ESR study of free radicals in mango

An electron spin resonance (ESR) spectroscopic study of radicals induced in irradiated fresh mangoes was performed. Mangoes in the fresh state were irradiated with γ-rays, lyophilized and then crushed into a powder. The ESR spectrum of the powder showed a strong main peak at g = 2.004 and a pair of peaks centered at the main peak. The main peak was detected from both flesh and skin specimens. This peak height gradually decreased during storage following irradiation. On the other hand, the side peaks showed a well-defined dose–response relationship even at 9 days post-irradiation. The side peaks therefore provide a useful means to define the irradiation of fresh mangoes.     

ESR study of nitro-substituted iminoxyl radicals

Iminoxyl radicals involved in the homolytic oxidation of nitrolic acids, with the general formula (NO₂)RC=NO^., have been detected by means of ESR. In the presence of a strong acid, the ESR spectra of the radical (NO₂)₂C=NO^. exhibits alternation of the line widths, owing to syn-anti isomerization.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1513–1516, July, 1991.     

Interaction between methyl and hydroxyl radicals: a theoretical study

In this paper, we conduct a computational quantum mechanic study of two molecules interaction between methyl (·CH₃) and hydroxyl (·OH) radicals. The molecular exploration has been focused on the possibility of finding non-bonding interactions (formation of complexes by weak-bond interaction among C–O–H atoms) and the potential energy reaction between those two molecules. It shows that the formed complexes presents in slightly lower potential energy than that of the reactants and/or products. The existence of these complexes could proceed to the further interaction, e.g. reaction of the molecules, as those complexes formed a particular configuration.     

Reaction of ethylene with hydroxyl radicals: a theoretical study

Ab initio calculations of portions of the C2H5O potential energy surface critical to the title reaction are presented. These calculations are based on QCISD geometries and frequencies and RQCISD(T) energies extrapolated to the complete-basis-set limit. Rate coefficients for the reaction of C2H4 with OH are calculated using this surface and the two transition-state model of Greenwald and co-workers [J. Phys. Chem. A 2005, 109, 6031] for the association of OH with C2H4. The present calculations reproduce most of the experimental data, including the temperature and pressure dependence of the rate coefficients, with only a small (0.4 kcal/mol) adjustment to the energy barrier for direct hydrogen abstraction. We confirm the importance of this channel above 800 K and find that a significant fraction of the total rate coefficient (approximately 10%) is due to the formation of vinyl alcohol above this temperature. Calculations of the vinyl alcohol channel are consistent with the recent observation of this molecule in low-pressure flames [Taatjes, C. A.; Hansen, N.; McIlroy, A.; Miller, J. A.; Senosiain, J. P.; Klippenstein, S. J.; Qi, F.; Sheng, L.; Zhang, Y.; Cool, T. A.; Wang, J.; Westmoreland, P. R.; Law, M. E.; Kasper, T.; Kohse-H?inghaus, K. Science 2005, 308, 1887] and suggest that this reaction should be included in hydrocarbon oxidation mechanisms.     

ESR study of carborane containing organofluorine radicals

It has been shown by ESR that the addition of photochemically generated boroncentered carboranyl radicals to branched fluoroalkenes affords stable spin-adducts. It has been found that the addition of boron-centered radicals with 1-substituted 2-fluoroalkenyl-o-carborane gives stable -carboranyl radicals. Negligible variations in the values of the constants of hyperfine coupling of the unpaired electron with the nucleus of the -¹¹B atom in radicals of various structures imply the stability of the carborane conformation with respect to the 2p _z-orbital of the unpaired electron. This may be caused by hyperconjugation between the boron atom and the unpaired electron as well as steric interaction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1316–1318, July, 1993.     

Effective attenuation lengths for photoelectrons in thin films of silicon oxynitride and hafnium oxynitride on silicon

We have used the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to simulate photoelectron intensities for thin films of SiO_1.6N_0.4 and HfO_1.9N_0.1 on silicon with excitation by Al Kα X‐rays. We considered Si 2p_3/2 photoelectrons from SiO_1.6N_0.4 and the substrate and Hf 4f_7/2 photoelectrons from HfO_1.9N_0.1. The simulations were performed for ranges of film thicknesses and photoelectron emission angles and for two common configurations for X‐ray photoelectron spectroscopy (XPS), the sample‐tilting configuration and the Theta Probe configuration. We determined photoelectron effective attenuation lengths (EALs) by two methods, one by analyzing photoelectron intensities as a function of film thickness for each emission angle (Method 1) and the other by analyzing photoelectron intensities as a function of emission angle for each film thickness (Method 2). Our analyses were made with simple expressions that had been derived with the assumption that elastic‐scattering effects were negligible. We found that EALs from both methods were systematically larger for the Theta Probe configuration, by amounts varying between 1% and 5%, than those for the sample‐tilting configuration. These differences were attributed to anisotropy effects in the photoionization cross section that are expected to occur in the former configuration. Generally, similar EALs were found by each method for each film material although larger EALs were found from Method 2 for film thicknesses less than 1.5 nm. SESSA is a useful tool for showing how elastic scattering of photoelectrons modifies EALs for particular materials, film thicknesses, and XPS configurations. Copyright © 2012 John Wiley & Sons, Ltd.     

ESR/DFT study of bis-iminophosphorane cation radicals

Bis-iminophosphoranes containing various types of linkers between two R3P==N moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3P==N moieties separated by a --C(CN)==C(CN)--group was obtained from its crystal structure.     

The β-carotene dilemma: ESR study with coordinated peroxyl radicals

The apparently unpredictable behaviour of β-carotene in the supplementation of the diet of smokers is discussed in the light of the reactions of peroxyl radicals with β-carotene in the absence of oxygen. The decay of tert-butylperoxyl radicals in the presence of β-carotene was studied at ambient temperature in non-polar solvents by ESR spectroscopy. The primary reaction in the absence of oxygen is interpreted as a spin-trapping effect of a peroxyl radical by β-carotene producing an intermediate labile free radical, which disappears after recombination with a second tert-butylperoxyl radical. The result is the transformation of β-carotene to a diamagnetic compound with two peroxy bonds. In the presence of chelating transition metals with unpaired d-electrons as electron donors the peroxy group of the oxidized β-carotene can be split to alkoxyl free radicals. The primary attack of tert-butylperoxyl radicals is completely inhibited in the presence of vitamin E followed by production of free aryloxy radicals and the presence of oxygen has no significant effect on this reaction. Spin-trapping of peroxyl radicals by the double bond of vitamin A leads to its oxidation in the absence of vitamin E. Transition metal ions such as Co, Cr, Fe, and Mn, known to be present in the aerosol of cigarette smoke, homolyse the peroxyl bonds of peroxidised β-carotene, which results in cell damage.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

ESR study on carboxymethyl chitosan radicals in an aqueous solution

Carboxymethyl chitosan (CMCTS) at a highly concentrated aqueous solution forms hydrogel by ionizing irradiation. To study on radiation-induced reaction mechanism of CMCTS in an aqueous solution, CMCTS radicals formed by reactions with OH radical were observed by ESR method. As a result of ESR spectral analysis, CMCTS radicals were identified as radicals on carboxymethyl groups.     

An ESR study of the cation radicals of azodibenzothiazolyls

Methyl and ethyl derivatives of thiobenzothiazole and their quaternary salts were prepared by the sealed glass tube method. Azinodibenzothiazoles were oxidized by lead(IV) acetate, giving cation radical salts with the boron tetrafluoride anion moiety. The well-resolved esr spectra observed were analyzed by comparing the data with those of a deuteriomethylated radical and with those from the MO calculations. Spin density distribution was consistently determined by computer simulation, including almost the same unpaired electron distribution on the four nitrogen atoms.     

ESR spin-trapping study of radicals produced by donor-acceptor interactions

Nitroso compounds, well known as spin traps, have not been applied for scavenging free radicals formed in reactions with the contribution of chloranil, so far. In our studies we show that spin traps such as nitrosodurene (ND), 2,6-dichloronitrosobenzene (Cl₂NB) react with radicals formed in the reaction between chloranil and durene, yielding stable spin adducts.

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, , . , 2,6-, , , , . , , , , .

     

ESR study of the reversible dimerization of phosphonylfullerenyl radicals

A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1936–1937, August, 1992.     

Reaction of hydroxyl radicals with azacytosines: a pulse radiolysis and theoretical study

Pulse radiolysis and density functional theory (DFT) calculations at B3LYP/6-31+G(d,p) level have been carried out to probe the reaction of the water-derived hydroxyl radicals (*OH) with 5-azacytosine (5Ac) and 5-azacytidine (5Acyd) at near neutral and basic pH. A low percentage of nitrogen-centered oxidizing radicals, and a high percentage of non-oxidizing carbon-centered radicals were identified based on the reaction of transient intermediates with 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), ABTS2-. Theoretical calculations suggests that the N3 atom in 5Ac is the most reactive center as it is the main contributor of HOMO, whereas C5 atom is the prime donor for the HOMO of cytosine (Cyt) where the major addition site is C5. The order of stability of the adduct species were found to be C6-OH_5Ac*>C4-OH_5Ac*>N3-OH_5Ac*>N5-OH_5Ac* both in the gaseous and solution phase (using the PCM model) respectively due to the additions of *OH at C6, C4, N3, and N5 atoms. These additions occur in direct manner, without the intervention of any precursor complex formation. The possibility of a 1,2-hydrogen shift from the C6 to N5 in the nitrogen-centered C6-OH_5Ac* radical is considered in order to account for the experimental observation of the high yield of non-oxidizing radicals, and found that such a conversion requires activation energy of about 32 kcal/mol, and hence this possibility is ruled out. The hydrogen abstraction reactions were assumed to occur from precursor complexes (hydrogen bonded complexes represented as S1, S2, S3, and S4) resulted from the electrostatic interactions of the lone pairs on the N3, N5, and O8 atoms with the incoming *OH radical. It was found that the conversion of these precursor complexes to their respective transition states has ample barrier heights, and it persists even when the effect of solvent is considered. It was also found that the formation of precursor complexes itself is highly endergonic in solution phase. Hence, the abstraction reactions will not occur in the present case. Finally, the time dependent density functional theory (TDDFT) calculations predicted an absorption maximum of 292 nm for the N3-OH_5Ac* adduct, which is close to the experimentally observed spectral maxima at 290 nm. Hence, it is assumed that the addition to the most reactive center N3, which results the N3-OH_5Ac* radical, occurs via a kinetically driven process.     

Photocyclization of iminyl radicals: theoretical study and photochemical aspects

The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G* calculations, using an active space of 14 electrons in 11 orbitals, indicate that S2 should be the spectroscopic state, and its relaxation leads directly to N-O bond breakage due to coupling between the imine pi* and the sigma* N-O orbitals. Subsequent calculations at the B3PW91/6-31+G* level suggest that the resulting iminyl radicals are able to cyclize to the five- or six-membered ring, depending on the presence of a phenyl group as a spacer, a process that has been verified experimentally. The photochemical aspects of the more common five-membered ring formation, such as excited-state quenching, quantum yield, excited-state sensitizers, laser flash photolysis experiments, Stern-Volmer plot, and luminescence measurements, were investigated. These studies indicate that singlet and triplet excited states undergo the same reaction. Emission lifetimes of ca. tau = 10.6 micros for compound 11 are suggestive of triplet parentage, while no fluorescence was detected, in agreement with the computed MEP energy profile.     

ESR study of free radicals chemically generated at high pressure

Free radicals generated by the thermal decomposition of benzoyl peroxide in polyacrylnitrile were studied by the ESR method at 700 MPa between 100 and 130°C. The formation and decay of macroradicals were investigated and the mechanism of macroradical generation is discussed.

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700 MPa 100 130°C. .

     

ESR study of N-acyl-N-alkyl nitroxide radicals from photolysis of N-nitrosoamides

A new method of generation of N-acyl-N-alkyl nitroxide radicals by photolysis of N-nitrosoamides is described. ESR parameters are reported for 17 radicals of the general formula R? CO? N(R′)O^.; the high resolution obtained allows conformational analysis which shows a preferred trans conformation around the CO? N bond and hindered rotation around the N? R′ σ-bond.     

ESR study of free radicals in some new hydrazones of terpenoids class

The effect of gamma-irradiation on some new hydrazones of terpenoids using electron spin resonance (ESR) is reported. Gamma-irradiation of three derivates of hydrazone and of compounds resulting from the condensation of these derivates with terpenoids produces stable free radicals at room temperature. The analysis of the ESR parameters (g-factors and hyperfine coupling) and the simulation performed lead us to conclude that free radicals are of form R–N–NH₂ (arising from hydrazine derivates) and R–N–N=R (arising from condensation compounds). The thermal stability of formed radicals is discussed and the activation energy involved in the process of recombination of free radicals is calculated.     

ESR study of free radicals produced in polyethylene irradiated by ultraviolet light

ESR studies of ultraviolet-irradiated polyethylene (PE) were carried out. Irradiation effects different from those of high-energy radiation are observed. Ultraviolet radiation is absorbed selectively, and especially in carbonyl groups in PE produced by oxidation. Radicals produced were identified as \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CH}_2 \hbox{---}$\end{document}. Some radicals giving a quintet signal stable at room temperature were also observed but remained unidentified. The radical \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} undergoes a mutual conversion with the acyl radical: