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1.
Using time resolved Fourier transform EPR spectroscopy the photoreduction of duroquinone by triethylamine in methanol solution was investigated. It is found that the spin-polarized (CIDEP) duroquinone triplet deactivates by electron transfer from triethylamine generating duroquinone radical anion and amine radical cation, and by hydrogen transfer from the solvent generating durosemiquinone radical and hydroxymethyl radical, respectively. All radicals are observed at different conditions and are spin-polarized by triplet mechanism and partially by ST0 radical pair mechanism. The time dependence of FT-EPR intensities of radical cation and radical anion on the amine concentration is investigated in the range of 1 to 100 mM triethylamine. The contribution of the triplet mechanism to the spin polarization of radicals changes with different triethylamine concentrations. The durosemiquinone radical is found to be transformed into duroquinone radical anion in the presence of triethylamine in the solution. CIDNP experiments indicate that the hydrogen back transfer between the durosemiquinone radical and hydroxymethyl radical pair has a significant influence on the time behaviour of duroquinone radical anion. The intensity of triethylamine radical cation is found to be decreased with the increase of triethylamine concentration, which is interpreted that the triethylamine radical cation is deprotonated by the amine. Based on the FT-EPR results, a new complete mechanism is proposed. 相似文献
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3.
Various free radicals formed during UV.-irradiation of aliphatic dicarboxylic acids in aqueous and methanolic solution are identified by ESR.-spectroscopy. Their structures point to α-cleavage and photoreduction as the dominant primary photochemical decay processes. The relative contributions of these reactions to the overall photodecomposition depend on solvent and degree of α-alkylation of the acid. Emission ESR.-spectra are found for radicals formed by C, CO-bond cleavage of α-dimethyl substituted acids. The polarization is referred to the triplet mechanism of CIDEP. and indicates this cleavage reaction occurs from a triplet molecular state. 相似文献
4.
The CIDEP spectra of transient radicals during photolysis of the duroquinone (DQ)/ethylene glycol (EG) system in acid, basic,
and micellar environments were measured with a home-made highly time-resolved ESR spectrometer. In the DQ/EG homogeneous solution,
the enhanced emissive CIDEP signal of the neutral durosemiquinone radical DQH• was observed. When the DQ/EG solution at pH 9 or the DQ/EG/TX-100/H2O micelle system was photolyzed, the CIDEP signal of the duroquinone anion radical (DQ•−) was obtained. When the DQ/EG solution at pH 2.5 was irradiated, the CIDEP signal of DQH• appeared. These experimental results indicate that the neutral radical DQH• was formed by proton transfer from EG to 3DQ*, and that DQ•− was formed by dissociation of DQH• accompanying polarization transfer. 相似文献
5.
TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U2 2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO2+ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity. 相似文献
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Application of the CIDEP phenomenon is extended to the identification of the specific triplet energy levels which react to produce radicals in pyrazine and its methyl derivatives. Advantage is taken of the proximity of 3nπ and 3ππ states in which the opposite signs of the zero-field coupling constants cause triplet mechanism polarization of opposite phase in the derived radicals. The electronic natures of the radicals observed parallel those of the triplet states from which they are formed. 相似文献
8.
White RC Gorelik V Bagryanskaya EG Forbes MD 《Langmuir : the ACS journal of surfaces and colloids》2007,23(8):4183-4191
Time-resolved magnetic resonance experiments (TREPR and CIDNP) are used to investigate previously unobserved redox chemistry of the surfactant dioctyl sulfosuccinate ester (AOT) using the photoexcited triplet state of anthraquinone 2,6-disulfonate (3AQDS*). Several different free radicals resulting from two independent oxidation pathways (electron transfer and hydrogen abstraction) are observed. These include the radical ions of AQDS and sulfite from electron-transfer processes, carbon-centered radicals from H-atom abstraction reactions, and an additional carbon-centered radical formed by electron transfer from the AOT sulfonate head group followed by the loss of SO3. The radicals exhibit intense chemically induced dynamic electron spin polarization (CIDEP) in their TREPR spectra. The intensity ratios of the observed TREPR signals for each radical depend on the water pool size and temperature, which in turn affect the predominant CIDEP mechanism. All signal carriers are accounted for by simulation, and CIDNP results provide strong supporting evidence for the assignments. 相似文献
9.
Makarov TN Savitsky AN Möbius K Beckert D Paul H 《The journal of physical chemistry. A》2005,109(10):2254-2263
Phosphinoyl radicals were produced in benzene solution by photolysis of three acylphosphine oxide photoinitiators, diphenyl-2,4,6-trimethylbenzoyl phosphine oxide (I), bis(2,6-dimethoxybenzoyl)-(2,4,4-trimethylpentyl) phosphine oxide (II), and bis(2,4,6-trimethylbenzoyl) phenylphospine oxide (III). The chemically induced dynamic electron polarization (CIDEP) of the radicals was measured by time-resolved electron paramagnetic resonance spectroscopy at different microwave frequencies/magnetic fields, in S- (2.8 GHz, 0.1 T), X- (9.7 GHz, 0.34 T), Q- (34.8 GHz, 1.2 T), and W-bands (95 GHz, 3.4 T). The CIDEP was found to be due to a triplet mechanism (TM) superimposed by a radical pair mechanism comprising ST(0) as well as ST(-) mixing. Contributions of the different CIDEP mechanisms were separated, and the dependence of the TM polarization on microwave frequency was determined. It agrees well with the numerical solution of the relevant stochastic Liouville equation, which proves the TM theory quantitatively. The applicability of previous approximate analytical formulas for the TM polarization is discussed. Parameters of the excited triplet state of III were estimated from the dependence of the TM polarization on microwave frequency. They are zero-field splitting constant 0.169 cm(-1) = D(ZFS) = 0.195 cm(-1), lifetime 40 ps = tau(T) = 200 ps, and initial population of its T(z)() spin sublevel 0.92 = w(z)() = 1. 相似文献
10.
Chemically induced dynamic electron polarization investigation of the triplet-radical system in the solution of the triplet quencher 总被引:1,自引:0,他引:1
The chemically induced dynamic electron polarization (CIDEP) of the triplet molecule/triplet quencher/2,2,6,6-te-tramethyl-1-piperidinyloxyl (TEMPO) systems were measured using the high time-resolved ESR spectrometer.The competition between the radical-triplet pair mechanism (RTPM) and triplet mechanism (TM) or radical pair mechanism (RPM) polarization in the solution of the triplet quencher was investigated,and the relationship between reaction rate of the radical-triplet pair and quenching rate of triplet was deduced. 相似文献
11.
The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm3 mol−1 for negative ions and ≈10 cm3 mol−1 for neutral radicals. 相似文献
12.
Chemically induced dynamic electron polarization (CIDEP) spectrum and transient absorptive spectrum are recorded in photolysis
of duroquinone (DQ) in 1,2-propanodiol (PG). Durosemiquinone neutral radical DQH• and PG ketyl radical
\textCH3[(\textC)\dot]\textOHCH2\textOH{\text{CH}}_{3}{{{\dot{\text{C}}}\text{OHCH}}}_{2}{\text{OH}} are produced through hydrogen transfer reaction from PG to 3DQ*. When stable radical TEMPO is added to DQ/PG solution, photolysis results in CIDEP on TEMPO, which can be interpreted
as a quartet precursor radical-triplet pair mechanism (QP-RTPM). There is competition between PG and TEMPO to quench 3DQ*. The CIDEP intensity of DQH• decreases with the increase of TEMPO concentration. The quenching dynamics in photolysis of DQ/TEMPO/PG system is analyzed
in detail. Based on the dynamics analysis and the measurement of the lifetime of 3DQ* by its transient absorbance decay, the quenching rate constant of 3DQ* by TEMPO in PG is obtained as 1.34 × 107 L mol−1 s−1. This quenching rate constant is closely diffusion-controlled. 相似文献
13.
A large group of polarized radicals derived pom polyatomic alcohols heretofore unexplored by CIDEP techniques are characterized
in viscous media. The TR ESR technique is shown to be capable of unravelling the mechanisms of the complicated dehydration
processes of the primary olyl radicals in neutral, acidic and basic media. 相似文献
14.
The spectral and kinetic properties of reduced radicals and the triplet state of neutral red (NR), a phenazine-based dye, have been investigated using pulse radiolysis technique. A mixed water-isopropanol-acetone solvent has been used to study the reduced radicals of NR for a wide pH range of about 1-13, due to limitation of solubility of the dye in aqueous solutions particularly above pH 8. From pH-dependent absorption studies it has been established that the reduced radicals of NR can exist in four different prototropic forms in solution. Three pKa values for the corresponding prototropic equilibria have been estimated. The formation and decay rate constants of reduced radicals have also been measured. The triplet state characteristics of the dye have been investigated in neat benzene solutions, both in the presence and in the absence of triplet sensitizers. The T1-->Tn absorption spectrum and decay kinetics of the triplet state have been measured. The triplet state energy (ET) of NR in benzene have been estimated to be within 36-42 kcal mol-1, using an energy transfer method. 相似文献
15.
K. A. Mclauchlan N. J. K. Simpson P. D. Smith 《Research on Chemical Intermediates》1991,16(2):141-163
It is demonstrated that the origin of the absorptive contribution observed in the spin polarized esr spectrum of the propan-2-olyl
radical when it is created by flash-photolysis of propan-2-one (acetone) in a hydrogen-donating solvent is not the triplet
mechanism of CIDEP, as previously assumed. It is shown to arise not in the photochemistry or photophysics of the parent molecule,
but rather to be a property of the radical pair itself. Arguments are presented to demonstrate that its origins may lie in
an unidentified novel polarization process, rather than the possible redistribution of level populations via very fast relaxation
processes. 相似文献
16.
A large group of polarized radicals derived pom polyatomic alcohols heretofore unexplored by CIDEP techniques are characterized in viscous media. The TR ESR technique is shown to be capable of unravelling the mechanisms of the complicated dehydration processes of the primary olyl radicals in neutral, acidic and basic media. 相似文献
17.
During the quinone-sensitized photooxidative degradation of polystyrene film and its solution in benzene, an initial rapid decrease of average molecular weight has been observed by GPC and viscosity measurements. The reaction rates are strongly increased by quinones such as p-quinone, duroquinone, anthraquinone, and chloranil. It has been suggested that this photosensitized degradation of polystyrene occurs by a singlet oxygen reaction which might be related to an energy transfer mechanism from excited triplet states of quinones to molecular oxygen. The photooxidative degradation of polystyrene in solution can be diminished by addition of typical singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene. 相似文献
18.
Michael R. Topp 《Chemical physics letters》1975,32(1):144-149
Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media. 相似文献
19.
The relative magnetic field effects on the total triplet—triplet annihilation (TTA) rate constant, on the rate constant for production of a singlet monomer and on the rate constant for production of a singlet excimer have been measured in a magnetic field range from 0 to 6000 gauss for the hydrocarbons pyrene, 3,4-benzopyrene, 1,2-benzanthracene and phenanthrene in solvents of different polarity between room temperature and 120 K. A qualitative discussion of the experimental results yields the following information on the mechanism of TTA: (i) The ratio of singlet to triplet products decreases with decreasing temperature or increasing viscosity of the solvent. (ii) The magnetic field effect depends much more on viscosity than on temperature. (iii) Singlet monomers and excimers are predominantly formed from different initial triplet—triplet pair configurations. (iv) Ionic radical pair states do not seem to play an important role in the TTA mechanism between equal molecules. 相似文献
20.
用高时间分辨电子自旋共振(TRESR)波谱仪, 研究对苯醌(PBQ)在不同的介质中光解苯半醌自由基的化学诱导动态电子自旋极化(CIDEP)。实验结果指出, 在乙二醇溶液中得到苯半醌自由基(PBQH)和以碳为中心的自由基R(OH)2。在乙二醇/水溶液中, 得到以PBQH的六条峰为主的谱线图, 然后在该体系中随着加入表面活性剂TX-100, AOT的浓度增加, 苯半醌负离子基(PBQ^-)的谱线强度增加, 而PBQH的谱线减弱直至消失。本文从自由基所处微环境性质出发, 讨论其极化传递机理和有关物理化学性质。 相似文献