共查询到10条相似文献,搜索用时 156 毫秒
1.
Atanu Barik K. Indira Priyadarsini Hari Mohan 《Research on Chemical Intermediates》2006,32(9):837-845
Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of
the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action.
Trichloromethylperoxyl radicals (CCl3O
2
·
) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique
with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 105 M−1s−1, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that
of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics
suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin
at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate
constant of 3 × 103 M−1s−1. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum
at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s−1. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl
radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other
phenolic compounds. 相似文献
2.
The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried
out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring
the absorbance of caffeine at λmax (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated
from the rates of oxidation of caffeine and the intensity of light at 254 nm which was measured by using peroxydisulphate
solution as a standard chemical actinometer. The reaction rates of oxidation of caffeine by PDP increase with increase in
[PDP] as well as with increase in light intensity, while they are independent of [caffeine]. The quantum yields of oxidation
of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by
PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable
mechanism has been suggested in which PDP is activated to phosphate radical anions (PO4
·2−) by direct photolysis of PDP and also by the sensitizing effect of caffeine. The phosphate radical anions thus produced react
with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in
a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric
acid and PO4
·2− radicals, the latter propagates the chain reaction. 相似文献
3.
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. 相似文献
4.
Zhong-Xian Guo Xin-Xiang Zhang Yi Gao Yuan-Zong Li Wen-Bao Chang Yun-Xiang Ci 《Mikrochimica acta》2003,141(3-4):183-189
This study describes a continuous flow procedure for monitoring atmospheric sulfur dioxide using a planar gas permeation
denuder (GPD) and a fiber optic spectrometer. When gaseous samples are directed through a GPD which consists of a gas-permeable
membrane of poly(vinylidene) difluoride and two perspex blocks with engraved channels of mirror image, the fraction of sulfur
dioxide passing the membrane is absorbed in a flowing stream of 5.0×10−4 mol L−1 5,5′-dithiobis(2,2′-dinitrobenzoic acid) (DTNB) in 0.025 mol L−1 phosphate buffer of pH 7.0, and reacts with DTNB to yield a yellow product. This product is monitored spectrophotometrically
at 410 nm with a miniature charge-coupled device (CCD) fiber optic spectrometer. The analytical range of sulfur dioxide is
easily adjusted via the flow rate of DTNB solution, normally from 200 ppb to 94 ppm (v/v). The procedure is hardly interfered
with by coexisting metal and non-metal species, except for hydrogen sulfide in the atmosphere. The error caused by hydrogen
sulfide at a sulfur dioxide level (in ppm) of 2% is less than ±10%. With the proposed procedure, the result for a mimetic
air sample is very close to that of the ultraviolet fluorescence method.
Correspondence: College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. e-mail: yli@chem.pku.edu.cn
Received August 12, 2002; accepted November 9, 2002 相似文献
5.
V. R. Gorelik V. F. Tarasov S. R. Shakirov E. G. Bagryanskaya 《Russian Chemical Bulletin》2008,57(7):1416-1427
The multispin systems consisting of spin-correlated radical pairs (SCRPs) and stable nitroxide radicals, localized in micelles
of sodium dodecyl sulfate (SDS), were studied by ESR and pulse laser photolysis techniques. In all the systems studied, the
stable nitroxide radicals exert no effect on the shape of the ESR spectra of the SCRPs (in particular, on the shape of their
antiphase structure) and on the decay kinetics of the ESR signal of the SCRPs. In the SDS micelles, the electron spin polarization
transfer from the nonequilibrium electron spin states of the molecular triplets (SCRP precursors) is the most efficient mechanism
of generation of the electron spin polarization in nitroxide radicals. The experimental data also show that the nitroxide
radicals and SCRP radicals are most probably distributed uniformly in the micellar phase.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1390–1401, July, 2008. 相似文献
6.
S. B. Lesnichin N. Kamdem D. Mauder G. S. Denisov I. G. Shenderovich 《Russian Journal of General Chemistry》2010,80(10):2027-2031
The study of 2,2′-bipyridyl adsorption on the surface of highly regular MCM-41 silica at 300 and 130 K was carried out by
the 15N NMR spectroscopy. It was shown that at 300 K the adsorbed molecules were involved in the processes of isotropic reorientation
accompanied by the formation and rupture of hydrogen bonds with the surface-located hydroxy groups. Each molecule of 2,2′-bipyridyl
forms no more than one hydrogen bond at a time, and their surface density is about one molecule per 1 nm2 of the surface. At 130 K 2,2′-bipyridyl forms a monolayer on the surface of silica including about 1.6 molecule per 1 nm2. In this monolayer each molecule forms a hydrogen bond with one hydroxy group and prevents the interaction of the other bipyridyl
molecules with one more hydroxy group. 相似文献
7.
The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance
of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by sulphate radical
anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found
to decrease the rate of oxidation of thymine suggesting that caffeic acid acts as an efficient scavenger of SO
4
•-
and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant
of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 1010 dm3 mol-1s-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light
intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results
of experimentally determined quantum yields (φexpt1) and the quantum yields calculated (φcl) assuming caffeic acid acting only as a scavenger of SO
4
•-
radicals show that φexpt1 values are lower than φcl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected
for SO
4
•-
scavenging by caffeic acid, are also found to be greater than φexpt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate
radical anions. 相似文献
8.
Eri Yoshida 《Colloid and polymer science》2010,288(3):341-345
In order to clarify the initiator factor dominating the molecular weight distribution of the resulting polymer, the nitroxide-mediated
photo-living radical polymerization of methyl methacrylate was performed using eight different kinds of azoinitiators: i.e.,
2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile),
racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile), meso-(2RS,2′SR)-azobis(4-methoxy-2,4-dimethylvaleronitrile),
dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(N-butyl-2-methylpropionamide). The bulk polymerization was carried out at room temperature for 3 h using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl
(MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as the photo-acid generator. All
the initiators provided a molecular weight distribution below 1.7 for the MTEMPO/initiator ratio of 2, although at the ratio
of unity, about half of the initiators produced the molecular weight distribution around 2.3–3.4. The UV analysis revealed
that the initiators having a higher ε value tended to more strictly control the molecular weight and provide a higher initiator
efficiency. The half-lives of the initiators had little effect on the molecular weight control and initiator efficiency. 相似文献
9.
Nature of the transient species formed during pulse radiolysis of thioacetamide in aqueous solutions
K. Kishore P. Dwibedy G. R. Dey D. B. Naik P. N. Moorthy 《Research on Chemical Intermediates》1998,24(1):35-45
Thioacetamide (TA) is an organic compound having thioamide group similar to that in thiourea derivatives. Its reactions with
eaq−, H-atom and OH radicals were studied using the pulse radiolysis technique at various pHs and the kinetic and spectral characteristics
of the transient species were determined. The initial adduct formed by the reaction of TA with OH radicals at pH 7 does not
absorb light in the 300–600 nm region but reacts with the parent compound to give a transient species with an absorption maximum
around 400 nm. At pH 0, the reaction of OH radicals with TA directly gives a similar transient species with absorption maximum
at 400 nm. Transient species formed by H-atom reaction with TA and pH 0 has no absorption in the 300–600 nm region but at
higher acidity a new transient species is formed which has absorption maximum at 400 nm. This transient absorption observed
in the case of both OH and H atom reaction with TA is ascribed to the formation of a resonance stabilized radical similar
to that obtained in the case of thiourea derivatives. The species produced by electron reaction viz. electron adduct was found to be a strong reductant and could reduce MV2+ with a high rate constant. H2S was produced as a stable product in the reaction of eaq− and its G-value was determined to be about 0.8. 相似文献
10.
The reactions of isolable dialkylgermylene and -stannylene with a galvinoxyl radical were found to give rather unusual cyclic compounds in high yields via intermediate adduct radicals. The structures of the cyclic compounds were determined by X-ray crystallography. We propose a concerted cyclization mechanism in which the abstraction of a hydrogen atom from the tert-butyl group by the galvinoxyl radical is accompanied by the simultaneous attack of the germyl radical center to the tert-butyl carbon atom. 相似文献