From boiling stones to smart crystals: supramolecular and magnetic isotope control of radical-radical reactions in zeolites

The chemistry of radicals adsorbed on zeolites is remarkable in that the products of radical-radical reactions, which are nonselective in solution, can be made selective and can be controlled by supramolecular effects and magnetic isotope effects. The photolysis of ketones adsorbed on zeolites can be manipulated so that either primary or secondary radicals produced by photolysis can be directed to selected radical-radical reactions which are unknown in solution.     

Hydrogen Isotope Effects in the Radiolysis of Water

The tritium fractionation between water and radicals formed in the radiolysis of dilute solutions of monomerous methyl methacrylate in H₂O-HTO and D₂O-DTO mixutres was studied. A parallel determination of the tritium content in molecular hydrogen was performed. Also, the isotopic composition of the initial molecular hydrogen was measured in the concentration range 10 to 90 Mol% D₂O of H₂O-HDO-D₂O mixtures. Hydrogen atoms, hydroxyl radicals and molecular hydrogen produced in the radiolysis of these solvents were found to be depleted in heavier hydrogen isotopes. The isotope effects on the composition of hydrogen atoms are discussed in terms of the rate isotope effects in proton transfer. The isotope effects on the composition of hydroxyl radicals are close to those on molecular hydrogen and their extent suggests ionic dissociation of water as a rate-determining step in the process of formation of both products.     

基于Py-GC/MS研究热解反应中自由基的捕获反应

在500℃下对类煤模型化合物1-萘甲醇与同位素示踪剂的共热解进行了机理研究。利用Py-GC/MS进行快速产物检测,同时联合自由基捕获剂来推断自由基的反应过程。结果表明,在500℃和0.2 min的条件下,1-萘甲醇的热解产物相对单一,但是自由基反应的引发、交换和湮灭极其快速,现有检测方法是对某一时间节点的平衡反应结果的分析。通过D₂O和H₂¹⁸O同位素示踪,成功捕获到了自由基交换后的D取代产物和¹⁸O取代产物。揭示了1-萘甲醇经过1-萘甲基自由基转化为1-萘甲醛的转化过程,发现萘环上的取代基在1-萘甲醇的热解过程中起着重要作用。同时对自由基及产物进行了定性和半定量分析,发现在加入自由基捕获剂后产物的丰度较之前降低了一个数量级,说明了自由基捕获剂抑制了产物的生成。     

Hydrogen-Atom Transfer Reactions of Transition-Metal Hydrides

Many reactions of transition-metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second-order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition-metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon-centered radicals or by metal radicals. Reasonably accurate values are now available for the M-H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.     

Opposite effects of gas flow rate on the rate of formation of carbon during the pyrolysis of ethane and acetylene on a nickel mesh catalyst

The importance of mass transfer in the formation of carbon on a nickel catalyst during the pyrolysis of ethane and acetylene was investigated. This included the formation of a composite of carbon fibre and amorphous carbon, using a non-porous nickel mesh catalyst in a tubular reactor at 750 °C. Increases in the gas flow rate through the Ni mesh decreased the formation of carbon during the pyrolysis of ethane. On the contrary, a similar increase in gas flow rate increased the formation of carbon during the pyrolysis of acetylene. Characterisation of the carbon products by SEM, TEM, TGA and FT-Raman spectroscopy also showed the opposite effects of gas flow rate on the type and structure of carbon formed on the nickel catalyst. These results can be explained by considering the relative ease with which various C-containing radicals and H radicals desorb from the nickel surface as the mass transfer resistance for radicals is decreased with increasing gas flow rate.     

Solvent Effect in Isotope Exchange Reaction of Strontium by Liquid Chromatography using a Cryptand (2B,2,2) Polymer

Abstract

Strontium isotopes were fractionated using liquid chromatography with a cryptand (2_B,2,2) polymer as a stationary phase. In this study, we observed that the single stage isotope enrichment factor of ⁸⁴Sr shows the following order, water[6.4×10^?3]>MeOH[4.9×10^?3]>DMF[1.1×10^?3]. The enrichment factors observed in this study did not show a mass‐dependent profile, indicating that the mass‐independent isotope effect affects isotope enrichment phenomena. The mass‐dependent and the mass‐independent isotope effects that influence the enrichment factor of ⁸⁴Sr are related to the donor number linearly, implying that the solvation strongly affects the isotope effect.     

NUCLEAR SIZE AND SHAPE EFFECT IN CHEMICAL ISOTOPE EFFECT OF GADOLINIUM USING DICYCLOHEXANO-18-CROWN-6

ABSTRACT

Gadolinium isotopes were fractionated in a liquid-liquid extraction system using dicyclohexano-18-crown-6. The isotope enrichment factors showed breakdowns of the conventional Bigeleisen-Mayer approximation for every condition of the initial phases; the concentrations of gadolinium and hydrochloric acid and the type of organic solvents. The nuclear mass effect and the field shift effect were estimated by use of the isotope pairs ¹⁵²Gd-¹⁶⁰Gd and ¹⁵⁴Gd-¹⁶⁰Gd. Since both isotopes, ¹⁵²Gd and ¹⁵⁴Gd, have the unique characteristics of the nuclear charge radii, <r²>'s, the unusual isotope effects due to the field shifts took place, while the mass effects of the other isotopes canceled the field shift effects. The maximum isotope effect was observed to be ε_160,152 =0·0252 ± 0·0024 whose initial aqueous phase was 0·023 M GdCl₃ in 12 M HC1, and its organic phase was 0·2 M DC18C6 in chloroform: this was 0·00315 ± 0·00030 in terms of the enrichment factor for unit mass.     

Nuclear Field Shift Effect of Chromium(III) in Repeated Extraction Using a Crown Ether

Abstract

Chromium isotopes were fractionated by continuous extraction using dicyclohexano‐18‐crown‐6 as an extractant. This system was designed for accumulating isotope effect into the aqueous phase. After a long period of operation (～2 weeks), the Cr isotope ratios in the residual aqueous phase were precisely measured by using a multi‐collector thermal ionization mass spectrometer with the total evaporation technique. The analytical results showed a clear mass‐independent isotope fractionation. The mass‐independent property was consistent with that obtained from a batch extraction. The isotopic mass balance was verified to be preserved in the continuous extraction. The Bigeleisen's 1996 theory gave the consistent explanation on the Cr isotope effect, in which the mass‐independent isotope effect was identified to be the nuclear field shift effect.     

Isotope Effects in Ion-Exchange Equilibria in Aqueous and Mixed Solvent Systems

Abstract

Isotope effects in ion-exchange equilibria in aqueous and mixed solvents are analyzed in terms of the general features of ion-exchange equilibria and of isotope effects in chemical equilibria. The special role of solvent fractionation effects in ion-exchange equilibria in mixed solvents is pointed out. The various situations arising in isotope fractionation in ion exchange in mixed solvents due to solvent fractionation effects are theoretically discussed. The experimental data on lithium isotope effects in ion-exchange equilibria in mixed solvents are shown to conform to the above situations. The limitations of ion-exchange equilibria in mixed solvents for isotope fractionation are pointed out.     

质谱分析导向的稳定同位素标记技术进展

介绍了近年来发展的一系列具有代表性的质谱分析导向的低残留稳定同位素标记技术:二甲氧基甲砜基嘧啶衍生化、低残留稳定同位素标记季铵化衍生化、新型胍剂化合物的衍生化等技术与方法。一些在常规电喷雾或基质辅助激光解吸电离条件下无法有效离子化的目标分子,在衍生化后产生新的正电荷中心或成为容易被电离物种而被质谱检测,从而提高了这类化合物的质谱学检测灵敏度。该类研究显著优势在于所开发的衍生化试剂本身不是离子型化合物,新产生的电荷中心或易于离子化的基团由衍生化反应引入。过量的衍生化试剂易于除去,不会对随后的质谱分析产生明显干扰。这一系列研究成果在代谢组学、食品安全分析、质谱成像以及单细胞分析等方面具有广泛的应用价值。     

一种吲哚基姜黄素类似物的合成及其抗氧化活性研究

吲哚-3-甲醛与2-羟基-5-甲氧基苯乙酮进行羟醛缩合反应,生成一种吲哚基姜黄素类似物1-(2-羟基-5-甲氧基苯基)-3-(3-吲哚基)-2-烯-1-酮,在不同的催化剂、温度、溶剂等条件下,探索研究合成反应所需的最优工艺条件,采用合适的方法分离提纯得到纯净产品。通过熔点测定、氢核磁共振分析法、质谱分析法等对合成产物进行分析,并研究其抗氧化生物活性。结果表明,该化合物均具有清除DPPH自由基、羟基自由基的能力,表现出明显的抗氧化活性。     

Intramolecular 1,2- and 1,3-Hydrogen Transfer Reactions of Thiyl Radicals

This review article summarizes recent experiments on 1,2- and 1,3-hydrogen transfer reactions in thiyl radicals from cysteine and related compounds. Pulse radiolysis in combination with time-resolved UV spectroscopy was applied to monitor the equilibration of initial thiyl radicals with carbon-centered radicals at both the C_α and C_β positions of cysteine. Experiments with thiyl radicals from penicillamine and cysteamine confirmed the formation of carbon-centered radicals at these positions. Complementary evidence for the intermediary formation of carbon-centered radicals was obtained from mass spectrometry and ¹H NMR spectroscopy experiments, both of which indicated covalent H/D exchange at original C H bonds when thiyl radicals were generated in D₂O. The 1,2- and 1,3-hydrogen transfer reactions can have profound consequences for the integrity of proteins when Cys residues are oxidized to Cys thiyl radicals, which subsequently equilibrate with carbon-centered radicals.     

Über die Pyrolyse von Pent-1-en-4,4,5,5,5-d5

On the Pyrolysis of Pent-1-ene-4,4,5,5,5-d₅ The distribution of deuterium in the main products of thermal decomposition of pent-1-ene-4,4,5,5,5-d₅ at 550 to 650°C was studied and interpreted. The results include conclusions on kinetic isotope effects, on relative reactivities of different C- H -bonds, on the proportion of terminal and nonterminal addition of methyl radicals, and on the importance of a radical isomerisation reaction. It was shown that the molecular decomposition (Retro-En-Reaction) cannot successfully compete with the radical way in the temperature range above 600°C     

有机同位素稀释质谱法在食品安全分析中的应用

有机同位素稀释质谱法具有高精度、高准确度、样品无需严格分离等优点,已广泛应用于食品安全领域.简要介绍了有机同位素稀释质谱技术的机理和特点,综述了其在食品安全领域中的应用,并对其发展趋势进行了展望.     

Extensions of the Melander–Westheimer Postulate: Isotope Effects in Reactions with Equilibrium Values Far from Unity

The symmetric dependence of experimental primary kinetic isotope effects on the free energy change, obtained for reaction series with zero equilibrium isotope effects, can be inverted to obtain the free energy profile along the reaction coordinate. Trends in the kinetic isotope effects for the general case of non-vanishing equilibrium effects can be understood by an extension of the same theory. Comparison is made with experiments for reactions of halogen atoms with hydrogen/deuterium molecules.     

Effects of Stabilized Criegee Intermediate and OH Radical Scavengers on Aerosol Formation from Reactions of β-Pinene with O 3

The formation of secondary organic aerosol (SOA) from reactions of O 3 with g -pinene, an exocyclic monoterpene prominent in the ambient atmosphere, was studied in an environmental chamber using a thermal desorption particle beam mass spectrometer for chemical analysis and a scanning mobility particle sizer for aerosol yield measurements. Potential reaction pathways for SOA formation were investigated in a series of experiments conducted using various scavengers for stabilized Criegee intermediates (SCI) and OH radicals, both of which are formed in the reaction. The major particulate products were compounds less volatile than pinic acid, a low-volatility dicarboxylic acid that was identified but was a minor component of the aerosol. The aerosol mass spectrum and yield were relatively insensitive to the identity of the SCI scavenger, indicating that association reactions of scavengers with SCI were not important in SOA formation. The mass spectrum of the aerosol also did not depend on the identity of the OH radical scavenger. SOA yields, on the other hand, were significantly larger when cyclohexane was used as an OH radical scavenger, compared to those measured for reactions conducted using alcohols or aldehydes. This dependence indicates that radical pathways play a major role in SOA formation in this reaction. Furthermore, the results show that reaction of OH radicals with scavengers used in laboratory studies can perturb the radical chemistry in such a way as to significantly impact SOA yields. We propose that this effect is due to increases in the ratio [hydroperoxy radicals]/[organic peroxy radicals] when alcohols or aldehydes are used as OH radical scavengers. This apparently enhances the rate of reaction of hydroperoxy radicals with key radical intermediates in SOA formation, effectively short-circuiting the reaction system into pathways leading to more volatile products.     

Contributions of Nuclear Size and Shape,Nuclear Mass,and Nuclear Spin to Enrichment Factors of Zinc Isotopes in a Chemical Exchange Reaction by a Cryptand

ABSTRACT

The isotope effect of zinc in the chemical exchange reaction using a macrocyclic ligand was not found to be ruled by the Bigeleisen-Mayer approximation, which suggested that the enrichment factor is proportional to the mass difference and is inversely proportional to the product of the masses of the isotopes. The separation factors of zinc isotopes in the chemical exchange reaction using cryptand(2_B,2,l) polymer were precisely measured by means of an ICP mass spectrometer equipped with nine collectors as ion detectors. The liquid chromatography of a column packed with the cryptand polymer was used for the separation of the zinc isotopes. The enrichment factor ε_67,66 for ⁶⁷Zn to ⁶⁶Zn was ?3.3329(3) × l0^?4.That for ⁶⁸Zn to ⁶⁶Zn was 1.846(1) × 10 ^?4 and that for ⁷⁰Zn to ⁶⁸Zn was 7.19(2) × 10^?4.They were not scaled with δm/mm′, where δm is the mass difference between the isotope pairs, and m and m′ represent the masses of the isotopes. The isotope effect of zinc is implicated with the isotope shift, and the hyperfine structure shift in the isotopomer of the zinc isotopes. The sum contribution of the vibrational energy shift from one isotope to the other and the nuclear mass shift to the enrichment factor of ⁶⁷Zn was ? 1.05 × 10^?3, and the contribution of the field shift caused by the nuclear size and shape of the isotope was 5.26 × 10^?4. The contribution of the nuclear spin or the hyperfine structure shift to the enrichment factor of ⁶⁷Zn was small: 1.94 × 10 ^?4     

Hydrogen-bond detection, configuration assignment and rotamer correction of side-chain amides in large proteins by NMR spectroscopy through protium/deuterium isotope effects

The configuration and hydrogen-bonding network of side-chain amides in a 35 kDa protein were determined by measuring differential and trans-hydrogen-bond H/D isotope effects by using the isotopomer-selective (IS)-TROSY technique, which leads to a reliable recognition and correction of erroneous rotamers that are frequently found in protein structures. First, the differential two-bond isotope effects on carbonyl (13)C' shifts, which are defined as Delta(2)Delta(13)C'(ND) = (2)Delta(13)C'(ND(E))-(2)Delta(13)C'(ND(Z)), provide a reliable means for the configuration assignment for side-chain amides, because environmental effects (hydrogen bonds and charges, etc.) are greatly attenuated over the two bonds that separate the carbon and hydrogen atoms, and the isotope effects fall into a narrow range of positive values. Second and more importantly, the significant variations in the differential one-bond isotope effects on (15)N chemical shifts, which are defined as Delta(1)Delta(15)N(D) = (1)Delta(15)N(D(E))-(1)Delta(15)N(D(Z)) can be correlated with hydrogen-bonding interactions, particularly those involving charged acceptors. The differential one-bond isotope effects are additive, with major contributions from intrinsic differential conjugative interactions between the E and Z configurations, H-bonding interactions, and charge effects. Furthermore, the pattern of trans-H-bond H/D isotope effects can be mapped onto more complicated hydrogen-bonding networks that involve bifurcated hydrogen-bonds. Third, the correlations between Delta(1)Delta(15)N(D) and hydrogen-bonding interactions afford an effective means for the correction of erroneous rotamer assignments of side-chain amides. Rotamer correction by differential isotope effects is not only robust, but also simple and can be applied to large proteins.     

Multiple isotope effects on the acyl group transfer reactions of amides and esters

Acyl group transfer reactions, especially those to amides and esters, are important in biochemistry. Multiple kinetic isotope effects for the atoms at the reactive center of these molecules have provided the most detailed bonding picture of the transition state to date. These kinetic isotope effect studies are reviewed for several reactions of formamide, methyl benzoate, and methyl formate. In these cases all the evidence is consistent with a stepwise mechanism, involving tetrahedral intermediates. In the case of p-nitrophenyl acetate, the change to an excellent leaving group causes the tetrahedral intermediates to become kinetically unstable; the kinetic isotope effects are best fitted to a concerted mechanism.     

Electron beam sterilization of cyclo olefin polymer leads to polymer degradation and production of alkyl radicals

We investigated the effects of electron‐beam (EB) sterilization on syringe barrels manufactured from cyclo olefin polymer (COP). The chemical structure of the polymer was determined by interpreting the ¹³C NMR and DEPT‐135 spectra of the COP resin. The antioxidants in the resin were identified by analyzing the liquid chromatography‐photo diode array‐mass spectrometry (LC‐PDA‐MS) data for the methanol extract of the resin and the gas chromatography‐mass spectrometry (GC‐MS) data for the supercritical methanol degradation products of the extract. NMR and LC‐PDA‐MS analyses revealed that EB sterilization produces degradation products in the COP main chain and reduces the quantity of the antioxidants in the COP resin. ESR spectra of the EB‐sterilized syringe barrels indicated the presence and location of alkyl radicals, which were generated in the COP main chain by EB sterilization. ESR analyses also indicated that the quantity of alkyl radicals decreased over time. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43498.