Magnetic field and magnetic isotope effects on the recombination kinetics of covalently-linked ketyl-phenoxyl triplet radical pairs

The recombination kinetics of three photogenerated covalently-linked ketyl-phenoxyl triplet radical pairs,³[PhC^.(OD)C₆H₄O(CH₂) _n OC₆H₄C₆H₄O^.] (n=3, 6, and 10), and of the corresponding deuterated derivatives were examined by the laser flash technique under an external magnetic field (up to 0.2 T) in a CDCl₃/CD₃OD (21) mixture. In zero magnetic field, radical pairs (RPs) with small exchange interactions (n=6 and 10) are characterized by high values of the magnetic isotope effect (MIE), which reach 3 for pairs withn=10. Under strong magnetic fields (up to 0.2 T), the values of MIE decrease to 1.2 to 1.1. The photochemical behavior of covalently-linked RPs is compared with that of similar unlinked RPs in micelles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 442–446, March, 1995.The authors are grateful to V. F. Tarasov, A. I. Shushin (N. N. Semenov Institute of Chemical Physics, RAS) and V. A. Nadtochenko (Institute of Chemical Physics in Chernogolovka, RAS) for helpful discussions.     

Modeling of recombination kinetics of radical pairs in magnetic field. Comparison with experimental dependences on the frequency of encounters in biradicals

The recombination kinetics of spin-correlated radical pairs (RPs) with three nonequivalent magnetic nuclei were calculated under conditions of enforced encounters between radicals at time-independent frequency n_dif. The simplest two-position model of a RP was used, which includes two states (contact state and distance-separated state) of the RP, differing in magnitude of isotropic spin-spin exchange interaction between radicals. The calculated kinetic curves were treated in terms of a three-exponential model. The dependences of corresponding rate constants (k _rec) on n_dif, external magnetic field strength (B ₀), and intensity, A _eff, of isotropic hyperfine coupling (HFC) were obtained. The k _rec-vs.-n_dif or k _rec-vs.-viscosity (n_dif varies simultaneously with the inverse lifetime of the contact state) plots pass through maxima whose positions are shifted from the n_dif region near the A _eff value at B ₀ = 0.5 G toward high n_dif values with an increase in B ₀. At n_dif ≫ A _eff, the k _rec-vs.-B ₀ plots pass through maxima in the region B ₀ = A _eff. The calculated dependences are compared with experimental data on recombination of biradicals. The results of calculations show that the experimentally observed maxima on the k _rec-vs.-B ₀ or k _rec-vs.-n_dif plots can be due to peculiar features of the spin dynamics induced by the hyperfine coupling rather than the exchange interaction effects, as is commonly accepted. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1103–1110, May, 2005.     

Experimental testing of molecular dynamic functions of pairs by the isotope selectivity of radical recombination

A method for experimental testing of the molecular dynamic functionf(t), which describes the generation of reencounters in a dynamic pair of molecular particles, has been developed. The method is based on the analysis of isotope separation in spin selective reactions of a radical pair. Thef(t) function derived from the solution of the diffusion equation with physically rigorous initial and boundary conditions describes experimental results much better than the well-known Noyes function. Conditions that make it possible to increase the efficiency of isotope separation in spin selective reactions are formulated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1639–1645, September, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-05227).     

Kinetics of fast reactions of triplet states and radicals under photolysis of 4,4′-dimethylbenzophenone in the presence of 4-halophenols in micellar solutions of sodium dodecyl sulfate in magnetic field

Quenching kinetics of the 4,4′-dimethylbenzophenone triplet state with para-substituted phenol derivatives RC₆H₄OH (R = H, F, Cl, Br, I) was studied by nanosecond laser photolysis in aqueous micellar solutions of sodium dodecyl sulfate. The kinetic data were processed in the framework of a model with the Poisson distribution of phenols between micelles. The partition constants of RC₆H₄OH between the aqueous and micellar phases and the rate constants of their escape from a micelle and quenching of the 4,4′-dimethylbenzophenone triplet state with phenols in micelles were obtained. The quenching proceeds with high rate constants through hydrogen atom transfer to form the ketyl and phenoxyl radicals (no radicals are formed in the case of 4-iodophenol), which then recombine in a micelle or escape into the outer aqueous volume. The application of an external magnetic field retards radical pair recombination in a micelle and increases the fraction of radicals escaped into the aqueous phase. The quantum yield of radical pairs decreases 2.5-fold, and the rate of their recombination in micelles increases 2.5-fold on going from 4-chloro- to 4-bromophenol. This is caused by the acceleration of triplet radical pair recombination in the solvent cage. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1391–1396, June, 2005.     

Kinetics of radical formation and decay in photooxidation of 4-halophenols sensitized by 4-carboxybenzophenone in aqueous solutions

Kinetics of formation and recombination of radicals formed by quenching of the triplet state of 4-carboxybenzophenone (CB) with para-substituted phenol derivatives RC₆H₄OH (R = OMe, H, Cl, Br, I) in aqueous solutions was studied by nanosecond laser photolysis. At pH ≥ 5.4, quenching proceeds with high rate constants ((1–3)⋅10⁹ L mol⁻¹ s⁻¹) through electron transfer to form the radical anion CB^⋅− and radical cation RC₆H₄OH^⋅+. The latter is transformed into the phenoxyl radical within ≤10 ns. At pH ≤ 8, the CB^⋅− radical anion is protonated in a phosphate buffer with the rate constant increasing from 4⋅10⁶ to 15⋅10⁶ s⁻¹ with a decrease in the pH from 8 to 5.4. The yield of radicals decreases from 100 to 13% as the atomic weight of halogen in the RC₆H₄OH molecule increases due to an increase in the probability of recombination of the primary triplet radical pair in the solvent cage and partial intersystem crossing in an encounter complex (³CB, RC₆H₄OH). The effect of heavy atom is also observed in the kinetics of volume recombination of the radicals, the magnitude of effect corresponds to the acceleration of the primary recombination of the triplet radical pair. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1397–1402, June, 2005.     

Effect of a magnetic field on the kinetics of the geminal recombination of triplet radical pairs adsorbed on glass with small pores

Conclusions A kinetic study was carried out for the geminal recombination of triplet radical pairs adsorbed in glass with small pores. The application of an external magnetic field is accompanied by a significant decrease in the fraction of radicals, which recombine in the cage. The magnetic effects in microporous glass are comparable in magnitude with the analogous phenomena in micelles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1202–1205, May, 1989.     

Effect of polymer additives on magnetic modulation of the yield of products of recombination of triplet radical ion pairs in electron phototransfer between N,N-dimethylaniline and pyrene. 1. Polyacrylamide in ethylene glycol

The effect of a magnetic field on the yield of products of recombination of triplet radical ion pairs in phototransfer of an electron between N,N-dimethylaniline and pyrene in ethylene glycol was investigated by laser strobe fluorometry. It was found that addition of a flexible-chain, linear polymer, polyacrylamide (PAAm), to the solution decreases the magnetic field effect (MFE) for concentrations higher than 130–140 g/liter. A linear correlation was established between the MFE and translational mobility of the reagents. A similar correlation was obtained for MFE in a viscous homogeneous liquid. A conclusion was drawn concerning the absence of a specific effect of PAAm on the cage effect based on the agreement of these correlations.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117334 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 2053–2058, September, 1992.     

Effect of polymer additives on magnetic modulation of the yield of products of recombination of triplet radical ion pairs in electron phototransfer between N,N-Dimethylaniline and pyrene. 2. Poly-N-tert-butylacrylamide in methanol

The effect of a magnetic field on the intensity of delayed fluorescence of pyrene excimers formed during evolution of pyrene — N,N-dimethylaniline radical ion pairs (RIP) in methanol containing glycerol or poly-N-tert-butylacrylamide was comparatively investigated by laser strobe fluorometry. Different methods of restricting the molecular mobility of the reagents cause different values of the magnetic effect with the same microviscosity measured with the spin probe orientational correlation time. This result contradicts the data obtained previously for mixtures with polyacrylamide and can be explained with the data on cross-linking of the polymer-containing solution. The viscosity of the microenvironment of the RIP partners is 10 times higher than the viscosity of the solvent. The presence of a maximum on the curve of the change in the magnetic effect in restriction of the molecular mobility was attributed to an increase in the lifetime of the radical pair and a change in the ratio of the relaxation rates and yield of radicals in bulk.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117334 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 2059–2065, September, 1992.     

胺基磁性纳米凝胶的光化学制备及形成机理

以Fe3O4纳米粒子为磁核,借助紫外光辐照含有烯丙基胺和N,N′-亚甲基双丙烯酰胺的水溶液,制备了胺基功能化的聚(烯丙基胺-共-N,N′-亚甲基双丙烯酰胺)磁性纳米凝胶(PAAm-Fe3O4),对其化学组成、表面电位、形貌、粒径分布及磁学性质进行了分析表征,并研究了光照时间和单体的滴加量对产物的粒径和粒径分布的影响.为探索聚合反应的引发方式,以烯丙基胺的类似物——苯胺为探针,借助激光光解-瞬态吸收装置研究了纳米Fe3O4粒子与有机电子供体的相互作用.结果表明,光化学方法实现了高分子凝胶层对单个Fe3O4粒子的有效包覆,通过控制光照时间和单体的滴加量可以获得在一定范围内尺寸可调且分布较窄的PAAm-Fe3O4.核壳结构的PAAm-Fe3O4近似球形,表面带正电性,磁含量接近88%,在室温下呈现准超顺磁性且饱和磁化强度达50emug?1.激光光解实验结果表明在光化学反应条件下Fe3O4与有机电子供体发生了电子转移反应,这可能是在Fe3O4表面引发有机胺单体的聚合并形成高分子壳的关键.最后,对PAAm-Fe3O4的形成机理进行了探讨.     

Magnetic field effects on the initiation of chain oxidation

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A laser photolysis study of the effect of a magnetic field on the kinetics of the recombination of radical and radicalion pairs in micelles

Conclusions The recombination of the radical pairs formed upon the transfer of a hydrogen atom from phenols and amines to triplet quinones in micelles is hindered upon the application of an external magnetic field and accelerated in the presence of atmospheric oxygen. The magnetic field effect is explained within the framework of a relaxation mechanism. The magnetic field has no effect on the kinetics of the deactivation of triplet exciplexes in micelles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 464–467, February, 1986.     

Efficiency of mercaptan chain transfer agents in emulsion copolymerizations. I. Influence on kinetics and microstructure. Modeling of radical desorption

The effect of chain transfer agents (CTA) on the emulsion copolymerization of styrene and butyl acrylate was studied in a bench scale 7 L reactor. On-line estimates of conversion were obtained through the joint use of calorimetric measurements and fast gravimetric data. Off-line measurements of partial conversions, molecular weight distribution (MWD), glass transition temperature (T_g), and particle diameter were also performed in order to investigate the effect of two mercaptans (tert-butanethiol and n-dodecanethiol) on both the kinetics of the polymerization process and the microstructure-dependent properties of the copolymer. The obtained experimental results were interpreted in terms of radical desorption and diffusive limitations of the CTA between the oil droplets and the particles. A model has been derived to compute the kinetic constants, the number of radicals per particle, and both the GPC/SEC diagrams and DSC thermograms related to MWD and T_g measurements, respectively. Several batch and semibatch examples are proposed to show that these important variables are satisfactorily fit by the model. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 157–168, 1998     

The effects of chain length dependent propagation and termination on the kinetics of free-radical polymerization at low chain lengths

The kinetics of the free-radical polymerization of methyl methacrylate at 60 °C in the presence of large amounts of n-dodecanethiol (DDM) were investigated as a test at short chain lengths of our recently proposed composite termination model [Smith GB, Russell GT, Heuts JPA. Macromol Theory Simul 2003;12:299]. The experimental data were found to show an unexpected and therefore very interesting trend: at high concentrations of DDM, the rate actually increases as chain lengths become smaller. No termination model can explain this unusual behaviour. However it was found that these results are very successfully described when, in addition to our composite termination model, chain length dependent propagation is included in modelling of the kinetics. These findings have important ramifications for all kinetic studies involving short polymeric radicals, including in low conversion living radical polymerization systems and studies determining the chain length dependence of the termination rate coefficient.     

Effects of chain transfer and recombination/disproportionation of inhibitor radicals on inhibited oxidation of lipids

The kinetics of inhibited oxidation of lipids was studied by computer simulation to evaluate the contributions of the recombination/disproportionation of inhibitor radicals and chain transfer to retardation effects. The influence of inhibitor regeneration on the induction periods and inhibited oxidation rate was demonstrated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1915–1920, October, 1999.     

Magnetic field effect on an exciplex between N-vinyl carbazole grafted on cellulose acetate film and 1,4-dicyanobenzene

The present work emphasises the investigation of photoinduced electron transfer reaction between an electron-donor fluorophore, N-vinyl carbazole (VCZ), grafted on a polymeric cellulose acetate film and an electron-acceptor, 1,4-dicyanobenzene, in the presence of an external magnetic field that serves as a powerful tool to identify the spin states where the initial electron transfer occurs and to modulate the course of the reaction as desire. Here initial electron transfer occurs in the singlet spin state, and the formation of exciplex increases in the presence of magnetic field. The maximum field effect is obtained in the solvent with medium dielectric constant (ε_max) around 8.0. The ε_max value indicates that the extent of charge transfer in this exciplex is less compared to other similar systems studied so far, owing to the characteristic binding of VCZ to the polymeric backbone through oxygen atom. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3910–3915, 1999     

磁场对光催化反应羟基自由基生成速率的影响

以对苯二酸作为Pt/TiO₂光催化反应所产生的羟基自由基(•OH)的捕获剂, 采用荧光技术考察外加磁场对羟基自由基(•OH)生成速率的影响, 结果表明, 与常规的光催化反应相比, 外加磁场可使样品表面羟基自由基的生成速率提高11.7%.     

脂肪酸甲胺铂配合物的结构表征及磁场对其抗癌活 性的影响

用元素分析、傅立叶变换红外光谱和核磁共振对6种脂肪酸胺(氨)铂配合物的结构和理化特性进行了研究。选用人黑色素瘤(LiBr)、白血病细胞K~5~6~2和肝癌(7721)作为体外模型。用扫描电子显微镜技术研究了磁场对肿瘤细胞膜表面微观结构的影响。用MTT法对合成的几种脂肪酸胺(氨)铂配合物的抗癌活性进行了研究。研究发现,抗癌活性与浓度梯度、胺(氨)配体、脂肪酸碳链的长度密切相关。磁场对药物与癌细胞相互作用的影响与磁场强度有关。     

A radical-anion chain mechanism initiated by dissociative electron transfer to a bicyclic endoperoxide: insight into the fragmentation chemistry of neutral biradicals and distonic radical anions

The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55+/-0.15 V and 20+/-3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals.