Degradation processes in the cellulose/<Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH₂–NO^•–CH₂∼ and/or ∼CH₂–NO^•–CH₃∼ at the first and methyl ^•CH₃ and formyl ^•CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO.     

N-Methylmorpholine-N-oxide ring cleavage registration by ESR under heating conditions of the Lyocell process

Thermal cleavage processes of N-methylmorpholine-N-oxide monohydrate (NMMO) were observed in pure NMMO as well as in cellulose/NMMO solutions by ESR at temperatures of the industrial Lyocell process ( approximately 370K). Generated radicals were attributed to the alkylnitroxyl type radicals -CH(2)-NO-CH(3) in NMMO and additional (and dominated) -CH(2)-NO-CH(2)- in cellulose/NMMO solutions. Formation of both radical types formed due to NMMO ring scission is suggested.     

Studies on the Stabilization of Modified Lyocell Solutions

Additives with functional properties makes the Lyocell process a versatile tool for the creation of new innovative materials beyond the textile sector. Occupying functional groups or active surfaces the additives emphasize the suitability of Lyocell fibers, but simultaneously enhance the complexity of chemical reactions in cellulose/N-methylmorpholine-N-oxide (NMMO) solutions, respectively. Concerning to the concentration acidic ion exchange resins, activated charcoals, carbon black etc. shift the start of decomposition to lower temperatures, decrease the viscosity, enhance the formation of amines as the main degradation products or cause autocatalytic reactions. New routes in stabilization of modified Lyocell solutions applying a polymeric stabilizer system are described. Using calorimetric, UV/VIS, ESR and HPLC analysis the degradation processes and thermal stability of modified Lyocell solutions compared to the unstabilized were studied. Moreover, as kinetic investigations show a distinguished behavior for modified solutions autocatalytic reactions can be suppressed by the stabilizing system. ESR kinetic study of radicals reveals that formation and recombination rates of radical reactions depend on cellulose concentration in Lyocell solutions and additional ingredients.     

Membrane-derived fluorinated radicals detected by electron spin resonance in UV-irradiated Nafion and Dow ionomers: effect of counterions and H2O2

Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed by UV irradiation of Nafion and Dow perfluorinated membranes partially or fully neutralized by Cu(II), Fe(II), and Fe(III). This method allowed the monitoring of ESR signals from the paramagnetic counterions together with the appearance of membrane-derived radical species. The most surprising aspect of this study was the formation of membrane-derived radical species only in the neutralized membranes, and even in the absence of H2O2 in the case of Nafion/Cu(II) and Nafion/Fe(III). In Nafion/Cu(II), ESR spectra from radicals exhibiting hyperfine interactions with three equivalent 19F nuclei (the "quartet") and with four equivalent 19F nuclei (the "quintet") were detected. In Nafion/Fe(II) exposed to H2O2 solutions, the formation of Fe(III) was detected. Upon UV irradiation, strong signals from the chain-end radical ROCF2CF2* were detected first, followed by the appearance, upon annealing above 200 K, of the quartet signal observed in Nafion/Cu(II). In subsequent experiments with Nafion and Dow membranes neutralized by Fe(III), the ROCF2CF2* radicals were formed even in the absence of H2O2, indicating that the role of H2O2 is oxidation of Fe(II) to Fe(III); moreover, in these systems small amounts of the chain-end radicals were detected even without UV irradiation. This result validates the method used to form the radicals: the role of UV irradiation is to accelerate the formation of a signal that is produced, albeit slowly, even in the dark, and possibly during fuel cell operation. The major conclusion is that cations are involved in degradation processes; the point of attack appears to be at or near the pendant chain of the ionomer. Therefore when studying membrane stability, it is important to consider not only the formation of oxygen radicals, such as HO*, HOO*, and O2*-, that can attack the membrane but also the specific reactivity of counterions.     

Detection of the reduced forms of radical adducts on the ESR trace using HPLC‐electrochemical detector‐ultraviolet absorption detector‐electron spin resonance‐MS

To detect and identify the electron spin resonance (ESR) silent forms of the α‐(4‐pyridyl‐1‐oxide)‐N‐tert‐butylnitrone (4‐POBN) radical adducts, an electrochemical detector (ECD) was employed as a reactor in the HPLC‐ECD‐UV absorption detector‐ESR‐MS (HPLC‐ECD‐UV‐ESR‐MS). The ECD was employed to regenerate the radical forms from the reduced forms. The reduced forms of the 4‐POBN/pentyl radical adducts were analyzed using the HPLC‐ECD‐UV‐ESR‐MS. On addition of the ECD applied potential of +0.3 V, a peak appeared on the ESR trace of the HPLC‐ECD‐UV‐ESR‐MS analyses, indicating that the radical forms are regenerated from the reduced forms. The HPLC‐ECD‐UV‐ESR‐MS analyses were also performed for the reaction mixtures of phenylhydrazine with CuCl₂. Two peaks (peaks I and II) were detected on the UV trace (300 nm) of the HPLC‐ECD‐UV‐ESR‐MS. The mass spectra showed that the peak I and peak II compounds are radical and reduced forms of the 4‐POBN/phenyl radical adducts under the ECD applied potential of 0.0 V. Peak I was only detected on the ESR trace under the ECD applied potential of 0.0 V. In addition to peak I, peak II appeared on the ESR trace under the ECD applied potential of +0.3 V, indicating that the reduced forms are oxidized to the corresponding radical forms.     

Formation of sugar radicals in RNA model systems and oligomers via excitation of guanine cation radical

In previous work, we have shown that photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of DNA and its model compounds at 143 K results in the formation of neutral sugar radicals with substantial yield. In this report, we present electron spin resonance (ESR) and theoretical (DFT) evidence regarding the formation of sugar radicals after photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of one-electron-oxidized RNA model compounds (nucleosides, nucleotides and oligomers) at 143 K. Specific sugar radicals C5'*, C3'* and C1'* were identified employing derivatives of Guo deuterated at specific sites in the sugar moiety, namely, C1'-, C2'-, C3'- and C5'-. These results suggest C2'* is not formed upon photoexcitation of G*+ in one-electron-oxidized Guo and deuterated Guo derivatives. Phosphate substitution at C5'- (i.e., in 5-GMP) hinders formation of C5'* via photoexcitation at 143 K but not at 77 K. For the RNA-oligomers studied, we observe on photoexcitation of oligomer-G*+ the formation of mainly C1'* and an unidentified radical with a ca. 28 G doublet. The hyperfine coupling constants of each of the possible sugar radicals were calculated employing the DFT B3LYP/6-31G* approach for comparison to experiment. This work shows that formation of specific neutral sugar radicals occurs via photoexcitation of guanine cation radical (G*+) in RNA systems but not by photoexcitation of its N1 deprotonated species (G(-H)*). Thus, our mechanism regarding neutral sugar formation via photoexcitation of base cation radicals in DNA appears to be valid for RNA systems as well.     

The role of melanin as protector against free radicals in skin and its role as free radical indicator in hair

Throughout the body, melanin is a homogenous biological polymer containing a population of intrinsic, semiquinone-like radicals. Additional extrinsic free radicals are reversibly photo-generated by UV and visible light. Melanin photochemistry, particularly the formation and decay of extrinsic radicals, has been the subject of numerous electron spin resonance (ESR) spectroscopy studies. Several melanin monomers exist, and the predominant monomer in a melanin polymer depends on its location within an organism. In skin and hair, melanin differs in content of eumelanin or pheomelanin. Its bioradical character and its susceptibility to UV irradiation makes melanin an excellent indicator for UV-related processes in both skin and hair. The existence of melanin in skin is strongly correlated with the prevention against free radicals/ROS generated by UV radiation. Especially in the skin melanin (mainly eumelanin) ensures the only natural UV protection by eliminating the generated free radicals/ROS. Melanin in hair can be used as a free radical detector for evaluating the efficacy of hair care products. The aim of this study was to investigate the suitability of melanin as protector of skin against UV generated free radicals and as free radical indicator in hair.     

取代基对氮和氧自由基稳定性影响的理论研究

用MNDO方法，全构型优化，研究了15个氮自由基4-RC6H4NH，和15个氧自由基4-RC6H4O（R＝－H；－OCH3，－Cl，－F，－CN，－COCH3，－NO2，－CH3，－CF3，－SCH3，－C6H5，－Nh2，－BH2，－PH2，－SiH3）的稳定化能．结果表明：－NH2，－CH3，－OCH3，－F基团对自由基起稳定化作用，－CF3，－NO2；－CN，－COCH3，－BH2基团对自由基起去稳定化作用．苯基对氧自由基有较大的稳定化作用，而对氮自由基的稳定性影响较小．－SCH3，－PH2，－SiH3，－Cl基团表现弱的去稳定化作用．计算和实验结果基本相符．     

Analysis of the substituent effect on the reactivity modulation during self-protonation processes in 2-nitrophenols

A voltammetric and spectroelectrochemical ESR study of the reduction processes of five substituted 4-R-2-nitrophenols (R = -H, -OCH(3), -CH(3), -CN, -CF(3)) in acetonitrile was performed. In the potential range considered here (-0.2 to -2.5 V vs Fc+/Fc), two reduction signals (Ic and IIc) were detected; the first one was associated with the formation of the corresponding hydroxylamine via a self-protonation pathway. The voltammetric analysis at the first reduction signal showed that there are differences in the reduction pathway for each substituted 4-R-2-nitrophenol, being the E1/2 values determined by the inductive effect of the substituent in the meta position with respect to the nitro group, while the electron-transfer kinetics was determined by the protonation rate (k(1)+ ) of the anion radical electrogenerated. However, at potential values near the first reduction peak, no ESR signal was recorded from stable radical species, indicating the instability of the radical species in solution. Nevertheless, an intense ESR spectrum generated at the second reduction peak was detected for all compounds, indicating the monoelectronic reduction of the corresponding deprotonated 4-R-2-nitrophenols. The spin-coupling hyperfine structures revealed differences in the chemical nature of the electrogenerated radical; meanwhile, the -CF(3) and -CN substituents induced the formation of a dianion radical structure, and the -H, -CH(3), and -OCH(3) substituents provoked the formation of an anion radical structure due to protonation by acetonitrile molecules of the initially electrogenerated dianion radical. This behavior was confirmed by analyzing the ESR spectra in deuterated acetonitrile and by performing quantum chemical calculations of the spin densities at each site of the electrogenerated anionic radicals.     

Supramolecular photomagnetic materials: photoinduced dimerization of ferrocene-based polychlorotriphenylmethyl radicals

New ferrocenyl Schiff-base polychlorotriphenylmethyl radicals have been synthesized and characterized. The imino group of one such radical undergoes an irreversible trans to cis structural isomerization induced by light. Such photoinduced isomerization has been monitored by UV/Vis and ESR spectroscopy and also monitored by HPLC. ESR frozen solution experiments at low temperature revealed that the cis isomer dimerizes, showing a strong antiferromagnetic interaction. Although numerous photochromic supramolecular systems have been described, such a photoinduced self-assembly process represents the first example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground state species is promoted by a photoinduced dimerization process driven by the formation of hydrogen bonds. DFT calculations on the minimized structure and on the rotational barriers have been performed to establish the origin of such behavior. The effect of the substituents and the media polarity on the photoisomerization of this imine chromophore have also been studied. It has been observed that the efficiency of the process is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents of the ferrocenyl Schiff-base polychlorotriphenylmethyl radicals as well as on the polarity of the solvent.     

Characterization of Autocatalytic Reactions in Modified Cellulose/NMMO Solutions by Thermal Analysis and UV/VIS Spectroscopy

The thermal stability of cellulose/N-methylmorpholine-N-oxide (NMMO) solutions were investigated using UV/VIS spectrometry with a temperature programming cuvette and caloric measurements by means of the Systag calorimeter RADEX (mini-autoclave). Both analytical methods allow to characterize the influences of stabilizers and additives. With the temporal course of the optical density, temperature and pressure thermal runaway reactions with gas evolution and accumulation of chromophoric degradation products were recognized. Kinetic model calculations compared with UV/VIS measurements demonstrate the existence of autocatalytic reactions in cellulose/NMMO solutions. Varying the heating rate autocatalysis can be proved by dynamic caloric measurements as well.     

Oxidation and structural changes in NMMO-regenerated cellulose films

Carbonyl and carboxyl groups introduced by oxidative processes during production and purification of celluloses determine intra- and intermolecular interactions and thus application-related bulk and surface properties of cellulosic materials. We report a comprehensive approach to the quantification of carboxyl and carbonyl groups in cellulose films upon reconstitution from NMMO solutions. Measurements of the excess conductivity were combined with the determination of the molecular weight distribution, quantification of the carboxyl and carbonyl group content, crystallinity and film swelling in aqueous solutions. TEMPO-oxidized, NMMO-regenerated cellulose films were additionally analysed as a reference system for extensive cellulose oxidation. Our reported data demonstrate that dissolution of cellulose in NMMO results in the formation of onic acids, chain degradation, increased ionization and film swelling, whereas TEMPO-oxidation introduced carbonyl groups as well as onic and uronic acids causing a significantly increased charging, ion accumulation and swelling even at higher crystallinity.     

Photocatalytic OH radical formation and quantification over TiO2 P25: Producing a robust and optimised screening method

Photocatalytic oxidation of coumarin to 7-hydroxycoumarin was used in order to identify the optimum conditions and the potential limitations of a photocatalytic screening method.     

Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl3F/alkane systems by ESR spectroscopy at cryogenic temperatures

A summary is presented of ESR results obtained in γ-irradiated disordered CCl₃F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all-trans conformation, i.e. ESR results on the system CCl₃F/heptane. The ESR spectrum of γ-irradiated CCl₃F/heptane consists of a triplet due to heptane radical cations in the extended all-trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the C---C skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C₂ conformation, i.e. ESR results on the system CCl₃F/octane. The ESR spectrum of γ-irradiated CCl₃F/octane indicates that octane radical cations are largely in the gauche-at-C₂ conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end C---H bond and one planar penultimate C---H bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.     

Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered pi- and sigma-bonds

[reaction: see text] Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or sigma-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2*) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1*) leads to only the sigma-dimer. Further encumbrances of 1* with a pair of alpha, beta- or beta, beta- tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3* - 6*) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-dimer by its intense near-IR band. The different temperature-dependent spectral (ESR and UV-vis) behaviors of these phenalenyl radicals allow the quantitative evaluation of the bond enthalpy of 12 +/- 2 kcal mol(-1) for sigma-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.     

氧化锌分散体光反应产生的自由基中间体

溶液中光诱导的电子转移反应已进行了大量的研究。而半导体粉末在水相或非水溶剂中的光化学研究也与自俱增^[1-3]。这种光化学与成像体系、太阳能转换以及光催化或污物的光降解有关。因此,越来越引起人们的重视。     

ESR studies of photosensitized degradation of poly(L‐lactic acid) via photoionization of dopant

The photosensitized degradation of poly(L ‐lactic acid) (PLA) via an anionic reaction process was studied using spectrophotometry, electron spin resonance (ESR), and gel permeation chromatography (GPC) measurements. PLA film doped with N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) was irradiated at 77 K using UV light (λ_c = 356 nm) by which the PLA matrix itself cannot be directly excited. After photoirradiation, a new broad absorption band appeared over the original spectrum due to TMPD⁺ ·, which was produced by two‐photon ionization. The ESR spectrum of the irradiated sample indicated the presence of the TMPD⁺ · radical and main‐chain scission radical of PLA. During the thermal annealing at 0 °C, the latter radical changed to another radical species by dehydrogenation of the alpha hydrogen of the PLA main chain. TMPD⁺ · was extremely stable at room temperature for 7 d. However, by thermal annealing at 40 °C, all the radicals decayed due to the enhanced molecular motions near T_g of PLA (58.7 °C). Spectral simulation for the obtained ESR spectra revealed the relative amounts of four radicals: TMPD⁺ ·, a main‐chain scission radical, a main‐chain tertiary radical, and an unknown radical. The last one was tentatively assigned to the PLA radical anion because of its short decay time. GPC measurements clearly indicated a decrease in the molecular weight of PLA after irradiation. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 706–714, 2001     

Degradation of polymer coating systems studied by positron annihilation spectroscopy. II. Correlation with free radical formation

The photo‐degradation of polymer coating systems due to irradiation by UV and Xenon light sources is studied using positron annihilation spectroscopy and electron spin resonance (ESR). Doppler broadened spectra of positron annihilation, as a function of slow positron implantation energy and ESR spectra, are measured in two types of polyurethane which were exposed, ex situ, to UV irradiation for up to 800 h. The UV irradiation systematically decreases the S parameter as a function of exposure duration and increases the ESR signals. Thus, significant S parameter decrease is correlated with the ESR signal increase resulting from photo‐degradation of polymers due to UV irradiation. Parallel in situ positron annihilation and ESR experiments are performed as a function of Xenon light exposure for up to 100 min. These results show that the photo‐degradation of the polyurethane coatings involves initial free‐radical formation, which is correlated with the subnanometer defects detected by positron annihilation spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1289–1305, 1999     

Studies on the Thermostability of Modified Lyocell Dopes

Modification of Lyocell dopes with special additives enables the creation of new innovative materials. Additives with functional groups and active surfaces may initiate complex chemical reactions in cellulose/N-methylmorpholine-N-oxide monohydrate (NMMO) solutions. The effect of carboxylic groups on the thermostability was investigated by incorporating of sugar acids, acidic ion exchange resins (IER) and superabsorbing polymers (SAP) into the solutions. Whereas the sugar acids show strict correlations between the carboxyl group content in solution and onset temperature, viscosity reduction, coloration and by-product formation, the additives show an induction time, which finally leads to accelerated degradation reactions. The cellulose/NMMO solutions were additionally characterized by means of UV/VIS spectroscopy.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.