Effect of substituents on spin density in benzimidazole nitronyl nitroxide radicals studied by electron spin resonance

Several novel benzimidazole‐3‐oxide‐1‐oxyl radicals with substituents at 5 and/or 6 position were synthesized. The ESR analysis of nitrogen hyperfine coupling constants (hfccs) revealed that substituents at 5 and 6‐position affect the spin density to greater extent than substituents on the phenyl ring at 2‐position. Density functional theory calculations of nitrogen hfccs were performed using several different Pople type basis sets, as well as double and triple zeta quality individual gauge for localized orbital (IGLO‐II, IGLO‐III) and electron paramagnetic resonance (EPR‐II, EPR‐II) basis sets. Experimental and theoretical hfccs are compared. Copyright © 2009 John Wiley & Sons, Ltd.     

Nature of plasma-induced radicals on crosslinked methacrylic polymers studied by electron spin resonance

Plasma-induced radicals of several crosslinked methacrylic polymers such as poly(ethyleneglycol dimethacrylate) (PEDMA), poly(2-hydroxyethylmethacrylate) (PHEMA) and polymethacrylamide (PMAAm) were studied by electron spin resonance (ESR). The observed ESR spectra did not exhibit a drastic difference in the spectral feature caused by the effect of crosslinking. All the spectral features can be represented by “nine-line spectra” as a major spectral component similar to those of linear methacrylic polymers such as polymethacrylic acid (PMAA). A pronounced effect of crosslinking, however, has emerged on the specific formation in the radical structure and the stability of radicals formed, especially in PEDMA. The formation of fewer kinds of radical in PEDMA is apparently caused by the high degree of crosslinking which leads to a suppression of the occurrence of depolymerization on plasma irradiation.     

Electron spin resonance study of radicals generated in cellulose/N-methylmorpholine solutions after flash photolysis at 77 K

The processes of radical formation in N-methylmorpholine-N-oxide monohydrate (NMMO) and cellulose/NMMO solutions were studied by ESR at 77 K under high-power UV (lambda = 248 nm) excimer laser flash photolysis. Radicals mainly generated were attributed to the nitroxide type radicals -CH2-NO*-CH2- and -CH2-NO*-CH3 at the first step and methyl *CH3 and formyl *CHO radicals at the second step of the photoreaction. Kinetic studies of these radicals revealed that formation and recombination rates of the radicals depend on the cellulose concentration in cellulose/NMMO solutions and the concentration of additional ingredients, e.g. Fe(II) and propyl gallate. Even at frozen state temperature, acceleration or quenching of radical reaction processes was found. The proposed scheme of UV light-induced NMMO degradation during irradiation based on ESR data correlates well with independently obtained results based on high-performance liquid chromatography (HPLC). The analysis of degradation products by HPLC, e.g. aminoethanol and acetaldehyde, supports the assumption concerning a radical-initiated ring opening of NMMO.     

An electron spin resonance study of azaallyl radicals

The ESR spectrum of the 2-aza-allyl radical, Me2CNCMe₂, has been recorded. A series of 1-aza-allyl radicals have been generated by reaction of 2-alkyl-4,4-dimethyloxazolines with t-butoxyl radicals and their ESR spectra studied. In this reaction 2-alkyl-4,4-dimethyloxazolin-5-yl radicals are also formed.     

Radiation-induced radicals in glucose-1-phosphate. I. Electron paramagnetic resonance and electron nuclear double resonance analysis of in situ X-irradiated single crystals at 77 K

Electron magnetic resonance analysis of radiation-induced defects in dipotassium glucose-1-phosphate dihydrate single crystals in situ X-irradiated and measured at 77 K shows that at least seven different carbon-centered radical species are trapped. Four of these (R1-R4) can be fully or partly characterized in terms of proton hyperfine coupling tensors. The dominant radical (R2) is identified as a C1-centered species, assumedly formed by a scission of the sugar-phosphate junction and the concerted formation of a carbonyl group at the neighboring C2 carbon. This structure is chemically identical to a radical recently identified in irradiated sucrose single crystals. Radical species R1 and R4 most likely are C3- and C6-centered species, respectively, both formed by a net hydrogen abstraction. R3 is suggested to be chemically similar to but geometrically different from R4. Knowledge of the identity of the sugar radicals present at 77 K provides a first step in elucidating the formation mechanism of the phosphoryl radicals previously detected after X-irradiation at 280 K. In paper II, the chemical identity, precise conformation, and possible formation mechanisms of these radical species are investigated by means of DFT calculations and elementary insight into the radiation chemistry of sugar and sugar derivatives is obtained.     

Determination of polycyclic aromatic hydrocarbons by electron spin resonance spectrometry

The use of electron spin resonance spectrometry with a modern instrument is described for the determination of polynuclear aromatic hydrocarbons (PAHs) down to nanogram levels, after adsorption on calcined silica/alumina. A single PAH or the total number of moles of PAHs can be determined. Implication for liquid chromatography are discussed.     

Stabilization and reactions of tetramethylurea radical cations in Freon matrices at 77 K: Intramolecular hydrogen transfer,ion-molecular reactions,and fragmentation by electron spin resonance study

ESR spectra for -irradiated at 77 K solutions /0.02–16%/ of tetramethylurea /TMU/ in CFCl₃ and Freon-113 have been studied. TMU^+. radical cations radiolytically produced in dilute solutions have been shown to undergo intramolecular hydrogen transfer upon photobleaching resulting in CH₂N= type radical. Evidence for intermolecular proton transfer in TMU^+. radical cations after annealing to phase transition temperature /110–120 K/ in Freon-113 was obtained. Primary radical cations of TMU^+. at their ground state take part in ion-molecular reaction via proton transfer. Molecular cations in their excited states may undergo fragmentation producing Me₂N radicals, which were trapped in liquid phase by t-BuNO as a spin trap.     

Nature of mechanoradical formation of substituted celluloses as studied by electron spin resonance

Mechanically induced free radical (mechanoradical) formation of several substituted celluloses such as carboxylmethyl cellulose, chitin, and chitosan was studied based on electron-spin resonance (ESR) in comparison with those of plasma-induced radicals. Room temperature ESR spectra had multicomponent spectra and were different in pattern from each other. The mechanoradical concentration gradually decreased after reaching the maximum value in each substituted polysaccharide, accompanied by a decrease in molecular weight in the course of vibratory milling. One of the most intriguing facts is that the component radicals are all glucose-based radicals as in the case of plasma irradiation, although it is known that mechanoradicals are formed by 1,4-glucosidic bond cleavage of polysaccharides.     

Tracking nitrogen-to-nickel ratio and prevalent paramagnetic species in synthetic diamonds by electron spin resonance at 90 K

Nitrogen and nickel are the main impurities in the type Ib synthetic diamonds obtained under high pressure and high temperature conditions using nickel-containing catalysts. Nitrogen enters the synthesized diamond from the air. The nickel/nitrogen ratio for the two types of diamonds, obtained with and without an aluminum getter that prevents the diamond crystal lattice from nitrogen penetration, was studied by electron spin resonance at 90 K.     

Identificatini and determination of aromatic nitro compounds by electron spin resonance spectrometry

Aromatic nitro compounds are quantitatively converted to the corresponding anion-radical form by electron tranfer at the surface of thermally activated magnesium oxide. The radicals are stable in the absorbed state, and the reaction is useful for the identification and determination of the parent compound. The method is applied to detect and quantify various nitrobenzenes, nitrotoluenes and teryl by electron spin resonance spectrometry. For 2,4,6-trinitrotoluene (TNT), the detection limit was ca. 10 ng; the r.s.d. at the 2 μg level was ± 1.8%. The analysis of hand-swab extracts that contained TNT at the trace levels demonstrateds a potentially important application of the method.     

Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.