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1.
Benzophenone ((C6H5)2CO) and decafluorobenzophenone ((C6F5)2CO) were applied to elucidate the photochemical reaction pathway of hydrogen peroxide (H2O2) with dimethylsulfoxide (DMSO). When a solution of benzophenone in DMSO was excited with the 355 nm laser light, three transient species were observed in the time-resolved electron paramagnetic resonance spectra: benzophenone ketyl (C6H5)2COH⋅, methyl⋅CH3, and methylsulfinic methyl⋅CH2SOCH3 radicals. However, when decafluoro-benzophenone was used with DMSO, only ketyl and methylsulfinic methyl radicals were observed under the same experimental conditions. When the reaction of benzophenone and DMSO was carried out in the presence of H2O2, different time profiles of⋅CH3 radicals were observed. In the reaction of decafluorobenzophenone-DMSO-H2O2, the time profiles of the radicals were not affected by the presence of H2O2. Thus, these results verify that⋅CH3 radicals are regenerated in a cyclic pathway, in which⋅CH3 radicals attack H2O2. The regeneration pathway allows us to observe f-pair polarization throughout the lifetime of⋅CH3 radicals, which last several microseconds, an order of magnitude longer than theT 1 relaxation time of⋅CH3 radicals. 相似文献
2.
Y. Apeloig D. Bravo-Zhivotovskii M. Yuzefovich M. Bendikov A. I. Shames 《Applied magnetic resonance》2000,18(3):425-434
Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me3SiMe2Si)3Si (1).1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me3Si)3Si radical. Thus, (Me3SiMe2Si)3Si⋅ has a half-life time of ca. 6 min at 20°C, while (Me3Si)3Si⋅ can be observed only at −25°C. Density-functional quantum-mechanical calculations show that1 and (Me3Si)3Si⋅ have the same thermodynamic stability. The high kinetic stability of1 is attributed to its backfold “umbrella”-type conformation where the β-silyl groups point “inwards” towards the radical center. This conformation protects the radical center of1 from dimerization and other reactions. The EPR spectrum of1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that1 is less pyramidal than (Me3Si)3Si⋅ but is more pyramidal than (i-Pr3Si)3Si⋅, with an estimated SiSiSi bond angle around the radical center of 118∘. Photolysis and thermolysis of [(Me3SiMe2Si)3Si]2 also involves the intermediacy of1. Photolysis of [(Me3SiMe2Si)3Si]2 leads to (Me3SiMe2Si)4Si, while thermolysis produced the less strained isomer of 1, (Me3SiMe2Si)3SiSi-Me2Si(Me3SiMe2Si)2SiMe3. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury. 相似文献
3.
Lithium trifluoromethane-sulfonate (Li-TFMS:CF3SO3Li) irradiated by γ-rays showed an electron spin resonance (ESR) powder spectrum having the rhombicg-factor ofg xx = 2.0259 ± 0.0005,g yy = 2.0112 ± 0.0005 andg zz = 2.0025 ± 0.0005 and a triplet hyperfine coupling constant ofA xx/gβ= 0.8 ± 0.15 mT.A yy andA zz are not obtained because of the broadened spectrum. The energy levels,g-factor,A xx/gβ and optical absorption spectrum of several conceivable radicals such as C⋅F2SO3Li, CF3-S-O⋅ and CF3-S-O-O⋅ have been calculated by softwares MOPAC-V2 and Gaussian-98 based on ROHF (Restricted Hatree-Fock for open shell molecule). The most probable radical was ascribed to CF3-SO⋅ from both calculated and experimental results. The response to γ-ray dose and the thermal stability have been studied in addition to the effect of UV illumination for possible use of the signal intensity in ESR dosimetry. The obtained number of free radicals per 100 eV (G-value) was 1.23 ± 0.40. 相似文献
4.
The inhibitory effects of lanthanide ions on the generation of free radicals from the reaction of Fe3+ and Fe2+ withtert-butyl hydroperoxide (tBuOOH) were investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap.
Peroxyl, alkoxyl, and carbon-centered free radicals generated from Fe3+-tBuOOH system were successfully trapped by DMPO, whereas peroxyl radicals were not trapped in Fe2+-tBuOOH system. Peroxyl and alkoxyl radicals are initial radical species generated from Fe3+ and Fe2+ systems, respectively. The carbon-centered radicals (⋅CH3) might be attributed to β-scission reaction of these alkoxyl radicals. The ESR signals of DMPO adducts of these radicals
were quenched in the presence of lanthanides (Ln3+ or [Ln(cit)2]3−), in concentration-dependent fashion. Moreover, the quenching effect of Ln3+ is closely related to the time the Ln3+ was added into the free-radical-triggered systems. The results reveal that there might be various mechanisms responsible
for inhibiting generation and transformation of the free radicals. If Ln3+ and iron react with peroxide simultaneously, the complex formation of Ln3+ withtBuOOH will be the main mechanism of the competitive inhibitory effect of Ln3+. Whereas if Ln3+ is added after iron, the inhibitory effect on the ESR signal of DMPO adducts might be interpreted preferentially by the coordination
and magnetic dipole-dipole interaction between Ln3+ and DMPO adducts. 相似文献
5.
K. G. Shaynurova S. A. Bakhramov A. M. Kokhkharov U. K. Makhmanov E. A. Zakhidov 《Journal of Applied Spectroscopy》2012,78(6):802-810
Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser
pulses (τ0.5 = 27 fs, λ0 = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-μm diameter core)
filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of ~1.5⋅1014 W/cm2 in the fundamental EH11 mode of the gas-filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather
high relative (≥95%) and absolute (~60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower
than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive
index n2 ≈ 4.5⋅10–23 cm2/(W⋅Torr) (N2) and n2 ≈ 2.8⋅10–23 cm2/(W⋅Torr) (He) have been found. A short-wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed
by the generated electron plasma at high pulse intensities (≥1014 W/cm2). 相似文献
6.
Manuel E. Medina Annia Galano Ángel Trigos 《Journal of Physical Organic Chemistry》2015,28(7):504-508
The density functional and the transition state theories were used to estimate detailed thermochemical and kinetic data for the oxidative damage to cholesterol induced by peroxyl radicals (ROO?) in lipid media. Two mechanisms of reactions were considered, the hydrogen transfer and the radical adduct formation, and it was found that hydrogen transfer is the only important route in this case, particularly at allylic sites. 7α products are predicted to represent more than 90% of the total initial damage, albeit 7β products are expected to be found in small but not negligible proportion. The chlorination degree was found to play an important role in the extent of the oxidative damage to cholesterol inflicted by the ROO? family. The estimated rate constants are 2.74 × 105, 1.32 × 105, 3.09 × 102, 6.07 × 101, and 8.75 × 10?1 M?1 s?1 for the reactions with ?OOCHCl2, ?OOCCl3, ?OOCH2Cl, ?OOH, and ?OOCH3, respectively. These values indicate that only chlorinated peroxyl radicals represent a significant hazard to the chemical integrity of cholesterol. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
7.
A. Yu. Panarin V. S. Chirvony K. I. Kholostov P.-Y. Turpin S. N. Terekhov 《Journal of Applied Spectroscopy》2009,76(2):280-287
We have optimized the procedure for preparation of nanostructured silver films on the surface of mesoporous silicon (PSi)
to use them as active substrates in surface-enhanced Raman scattering (SERS) spectroscopy. The greatest enhancement of the
SERS signal was observed for samples obtained when the silver was deposited on PSi from an aqueous AgNO3 solution with concentration 1⋅10–2 M over a 10–15 minute period. The detection limit for rhodamine 6G on SERS-active substrates prepared by the optimized procedure
was 1⋅10–10 M. The enhancement factor for the SERS signal on these surfaces was estimated as ≈2⋅108. We have shown that SERS-active substrates based on mesoporous silicon are promising for detection and study of complex organic
compounds, in particular tetrapyrrole molecules.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 298–306, March–April, 2009. 相似文献
8.
The laws governing polarization of luminescence in the nanostructure Si/CaF2 upon polarization of the spins of the fluorine nuclei by means of optical excitation of charge carriers are considered theoretically.
The possibility of studying experimentally the properties of nuclear spins in analyzing luminescence is shown. The polarization
of luminescence is most informative in the range of excitation rates of charge carriers from 3⋅107 to 3⋅ 108 sec−1 with the CaF2 layer of thickness from 0.6 to 0.8 nm and optical excitation polarization degree of 0.1.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 524–529, July–August, 2005. 相似文献
9.
J. Striova M. Camaiti E. M. Castellucci A. Sansonetti 《Applied Physics A: Materials Science & Processing》2011,104(2):649-660
Several pigments (malachite CuCO3⋅Cu(OH)2, azurite 2CuCO3⋅Cu(OH)2, yellow ochre (goethite α-FeOOH, gypsum CaSO4⋅2H2O), St. John’s white CaCO3 formed from slaked lime) and respective mural paintings specimens were subjected to the free-running Er:YAG laser radiation
in order to study their damage thresholds, in a broad range of laser fluences, both in dry and wet conditions. The specimens’
damage thresholds were evaluated by spectroscopic methods, colorimetric measurements and microscopic observation. The pigments
containing –OH groups were found to be more sensitive than St. John’s white; hence the most sensitive paint layers in dry
conditions are those containing malachite, azurite (both 1.3 J/cm2) and yellow ochre (2.5 J/cm2) as compared to the ones containing St. John’s white (15.2 J/cm2). The presence of wetting agents (w.a.) attenuated the pigments chemical alteration. The damage thresholds of all the paint
layers, in presence of w.a., were found to be around 2.5 J/cm2. The alteration was caused by thermo-mechanical damage and by binding medium ablation of a fresco and a secco prepared specimens,
respectively. 相似文献
10.
A. I. Shames R. Nucci S. D’Auria F. Febbraio C. Vaccaro E. Lozinsky M. Rossi G. I. Likhtenshtein 《Applied magnetic resonance》2000,18(4):515-526
The conformational transitions of thermophilic β-glycosidase fromSulfolobus solfataricus and the mechanism of its thermal and chemical activation were studied by electron paramagnetic resonance (EPR) of nitroxide
spin labels immobilized on the protein matrix. For this purpose, β-glycosidase was covalently modified by maleimide nitroxide
spin label (MAR⋅) and iodoacetamide nitroxide spin label (IAR⋅), both specific for -SH groups. The degree of modification was found to be independent of the temperature as well as of the
presence of two enzyme activators, sodium dodecyl sulphate (SDS) and butanol. In addition, a dansyl-piperidine nitroxide radical
probe (DR⋅), which has an affinity to the hydrophobic surfaces of proteins, was used in this study. The noncovalent binding of DR⋅ results in immediate formation of a probe-enzyme complex. At room temperature, the rotation frequency of the immobilized
labels decreases in order of IAR⋅ > MAR⋅ ≥ DR⋅. The temperature measurements of rotation correlation frequencies (v
c
) display values ranging from 6·107 to 2·108 s−1 and indicate a discontinuity with the inflection point at temperatureT
in in a range from 312 to 313 K. The observed enthalpies (ΔH
≠) and entropies (ΔS
≠) of the activation of spin label rotation were derived from the Arrhenius plots. The activation parameters were found to
be typical for rigid model systems. The addition of SDS and butanol produced a slight shift of the inflection point and changes
of spin-label mobility. A correlation between conformational transitions and enzyme thermal activation was discussed. 相似文献
11.
Akira Nakajima Yukari Masuda Emiko Matsuda Kunihiko Tajima Hiroshi Sameshima Tsuyomu Ikenoue 《Applied magnetic resonance》2013,44(9):997-1005
Electron spin resonance (ESR) of 5-(2,2-dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) spin adducts of free radicals derived from the UV irradiation of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) under rapid scanning condition was examined. The ESR signal obtained was the superposition of two spectra, the spin adduct of the alkoxyl radical [CYPMPO-ORa, Ra = C(CH3)2-C(+NH2Cl?)NH2] and the peroxyl radical (CYPMPO-OORa). The decay rate of CYPMPO-OORa was 8,400 times that of CYPMPO-ORa. The order of the k SB/k ST values (the slope of the Stern–Volmer’s plot) for the peroxyl radical (RaOO·) was l-ascorbic acid > caffeic acid > rutin ~ Trolox ~ (+)-catechin ~ glutathione (reduced), which was almost the same order as that for the alkoxyl radical (RaO·). Though the k SB/k ST value of each antioxidant for the peroxyl radical was about half of that for the alkoxyl radical, the ratios of the values of antioxidants to that of trolox (the relative ORAC values) were almost the same between the peroxyl and alkoxyl radicals. The relative oxygen radical absorbance capacity (ORAC) values of polyphenols estimated by the ORAC-ESR assay using both peroxyl and alkoxyl radicals were smaller than those by the ORAC-FL assay. 相似文献
12.
A Fourier transform electron paramagnetic resonance (FT-EPR) study was made of the photochemistry of 3,4-methylenedioxyphenol (sesamol, SEOH)) in aqueous solution. FT-EPR measurements show that in alkaline (pH 11) solution, pulsed-laser excitation of SECT leads to photoionization giving the hydrated electron and SEO⋅ free radical. Resonance signals from these paramagnetic species develop with instrument-controlled rise time. They exhibit a low-field emission/ high-field absorption (E/A) CIDEP pattern with the transition from emission to absorption occurring at the resonance of the hydrated electron. It is shown that the spin polarization stems from contributions from the ST0 radical pair mechanism (E/A) and triplet mechanism (A). From this it is concluded that photoionization of sesamol occurs via the triplet excited state. In neutral and acidic (pH 4–7) aqueous solution, photoexcitation generates SEO⋅ and cyclohexadienyl-type radicals. In this case, radicals grow in over a period of 1–2 μs and FT-EPR spectra display an E/A pattern with the inversion point in the center. The lowering of the pH of the solution apparently is accompanied by a strong reduction in the relative importance of photoionization. From the FT-EPR data it can be deduced that in neutral and acidic solutions the dominant reaction channel is H-atom transfer. In this respect, the photochemistry of sesamol differs from that of phenol andp-cresol. For these phenols the change in pH does not affect the appearance of the FT-EPR spectra. Apparently, the change in electronic structure caused by the methylenedioxy substituent strongly affects the excited state reactivity of sesamol. 相似文献
13.
Mechanism and kinetics of the oxidative damage to ergosterol induced by peroxyl radicals in lipid media: a theoretical quantum chemistry study
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Manuel E. Medina Cristina Iuga Ángel Trigos 《Journal of Physical Organic Chemistry》2016,29(4):196-203
In this work, we have studied the mechanisms and kinetics of the initial damage to ergosterol induced by ?OOH and ?OOCH3 peroxyl radicals in a lipid media, using quantum chemistry and computational kinetics methods. The initial damage to ergosterol induced by these radicals occurs predominantly through the hydrogen transfer mechanism (HT) from the allylic position C14 of ergosterol. For the reaction of ergosterol with ?OOH, the HT‐9 pathway represents ~90.8% of the overall rate constant, while in the case of ?OOCH3, the HT‐14 pathway represents more than ~97.2% of the overall rate constant. The calculated overall reaction rates for the initial damage to ergosterol caused by the ?OOH and ?OOCH3 are 2.05 × 106 and 6.26 × 104 M?1 s?1, respectively, indicating that the oxidative damage to ergosterol initiated by these radicals, and probably other alkyl‐peroxyl radicals, could be significantly dangerous to their integrity. Taking into account the calculated values of the overall rate coefficients, we can conclude that ergosterol is more susceptible to damage produced by peroxyl radicals than cholesterol and fatty acids. This suggests that fungal cells might be more sensitive to these radicals than animal cells, coinciding with the fact that one of the targets in combating fungi is precisely ergosterol. Finally, theoretical calculations confirm the antioxidant potential of ergosterol and could help explaining the nutraceutical activity of edible and medicinal mushrooms. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
14.
A. V. Stepanov D. V. Shestakov 《Russian Journal of Physical Chemistry B, Focus on Physics》2012,6(4):471-474
A method for studying the reactions of surface alkoxy radicals with O2 at temperatures of 230 to 300 K is described. Alkoxy radicals were generated directly in the cavity of an EPR spectrometer. Surface organic radicals, prepared from paraffin wax ((CH3)2(CH2) n , n = 16–20), were applied to Aerosil particles from a solution in heptane. The Aerosil sample was placed in the cavity of the EPR spectrometer in a cylindrical cup with a central hole for pumping out gases and exposed to H atoms. In this way, it is possible observe a steady increase in the EPR signal from the surface radicals. To measure the rate constant at tropospheric temperatures, the reaction tube was placed in a Teflon jacket, through which cool nitrogen vapor was pumped. The temperature in the reactor was varied from 230 to 300 K. The recorded EPR spectra belong to the (RO) s radical. After obtaining a stable EPR signal from the surface radicals, treatment with H atoms was stopped, additional flow of O2 was introduced ([O2] = 1014–1016 cm−3), and the reaction of O2 with the surface organic radicals was studied by monitoring the EPR signal decay. The temperature dependence of the rate constant for the (RO) s + O2 → HO2 + ketone was obtained within T = 230–300 K. The extrapolation of the data to real tropospheric conditions ([O2] = 1018 cm−3) was performed. 相似文献
15.
The efficiency of13C isotope separation by multiphoton dissociation using a CO2 laser has been shown to increase by ∼33% upon addition of small amounts of HI to the starting mixture. The reason for this
increase is that the CF3 radicals are scavenged by HI more rapidly than they recombine with free iodine atoms to reform the starting material. For
Torr and for 0.5J/cm2 roughly 65% of the CF3 and I radicals reform the starting material in the absence of HI. The results of this study indicate that at −80°C the rate
constants for CF3+CF3→C2F6 and CF3+I→CF3I are about equal and are roughly 8 times higher than that for CF3+HI→CF3H+I. 相似文献
16.
Accurate conductivity measurements as a function of hydrostatic pressure (1 – 5000 bars) and temperature (20 – 150 °C) have
been performed on a cationic inorganic glass and a cationic conducting polymer.
In both cases, the conductivity decreases with increasing pressure and the variation of Inσ at constant temperature as a function
of pressure gives straight lines with slopes which allow an “activation volume”, ΔV*, to be obtained by the relationship (∂lnσ/∂P)T=− (ΔV*/RT). In the case of silver metaphosphate glass, studied below its glass transition temperature, the activation volume
(5 cm3⋅mol−1) is temperature independent and equal to the molar volume of the silver cation. Since the transport mechanism implies a free
energy barrier, this volume is a real activation volume, corresponding to the difference in volume between a mole of the moving
species in its activated transition state and its volume at normal equilibrium.
In the case of the sodium conductive polymer, studied above its glass transition temperature, the previous thermodynamic definition
does not hold any more because the ionic transport follows a V.T.F. behaviour rather than an Arrhenius law. Consequently,
ΔV* is an “apparent activation volume” without a simple physical meaning. Experimental values are higher (20 to 30 cm3⋅mol−1) and decrease with temperature. In this polymer, the mobility of the charge carriers is interpreted in terms of free volume
mechanism. From the variations of the apparent activation volume with temperature, the critical free volume Vf* for an elementary displacement is estimated. For the Na+ conductive ionomer Vf* is estimated to be equal to 13 cm3⋅mol−1. This large value would indicate the participation of macromolecular chain segments in the ionic transport.
Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998. 相似文献
17.
N. G. Galkin V. M. Astashynski E. A. Chusovitin K. N. Galkin T. A. Dergacheva A. M. Kuzmitski E. A. Kostyukevich 《Journal of Applied Spectroscopy》2009,76(6):840-846
Consecutive plasma-epitaxial synthesis on silicon wafers is used for the first time to fabricate monolithic nanoheterostructures
with embedded nanocrystals (NC) of chromium disilicide (Si–NC CrSi2–Si). It is found that, initially, nanoislands form on the surface and within a coating layer of silicon, followed by the
formation of small (10–15 nm) nanocrystals of semiconducting chromium disilicide (CrSi2) at a high occupation density ((2–3)⋅1011 cm–2). During formation of silicon-silicide-silicon heterostructures, CrSi2 nanocrystallites “float up” into the near surface area of the covering silicon layer. 相似文献
18.
G. A. Kotel’nikov 《Russian Physics Journal》2009,52(1):53-58
The kinematics of Cherenkov’s effect is considered for an electron moving in the Minkowski space with universal time and velocity
of light c = c
0 (1 + v
2/c
02)1/2, where c
0 = 3⋅10 m/s is the invariant constant and v is the particle velocity.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 47–50, January, 2009. 相似文献
19.
Jun Yin 《Journal of statistical physics》2010,141(4):683-726
We derive an upper bound on the free energy of a Bose gas at density ϱ and temperature T. In combination with the lower bound derived previously by Seiringer (Commun. Math. Phys. 279(3): 595–636, 2008), our result proves that in the low density limit, i.e., when a
3
ϱ≪1, where a denotes the scattering length of the pair-interaction potential, the leading term of Δf, the free energy difference per volume between interacting and ideal Bose gases, is equal to 4pa(2r2-[r-rc]2+)4\pi a(2\varrho^{2}-[\varrho-\varrho_{c}]^{2}_{+}). Here, ϱ
c
(T) denotes the critical density for Bose–Einstein condensation (for the ideal Bose gas), and [⋅]+=max {⋅,0} denotes the positive part. 相似文献