Application of benzophenones to elucidate the photochemical reaction pathways of hydrogen peroxide with dimethylsulfoxide

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.     

Polysilyl radicals: EPR study of the formation and decomposition of star polysilanes

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.     

Radical formation in lithium trifluoromethane-sulfonate for ESR dosimetry

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.     

Characterization of free radicals generated from the reaction of Fe3+/Fe2+ withtert-butyl hydroperoxide and the effect of lanthanide ions

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⁺ with^tBuOOH 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.     

Nonlinear transformation of a spectrum of femtosecond laser pulses in a gas-filled microcapillary

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser pulses (τ_0.5 = 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).     

Theoretical study on the oxidative damage to cholesterol induced by peroxyl radicals

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.     

Formation of SERS-active silver structures on the surface of mesoporous silicon

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.     

Polarization of Luminescence in the Nanostructure Si/CaF<Subscript>2</Subscript> Upon Polarization of the Nuclear Spins of Fluorine

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.     

Chemical, morphological and chromatic behavior of mural paintings under Er:YAG laser irradiation

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.     

EPR spin labeling study of conformational transitions of β-glycosidase from the hyperthermophilic archaeonSulfolobus solfataricus expressed inEscherichia coli

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.     

Abilities of Antioxidants to Eliminate the Peroxyl Radical Derived from 2,2′-Azobis(2-Amidinopropane) Dihydrochloride (AAPH)

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.     

FT-EPR study of the pH dependence of the photochemistry of sesamol in aqueous solution

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.     

Mechanism and kinetics of the oxidative damage to ergosterol induced by peroxyl radicals in lipid media: a theoretical quantum chemistry study

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.     

A study of the temperature dependence of the rate constant for the reaction of O<Subscript>2</Subscript> with surface alkoxy radicals

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.     

Improved isotope separation in laser dissociation of trifluoromethyl halides: Scavenging of CF3 with HI

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.     

Influence of pressure on ionic transport in amorphous electrolytes: Comparison between glasses and salt polymer complexes

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 V_f* for an elementary displacement is estimated. For the Na⁺ conductive ionomer V_f* 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.     

Optical properties of silicon-silicide nanoheterostructures grown by consecutive plasma-epitaxy synthesis

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.     

Kinematics of Cherenkov’s effect with noninvariant velocity of light

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.     

Free Energies of Dilute Bose Gases: Upper Bound

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-r_c]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.