Covalent Spin Research Articles

Synthesis and Application of Stable Nitroxide Free Radicals Fused with Carbocycles and Heterocycles

Stable nitroxide free radicals have traditionally been associated with 2,2,6,6- tetramethylpiperidine-1-oxyl (TEMPO) or its 4-substituted derivatives as relatively inexpensive and readily accessible compounds with limited possibilities for further chemical modification. Over the past two decades, there has been a resurgence of interest in stable free radicals with proper functionalization tuned for various applications. The objective of this review is to present recent results with synthetic methodologies to achieve stable nitroxide free radicals fused with aromatic carbocycles and heterocycles. There are two main approaches for accessing stable nitroxide free radicals fused with arenes, e.g., isoindoline- like nitroxides: further functionalization and oxidation of phthalimide or inventive functionalization of pyrroline nitroxide key compounds. The latter also offers the constructions of versatile heterocyclic scaffolds (furan, pyrrole, thiophene, 1,2-thiazole, selenophene, pyrazole, pyrimidine, pyridine, pyridazine, 1,5-benzothiazepine) that are fused with pyrroline or tetrahydropyridine nitroxide rings. The possible applications of these new stable nitroxide free radicals, such as covalent spin labels and noncovalent spin probes of proteins and nucleic acids, profluorescent probes, building blocks for construction of dual active drugs and electroactive materials, and substances for controlled free radical polymerization, are discussed.

Low Temperature Neutron Diffraction Studies in [Mn3(suc)2(ina)2]n: An Homometallic Molecular 3D Ferrimagnet

Neutron diffraction techniques have been used to determine the low temperature crystal structure and to shed light on the magnetic behavior of the [Mn(3)(suc)(2)(ina)(2)](n) (suc = succinate and ina = isonicotinate) complex. The ferromagnetic signal observed below T(c) ≈ 5 K in this compound is due to a noncompensation of homometallic spins in the 3D framework. The Mn(II) magnetic moments obtained from neutron diffraction refinements are slightly lower than those observed for isolated Mn(II) ions; this can be due to covalent spin delocalization or geometrical magnetic fluctuations. A small discrepancy between the value of the magnetic moments of each Mn(II) site is also observed [Mn(1) 4.1(2) μ(B) and the Mn(2) 3.9(1) μ(B)]. These differences between the theoretical and observed manganese magnetic moments are not unexpected in this large spin metal complex, and qualitatively reasonable given the synergistic interaction between the metal ions through oxo-bridge. The competition among different interactions, principally those covalent through organic ligands and dipolar interaction, drive to a final 3D ferrimagnetic order.

Neutron scattering evidence for isolated spin-12ladders in(C5D12N)2CuBr4

Inelastic neutron scattering was used to determine the spin Hamiltonian for the singlet ground-state system of fully deuterated BPCB, ${({\text{C}}_{5}{\text{D}}_{12}\text{N})}_{2}{\text{CuBr}}_{4}$. A two-leg spin-1/2 ladder model, with ${J}_{\ensuremath{\perp}}=(1.09\ifmmode\pm\else\textpm\fi{}0.01)\text{ }\text{meV}$ and ${J}_{\ensuremath{\parallel}}=(0.296\ifmmode\pm\else\textpm\fi{}0.005)\text{ }\text{meV}$, accurately describes the data. The experimental limit on the effective interladder exchange constant is $|{J}_{\text{int}}^{\text{eff}}|\ensuremath{\lesssim}0.006\text{ }\text{meV}$, and the limit on total diagonal, intraladder exchange is $|{J}_{F}+{J}_{{F}^{\ensuremath{'}}}|\ensuremath{\le}0.1\text{ }\text{meV}$. Including the effects of copper to bromide covalent spin transfer on the magnetic form factor, the experimental ratios of intraladder bond energies are consistent with the predictions of continuous unitary transformation.

F+ tunable laser activity and interaction of atomic halogens (F, Cl and Br) at the low coordinated surface sites of SrOAb initio and DFT calculations.

The twofold potential of F+ color centers at the low coordinated surfaces of SrO for providing tunable laser activity and adsorption properties for atomic halogens (F, Cl and Br) has been investigated using ab initio electronic structure calculations. SrO clusters of variable sizes were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to F+ were allowed to relax to equilibrium. Based on Stokes shifted optical transition bands and horizontal shifts along the configuration coordinate diagrams, the F+ laser activity was found to decrease as the coordination number of the surface ions decreases from 5 (flat) to 4 (edge) to 3 (corner). An attempt has been made to explain this result in terms of Madelung potentials and optical-optical conversion efficiencies. All relaxed excited states are deep below the conduction bands of the perfect ground states, implying that F+ is a laser-suitable defect. The most laser active flat surface is the least probable for relaxed excited state orientational destruction of F+. The excited state at the edge has the highest energy, implying exciton (energy) transfer to the flat and edge sites. F+ relaxation and defect-formation energies increase with increasing surface coordination number. The Glasner-Tompkins relation between the fundamental optical absorption of F+ in solids and the fundamental absorption of the host crystals can be generalized to include the low coordinated surfaces of SrO. The F+ color center changes the nature of halogen-surface interaction (adsorption energies) from physical adsorption to chemical adsorption. The halogen-surface interactions increase with increasing electronegativity of the halogen. The calculated adsorption energies can be explained in terms of surface electrostatic potentials, and the covalent spin pairing mechanism plays a dominant role in determining adsorbate-substrate interactions.

F2+ defect based model for tunable laser activity and interaction of atomic halogens (F, Cl and Br) at the low coordinated surfaces of LiH: ab initio calculations

Abstract The twofold potential of F 2 + color center at the low coordinated surfaces of LiH in providing tunable laser activity and adsorption properties for atomic halogens is examined using ab initio calculations. Based on Stokes shifts of the examined clusters, the F 2 + laser activity was very weak, but increases significantly as the coordination number decreases from 5 (flat) to 4 (edge) to 3 (H − corner). The corner site is the least probable for relaxed excited state orientational destruction of F 2 + . The exciton (energy) transfer takes place from the corner to the flat or edge sites. The Glasner–Tompkins relation is generalized to include the low coordinated surfaces of LiH. The F + 2 color center change the nature of halogen–surface interaction from physical adsorption to chemical adsorption. The halogen–surface interactions were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred between the halogen and the surface. The “covalent spin pairing” mechanism play the dominant role in the course of adsorbate–substrate interactions.

Magnetic structure and covalence in tetrachlorobis(N-phenylacetamidinato)rhenium(IV) by neutron diffraction

Powder neutron diffraction measurements have been made at 2 and 20 K for the complex cis-[ReCl4(NHCMeNHPh)2]. The changes in diffracted intensities on magnetic ordering at 9.7(1) K were fitted in the Shubnikov group Pc′c′n to an antiferromagnetically ordered magnetic structure in which the spins are aligned along c in this orthorhombic crystal. A moment of 2.58(6) µB resides on the rhenium site in each molecule, while each chlorine site has a spin population of 1.03(8) µB due to covalent spin delocalisation. The N–C–N region of the other two ligands each has a spin population of –1.86(10) µB, indicating a substantial spin polarisation resulting from electron correlation effects. The antiferromagnetic structure is consistent with a reinterpretation of earlier single crystal magnetisation data. The very large amounts of spin transfer observed within the molecule are not unexpected in this heavy metal complex, and qualitatively reasonable given a synergistic interaction between the two ligand types. The previous observation of a metamagnetic transition in this crystal is explained as being driven by a change in canting angles within the molecule, from alignment closely along c to an arrangement in which opposite halves of the molecule are differently canted, with small but significant +b and –b components. The direction of magnetisation within the molecule is thus not all parallel.

Electron nuclear double resonance study of the Mn2+ environs in the oxalate-ATP complex of pyruvate kinase.

Electron nuclear double resonance (ENDOR) and the related pulse technique of pulse field sweep EPR (PFSEPR) were used to probe the site I environment of Mn2+ in the oxalate-ATP complex of pyruvate kinase. Assignment of features and an estimate of hyperfine couplings have shown proximity of protons to the metal ions through their dipolar interaction and proximity of 31P and 17O because of a contact interaction from direct Mn(2+)-ligand covalent spin transfer. Since Mn2+ is a spin5/2 ion whose Ms = +/- 1/2, +/- 3/2, and +/- 5/2 electron spin states can all contribute to EPR and ENDOR, we have developed experimental and theoretical strategies for elucidating hyperfine couplings to the Mn2+ electron spin states. Solvent-exchangeable proton ENDOR features were evident with couplings very similar to the hyperfine couplings of H2O in [Mn(H2O)6]2+. ENDOR of exchangeable, more distant protons originated from a dipolar coupling such as could be expected from protons residing 5.5 A from Mn2+ and hydrogen-bonded to a nonliganding oxygen or nitrogen. Nonexchangeable proton ENDOR features indicated dipolar coupling to proton(s) from the protein residing at approximately 4.5 A from the Mn2+. The approximately 4-MHz 31P phosphate hyperfine couplings in Mn(II)-nucleotide models and in pyruvate kinase were similar, but a detailed ENDOR and PFSEPR comparison revealed that the hyperfine coupling to the ATP gamma-phosphate in pyruvate kinase was approximately 10% less than coupling to phosphates of Mn(II)-nucleotides. [In pyruvate kinase only the gamma-phosphate has been shown to bind to Mn2+ at site I (Lodato & Reed, 1987).](ABSTRACT TRUNCATED AT 250 WORDS)

Structural, electronic, and magnetic properties of clean and Ag-covered Fe monolayers on W(110).

The structural, electronic, and magnetic properties of magnetic Fe transition-metal atoms as an overlayer on a nonmagnetic transition metal, W(110), and the effects of a Ag covering on the magnetism of Fe/W(110), are determined by means of the highly precise self-consistent all-electron full-potential linearized-augmented-plane-wave method based on the local-spin-density approximation. The interlayer spacings of Fe-W and Ag-Fe are determined from total-energy calculations. We find that the Fe atoms are relaxed downward (compared to the average of the Fe-Fe and W-W bulk bond lengths) by 9.5% and 4.0% for clean and Ag-covered Fe/W(110), respectively. We find that the hybridization of the W and Fe d bands plays an important role in determining the magnetism of the Fe/W(110) systems. The magnetic moment (2.18${\ensuremath{\mu}}_{B}$) and the magnitude of the Fermi-contact hyperfine field (-148 kG) of the Fe in relaxed Fe/W(110) are greatly reduced compared to those of the unrelaxed Fe/W(110) (by 0.38${\ensuremath{\mu}}_{B}$ and 46 kG, respectively). The Ag covering increases the magnitude of the Fermi-contact term of the Fe by 29 kG by encouraging the indirect covalent spin polarization of the s-like electrons. If one includes estimates of the dipolar and unquenched orbital-angular-momentum contributions, then our calculated values of the hyperfine field are found to be in remarkable agreement with recent conversion-electron M\ossbauer-spectroscopy experimental values.

Covalent bonding and spin density in cis-[Fe(bipy)2Cl2][FeCl4](bipy = 2,2′-bipyridyl) studied by polarised neutron diffraction

Polarised neutron-diffraction experiments on the Pccn polymorph of deuteriated cis-[FeIII-(bipy)2Cl2][FeIIICl4](bipy = 2,2′-bipyridyl) gave 402 unique magnetic structure factors collected with magnetic fields almost parallel to the c and b crystal directions. These data were analysed using a valence-orbital model involving anisotropic spin populations on all non-hydrogen atoms. The model refined to R′= 0.052 and χ= 1.56. Most of the spin resides on the iron atoms. In the cation the populations are Fe 3d 4.38(15), 4p–0.60(15), in the anion Fe 3d 4.13(15), 4p 0.00(17). Approximately 0.23 spins reside on each chlorine atom and 0.34 on each bipyridyl molecule of the cation. In the [FeCl4]– anion the division between 3d t2 and 3d e populations on the iron(III) atom, 3pσ and 3pπ populations on the chlorine atoms, and overlap density in the Fe–Cl bonds show the spin delocalised onto the chlorine atoms via both e* and t2* molecular orbitals with approximately equal amounts of σ and π spin transfer. There is significant 4p participation in the t2* molecular orbital. In the [Fe(bipy)2Cl2]+ cation the Fe–Cl bonding is similar to that in the anion. The bidentate 2,2′-bipyridyl ligand has much more spin in the ring bonded by the shorter Fe–N bond, 0.25(3), than on the other ring, 0.09(3), with σ donation into 3dσ and π back-donation from the iron 3d t2g orbitals. The distribution agrees qualitatively, but not quantitatively, with the charge-density results and theoretical predictions. Comparison with charge-density results shows that besides covalent spin transfer there are spin-polarisation effects, arising from electron–electron correlation, of similar size. These effects complicate simple molecular orbital descriptions of the bonding in both cation and anion of the complex, but agree with theoretical conclusions developed for the [CoCl4]2– ion.

57Fe,61Ni, and121Sb M�ssbauer study of oxidic spinel ferrites CoxNi5/3?xFeSb1/3O4

Antimony substituted nickel ferrites Co x Ni5/3−xFeSb1/3O4 withx=0,1/3,1,4/3, and 5/3 were prepared by the common ceramic technique, and the single-phase structure was confirmed by x-ray powder diffraction.57Fe Mossbauer spectra were recorded at 4.2 K with and without an applied magnetic fieldHlong=60 kOe in order to determine the cation distribution and the spin structure. Utilizing the cation distributions and the Yafet-Kittel spin structures for B-site Fe3+ ions, derived from Mossbauer experiments the calculated magnetic moments are in excellent agreement with the experimental values at 4.2 K. The existence of Ni2+ ions in crystallographic A-sites was confirmed by61Ni Mossbauer spectroscopy. The magnitude of the supertransferred hyperfine field at121Sb nuclei is determined mainly by a covalent spin charge transfer from A-sites, occupied by Fe3+, Ni2+, and Co2+ ions, into the 5s orbital of Sb5+ ions. The different contributions of various magnetic A-site ions to H STHF Sb may be understood by combining the energy differences between the ionic configurations, connected with the electron transfer process and the related exchange integrals.

Spin density and bonding in (ND 4 ) 2 Ni(SO 4 ) 2 • 6D 2 O

Polarized neutron diffraction experiments on the deuterated ammonium nickel Tutton salt at 2.00 and 4.20 K and a magnetic field of 4.60 T yielded 483 unique magnetic structure factors based upon the more intense Bragg nuclear reflections. A supporting experiment on the hydrogenous compound yielded 100 unique magnetic structure factors. These structure factors were used in a least-squares refinement of a model based upon atomic orbital populations located on the nickel, oxygen and deuterium atoms and in the Ni—O bond centre of the Ni(OD 2 ) 2+ 6 octahedron. The refinement (R w = 0.061, X = 1.98) gave averaged populations as follows: Ni : t 2g = – 0.06, e g = 1.64,4s = 0.38; O : (sp 2 ) 1 = 0.04, (sp 2 ) 2, 3 = 0.01, p π = 0.00; D: Is = 0.00; Ni—O: ‘overlap ’ = —0.05 spins. The Ni—OD 2 interactions show more covalent spin transfer than was seen for the similar Mn—OH 2 system and the ‘overlap’ population is correspondingly higher. These interactions resemble strongly those in the Ni—ND 3 bond of Ni(ND 3 ) 4 (NO 2 ) 2 . A feature of the difference from the Mn(OH 2 ) 2+ ion is the absence of spin in the hydrogen atoms, and this correlates with the essentially zero spin in the Ni-t 2g orbitals. Evaluation of the mixing parameters of a simple m. o. model, A σ = 0.49, S σ = 0.09, leads to values which resemble those for the Ni—ND 3 bonding in Ni(ND 3 ) 4 (NO 2 ) 2 . Spin polarization effects are negligible

Production of superoxide during the metabolism of nitrazepam.

Nitrazepam is metabolized in both humans and rats to 7-amino-nitrazepam OFFicating that this drug is reduced to a number of metabolic intermediates including several free radical species. When rat-hepatic microsomes are incubated with NADPH in the presence of nitrazepam, its nitro anion free radical was observed under anaerobic conditions. In the presence of oxygen, this free radical reduced oxygen giving nitrazepam and superoxide. 7-Nitroxyl-nitrazepam was produced by the chemical oxidation of 7-amino-nitrazepam using m-chloroperbenzoic acid. Reaction of this reactive free radical with hepatic microsomes led to the covalent spin labelling of microsomal protein. This phenomenon was also observed by the enzymic oxidation of 7-amino-nitrazepam with hepatic microsomes, obtained from a phenobarbital-induced rat, in the presence of a NADPH-generating system. With the generation of superoxide and hydrogen peroxide (arising from the dismutation of superoxide), it is not surprising that nitrazepam-enhanced lipid peroxidation was demonstrated by monitoring the production of lipid peroxyl radicals using spin-trapping techniques.

Spin-label study of histone H1-DNA interaction. Involvement of the central part of the molecule in reconstituted chromatin.

As shown in a previous paper [J. J. Lawrence et al. (1980) Eur. J. Biochem. 107, 263-269], covalent spin labeling of basis residues in histone H1 allows the study of the interaction of this protein with DNA. Using a step gradient dialysis procedure to reconstitute chromatin from labeled H1 and stripped chromatin, it is shown that the process of interaction of the lysine residues and DNA is the same whether histone H1 is bound to linear purified DNA or to H1-depleted chromatin. In contrast, spin labeling of the unique tyrosine of histone H1 located in the globular part of the molecule shows that this part is more involved in the interaction with chromatin than it is with linear DNA (as judged from the lengthening of the rotational correlation time). These data are interpreted as reflecting different roles for the N and C termini of the molecule of H1 and the central globular part. A model, based on these observations together with examination of the primary structures of histones H1, is proposed which accounts for the H1 involvement in the chromatosome structure.

Spin-label study of histone H1-DNA interaction. Comparative properties of the central part of the molecule and the N and C-amino tails.

Covalent spin labelling of lysine and arginine residues in histone H1 under specified conditions allows the study of histone-DNA interaction. When the labelled histone is free in solution, the electron spin resonance spectrum is characteristic of a label moving in a viscous medium while the label becomes fully immobilized upon the interaction of histone and DNA. This property is used to study histone-DNA interactions under various conditions of ionic strength and thermal denaturation. On the other hand, spin label probes in the globular part of the molecule shows no strong immobilization upon binding to linear DNA, suggesting a special mechanism for the binding of histone H1 to chromatin.

Transferred hyperfine interactions for Fe2+in KMgF3

EPR techniques have been used to investigate the (FeF6)4 complex in KMgF3:Fe2+ which is a Jahn-Teller system. From the observed 19F hyperfine structure the covalent spin transfer parameters are deduced and hence the contribution from covalency to the observed reduction in the g-value is calculated. It is found that covalency effects are fairly small and that most of the reduction is attributable to Jahn-Teller effects.

Selective enzymatic radioactive and spin labelling of phospholipids in biological membranes: application to study of temperature-induced changes of microsomal phosphatidylinositols and mitochondrial polyglycerophosphatides

A new method for the covalent radioactive and spin labelling of lipids within isolated biological membranes has been described in detail and applied to studies of temperature-induced changes of microsomal and mitochondrial membranes. The method is based on the enzymatic use of radioactive substrates carrying covalently bound doxyl derivatives of stearic acid in the biosynthesis of phospholipids in isolated membranes. Radioactive-and spin-labelled lipids bound to the microsomal and mitochondrial membranes were then used as internal probes in monitoring temperature-induced changes of these membranes. Since the analysis of isolated radioactive-and spin-labelled lipids revealed the exact composition of membrane-bound labelled lipids, specific temperature-induced changes were correlated with specific lipids of examined membranes. Phosphatidylinositol of microsomal membranes and polyglycerophosphatides (phosphatidyl-glycerol and cardiolipin) of mitochondrial and inner mitochondrial membranes were found to be involved in the apparent formation of lipid clusters at around 20-30 degrees C. Cardiolipin was found to be involved in the fluidization of inner mitochondrial membranes. These findings are discussed in view of the present state of knowledge of the organization of lipids in biological membranes.

Magnetic ordering and bond geometry in garnets with Fe3+ ions in the tetrahedral sublattice

en

The interaction of a proteolipid from sarcoplasmic reticulum membranes with phospholipids. A spin label study

A homogeneous proteolipid fraction ( M r ~ 12,000) was isolated from sarcoplasmic reticulum membranes of rabbit skeletal muscle. Covalent spin labeling of the protein and incorporation into egg yolk lecithin dispersions revealed its location in the low-polarity inferior of the phospholipid bilayer. By introducing stearic acid spin labels 2-(3-carboxypropyl)-4,4-dimethyl-2-tridecyl-3-oxazolidinoxyl and 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinoxyl into bilayers reconstituted from membrane phospholipids and proteolipid we found that the proteolipid reduces the motional freedom of the alkyl chains in the centre of the bilayer whereas the glycerol backbone region remains unaffected. Macroscopically ordered preparations of stearic acid spin-labeled phospholipids containing proteolipid showed no marked effect of the protein on the predominant orientation of the lipid alkyl chains perpendicular to the bilayer. The results suggest a two-dimensional arrangement of the polypeptide chains in the hydrocarbon interior of the bilayer.

17O ENDOR measurements on the (NiO6)10-complex

17O transferred hyperfine interactions, measured by use of the ENDOR technique are reported for Ni2+ impurities in single crystals of magnesium oxide. The interpretation of these measurements in terms of covalent spin transfer is discussed, and the results are compared with those for other nickel complexes.

Supertransferred Hyperfine Fields and Covalency at Diamagnetic Cations in Magnetic Insulators

The sign and magnitude of the antimony hyperfine field has been measured in Sb-substituted nickel ferrite using the $^{121}\mathrm{Sb}$ M\"ossbauer effect. We obtain a value of -311 \ifmmode\pm\else\textpm\fi{} 4 kG for this field at 100 K. This result is compared with results obtained for Sn in yttrium iron garnet and, among the possible mechanisms, $3d\ensuremath{-}5s$ covalent spin transfer appears to make the predominant contribution to the hyperfine field.