电极过程自由基中间体的ESR研究——取代苯基重氮盐的电解还原

本文用自由基捕捉剂苯亚甲基叔丁基氮氧化物(PBN)与ESR相结合的方法研究了十种取代苯基重氮盐RC₆H₄N₂⁺BF₄^-(R=F,Cl,Br,NO₂,OCH₃及CH₃)在乙腈中电解还原产生的活泼自由基。结果表明:1.RC₆H₄N₂⁺BF₄^-电解还原时产生RC₆H₄自由基,并能被捕捉剂PBN所捕捉,以形成稳定的自由基加合物[RC₆H₄-PBN]^·。2.由[RC₆H₄-PBN]^·ESR谱的超精细裂分常数算出的二面角θ_β值大小与处于不同位置的给定取代基R的关系为:θ_β(O-R) < θ_β(m-R) < θ_β(P-R)     

本体聚合反应过程中聚甲基丙烯酸甲酯增长自由基的ESR研究

采用TM₁₁₀谐振腔和φ2mm样品管,在17℃室温条件下成功地记录了MMA本体聚合反应过程中增长自由基的ESR谱。当把DMA加入到MMA和BPO中后,立即抽取0.17ml混合液到φ2mm样品管并记谱。以后每隔2分钟记谱一次,波谱从13(5+8)条线逐渐变成9(5+4)条线。我们用阻碍振荡模型和构象重叠模型作了模拟。从全部谱图看,前者似更合理些。ESR实验表明:在聚合过程前期,自由基浓度基本保持不变,但从聚合中期的某一时刻开始,浓度剧增,它正好同步地与本体聚合反应的自加速效应相对应,而且其变化规律和单体转化率相平行。最后,我们用微波功率饱和方法观测到9线谱的协同自旋跳跃所产生的卫线,证明了主导的电子自旋晶格弛豫机理来自电子一核自旋间的偶极偶合角调制。     

Spectroscopic investigations on polypropylene‐carbon nanofiber composites. I. Raman and electron spin resonance spectroscopy

Isotactic polypropylene‐vapor grown carbon nanofiber composites containing various fractions of carbon nanofibers, ranging from 0 to 20 wt %, have been prepared. Raman spectroscopy was used to analyze the effect of the dispersion of carbon nanofibers within polypropylene and the interactions between carbon nanofibers and macromolecular chains. The as‐recorded Raman spectra have been successfully fitted by a linear convolution of Lorentzian lines. Changes of the Raman lines parameters (position, intensity, width, and area) of polypropylene and carbon nanofibers were analyzed in detail. The Raman spectra of the polymeric matrix—at low concentrations of nanofibers—show important modifications that indicate strong interactions between carbon nanofibers and the polymeric matrix reflecting by vibrational dephasing of macromolecular chains. The Raman spectrum of carbon nanofibers is sensitive to the loading with carbon nanofibers, showing changes of the resonance frequencies, amplitudes, and width for both D‐ and G‐bands. Raman data reveals the increase of the disorder, as the concentration of carbon nanofibers is increased. The presence of the typical ESR line assigned to conducting electrons delocalized over carbon nanofibers is confirmed and the presence of a spurious magnetic line due to catalyst's residues is reported. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1644–1652, 2009     

The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structure

Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number‐average molecular weight, M_n, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N‐tert‐butyl‐α‐phenylnitrone (PBN), N‐methyl‐α‐phenylnitrone (PMN), and N‐methyl‐α‐(4‐bromo‐phenyl) nitrone (pB‐PMN), addition rate coefficients, k_ad,macro, in a similar magnitude to the styrene propagation rate coefficient, k_p, are found with spin capturing constants C_SC (with C_SC = k_ad,macro/k_p) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol⁻¹ were deduced for k_ad,macro, congruent with a decreasing C_SC with increasing temperature. Almost constant M_n over up to high monomer to polymer conversions is found when C_SC is close to unity, while increasing molecular weights can be observed when the C_SC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098–1107, 2009     

Effective 2,6-substitution of piperidine nitroxyl radical by carbonyl compound

Nitroxyl radicals (nitroxides) with unpaired electron are widely used as antioxidants, contrast agents, and spin probes. Although piperidine nitroxyl radicals have many applications, these are mainly tetramethylpiperidine compounds, and only a few reports consider the substitution of N-O surround as a reaction site, such as 2,2,6,6-tetrasubstituted piperidine nitroxyl radicals. Our results revealed that the 2,6-position of the 2,2,6,6-tetramethylpiperidin-4-one compound was substituted by cyclohexyl groups to produce 2,2,6,6-tetrasubstituted piperidin-4-one derivatives under mild reaction conditions. An interesting result was obtained by using ¹⁵N-labeled NH₄Cl instead of ¹⁴NH₄Cl: it gave ¹⁵N-labeled 2,2,6,6-tetrasubstituted piperidin-4-one-1-oxyls with a high ¹⁵N content. In conclusion, the new method for the synthesis of nitroxyl radicals readily yields 2,2,6,6-tetrasubstituted piperidin-4-one under mild conditions.     

An in situ temperature‐programmed XPS study of the surface chemical reactions of thiophene with Ag/titania

The Ag/titania sorbent for the ultradeep desulfurization of liquid fuels was characterized by electron spin resonance and was found to contain nearly the stoichiometric titania, without significant concentration of Ti³⁺ or the reactive oxygen species. The surface chemical reactions of thiophene adsorbed on the Ag/titania were studied by temperature‐programmed XPS from 25 to 525 °C upon in situ thermal annealing in high vacuum and in situ oxidation by oxygen gas. The titania support is not chemically reactive under those conditions. Silver oxide in the Ag/titania sorbent is converted to Ag₂ S without formation of the transient surface sulfates or sulfites and is further oxidized by molecular oxygen. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and structure of dinuclear dimethylene- or 1,4-phenylene-linked bis(decamethyltitanoceneoxide) (Ti) complexes

The singly tucked-in titanocene [Ti(η⁵-C₅Me₅)(η⁵:η¹-C₅Me₄CH₂)] (1) reacts smoothly with ethylene glycol or hydroquinone to give bis(titanoceneoxide) (Ti^III) complexes [CH₂OTi(η⁵-C₅Me₅)₂]₂ (2) and [(η⁵-C₅Me₅)₂TiOC₆H₄OTi(η⁵-C₅Me₅)₂] (3) containing dimethylene and 1,4-phenylene link, respectively. EPR spectra of 2 in 2-methyltetrahydrofuran glass and 3 in toluene glass revealed that the unpaired d¹ electrons are in interaction to form triplet state molecules. The Ti-Ti distance derived from the zero-field splittings D for the two conformations of 2 (7.42 Å and 7.66 Å) are in good agreement with the Ti-Ti distance of 7.2430(7) Å from the X-ray diffraction single-crystal analysis. For 3, however, the Ti-Ti distance derived from D (7.65 Å) is by 1.47 Å shorter than the crystallographic distance of 9.1230(8) Å that indicates an enhancement of the through-space dipole-dipole interaction due to the presence of a conjugated quinonide link.     

Thermal desorption/gas chromatography/mass spectrometry approach for characterization of the volatile fraction from amber specimens: A possibility of tracking geological origins

Research on the chemical composition of fossil resins has evolved during the last decades as a multidisciplinary field and is strongly oriented toward the correlation with their geological and botanical origin. Various extraction procedures and chromatographic techniques have been used together for identifying the volatile compounds contained in the fossil resin matrix. Hyphenation between thermal desorption (TD), gas chromatography (GC) and mass spectrometry detection (MS) has been chosen to investigate the volatile compounds fraction from ambers with a focus on Romanite (Romanian amber) and Baltic amber species. A data analysis procedure was developed for the main purpose of fingerprinting ambers based on the MS identity of the peaks generated by the volatile fraction, together with their relative percentual area within the chromatogram. Chromatographic data analysis was based entirely on Automated Mass Spectral Deconvolution & Identification System (AMDIS) software to produce deconvoluted mass spectra which were used to build-up a mixed mass spectra and relative retention scale library. Multivariate data analysis was further applied on AMDIS results with successful discrimination between Romanite and Baltic ambers. A special trial was conducted to generate pyrolysis “like” macromolecular structure breakdown to volatile compounds by gamma irradiation with a high absorbed dose of 500 kGy. Contrary to our expectations the volatile fraction fingerprints were not modified after irradiation experiments. A complementary non-destructive new approach by ESR spectroscopy was also proposed for discriminating between Romanite and Baltic ambers.     

Spinach--a software library for simulation of spin dynamics in large spin systems

We introduce a software library incorporating our recent research into efficient simulation algorithms for large spin systems. Liouville space simulations (including symmetry, relaxation and chemical kinetics) of most liquid-state NMR experiments on 40+ spin systems can now be performed without effort on a desktop workstation. Much progress has also been made with improving the efficiency of ESR, solid state NMR and Spin Chemistry simulations. Spinach is available for download at http://spindynamics.org.     

Electron Spin Resonance(ESR)Spectroscopy of the Transition-metal Complexes and Clusters I.Systematic Investigations of Real-and Complex-Orbital Methods for Calculating the d-Orbital Energies of a Low-spin(S=1/2)nd~5(t_2~5,~2T_2)(n=3

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