magnetic transitions in 4-hydroxyphenyl-a-nitronyl nitroxide 4-HOPNN at T f 700 and T ; 100 mK. The different C1 C2 position of the hydroxyl group in these positional isomeric compounds causes different crystal packings and magnetic dimensionalities. The results reveal different magnetic transitions in these two a-nitronyl nitroxides radicals suggesting a 2D-3D magnetic crossover in 4-HOPNN.
International Journal of Quantum Chemistry, 1996
We describe by approximate MO calculation a number of species for which high spin states are either the ground state or lie very low in energy. These are models for the spin coupling in projected ferromagnetic organic materials. The theory guiding their construction is based on planar conjugated systems, while experimentally realized systems are often far from planarity. We can by appeal to steric decoupling explain the failure of the prediction that methoxy subsitution on metaphenylene-coupled nitroxides should stabilize the triplet. However, we find in general that the qualitative rules derived from discussion of planar systems are robust; drastic departures from planarity are required before they lose their value. 0 1996 John Wiley & Sons, Inc. netic (i.e., spin-aligning) coupling between sites. A number of simple theoretical methods and principles have helped to guide and rationalize experimental design. These include the Huckel analysis of Longuet-Higgins [2] and the valence-bond treatment of Ovchinnikov [3], which permit the prediction of possible high-spin states in planar conjugated hydrocarbon species. The topology of rr systems is the foundation of all these analyses, and lntroduction he possibility of producing ferrmagnetic or-T ganic materials has attracted great interest [l]. The preparation of these materials requires sites with high local spin densities and ferromag-
ACS omega, 2018
Two stable nitronyl nitroxide free radicals {R 1 = 4′-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R 2 = 2-(2′-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac) 2 (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal−organic complexes Co-(hfac) 2 (NNPhOMe) 2 , 1; Co(hfac) 2 (NNT) 2 •(H 2 O), 2; Mn(hfac) 2 (NNPhOMe)• x(C 7 H 16), 3; and Mn(hfac) 2 (NNT) 2 , 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P1̅ , space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P2 1 /c space group in which the N−O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/K B) are derived using isotropic spin Hamiltonian H = −2J∑(S metal S radical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be −328, −1.25, −248, and −256 K, for the 1, 2, 3, and 4 metal−organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal−radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.