Laser Flash Photolysis Mechanism of Anthraquinone-2-Sodium Sulfonate in Pyridine Ionic Liquid/Water Mixed System

The photochemical reaction process of anthraquinone-2-sodium sulfonate (AQS) in the mixture of water (H₂O) and N-butylpyridinium tetrafluoroborate ([BPy][BF₄]) was studied using the laser flash photolysis technique. Experimental results show that the excited triplet of AQS (³AQS^*) could react rapidly with H₂O and the transient absorption spectra greatly changed by increasing the volume fraction of the ionic liquid (V_IL) in [BPy][BF₄]/H₂O mixtures. The absorbance at 510 nm increased gradually with increasing V_IL when 0< V_IL< 0.1. By contrast, the absorbance decreased gradually when V_IL>0.1. Otherwise, the absorbance of the band near 380 nm steadily increased. The apparent kinetic parameters of transient species B and ³AQS^* are obtained approximately. ³AQS^* abstracting hydrogen from [BPy]⁺ was also explored. It was deduced that the 350-420 nm band was the superposition of the peaks of ³AQS^* and AQSH^·. The two reactions of ³AQS^* with [BPy][BF₄] and H₂O are a pair of competitive reactions. We also concluded that the entire reaction processes slow down in the case of high [BPy][BF₄] concentrations.     

Electrostatic Perturbations in the Substrate-Binding Pocket of Taurine/α-Ketoglutarate Dioxygenase Determine its Selectivity

Taurine/α-ketoglutarate dioxygenase is an important enzyme that takes part in the cysteine catabolism process in the human body and selectively hydroxylates taurine at the C¹-position. Recent computational studies showed that in the gas-phase the C²−H bond of taurine is substantially weaker than the C¹−H bond, yet no evidence exists of 2-hydroxytaurine products. To this end, a detailed computational study on the selectivity patterns in TauD was performed. The calculations show that the second-coordination sphere and the protonation states of residues play a major role in guiding the enzyme to the right selectivity. Specifically, a single proton on an active site histidine residue can change the regioselectivity of the reaction through its electrostatic perturbations in the active site and effectively changes the C¹−H and C²−H bond strengths of taurine. This is further emphasized by many polar and hydrogen bonding interactions of the protein cage in TauD with the substrate and the oxidant that weaken the pro-R C¹−H bond and triggers a chemoselective reaction process. The large cluster models reproduce the experimental free energy of activation excellently.     

Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

The reactions of the indenyl radicals with acetylene (C₂H₂) and vinylacetylene (C₄H₄) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉H₇^.) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆H₅^.) and acetylene forming phenylacetylene (C₆H₅CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol⁻¹) and slow, contrary to the exoergic (−38 kJ mol⁻¹) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.     

Polymerization of novel trifluorovinyl ethers: Insight into the mechanisms of termination and chain transfer

New trifluorovinyl ether polymers were synthesized with the view toward overcoming the high chemical and thermal stabilities commonly associated with fluoropolymers. Trifluorovinyl ether copolymers, with fluorinated pendant groups, have previously been prepared to overcome limitations in processibility. To further enhance solubility in common organic solvents and to improve processibility, we prepared three new trifluorovinyl ether monomers, having hydrocarbon ether pendant groups, for polymerization: 1-[2-(2-ethoxy ethoxy)ethoxy]-1,2,2-trifluoroethene (Et-TFVE), 1-[2-(2-t-butoxy ethoxy)ethoxy]-1,2,2-trifluoroethene (t-Bu-TFVE), and 1-(2-phenoxy ethoxy)-1,2,2-trifluoroethene (Ph-TFVE). Homopolymers of these three monomers were prepared by aqueous emulsion polymerization with the use of a redox initiator. Poly(Et-TFVE) and poly(Ph-TFVE) were prepared with a range of molar masses, the highest of which had weight average molar masses of 33,800 g mol⁻¹ and 59,000 g mol⁻¹, respectively. As a result of monomer reactivity and structure, the polymerization mechanism was complicated, resulting in β-scission termination/chain transfer for all three polymers and hydrogen abstraction chain transfer for poly(Et-TFVE) and poly(t-Bu-TFVE). To the best of our knowledge, this is the first example of hydrogen abstraction from the pendant group of the trifluorovinyl ether itself. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3301–3308, 1999     

二苯酮与氯代酚光化夺氢反应量子产率的测定

用自行研究制的脉冲激光光声微量量热计准确测定了二苯酮与５种氯代苯酚光化夺氢反应的量子产率,探讨了取代基对夺氢反应量子产率的影响。结果表明,随着氯代酚中氯原子数目的增多,空间位阻增大,致使发生夺氢反应后生成的自由基较难复合,从而提高了夺氢反应的量子产率。该法为人们从能量学的角度深入了解光化夺氢的机理提供了重要信息。     

Ab initio study on the dynamical properties of the hydrogen abstraction reaction NH2 + OH → NH + H2O

 The geometry of the transition state of the title reaction was optimized at the unrestricted Hartree–Fock, the spin-unrestricted second-order M?ller–Plesset, and the spin-unrestricted quadratic configuration interaction with all single and double substitutions levels of theory. The changes in the geometry, the bound vibrational modes, and the potential energy along the minimum energy path are discussed. Variational transition-state theory rate constants calculated with the tunneling and curvature effect correction agree very well with the experimental values. Received: 23 April 1999 / Accepted: 9 June 1999 / Published online: 15 December 1999     

9,10-二氰基蒽光敏夺氢反应的研究

本文研究了缺电子敏化剂9,10-二氰基蒽(DCA)对苄醇类化合物(二苯甲醇、苯甲醇)及甲苯类化合物(甲苯、对-二甲苯)的光敏化夺氢反应,证明上述两类反应是经由两种不同机制进行的。     

Cobaloximes as Environmentally Advantageous Alternatives to Organotin Hydrides in Iodine Atom Abstraction Routes to Benzyl Radicals

High yields of alkyl radicals derived from alkylcobaloximes have been achieved using tungsten light (or in some cases ultrasound) radiation in both organic and aqueous media. These improved yields are obtained when pyridine (the usual base ligand) is replaced by suitably bulky lone-pair donors or water. The alkyl radicals so generated take part in iodine abstraction reactions with benzyl iodide giving benzyl radicals which may be trapped in near-quantitative yield with 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) or in good yield with lepidinium (4-methylquinolinium) trifluoroethanoate or lepidinium camphor-10-sulphonate. The usual drawbacks of cost and tedious work-up procedures associated with the more commonly used organotin hydride reagents are avoided.     

Revisiting Alkane Hydroxylation with m-CPBA (m-Chloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes

Mechanistic studies are performed on the alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid) catalyzed by nickel(II) complexes, Ni^II(L). In the oxidation of cycloalkanes, Ni^II(TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [Ni^II(L)] but is independent on [m-CPBA]; v_obs=k₂[substrate][Ni^II(L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect (KIE) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, Ni^II(L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the Ni^II(L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O₂ and CCl₄ on the product distribution pattern, possible contributions of (L)Ni^II−O^. and the aroyloxyl radical as the reactive oxidants are discussed.