Kinetic Magnetic‐Field Effect Involving the Small Biologically Relevant Inorganic Radicals NO and O2.−

Oxidation of dihydrorhodamine 123 (DHR) to rhodamine 123 (RH) by oxoperoxonitrite (ONOO^?), formed through recombination of NO and O₂^.? radicals resulting from thermal decomposition of 3‐morpholinosydnonimine (SIN‐1) in buffered aerated aqueous solution at pH 7.6, represents a kinetic model system of the reactivity of NO and O₂^.? in biochemical systems. A magnetic‐field effect (MFE) on the yield of RH detected in this system is explored in the full range of fields between 0 and 18 T. It is found to increase in a nearly linear fashion up to a value of 5.5±1.6 % at 18 T and 23 °C (3.1±0.7 % at 40 °C). A theoretical framework to analyze the MFE in terms of the magnetic‐field‐enhanced recombination rate constant k_rec of NO and O₂^.? due to magnetic mixing of T₀ and S spin states of the radical pair by the Δg mechanism is developed, including estimation of magnetic properties (g tensor and spin relaxation times) of NO and O₂^.? in aqueous solution, and calculation of the MFE on k_rec using the theoretical formalism of Gorelik at al. The factor with which the MFE on k_rec is translated to the MFE on the yield of ONOO^? and RH is derived for various kinetic scenarios representing possible sink channels for NO and O₂^.?. With reasonable assumptions for the values of some unknown kinetic parameters, the theoretical predictions account well for the observed MFE.     

[Re(CO)3Cl]‐Chelation‐Mediated Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of 2,2′‐Bipyridine

The electronic coupling between two amine redox sites bridged through the 5,5′‐positions of the [Re(CO)₃Cl]‐chelated 2,2′‐bipyridine was studied by the electrochemical, spectroscopic, and EPR analysis. Interestingly, multiple near‐infrared bands were observed in this new organic mixed‐valent system. The results are interpreted with the aid of DFT and TDDFT calculations.     

Improving Physical Properties via C?H Oxidation: Chemical and Enzymatic Approaches

Physicochemical properties constitute a key factor for the success of a drug candidate. Whereas many strategies to improve the physicochemical properties of small heterocycle‐type leads exist, complex hydrocarbon skeletons are more challenging to derivatize because of the absence of functional groups. A variety of C H oxidation methods have been explored on the betulin skeleton to improve the solubility of this very bioactive, yet poorly water‐soluble, natural product. Capitalizing on the innate reactivity of the molecule, as well as the few molecular handles present on the core, allowed oxidations at different positions across the pentacyclic structure. Enzymatic oxidations afforded several orthogonal oxidations to chemical methods. Solubility measurements showed an enhancement for many of the synthesized compounds.     

Structures and stabilities of CBHz (z ≤ 8) and CxB3-xHz (x = 1, 2, z ≤ 14): prediction of novel organo-boron radicals

Hydrogenated boron-carbon clusters, i.e. organo-borons, have received considerable attention both theoretically and experimentally. Herein, using a topology searching strategy, we systematically explore the structures and stabilities of small organo-borons with CBH_z (z ≤ 8) and C_xB_3-xH_z (x = 1, 2,z≤ 14) stoichiometry, with particular interests in the intrinsic stabilities of the organo-boron radicals. At the CCSD(T)/aug-cc-pVQZ//B3LYP/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVTZ levels, the stabilities of these global minimum organo-boron species were evaluated by considering dissociation pathways and the binding energy per atom. Aside from the five already studied radicals (CBH₂, CBH₄, C₂BH₂, C₂BH₄ and CB₂H₃), we predict six novel radicals, i.e. CBH₆, C₂BH₆, C₂BH₈, CB₂H, CB₂H₅ and CB₂H₇, which could be detected under suitable circumstances. However, observation of the highly hydrogenated CB₂H₉ radical is much less likely due to its minute stability towards H-extrusion. The computationally determined stable/meta-stable maximum hydrogenation numbers for CB, C₂B and CB₂ (6, 8 and 8, respectively) are in excellent agreement with a simple electron-counting model for C_xB_y chains. The newly predicted organo-boron radicals await future laboratory verification.     

The role of guaiacyl moiety in free radical scavenging by 3,5-dihydroxy-4-methoxybenzyl alcohol: thermodynamics of 3H+/3e− mechanisms

ABSTRACT

Participation of guaiacyl moiety of 3,5-dihydroxy-4-methoxybenzyl alcohol (DHMBA) in inactivation of free radicals was investigated using the DFT method. The thermodynamics of triple (3H⁺/3e^?) free radical scavenging mechanisms was investigated. The Gibbs free energies of reactions of inactivation of selected 10 free radicals indicate DHMBA as a potent scavenger. Obtained results allow us to suggest that the contribution of guaiacyl moiety to antioxidant activity of phenolic compounds should be taken into account, what has been scarcely considered until now.     

Radical grafting from carbon fiber surface: graft polymerization of vinyl monomers initiated by azo groups introduced onto the surface

This paper describes the radical graft polymerizations of vinyl monomers from carbon fiber surface initiated by azo groups introduced onto the fiber surface. The carbon fiber used in this experiment was the polyacrylonitrile type. The introduction of azo groups onto the carbon fiber surface was achieved by the reaction of 4,4'-azobis (4-cyanopentanoic acid) with isocyanate groups which were previously attached onto the surface by the treatment of the fiber with tolylene 2,4-diisocyanate. The amount of surface azo groups introduced onto nitric acid-treated carbon fiber was determined to be 0.60 x 10^-5 mol 9^-1 by nitrogen analysis. The radical graft polymerization of methyl methacrylate (MMA) was tried. Though the thermal polymerization of MMA proceeded slightly in the absence or in the presence of untreated carbon fiber, the rate of the polymerization was considerably low. In contrast, the graft polymerization of MMA was initiated in the presence of the carbon fiber having surface azo groups, and part of resultant poly(MMA) grafted onto the surface. The percentage of grafting increased with an increase in polymerization time and reached 42.8% after 24 h. The graft polymerizations of other monomers, such as styrene, vinyl acetate, and acrylic acid, were also initiated by the surface azo groups attached onto the carbon fiber, and the corresponding polymer effectively grafted onto the surface.     

聚醋酸乙烯酯的光引发可控合成——介绍一个高分子化学综合实验

本实验开展了醋酸乙烯酯的可逆加成-断裂链转移自由基聚合(RAFT),分别运用偶氮二异丁腈(AIBN)引发和可见光引发两种方式进行了聚合反应,运用核磁共振和凝胶色谱等多种手段对所得聚合物结构进行了表征与分析。通过比较AIBN引发与光引发所获得聚合物端基结构的异同,加深了学生对RAFT聚合方法原理的理解。同时,运用该方法实现了聚合物两端端基结构的高度功能化,深入体会聚合物合成设计概念。本综合实验教学不仅通过对比法加深了学生对实验原理和专业知识的理解,提升了学生创新研究能力,而且训练了学生的实验操作技能、大型仪器使用能力和结果分析能力,提升了综合素质。