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Schematic view of the magnetic couplings according to the McConnell spin density mechanism. Arrows indicate the total spin of each molecular subunit symbolized by a square. Dark and grey TMorbitals∫ denote the positive and negative spin densities, respectively, and smaller TMorbitals∫ illustrate the spin polarization.
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The nitronyl nitroxide 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1) crystallises in the tetragonal P42(1)m space group with a=7.4050(7), c=8.649(1) A. In the crystal the molecules form layers parallel to the ab plane in which they are orthogonal to each other. In the layers there are close contacts, 2.953(2) A, between the NO groups an...
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... data by using a literature model for a 2D square- planar lattice of antiferromagnetically coupled spins S 1/2. [7] The best fit gave J À 10 cm À 1 and g 2.01. As mentioned before, the closest spin ± spin interaction is between the central carbon atom C2 and the oxygen atom in a neighbouring molecule (O1 À C2 2.953(2) ä). Both simple molecular orbital considerations and experimental observa- tions indicate that the spin density on C2 is negative and approximately 5 ± 15 % of the positive spin on O1. [8] Further- more, the consequence of the crystallographic symmetry is to make the p z orbitals (taking the xy plane as the molecular plane) on C2 and O1 orthogonal in the quantum-chemical sense (overlap 0). In terms of Figure 1, the radical 1 is clearly a case IV, and the antiferromagnetism thus readily explained. However, it has been put forward that it may be misleading to take into account only the shortest spin ± spin interaction. [9] It may thus be argued that the larger spin densities on neighbouring O and N atoms may make up for the longer distance (O1 À N1 3.251(2)) and play a role in determining the overall magnetic interaction. To prove the TMcase IV∫ mechanism we would have to grow crystals of a different phase with the radicals 1 slightly tilted, in order to break the orthogonality, but retaining the C2 À O1 interaction as the closest spin ± spin contact. The system should then revert to a TMcase III∫ ferromagnetic coupling similar to that found in, for example, 5-pyrimidinyl-nitronyl nitroxide. [10] Density functional theory calculations : An experiment such as that mentioned above could only be carried out with the help of a good portion of luck and a lot of patience. However, a second best possibility was available, density functional theory calculations. We chose a three-radical system (see Figure 5) where we first calculated the magnetic coupling for a system with the X-ray geometry to J À 25 cm À 1 , a reasonable agreement with the experimental value ( À 10 cm À 1 ) considering the differences between the model system and the real system. The two peripheral radicals were then rotated 10 ...
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Citations
... They may occur either through a direct interaction between the NÀO group [30] or a McConnell-type interaction. [31] In the present case, a McConneltype interaction is unlikely as it requires that the phenyl groups bearing the radical come close in a stacking mode, which is sterically prevented by the bulky neopenthyl substituents. We favor a direct interaction of the NÀO group in a headto-tail mode (Figure 4) that generally results in strong antiferromagnetic interaction and a diamagnetic behavior as is observed here for 10. ...
Phthalocyanines derivatives does not cease to aroused attention due to their numerous properties and applications (e.g.,sensor, PDT). This makes them unique scaffold for the design of new material. In this expectative, we were interested to develop the synthesis of an imino nitroxyde-substituted phthalocyanine via the Ullman's procedure; a challenge due to intrinsic low solubility of most phthalocyanine derivative in much solvents. To overcome this solubility problem we designed phthalocyanine with bulky neopentyl substituents in peripheral position as counterpart to the imino nitroxyde moieties. The imino nitroxyde-substituted phthalocyanine was obtained by condensation of a monoformyl-substituted phtahocyanine with 2,3-bis(hydroxylamino)-2,3-dimethylbutane in refluxing THF-MeOH (2:1) mixture in the presence of p-toluenesulfonic acid monohydrate, follow by oxidation with PbO2. Characterization was performed by electrochemistry, UV-Vis and EPR spectroscopy in solution as well as SQUID in solid state.
... This should lead to antiferromagnetic (AF) exchange interactions between RAs. If two paramagnetic species in heterospin salts mainly contact each other in the regions of unlike spin density, ferromagnetic (FM) interactions are possible187188189190191192193194.Figure 15. Spin density (ρ) distribution over the VdW surface of RA of compound 22 (left) and cation [CrCp* 2 ] + (right) from the UB3LYP/6-31+G(d) calculations. ...
... This should lead to antiferromagnetic (AF) exchange interactions between RAs. If two paramagnetic species in heterospin salts mainly contact each other in the regions of unlike spin density, ferromagnetic (FM) interactions are possible187188189190191192193194.Figure 15. Spin density (ρ) distribution over the VdW surface of RA of compound 22 (left) and cation [CrCp* 2 ] + (right) from the UB3LYP/6-31+G(d) calculations. ...
Recent progress in the design, synthesis and characterization of chalcogen-nitrogen π-heterocycles, mostly 1,2,5-chalcogenadiazoles (chalcogen: S, Se and Te) and their fused derivatives, possessing positive electron affinity is discussed together with their use in preparation of charge-transfer complexes and radical-anion salts-candidate building blocks of molecule-based electrical and magnetic functional materials.
... Since then, both ferromagnetic and antiferromagnetic behaviors have been found in the crystals of nitronyl nitroxides [13][14][15] . Previous results indicate that the magnetic exchange between neighboring nitronyl nitroxides in the solid state is, to some extent, dependent on the features of the substituent linked to the central carbon atom of the O-N-C-N-O unit of nitronyl nitroxide and the mode of crystal packing [16] . The substituent is aromatic ring in most cases and sometimes alkyl moiety. ...
... Fig.4(G) and (H)] was considered by McConnell as "unlikely", the exchange coupling is antiferromagnetic when the closest contact of spin density between molecules is of the positive-negative type. One example of this case has been found just recently [16] . For case III[ Fig.4(E) and (F)] it is indicated that ferromagnetic coupling should be due to the fact that atoms of positive spin density exchange coupled most strongly to the atoms of negative spin density in neighboring molecules. ...
A supramolecular 1D ferromagnetic system was studied experimentally as well as theoretically. Hybrid density functional theory(DFT) calculations were based on the X-ray analysis. The results of DFT calculations and McConnell mechanism have contributed to the understanding of the factors governing the exchange coupling of magnetism in the crystal packing. Both the experimental evidence and theoretical calculation indicate that spin density in 2-iodo nitronyl nitroxide(INN) radicals confirms 1D ferromagnetic chain with inter-chain antiferromagnetic interaction.
... Arrows indicate the total spin of each molecular subunit (symbolized by a square), dark 'orbitals' positive spin density, red 'orbitals'negative spin density, smaller 'orbitals' spin polarization. Adapted from Ref. [13]. The ground state of the system (A + B) cannot be described by a single configuration (left) but is a mixture of this configuration and another one (right). ...
... The explanation is that the molecules are oriented perpendicularly in such a way that the crystallographic symmetry makes the p z -orbitals (the xy-plane is the molecular plane) on the two atoms orthogonal in the quantum chemical sense (overlap = 0). In terms of Fig. 2, the cyano-nitroxide radical 4 is a highly unusual example of a case IV antiferromagnetic coupling [13]. ...
The use of small molecule density functional theory calculations to enhance and complement experimental work in the area of molecule-based magnetic materials is highlighted through a review of the author's own work. Focus is on the spin density of radicals and the consequences this have on the magnetic coupling between interacting spins in solid-state compounds. Both examples of the McConnell I mechanism, based on an analysis of possible orthogonality and sign of the interacting spins, and of the McConnell II mechanism based on charge transfer are encountered. It is concluded that such relatively small and easy calculations on the molecular 'bricks' can often help in the analysis of spin interactions in the resulting material. They can give direct indications of the McConnell I type of exchange interaction or they can give hints about possible pathways for the McConnell II mechanism. However, care should be taken not to over-interpret the results and one should be aware of the limitations of the methods.
Manganese(III) complexes were synthesized by an one-electron transfer from Mn(II) ions towards imino nitroxide radicals 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl (IMImH) in methanol. Following manganese ions pass in the +III oxidation state, while imino nitroxide radicals are irreversibly reduced as found in complexes. Depending on synthesis conditions, two complexes differing by their counter-anions were isolated as single crystals. These are [Mn(IMHIm)2(MeOH)2]ClO4.H2O (1) and [Mn(IMHIm)2(MeOH)2]PF6 (2) which crystallize in monoclinic P21/n and triclinic P 1 space groups, respectively. The two complexes show a Jahn-Teller distortion typical for Mn(III) centres and only reduced radicals are coordinated as seen from the N-O bond lengths and electroneutrality. In addition, the crystal structure analyses reveal two networks of intermolecular hydrogen bonding. One involves counter-anions, water molecules and reduced radicals, and the other one coordinated methanol molecules and imidazole moieties. These intermolecular interactions are a driving-force stabilizing the two complexes. They also suggest the tautomer to be the amino imine-oxide form after the reduction of the radical and reveal the deprotonation of the imidazole ring required for electroneutrality. This assessment is supported by single-crystal X-ray diffraction, EPR and Raman spectroscopies, as well as magnetic and electrochemical studies.
The recent progress in the synthesis of the title heterocycles exemplified by closed‐ and open‐shell 1,2,5‐chalcogenadiazoles, 1,2,3‐dichalcogenazoles and their hybrids (chalcogens: S, Se, and Te), and in actual or potential applications of these compounds in materials science and biomedicine, are discussed. Synthetic importance of the chalcogen exchange, both direct and indirect, is emphasized, as well as significance of the heavier chalcogen derivatives in the design and synthesis of smart molecular materials.
Six different diradical systems based on azulene coupler and nitronyl nitroxide radical moieties have been studied theoretically. Among them, three ferromagnetic diradicals are chosen for further calculation to assess their usability in different biological applications such as two-photon absorption photodynamic therapy (TPA-PDT) and magnetic reso nance imaging contrast agents (MRICA). The TPA cross-section values have been calcu lated to judge their efficiency as TPA-PDT photosensitizing agents. Calculated values of TPA cross-section lie in the range of commonly used photosensitizers for PDT. It is seen that the ferromagnetic diradicals can act as very good MRI contrast agents due to their high magnetic anisotropy characteristics. In connection to the magnetic anisotropy, the zero-field splitting (ZFS) parameters are also computed using density functional theory (DFT) based methods. It is found that the proposed diradicals can show good relaxation behaviour and consequently act as efficient MRI contrast agents. Thus, one can anticipate the multifunctional biological activity of these diradicals.
Two nitronyl nitroxide radical NIT 2PhMe 2 (1), 2, 5-dimethyl-l, 4-bis(nitronyl nitroxide) benzene and NITPhNMe 3 · I · H 2O (2), hydrous iodide m-N, N' , N" -trimethyl-benzenaminium nitronyl nitroxide, have been synthesized and structurally characterized. The compound 1 crystallized in monoclinic, space group P2 1/c with a=0.6137(4) nm, b = 1.766 2(12) nm, c = 1.180 9(6) nm, ß=117.66(3)°, V=1.1337(13) nm 3, M r=416.52, Z=2 and F (000)=456. The compound 2 also crystallized in monoclinic, space group P2 1/c with a=1.143 47 (11) nm, b = 1.119 09 (11) nm, c = 1.527 38 (15) nm, ß=101.766 (2)°, V=1.9134 (3) nm 3, M r=436.31, Z=4 and F (000)=884. Through the weak hydrogen bond, 1 and 2 form three-demensional supramolecular networks, respectively. CCDC: 749328, 1; 749329, 2.
