Radical R1 Research Articles

Spectroscopic analysis of residual radicals of gamma sterilized UHMWPE with fractional order differential operators

The purpose of this research was to evaluate the Grunwald-Letnikov definition of fractional order derivatives for determining the fractional derivative of ESR signals from UHMWPE free radicals using the fitted Gaussian distribution method. Specifically, the study focused on analyzing two long-lasting oxygen-induced residual radicals di- or tri-enyls with a carbon center radical (R1) and the oxygen-containing radical (R2). The impact of the derivative order on ESR spectral parameters, such as the Landé g-factor and peak-to-peak separation, was analyzed, and new spectral parameters were established for both radicals. The samples were measured in an ESR tube using ESR power saturation techniques at room temperature. Following the measurements, the fitted Gaussian distribution of the ESR signals was used to determine the fractional derivative using the Grunwald-Letnikov definition. Two estimators (I and II) were developed for both radicals, and their values were found to be 9.19 and 4.27 for Radical R1 and 11.51 and 2060.62 for Radical R2. Our results showed that the fractional derivative approach provided accurate and reliable estimations for the radicals. This method can be easily implemented for various ESR signals and can be useful in many applications, including materials science and biomedical research. The accuracy and reliability of our estimators were confirmed by comparing them with the results obtained from the conventional method. Our findings have important implications for materials science and biomedical research, where ESR signals are widely used. This method can help researchers to obtain more accurate and reliable estimations of radicals, which can ultimately lead to more accurate and reliable conclusions in their studies. Additional research is required to assess the effectiveness of this approach on various ESR signals and to investigate its potential use in other fields of study.

Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

The bay-capping mechanism on PAH armchair edges and the kinetics of acetylene addition to 6–6–5 and 5–6–5 bays have been explored by ab initio/RRKM-ME calculations. The bays on the edges were modeled by C21H11 and C20H9 radicals produced by H abstractions from 7H-benzo[c]cyclopenta[e]pyrene and dicyclopenta[cf]pyrene. The C20H9 + C2H2 reaction is shown to have a low entrance barrier and to rapidly form the capped product, indaceno[2,1,8,7-cdefg]pyrene, along with ethynyl substituted dicyclopenta[cf]pyrene at temperatures above 1400 K. The reactivity of C21H11 is shown to be governed by the location of the unpaired electron; the π radical R1 formed by H abstraction from the CH2 group in 7H-benzo[c]cyclopenta[e]pyrene reacts with C2H2 very slowly owing to a high entrance barrier, with the bay-capping rate constant approaching 10−16 cm3 molecule−1 s−1 only at temperatures above 2000 K. This result reaffirms that the growth of π aryl radicals via acetylene addition is inefficient and reflects the generally low reactivity of such radicals where the spin density is highly delocalized over the entire polyaromatic system. Alternatively, the σ C21H11 radical R2 produced by H abstraction from the five-membered ring at the bay rapidly reacts with C2H2 forming the bay-capped product, with the rate constant on the order of 10−12 cm3 molecule−1 s−1 at T ≥ 1500 K. Rate constants for the capping reactions at the 6–6–5 and 5–6–5 bays are compared with those at the 6–0–6, 6–6–6, and 6–5–6 bays. The site-specific bay-capping rate constants have been utilized in kMC simulations of the PAH growth and the results showed measurable differences when the 6–6–5 and 5–6–5 bay-capping reactions are taken into account, including an increase of the growth rate and the formation of closed-shell PAH and a rise of the number of embedded five-membered rings accompanied with a slight decrease of their overall amount.

ANTIOXIDANT ACTIVITY AND CAPACITY OF SILVER NANOPARTICLES BIOSYNTHESIS OF COMMON FRUITS AQUEOUS EXTRACTS OF THE SUNDARBAN FOREST

Fruits provide numerous health-promoting polyphenols and antioxidants. The Sundarbans mangrove forest of Bangladesh produces various fruits, and off them ten to twelve are known to be consumed or used as an ingredient in food preparations. These fruits were used in this study to evaluate the total polyphenols (TPH) contents following the method of Folin-Ciocalteu’s; and the antioxidant activity by measuring 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals scavenging, reducing power and total antioxidant capacity. Additionally, capacity of silver nanoparticles (AgNPs) biosynthesis of the fruits was determined spectrophotometrically. Sonneratia apetala fruit showed the highest TPH content (45.7 mg of gallic acid equivalent (GAE)/g powder). The fruit also showed the largest antioxidant activity attributed to the DPPH free radicals scavenging, reducing power, and total antioxidant capacity. Additionally, S. apetala fruit showed the highest capacity in the AgNPs biosynthesis (OD, 0.41; 10 mg powder/mL). Avicennia officinalis, Ceriops decandra, Heritiera fomes, and S. apetala fruits showed scavenging of 50% DPPH free radicals at 81, 34.7, 61.2, and 33.5 µg powder/mL, respectively. Contents of total polyphenols in these fruits displayed strong positive correlations with reducing power (r2 = 0.97), total antioxidant capacity (r2 = 0.85) and scavenging DPPH free radicals (r2 = 0.85) whereas that for the AgNPs biosynthesis capacity was small (r2 = 0.32). Thus, the aqueous extract of S. apetala fruit is the most potential in antioxidant activity and biosynthesis of AgNPs.

Superoxide free radical spin-lattice relaxivity: A quench-assisted MR study.

QuEnch-assiSTed (QUEST) MRI provides a unique biomarker of excessive production of paramagnetic free radicals (oxidative stress) in vivo. The contribution from superoxide, a common upstream species found in oxidative stress-based disease, to the QUEST metric is unclear. Here, we begin to address this question by measuring superoxide spin-lattice relaxivity (r1) in phantoms. Stable superoxide free radicals were generated in water phantoms of potassium superoxide ( . To measure r1, 1/T1 of different concentration solutions of KO2 in the presence and absence of the antioxidant superoxide dismutase were measured. The 1/T1 confounding factors including acquisition sequence, pH, and water source were also evaluated. The T1 -weighted signal intensity increased with KO2 concentration. No contribution from pH, or reaction products other than superoxide, noted on 1/T1 . Superoxide r1 was measured to be 0.29 mM-1 s-1 , in agreement with that reported for paramagnetic molecular oxygen and nitroxide free radicals. Our first-in-kind measurement of superoxide free radical r1 suggests a detection sensitivity of QUEST MRI on the order of tens of μM, within the reported level of free radical production during oxidative stress in vivo. Similar studies for other common free radicals are needed.

EVALUATION OF THE MODEL PREDICTION TOXICITY (LD50) FOR SERIES OF 42 ORGANOPHOSPHORUS PESTICIDES

Structure-Toxicity Relationships have been studied for a set of 42 organophosphorous pesticides (OPs) through multiple linear regression (MLR) and artificial neural networks (ANN). A model with three descriptors, including: total lipophilicity [log (P)], widths radicals R1 [(LR1)] and R2 [(LR2)] has achieved good results in phase Training and phase prediction of toxicity [log LD50 (lethal dose 50, Oral rat)]. The linear model (MLR: n=40, r²=0.86, s=40 and q2 = 0.66) and non-linear model with a configuration [3-6-1] (ANN: r²=0.95, s=0.73 and q2 = 0.17) have proved very successful and complementary. The selected descriptors indicate the importance of lipophilicity and widths radicals R1 and R2 in the contribution of the toxicity of pesticides derived from OPs used in this study. This information is relevant for the design of a new model of non-toxic pesticides OPs.

Photoinduced transformation of glycerol in titania suspensions. (An EPR spin trapping study of radical intermediates)

Formation of radical intermediates upon photocatalytic (UVA) glycerol transformation in aerated aqueous TiO2 (P25) suspensions was followed by EPR spin trapping technique. Character and concentration of the spin-adducts generated by the addition of the radical species to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap are significantly affected by the UVA radiation dose and by the initial composition of the photocatalytic system. The DMPO spin-adducts of hydroxyalkyl radicals R1 and R2 detected in the aerated aqueous P25 suspensions containing glycerol are compatible with the H-abstraction processes attributed to the interaction of physisorbed glycerol molecules with the trapped holes or to the reaction of free hydroxyl radicals with glycerol in solution. Addition of fluoride at various concentrations has no effect on the character and concentration of DMPO spin-adducts observed upon in situ UVA photoexcitation of photocatalytic system P25/DMPO/glycerol under given experimental conditions (pH > 7).

ESR spectroscopy of alanine impacted by high energy irradiations for wide range dosimetry

The ESR spectrometry, based on alanine dosimeters, is a standard tool for dosimetry of different type ionizing radiations. The calibrated dosimeters and ESR readers operate rather reliably in the dose range up to 200 kGy. In this study, the ESR spectrometry of stable free radicals in alanine, induced by penetrative X- and γ-rays as well as neutron and stopped proton irradiations covering the dose range from a few Gy to a few MGy, has been performed in order to create the generalized calibration function for the wide range dosimetry. The simulations based on the cumulative generation of ESR active stable radicals R1, R2 and R3 enabled us to derive the generalized calibration model ascribed to the irradiation fluence from 1010to 2 × 1016particle/cm2normalized using 1 MeV neutron equivalent.

ChemInform Abstract: Labile Alkoxyamines: Past, Present, and Future

AbstractReview: 88 refs.

Assessment of residual radicals in γ-sterilized shelf-aged UHMWPE stabilized with α-tocopherol

In this study, we investigated the effects of α-tocopherol (vitamin E, (α-T)) on residual free radicals in ultra-high molecular weight polyethylene (UHMWPE). More specifically, its effects on the carbon-centered polyenyl radical R1 [(–˙CH–[CHCH–]m–) with m > 3] and the oxygen-centered di-enyl or tri-enyl radical R2 in shelf-aged γ-irradiated UHMWPE stabilized (blended) with α-T were examined. For the UHMWPE/α-T blends, the α-T concentrations were 5000 ppm, 10,000 ppm, 100,000 ppm, and 150,000 ppm; the blends were γ-irradiated (∼30-kGy) at room temperature (23 °C) and shelf-aged under light for five years in air. Electron spin resonance (ESR) power saturation techniques had been employed at room temperature (23 °C), which revealed that α-T was more effective in reducing residual radicals when irradiation was performed in air as compared to a nitrogen environment. While the total quantities of residual radicals reduced for the α-T concentration of up to 10,000 ppm, unexpected increasing trends in radicals were observed for 100,000 ppm and 150,000 ppm of α-T concentrations. The average crystallite sizes were found to increase from approximately 40 nm to 65 nm at L (110) and from approximately 28 nm to 42 nm at L (200), as suggested by wide angle X-ray scattering (WAXS) data, which indicated that total amount of residual radicals had strong dependence on the average crystallite sizes, as opposite to % crystallinity of these UHMWPE/α-T blends. In addition, behavior of oxygen-centered di-enyle or tri-enyl radicals (R2) was investigated using Raman spectroscopic methods and it was reported experimentally for the first time, to our knowledge, that radical R2 resides at the interphase region of UHWMPE and has direct proportion with % interphase contents of UHMWPE. Our result may be relevant when considering the long-term resistance of UHMWPE/α-T blends to oxidation processes.

Atmospheric Oxidation Mechanism of 2,7-Dimethylnaphthalene is Different from That of Monocyclic Aromatic Benzenes. A Theoretical Study

The atmospheric oxidation mechanism of 2,7-dimethylnaphthalene (27DMN) initiated by OH radical is investigated at levels of BB1K and G3MP2-RAD//BH&HLYP. The reaction is mainly initiated by OH addition to the C(1) position to form radical adduct R1. In the atmosphere, R1 reacts with O(2) via two comparable pathways as direct H abstraction to form 27DMN-1-ol and as O(2) addition to the C(2) position to form R1-2OO radicals, both being slow with rate constants of 10(-18)-10(-17) cm(3) molecule(-1) s(-1). The R1-2OO-s conformer is found to be important in 27DMN oxidation whereas the role of the R1-2OO-a conformer is negligible. Radicals R1-2OO-s have three comparable pathways: ring closure to tricyclic intermediate R1-29OO-s, intramolecular H shift from -OH to -OO to form dicarbonyl products, and reactions with atmospheric NO and/or HO(2), etc. The ring closure to R1-29OO-s is endothermic and reversible whereas similar ring closures in benzene and toluene oxidations are exothermic and irreversible. The intramolecular H shift becomes prominent because of the reversibility of ring closure in 27DMN oxidation and is responsible for rapid formation of dicarbonyl compound (C(12)H(12)O(2)) in simulation chamber studies. The oxy radical (R1-2O herein) would not undergo C(1)-C(2) cleavage to form dicarbonyl, as suggested in previous studies; instead, R1-2O would close the ring to form epoxide radical R1-23O. Radical R1-29OO-s would recombine with the atmospheric O(2) and isomerize to diepoxide radical R1-23O-89O-s at comparable rates, and study on their further reactions is desirable.

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5' and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4'. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH(2)OH hydroxymethyl group is a better radical trapping site than other positions.

Free Radicals in l-Arginine·HCl·H2O Single Crystals X-irradiated at 66K- EPR, ENDOR, EIE and DFT Studies

X-irradiation of L-arginine hydrochloride monohydrate crystals at 66 K led to at least five radicals detectable with K-band Electron Paramagnetic Resonance (EPR), Electron-Nuclear DOuble Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. Radicals R1a and R1b were identified as carboxyl-centered radicals from one-electron reduction, (H(2)OOC*)CH (NH(3))(+)(CH(2))(3)NHC(NH(2))(2)(+) in two different conformations arising from the two geometrically distinct molecules of the asymmetric unit in the crystal. Density-Functional Theory (DFT) calculations on cluster models constructed separately for each molecule of the asymmetric unit support the assignments. Radical R2 was identified as the decarboxylation radical, *CH(NH(3))(+)(CH(2))(3)NHC (NH(2))(2)(+). Radical R3, with two proton couplings and one nitrogen coupling, was identified as a radical with the unpaired electron localized on the guanidyl group, (-)(OOC)CH(NH(3))(+)(CH(2))(3)NHC*(NH(2))(2)(+). R3 is a product of one-electron reduction different from radicals R1a and R1b. DFT calculations on a cluster model reproduced the experimental values very well and thus supported the assignment of R3. Geometry optimization indicated that the guanidyl group transformed from planar to pyramidal upon trapping the electron. Radical R4 was identified as a side chain dehydrogenation radical, (-)(OOC)CH(NH(3))(+)(CH(2))(2)*CH NHC(NH(2))(2)(+). It was not possible to collect sufficient data to identify radical R5, although it clearly exhibited hyperfine coupling to one nonexchangeable beta-proton.

Studies on structures and properties of protonated pyridoxine, (PN+H) [rad] radicals and (PN+H) − anions with density functional theory

The hydrogenated pyridoxine (PN) species in cationic, neutral, and anionic states have been studied. The structures, energetics, and theoretical electron affinities are predicted by means of the B3LYP/DZP++ method. These radical and anionic species come from consecutive electron attachment to the corresponding protonated (PN+H) + cations in low pH environments. The most reliable proton affinities of PN (protonation at N1) are predicted to be 232.3 kcal/mol. The radicals range within 11.5 kcal/mol in relative energy and radicals r1 (attached H to N1 site of PN) are the lowest energy. Structures r7 and r8 are predicted to extrude water molecule. The theoretical adiabatic electron affinities (AEA), vertical electron affinities (VEA), and vertical detachment energies (VDE) are estimated. The ranges of AEA for six radicals are from 0.44 (or 0.35) to 1.76 eV. Taking into account the solvent effect, protonation of PN in aqueous solution is more stable and neutral hydrogenated PN more readily takes up solvated electrons.

EPR, ENDOR and DFT study of l-lysine·HCl·2H 2O single crystals X-irradiated at 66 K

Electron Paramagnetic Resonance (EPR), Electron-Nuclear DOuble Resonance (ENDOR) and ENDOR-Induced EPR (EIE) results indicated at least three radicals produced in l-lysine·HCl·2H 2O crystals irradiated at 66 K. EPR from radical R1 dominated the spectra and it was identified as the carboxyl anion, (H 2OOĊ) CH (NH 3) + (CH 2) 4 (NH 3) +. Density-Functional Theory (DFT) calculations supported the assignment and indicated that the carboxyl group transformed from planar to a pyramidal configuration following electron trapping. Two small couplings detected from R1 were ascribed to protons transferred to the carboxyl group from neighboring molecules through intermolecular hydrogen bonds. Radical R2 was identified as the product of decarboxylation, ĊH (NH 3) +(CH 2) 4 (NH 3) +. Although it was not possible to obtain R2's α-coupling tensor from the experiment, EPR simulations and DFT calculations provided strong support for the identification. Radical R3 exhibited several β-couplings but could be detected only in one plane and could not be identified.

Kinetics of the Thermal Disappearance of Radicals Formed During the Radiolysis of Aspartic Acid

The radiolysis of the isomers L-, D- and DL- of the aspartic acid, in solid polycrystalline state, was investigated at room temperature. The analysis of their ESR spectra indicated the formation of at least two radicalic entities. The radical, identified as R3, resulting from the deamination of the acid, exhibits the highest concentration and thermal resistance. Possible mechanisms of formation of three radical species are suggested, based also on literature data. The kinetics of the disappearance of radical R3 indicated a complex mechanism. Three possible variants were suggested for this mechanism.

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.

Influence of the Metallofragment on the Regioselectivity in the Addition of the Radicals •CCl3 and •But to (η2‐C60)Ir H(CO) (PPh3)2, (η2‐C60)Ir H(CO)[(+)DIOP], (η2‐C60)Os(CO)(PPh3)2(CNBut) and on the Stability of the Spin‐adducts

In this paper we have present the results of study for electronic influence of the metal fragment on the regioselectivity of addition of •CCl3 (R1 ) and •Bu t (R2 ) radicals to the complexes (η2‐C60)IrH(CO)(PPh3)2 (I), (η2‐C60)IrH(CO)[(+)DIOP] (II), (η2‐C60)Os(CO)(PPh3)2(CNBut) (III) and on the stability of spin‐adducts (SA). It was found that the addition of R1 and R2 to I–III unlike methano‐ or pyrrolidinofullerenes the ESR spectra exhibit signals from some regio‐isomers which substantial different by the g‐factor. The assignment of the individual signals to definite regio‐isomers was made. The rate constants of the addition of radicals R1 and R2 to carbon atoms of individual bonds in I–III were determined using the method of competing reactions.

Visualizing the Dose Distribution and Linear Energy Transfer by 1D and 2D ESR Imaging: A Potassium Dithionate Dosimeter Irradiated with C6+ and N7+ Ions

We report the application of one- and two-dimensional (1D and 2D) spectral-spatial electron spin resonance imaging (ESRI) for visualizing the dose distribution and linear energy transfer (LET) in a potassium dithionate, K2S2O6 (PDT), dosimeter irradiated with the heavy ions C(6+) and N(7+). The ESR spectrum in the irradiated PDT consists of a superposition of two isotropic signals assigned to two *SO3(-) radicals, R1 and R2, with no hyperfine splittings and slightly different g values. The 1D ESRI profiles clearly indicate the spatial penetration of the beams and the location of the sharp maximum dose, the "Bragg peak", detected for each beam. The depth penetrations are different: approximately 2.3 mm for C(6+) and approximately 1.8 mm for N(7+) beams, +/-0.1 mm; beyond these limits, no radicals were detected. 2D spectral-spatial ESRI images reflect both the dose distribution and the spatial dependence of the relative intensities of radicals R1 and R2, an effect that is assigned to the depth variation of the LET. This study has demonstrated that ESRI is a promising new method for dose and LET determination. Of particular interest are applications in the field of radiotherapy with heavy ions, because in this case the Bragg peak is pronounced and the dose can be focused at specific depths while the surrounding areas are protected.

Free Radical Conformations and Conversions in X-Irradiated Single Crystals of l-Cysteic Acid by Electron Magnetic Resonance and Density Functional Theory Studies

Single crystals of L-cysteic acid monohydrate were X-irradiated and studied at 295 K using EPR, ENDOR, and EIE techniques. Three spectroscopically different radicals were observed. These were a deamination radical reduction product (R1), and two oxidation products formed by hydrogen abstraction (radicals R2, R3). R2 and R3 were shown to exhibit the same chemical structure while exhibiting very different geometrical conformations. Cluster DFT calculations at the 6-31G(d,p) level of theory supported the experimental observations for radicals R1 and R2. It was not possible to simulate the R3 radical in any attempted cluster; hence, for this purpose a single molecule approach was used. The precursor radicals for R1, R2, and R3, identified in the low-temperature work on L-cysteic acid monohydrate by Box and Budzinski, were also investigated using DFT calculations. The experimentally determined EPR parameters for the low-temperature decarboxylated cation could only be reproduced correctly within the cluster when the carboxyl group remained in the proximity of the radical. Only one of the two observed low-temperature carboxyl anions (stable at 4 and 48 K) could be successfully simulated by the DFT calculations. Evidence is presented in support of the conclusions that the carboxyl reduction product already is protonated at 4 K and that the irreversible conversion between the two reduction products is brought forward by an umbrella-type inversion of the carboxyl group.

Examination of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene

Electron spin resonance (ESR) measurements were conducted to determine the thermal stability and structure of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene (UHMWPE). Employing a microwave saturation technique in conjunction with low-temperature (−133 °C) ESR, it was revealed that two most likely radicals, polyenyl (R1) and oxygen-centred di- or tri-enyl (R2) radicals, constitute the so-called oxygen-induced radicals (OIR). The reported single line due to OIR has in fact two resonance structures, one can be detected as an isolated signal (due to polyenyl, R1) at low microwave power (near 0.01 mW) and the other (oxygen-centred di- or tri-enyl) is prominent at high microwave power (>10 mW). The g-values of the radicals R1 and R2 were found to be 2.0044 and 2.0056, respectively. While R1 is a well-defined singlet with Δ H pp = 5.5 G, R2 has weak hyperfine lines (6) with proton coupling constant a H = 4.8 G. Annealing tests near 130 °C and 140 °C further suggest that R1 resides in crystalline regions, and R2 near crystalline surface with a dangling bond or in the voids or imperfections within the crystalline regions favouring oxygen attachment.