Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicals

Electrospray ionization (ESI) mass spectra have been recorded for a range of substituted nitronyl nitroxide and iminyl nitroxide monoradicals and biradicals. Secondary species formed in the ESI source were observed as the dominant ions in both the iminyl nitroxide and nitronyl nitroxide spectra. Daughter ion spectrometry was used to establish fragmentation mechanisms for the nitronyl nitroxide and iminyl nitroxide moieties as well as the secondary species under ESI conditions.     

氮氧自由基磁偶合体系的研究进展

介绍了含氮氧自由基磁偶合体系及金属－自由基类分子铁磁体的结构特点及其磁偶合机制.对其发展趋势和前景作一展望.     

Interpretation of cw-ESR spectra of p-methyl-thio-phenyl-nitronyl nitroxide in a nematic liquid crystalline phase

In this paper we report on the characterization by continuous wave electron spin resonance spectroscopy (cw-ESR) of a nitronyl nitroxide radical in a nematic phase. A detailed analysis is performed by exploiting an innovative modeling strategy alternative to the usual spectral simulation approach: most of the molecular parameters needed to calculate the spectrum are evaluated a priori and the ESR spectrum is obtained by direct application of the stochastic Liouville equation. Allowing a limited set of fitting parameters it is possible to reproduce satisfactorily ESR spectra in the temperature range 260 K-340 K including the nematic-to-isotropic phase transition (325.1 K). Our results open the way to a more quantitative understanding of the ordering and mobility of nitronyl nitroxide radicals in nanostructured environments.     

Effect of imino nitroxyl and nitronyl nitroxyl groups on the photochromic reactivity of diarylethenes

Diarylethene derivatives having imino nitroxide and nitronyl nitroxide have been prepared to examine the effect of the radical substituents on the photochromic reactivity of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene. These radical substituents reduce the quantum yields of both cyclization and cycloreversion reactions. The nitronyl nitroxyl moiety is more effective to suppress the reactivity in comparison with the imino nitroxide moiety. [reaction: see text]     

Theoretical calculations of the pressure effect for the β-phase of p-NPNN

Hybrid DFT (HDFT) calculations were carried out for model clusters of paranitrophenyl nitronyl nitroxide (p-NPNN) whose geometrical structures were based on its β-phase crystal structure. The effective exchange integrals between the neighboring p-NPNN molecules were calculated for the qualitative understanding of the pressure-induced transition form ferro- to antiferro-magnetic state. In the high pressure-induced condition, it could be presumed that the p-NPNN molecules in the crystal was gradually shifted along several lines or was rotated around appropriate axes. From these model calculations, it was found that the ferromagnetic exchange interaction between the neighboring dimers changed to antiferromagnetic one by the structure deformations.     

The polyoxometalates as precursors in molecular materials

The potentialities of the use of polyoxometalates as precursors for conducting and/or magnetic molecular materials are illustrated with a few examples of their chemical and electrochemical assemblies with organic donors derived from TTF, nitronyl nitroxide radicals and metallocenium radical cations.     

Evaluation of the β value of the phenylene unit by probing exchange interaction between two nitroxides

Oligophenylene molecular rods with bicyclo[2.2.2]octane having two nitronyl nitroxide radicals were synthesized to investigate the decay constant of p-phenylene. By the measurement and simulation of the ESR spectra of the biradicals with different rod length, it was found that the exchange interaction was decreased with the decay constant β of 0.51 ± 0.01 ?(-1). This result indicates that the spin-spin exchange interaction between neutral radicals has a decay constant similar to the molecular conductance.     

TRAPPING OF NITRIC OXIDE FORMED DURING PHOTOLYSIS OF SODIUM NITROPRUSSIDE IN AQUEOUS AND LIPID PHASES: AN ELECTRON SPIN RESONANCE STUDY

Abstract— Photolytic decomposition of sodium nitroprusside (SNP), a widely used nitrovasodilator, produced nitric oxide (NO), which was continuously monitored by electron spin resonance (ESR) spectroscopy. The NO present in the aqueous or the lipid phase was trapped by either a hydrophilic or a hydrophobic nitronyl nitroxide, respectively, to form the corresponding imino nitroxide. The conversion of nitronyl nitroxide to imino nitroxide was monitored by ESR spectrometry. The quantum yield for the generation of NO from SNP, measured from the rate of decay of nitronyl nitroxide, was 0.201 ± 0.007 and 0.324 ± 0.01 (¯± SD, n = 3) at 420 nm and 320 nm, respectively. The action spectrum for NO generation was found to overlap the optical absorption spectrum of SNP closely. A mechanism for the reaction between SNP and nitronyl nitroxide in the presence of light is proposed and computer-aided simulation of this mechanism using published rate constants agreed well with experimental data. The methodology described here may be used to assay NO production continuously during photoactivation of NO donors in aqueous and lipid environments. Biological implications of this methodology are discussed.     

A density functional study on magnetic exchange interaction between Mn(II) ion and nitronyl nitroxide radical in trans- and cis-metal-radical complexes

In the recent few years, transition metal complexeswith radical ligands have received much attentionaiming at a so-called metal-radical approach for novelmolecular magnet design[1]. One of the more popularfamilies is concerned in nitronyl nitroxide radica…     

Magnetism in organic radical ion salts based on nitronyl nitroxide derivatives substituted with heterocyclic aromatic hydrocarbons

Radical cation and anion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benzimidazolyl nitronyl nitroxide (2-BIMNN), have been prepared and their magnetic properties studied by SQUID magnetometry. The radical salts exhibit one-dimensional (1-d) antiferromagnetic (AFM) intermolecular interactions with the exchange coupling J/k between −0.8 and −6.3 K, which are significantly reduced from those observed in the two neutral radicals, while 2-IMNN shows an AFM interaction with J/k=−88 K within the molecular dimers and 2-BIMNN has quasi 1-d ferromagnetic (FM) intermolecular interactions with J/k=+22 K (intrachain) and zJ′/k=+0.24 K (interchain). The magnetic properties of the nitronyl nitroxide and iminonitroxide derivatives having molecular structure related to 2-IMNN have also been investigated. In 2-benzimidazolyl iminonitroxide (2-BIMIN), the FM interaction observed in 2-BIMNN is replaced by strong 1-d AFM interaction with J/k=−11.7 K.     

Photochromism of diarylethenes with two nitronyl nitroxides: photoswitching of an intramolecular magnetic interaction

Photochromic diarylethenes that have p-phenylene-substituted benzothiophene aryl groups with and without nitronyl nitroxide radicals at both ends of the molecules were synthesized. The absorption maxima of the closed-ring isomers showed a hypsochromic shift with the increase in the pi-conjugated chain length. The unique behavior was attributed to the stabilization by the resonant quinoid structures. Both photocyclization and photocycloreversion quantum yields of the diarylethene with nitronyl nitroxide radicals were found to increase with the increase in the pi-conjugated chain length. Photoswitching of the magnetic interaction between two nitronyl nitroxide radicals was studied by means of ESR spectroscopy. The change in exchange interaction between open- and closed-ring isomers of 1,2-bis[6-[4-[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]phenyl]-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene was determined to be more than 30-fold.     

Density functional theory study of the magnetic properties of rare earth complexes: the magnetic coupling mechanism in YIII and GdIII complexes with nitronyl nitroxide

The magnetic coupling interactions of the nitronyl nitroxide radicals bound to diamagnetic (Y^III) and paramagnetic (Gd^III) rare earth ions in two model magnetic systems based on novel rare earth organic radical complexes Ln(hfac)₃(NITPhOCH₃)₂ (Ln = Y^III 1, Gd^III 2; hafc = hexafluoroacetylacetonate; NITPhOCH₃ = 4′-methoxyo-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been investigated by density functional theory (DFT). The magnetic coupling mechanisms were also explored from the viewpoint of molecular orbital and spin density populations. DFT calculations show that the empty 4d-orbitals of Y^III and 5d-orbitals of Gd^III play an important role in the antiferromagnetic coupling between the two nitronyl nitroxide radical ligands, and that the ferromagnetic coupling between the Gd^III ion and the radical magnetic centers can be attributed to the nearly complete localization of the isotropic 4f-shell and singly occupied magnetic orbital (Π^*) of the nitronyl nitroxide.     

Radical-copper wheels: structure and magnetism of hexanuclear hybrid arrays

Complexation of copper(II) bromide and chloride with 4-pyrimidinyl nitronyl nitroxide (4PMNN) as a bridging ligand gave discrete hexanuclear complexes carrying 12 spins, [CuX(2).(4PMNN)](6) [X = Br (1), Cl (2)], which crystallize in a trigonal space group. The crystallographic parameters are C(11)H(15)Br(2)CuN(4)O(2).0.3H(2)O, a = 28.172(2), c = 12.590(2) A, V = 8653(2) A(3), and Z = 18 for 1, and C(11)H(15)Cl(2)CuN(4)O(2).0.3H(2)O, a = 28.261(2), c = 12.378(1) A, and Z = 18 for 2. The hexanuclear arrays construct a perfect column perpendicular to the molecular plane. The diameter of the resultant honeycomblike channel is ca. 11.5 A defined by the interatomic distance of two opposing copper ions. Their magnetic behavior is interpreted as the simultaneous presence of ferro and antiferromagnetic couplings. The ferromagnetic couplings are attributed to the interactions between a copper spin and the axially coordinated nitronyl nitroxide spin and between nitronyl nitroxide groups through van der Waals contacts. The antiferromagnetic coupling is due to the interaction between copper ions across the pyrimidine bridges.     

具有光控功能的两种氮氧自由基体系磁偶合作用的理论研究

采用密度泛函结合对称性破损态(DFT-BS)方法, 通过计算具有光控磁性分子开关功能的二氮氧自由基二芳基乙烯化合物的磁偶合常数, 合理解释了其分子结构发生开环和闭环变化时, 分子磁性发生的改变．同时设计了二氮氧自由基二甲基二氢化芘分子光控开环和闭环模型, 并用同样的方法计算了模型分子的磁偶合常数, 发现这些模型分子的磁行为类似于二氮氧自由基二芳基乙烯化合物, 有可能也具有光控磁性分子开关功能．     

Ab initio investigations of the electric field dependence of the geometric and electronic structures of molecular wires

Theoretical investigations on the typical molecular wire, polyacetylene, which bridges two chemically inert electrodes, have been carried out at the Hartree-Fock level by incorporating the external electric field into the calculations. The results demonstrate that both the geometric and the electronic structures of the conjugated molecular wires are sensitive to the electric field. When the electric field increases, the carbon-carbon single bonds become shorter and the double bonds become longer, leading to a higher conjugation. The electric field reduces the HOMO-LUMO gap and increases the dipole moment. The spatial distributions of the molecular orbitals are used to analyze the electrical properties of the molecular wire. All of these features are more pronounced with increasing conjugation chain length. Quantitative correlations between most of these features and the electric field have been discussed as well.     

New type of spin-labeled aminoenal

Ethynyl-substituted nitronyl nitroxide is involved in the 1,3-dipolar cycloaddition with cyclic nitrones without affecting the nitronyl nitroxide fragment. The reaction of 2,4,4,5,5-pentamethyl-4,5-dihydro-1H-imidazole 3-oxide produces the corresponding 1,2,3,7a-tetrahydroimidazo[1,2-b]isoxazole derivative. The analog of the latter derived from 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide is transformed into spin-labeled aminoenal. The solid phase of aminoenal is formed by packing unsymmetrical trimers resulting from the self-assembling of the molecular fragments based on a hydrogen bond network. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1975–1979, October, 2007.     

Systematic approach to design organic magnetic molecules: strongly coupled diradicals with ethylene coupler

The intramolecular magnetic coupling constant (J) values of diradical systems linked with two monoradicals through a coupler (para-substituted phenyl acetylene (Model I), meta-substituted phenyl acetylene (Model II), ethylene (Model III)) were investigated by unrestricted density functional theory calculations. We divided eight monoradicals into α-group and β-group according to Mulliken spin density values of the connected atoms. The overall trends in the strength of magnetic interactions of diradicals were found to be identical in three different model systems. The diradicals with para-substituted phenyl acetylene coupler resulted in almost twice stronger intramolecular magnetic coupling interactions of the corresponding diradicals as compared to the meta-substituted one with opposite magnetism. NN-Ethylene-PO (nitronyl nitroxide radical coupled to phenoxyl radical via ethylene coupler) was calculated to have the strongest magnetic coupling constant with ferromagnetism, and to be even stronger (more than twice) than NN-ethylene-NN (nitronyl nitroxide diradical with ethylene coupler), which was reported to have strong antiferromagnetic interactions in a previous experiment. It was found that the spin density values of the connected atoms are closely related to the determination of magnetic interactions and J values. The spin states of the ground state in diradical systems were explained by means of the spin alternation rule.     

极化中子衍射及在电子自旋密度研究中的应用

极化中子衍射方法常用于研究含未配对电子化合物中电子自旋密度的分布．分子中电子自旋密度分布从一个独特的角度反映化合物的磁性质．本文介绍极化中子衍射方法的背景知识和基本原理．包括中子源、中子和X射线衍射、极化中子衍射,以及一些常用的实验数据处理方法．选用几个实例总结了用极化中子衍射方法得到的电子自旋密度分布在无机和有机化学中的应用．通过单分子磁体[Fe8O2（OH）12（tacn）6]^8＋和氰基桥联化合物K2[Mn（H2O）2]3[Mo（CN）7]2·6H2O,说明如何用该方法研究金属原子间的磁相互作用;并通过Ru（acac）3这个只含一个未配对电子的化合物来说明如何获得化合物中金属和配体上小的自旋密度;最后介绍了该方法在nitronylnitroxide自由基研究中的应用．     

Synthesis and ESR studies of nitronyl nitroxide-tethered oligodeoxynucleotides

We report on the development of a nucleoside labeled with a nitronyl nitroxide group, ^NNU, and the synthesis of oligodeoxynucleotides containing ^NNU. The spin signals of ^NNU-containing oligodeoxynucleotides varied with the degree of hybridization of the complementary strand and the distance between nitronyl nitroxide spins.     

Asymmetric deformation density analysis in carbon nanotubes

Electrons and holes as charge carriers appear when a molecular system is exposed to external electric field. For nonsymmetric molecules, the distribution of charge carriers is also nonsymmetric. Although symmetric distribution of charge carriers is expected when the molecular system is symmetric, as the present work shows, they are not symmetric unexpectedly; that is, electrons and holes are not each other's mirror images. In this respect, asymmetric deformation density analysis is introduced to measure the extent of asymmetric distribution of electrons and holes as charge carriers when the symmetric system is exposed to symmetric external potential. Segments of (5,0) carbon nanotubes with different lengths are selected as symmetric systems, and a linear electric field is applied along the principal axis as symmetric potential. Results show that, at high electric fields, electrons tend to localize at the ends, while holes tend to occupy the middle area of carbon nanotube segments. While charge carriers play a vital role in molecular conductivity, asymmetric distribution of electrons and holes in symmetric systems has not yet been reported.