Large Spin Density Research Articles

Transition states of the 3MLCT to 3MC conversion in Ru(bpy)2(phen derivative)2+ complexes

Electronic structures of ruthenium complexes [Ru(bpy)2(L)]2+ were studied by DFT calculations, where bpy is 2,2′-bipyridine and L’s are 1,10-phenanthroline (phen) derivatives. They are parent (1) and eight complexes with L=4-phenyl-phen (2), 4,7-diphenyl-phen (3), 2,9-dimethyl-4,7-diphenyl-phen (4), 2,9-dimethyl-phen (5), 3,4,7,8-tetramethyl-phen (6), 5-amino-phen (7), 5-methyl-phen (8), and 3-phenylethynyl-phen (9). Those complexes except 7, and 9 were prepared and characterized.Three triplet (called here 3MLCT, 3MC1 and 3MC2) states were obtained in all the complexes. 3MC1 and 3MC2 states were found to have the elongated Ru–N bonds and large spin densities on Ru2+. The geometry optimization of the triplet state of ruthenium complexes leads to the elongation of two trans Ru–N bonds confirming the 3MC character. The correlation between quantum yields and (3MLCT – 3MC2) energy gaps was examined and discussed. Successive conversion paths of (3MLCT→3MC1) and (3MC1→3MC2) of 1 and 5 were determined. Their activation energies were calculated to be very small.

Catalytic Mechanisms of Sulfur-Doped Graphene as Efficient Oxygen Reduction Reaction Catalysts for Fuel Cells

Density functional theory (DFT) was applied to study sulfur-doped graphene clusters as oxygen reduction reaction (ORR) cathode catalysts for fuel cells. Several sulfur-doped graphene clusters with/without Stone–Wales defects were investigated and their electronic structures, reaction free energy, transition states, and energy barriers were calculated to predict their catalytic properties. The results show that sulfur atoms could be adsorbed on the graphene surface, substitute carbon atoms at the graphene edges in the form of sulfur/sulfur oxide, or connect two graphene sheets by forming a sulfur cluster ring. These sulfur-doped graphene clusters with sulfur or sulfur oxide locating at graphene edges show electrocatalytic activity for ORR. Catalytic active sites distribute at the zigzag edge or the neighboring carbon atoms of doped sulfur oxide atoms, which possess large spin or charge density. For those being the active catalytic sites, sulfur atoms with the highest charge density take a two-electron tran...

Electronic Theory of Ultrafast Spin Dynamics in NiO

Electronic Theory of Ultrafast Spin Dynamics in NiO NiO is a good candidate for ultrafast magnetic switching because of its large spin density, antiferromagnetic order, and clearly separated intragap states. In order to detect and monitor the switching dynamics, we develop a systematic approach to study optical second harmonic generation (SHG) in NiO, both at the (001) surface and in the bulk. In our calculations NiO is modeled as a doubly embedded cluster. All intragap d-states of the bulk and the (001) surface are obtained with highly-correlational quantum chemistry and propagated in time under the influence of a static magnetic field and a laser pulse. We find that demagnetization and switching can be best achieved in a subpicosecond regime with linearly rather than circularly polarized light. We also show the importance of including an external magnetic field in order to distinguish spin-up and spin-down states and the necessity of including magnetic-dipole transitions in order to realize the process in the centrosymmetric bulk. Having already shown the effects of phonons in the SHG for the bulk NiO within the frozen-phonon approximation, and following the same trail of thoughts, we discuss the role of phonons in a fully quantized picture as a symmetrylowering mechanism in the switching scenario and investigate the electronic and lattice temperature effects.

Photolysis of cycloxydim, a cyclohexanedione oxime herbicide. Detection, characterization and reactivity of the iminyl radical

Cyclohexanedione oxime herbicides have been reported to be readily photodegraded in the environment but the reaction mechanism has never been studied in detail. Here we investigated the photolysis of cycloxydim (CD) in acetonitrile and water, solvents in which CD is present as two distinct tautomeric forms: keto form in water and enol form with an intramolecular hydrogen bond between the enolic proton and the nitrogen atom of the oxime in acetonitrile O-H···N. CD (E isomer) undergoes photoisomerization in water but not in acetonitrile. This difference is attributed to the inhibiting effect of the intramolecular hydrogen bond existing in acetonitrile. In both solvents, irradiation of CD leads to the cleavage of the N-O bond as evidenced by the imine formation. The iminyl radical could be detected in acetonitrile by nanosecond laser-flash photolysis (λ(max) = 280, 320 and 480 nm, τ ~ 100 μs). This radical is unreactive toward oxygen but readily abstracts an H atom from methanol (k = 1.8 × 10(5) M(-1) s(-1)). Quantum calculations confirm the assignment of the transient species to the iminyl radical by showing that (i) the most stable structure of the iminyl carries a large spin density on the ring carbon and on the nitrogen atoms, (ii) the enolic H atom is transferred to the nitrogen atom and (iii) the intramolecular hydrogen bond OH-N is responsible for both the iminyl long wavelength absorption and its high hydrogen abstraction reactivity.

High-Spin Ribbons and Antiferromagnetic Ordering of a MnII-Biradical-MnIIComplex

A binuclear metal coordination complex of the first thiazyl-based biradical ligand 1 is reported (1 = 4,6-bis(1,2,3,5-dithiadiazolyl)pyrimidine; hfac =1,1,1,5,5,5,-hexafluoroacetylacetonato-). The Mn(hfac)2-biradical-Mn(hfac)2 complex 2 is a rare example of a discrete, molecular species employing a neutral bridging biradical ligand. It is soluble in common organic solvents and can be easily sublimed as a crystalline solid. Complex 2 has a spin ground state of S(T) = 4 resulting from antiferromagnetic coupling between the S(birad) = 1 biradical bridging ligand and two S(Mn) = 5/2 Mn(II) ions. Electrostatic contacts between atoms with large spin density promote a ferromagnetic arrangement of the moments of neighboring complexes in ribbon-like arrays. Weak antiferromagnetic coupling between these high-spin ribbons stabilizes an ordered antiferromagnetic ground state below 4.5 K. This is an unusual example of magnetic ordering in a molecular metal-radical complex, wherein the electrostatic contacts that direct the crystal packing are also responsible for providing an efficient exchange coupling pathway between molecules.

Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis, EPR/DFT Studies and Catalytic Activity

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d z 2〉 and ∣3d x 2-y 2〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4≈6≫5).

Nonlinear Anomalous Hall Effect and Negative Magnetoresistance in a System with Random Rashba Field

We predict two spin-dependent transport phenomena in two-dimensional electron systems, which are induced by a spatially fluctuating Rashba spin-orbit interaction. When the electron gas is magnetized, the random Rashba interaction leads to the anomalous Hall effect. An example of such a system is a narrow-gap magnetic semiconductor-based symmetric quantum well. We show that the anomalous Hall conductivity reveals a strongly nonlinear dependence on the magnetization, decreasing exponentially at large spin density. We also show that electron scattering from a fluctuating Rashba field in a two-dimensional nonmagnetic electron system leads to a negative magnetoresistance arising solely due to spin-dependent effects.

CO oxidation on metal-free nitrogen-doped carbon nanotubes and the related structure–reactivity relationships

The reduction or elimination of noble metal used in carbon monoxide (CO) oxidation remains a challenge. Based on systematic research with methods using density functional theory, metal-free nitrogen-doped carbon nanotubes (NCNTs) are found to be effective catalysts for CO oxidation. The reaction route involves the direct oxidization of CO to CO2 by O2 adsorbed on the surface of NCNTs, as in CO + O2 → CO2 + O. The remaining O atom can further oxidize CO to CO2. The barrier heights of the rate-determining step range from 0.477 to 0.619 eV for different catalysts investigated in the present study. These values are quite close to those processes using noble metal catalysts. The catalytic ability of NCNTs is influenced by the tube length, diameter and number of doped N atoms. The barrier height for the rate-determining step slightly oscillates with increased tube length, and progressively increases with expanded tube diameter. The structure–reactivity relationships of NCNTs in CO oxidation are revealed. A lower negative charge of the doped N atom corresponds to more reactive NCNTs. NCNT–O2 with large spin densities on the O atom also show more powerful reactivity for CO oxidation. The present paper provides a way for the development of metal-free catalysts for CO oxidation.

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Theoretical Study of Magnetic Properties of Oxovanadium(IV) Complex Self-Assemblies with Tetradentate Schiff Base Ligands

The theoretical study of the magnetic properties of oxovanadium(IV) complex self-assemblies with tetradentate Schiff base ligands is discussed on the basis of DFT calculations. Large negative spin densities are found on the axial oxygens of the various oxovanadium(IV) complexes. The relationship between the effective exchange parameters J(ab) and the geometrical parameters for these complexes was studied by changing the position of the neighboring molecules for the purpose of clarifying the mechanism of the ferromagnetic coupling. The intermolecular ferromagnetic interaction of the oxovanadium(IV) complexes with tetradentate Schiff base ligands is significantly affected by the formation of polymeric octahedral structures in the solid state. The overlap between the 2p orbitals of the axial oxygen and the 3d orbitals of the adjacent vanadium is effective for the ferromagnetic coupling. On the other hand, the effect of overlap between the vanadium 3d(xy) orbitals is too small to lead to magnetic coupling. It was revealed that the intermolecular ferromagnetic interaction of the polynuclear oxovanadium(IV) complexes is significantly affected by the spin polarization on the axial oxygen.

Electronic Stabilization Effect of a Spin‐Delocalized Neutral Radical: Synthesis of an 8‐Cyano‐6‐oxophenalenoxyl Derivative and Quantitative Evaluation of the Electronic Spin Structure in terms of Resonance Structures

A new 2,5-di-tert-butyl-6-oxophenalenoxyl (6OPO) derivative with a cyano group at the 8-position, where a large spin density resides, has been synthesized. This neutral radical exhibits high stability in the solid state in air despite the low steric protection on the 8-position; the stability is comparable to that of a corresponding 8-tert-butylated 6OPO derivative. EPR/(1)H-ENDOR/TRIPLE (electron paramagnetic resonance/(1)H-electron-nuclear double resonance/TRIPLE) spectroscopy and cyclic voltammetry showed an extended spin delocalization on the cyano group and a significant increase in electron-accepting ability relative to that of the 8-tert-butylated 6OPO derivative. DFT calculations indicated the extension of a singly occupied molecular orbital (SOMO) onto the cyano group and the lower-lying SOMO and LUMO in comparison with those of the 8-tert-butylated 6OPO derivative, which was consistent with experimental results. Furthermore, the extended nature of π conjugation onto the cyano group was quantitatively evaluated by calculating the contributing weights of resonance structures in terms of a molecular orbital (MO)-based valence-bond (VB) method. Herein, the synthesis and physical properties of the 8-cyano-6OPO derivative are described, emphasizing that the high stability arises from the electronic effect of the cyano group. Also, the usefulness of the quantitative resonance structure analysis is shown.

ChemInform Abstract: Radical Cations of cis- and trans-1,3-Di- and 1,3,5- Trimethylcyclohexanes. Matrix Influence on Two Nearly Degenerate SOMOs

Radical cations of cis and trans isomers of 1,3-di- and 1,3,5-tri-methylcyclohexane, stabilized in various γ-irradiated solute–halocarbon matrices at 77 K and above, have been investigated by means of EPR spectroscopy. The hfs constants have been compared with those calculated using semiempirical methods, INDO (spin density) and MNDO (geometry optimizations). The SOMOs of the methyl-substituted cyclohexane cations are all similar to either of the orbitals, ag or bg, following a Jahn–Teller split of the eg orbital (HOMO) of cyclohexane. The selection of SOMO depends on matrix and isomer. The trans isomers have one axial methyl group and the cations were found to take an ag-like SOMO in all matrices used, with large spin density on the equatorial hydrogen attached to the axially substituted ring-carbon. Using CF3CCl3 as matrix the EPR signal due to the trans-1,3,5-trimethylcyclohexane cation contains (in addition) a spectrum of the bg-like SOMO superimposed. The cation of cis-1,3-dimethylcyclohexane shows contributions from both SOMOs in the matrices CF3-cC6F11, CF2ClCFCl2 and CF3CCl3, but exclusively a bg-like SOMO in cC6F12. A bg-like SOMO is observed for the cis-1,3,5-trimethylcyclohexane cation in all matrices. The abundance ratios, [ag]/[bg], for the cases where the matrix-stabilized cation shows contribution from both bg- and ag-like SOMOs simultaneously, are found to be ca. 1/4 from line shape simulations. No prominent temperature dependence of this ratio is detected.

Magnetic property and crystal structure of bis [ N-(4-chlorophenyl) salicylideneaminato] oxovanadium (IV)

Crystallographic analyses and magnetic measurements for two polymorphs, the prism form and the needle form of bis [ N-(4-chlorophenyl) salicylideneaminato] oxovanadium (IV) are carried out. The polymeric structure is observed for the needle form against the monomeric structure for the prism form. The ferromagnetic interaction is observed for the needle form, while the appreciable magnetic coupling is not detected for the prism form. The large negative spin density is found on each of the axial oxygen of these crystals. The intermolecular ferromagnetic interaction of these complexes is greatly affected by the formation of polymeric V O⋯V O⋯ structures in the solid state.

Different electronic structure of phosphonyl radical adducts of N-heterocyclic carbenes, silylenes and germylenes: EPR spectroscopic study and DFT calculations

Stable N-heterocyclic carbenes and germylenes were allowed to react with a phosphonyl radical, (i-PrO)2(O)P˙ (7), generated by photolysis of [(i-PrO)2(O)P]2Hg. The products were identified by EPR spectroscopy. An unsaturated carbene (1) and germylene (3) react with 7 at the divalent atom to give unstable radical products (τ½ = 0.2 s). A benzo-annulated carbene (4) and a saturated germylene (6) react with 7 to give more active radicals. An unsaturated (2) and a saturated silylene (5) undergo rapid reaction (in the dark) with [(i-PrO)2(O)P]2Hg to yield unusual silyl phosphites. In these cases only secondary radicals were observed. DFT (PBE0/TZVP//B3LYP/6-31+G(d)) calculations of the radical adducts of the different (C, Si, Ge) unsaturated N-heterocyclic divalent species with the phosphonyl radical show that the unpaired electron is delocalized over the five-membered ring; the spin density on the central atoms decreases in the order C, 39% > Si, 14% > Ge, 2%. These trends can be understood in terms of a zwitterionic structure of the radical adducts. The calculations of the radical adducts of 4, 5 and 6 with 7 indicate larger spin density on the central atom, 47%, 58% and 42% on C, Si, Ge, respectively.

Collectivization of Electronic Spin Distributions and Magneto-Electronic Properties of Atomic Clusters of Ga and In with As, V, and Mn

Exponential increase in nano- and sub-nanoscale research has involved investigations of the microscopic nature and origin of magnetism, including a search for the smallest atomic clusters that possess collectivized electronic spin density distributions. In this work the Hartree-Fock (HF) and restricted open-shell HF (ROHF) methods have been used to synthesize virtually (i.e., fundamental theory-based, computationally) a set of small pyramidal clusters of Ga or In atoms with As, V, and Mn atoms. The majority of the clusters are not destabilized by inclusion of vanadium or manganese atoms. The cluster charge and spin density distributions are collectivized. The In-based clusters with V or Mn atoms possess relatively large spin density values that may be of interest for spintronic materials development

Preparation, Properties, and Reduction of a Novel TCNQ-Type Thienoquinoid

[Structure: see text] TCNQ-type bithienoquinoid condensed with a alpha-dicarbonyl group was synthesized. The compound revealed a reversible reduction wave at +0.32 V vs SCE. The radical anion species was cleanly generated by a reduction with Na-Hg and characterized: lambdamax=1027 nm in vis-NIR and a large spin density of the beta-carbon atoms on the fused ring by EPR spectroscopy. The radical anion salts were isolated by a reduction with iodide and showed moderate conductivity.

NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities.

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im+ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a(2u)(pi)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40 degrees C) and thus the negative spin density at the pyrroline-alpha-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25 degrees C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct.

Clustering of molecular spins in the crystals of nitronylnitroxide and iminonitroxide triradicals based on benzene-1,3,5-triyl frameworks

We propose a strategy of crystal-engineering for heteromolecular crystals composed of organic open-shell molecules, in which two kinds of open-shell molecular entity are co-crystallized and, at the same time, distinct molecular entities with differing spin multiplicities are adjacent to each other in a crystalline solid state. Stable organic triradicals of nitronylnitroxide and iminonitroxide based on the benzene-1,3,5-triyl framework, trisNN (3) and trisIN (4), have been designed and synthesized. These molecules consist of a m-phenylene-substituted biradical and a monoradical moiety united by an ester bridge. From X-ray crystallography, these molecules have been found to possess an approximately isosceles triangular geometry and fully head-to-tail molecular packing in three dimensions has been achieved in the crystalline solid state. Thus, the co-crystallization is attained by binding the two π-conjugated radicals by σ-bonds, retaining the magnetic degree of freedom for the two distinct open-shell moieties. The alternating aggregation of the heteromolecular assembly is achieved by tuning the molecular geometry of the building-block molecules, leading to the close packing of isosceles triangular molecules in the head-to-tail fashion. Local contacts of the nitroxide groups with large spin densities between the nearest-neighbor molecules give a six-spin molecular cluster on a tetrad of the bi-, mono-, mono-, and biradical moieties in the head-to-tail stacking. Temperature dependence of magnetic susceptibility of the crystalline solids of 3 and 4 has been analyzed on the basis of their crystal structures. The six-spin cluster model has reasonably explained the susceptibility.

The17O Hyperfine Interaction in V17O(H217O)52+and Mn(H217O)62+Determined by High Field ENDOR Aided by DFT Calculations

The 17O hyperfine interaction of the water ligands and the V=O oxygen in the vanadyl aquo complex and of the water ligands in the Mn2+ aquo complex in a frozen solution were determined by W-band (95 GHz) electron-nuclear double resonance (ENDOR). Orientation selective ENDOR spectra of the vanadyl complex exhibited two distinct signals assigned to the vanadyl oxygen and the water ligands. The assignment of the signals was done based on the orientation of the principal axis system of the hyperfine interaction and through comparison with the hyperfine interaction predicted by DFT calculations. The latter showed good agreement with the experimental values thus providing clear evidence that the vanadyl oxygen is exchangeable. The interaction of the vanadyl oxygen, especially its anisotropic part, was significantly larger than that of the water oxygens due to a relatively large negative spin density on the oxygen p orbitals. The 17O hyperfine interaction of the water ligand in the Mn2+ complex was found to be similar to that of the water ligand in the vanadyl complex and was in good agreement with earlier single-crystal data. Here, due to the large thermal polarization, it was also possible to determine the absolute sign of the hyperfine coupling by selecting different EPR transitions.

A DFT study of the reactivity of OH groups in quercetin and taxifolin antioxidants: The specificity of the 3-OH site

Over the past decade, the chemical behaviour of flavonoids as antioxidants has become the subject of intense experimental research. In this paper, we use a quantum-chemical approach to shed light on the reactivity of two flavonoids, quercetin and taxifolin. We particularly focus on the 3-OH site and the role played by the 2,3-double bond in the reactivity of that site. In order to establish the most efficient theoretical methodology, different methods, either Hartree–Fock-based or derived from density functional theory, and different basis sets (from 6-311G(d) to 6-311++G(2d,p)) were tested on phenol and catechol, for which experimental bond dissociation enthalpy (BDE) values are available. It appears that (U)B3P86/6-311+G(d,p) is the most relevant method for BDE prediction of these phenolic compounds and it has, therefore, been used for an extensive study of the two flavonoids. The analysis of the theoretical BDE values, for all OH sites of quercetin and taxifolin, clearly shows the importance of the B-ring and the 3-OH group only when the 2,3-double bond is present (i.e. in quercetin). We have also considered the importance of keto–enol tautomerism (present in quercetin but not in taxifolin) to rationalize the difference in reactivity between the two compounds. Our analysis also includes the Mulliken spin density distribution for the radicals formed after H-removal on each OH site of both flavonoids. The results clearly show that the 3-OH quercetin radical possesses a large spin density on the C-2 atom, which explains the C-ring opening process observed in different redox systems, including metabolization.