X-irradiated Single Crystals Research Articles

Formula omitted] radicals for EPR dosimetry: X- and Q band EPR study and LET dependency of crystalline potassium dithionate

The structures of the free radicals formed by the irradiation of potassium dithionate (K2S2O6) with 60Co γ-rays and 14N7+ ions were investigated by EPR to further examine a recently proposed LET effect in this material. Two types of SO3− radical ions were identified in X-irradiated single crystals by measurements at X- and Q bands. One of these (S1) exhibited 33S hyperfine couplings A⊥ = 12.49, A|| = 15.60 mT, the other (S2) A⊥ = 11.29, A|| = 13.92 mT. The g-factors were nearly isotropic, g⊥ = 2.0010, g|| = 2.0003; and g⊥ = 2.0026, g|| = 2.0008, respectively. The 33S hyperfine coupling tensors and g-tensors were axially symmetric about the trigonal <c>-axis, coinciding with the direction of the S–S bonds of the two non-equivalent S2O62− ions in the crystal. A model for the radiation damage was proposed in which the SO3− radical ions retain the orientation of the SO3 groups, aligned along the trigonal axis. The structure of a third main radical species (S3) with g⊥ = 2.0026, g|| = 2.0052 could not be unambiguously assigned, due to undetected 33S features. The relative integrated intensities of S1, S2 and S3 depended on the radiation quality and were approximately estimated as 0.18: 0.65:0.17 for 60Co γ-rays and 0.47: 0.38: 0.15 for 14N7+ ions. Additional weak lines on the low field side of the main signal were tentatively attributed to SO2− radical ions. An even weaker strongly anisotropic pair of lines was attributed to SO3− radical pairs separated by 0.93–0.95 nm along the trigonal axis.

Primary Oxidation Products of 5-Methylcytosine: Methyl Dynamics and Environmental Influences

The primary oxidation product in X-irradiated single crystals of 5-methylcytosine hemihydrate and 5-methylcytosine hydrochloride has been studied at 10 K, using electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) spectroscopies. The radical is characterized by large couplings to the methyl protons and appears to be deprotonated at N1 in both crystal systems. In the hydrochloride crystal the methyl group is completely frozen at 10 K, whereas in the hemihydrate crystal it undergoes tunneling rotation. For the hemihydrate crystal, four ENDOR lines associated with transitions within the A and E rotational states were followed in three planes of rotation. Large ENDOR shifts as measured by saturation of the high- and low-field parts of the EPR spectrum indicate that the rotation is rather slow. Sidebands due to mixing of A and E rotational states are expected for slow rotation and were observed in both the EPR and the EIE spectra. The ENDOR shifts and the sideband frequencies indicate a tunneling splitting between 40 and 60 MHz. Estimates of the barrier to rotation in both crystalline systems were calculated using cluster and single-molecule density functional theory methods, and the results are consistent with those obtained by analysis of the experimental results.

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.

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.

M(CO) 4PPh 3] [rad]− radicals (M = Cr, Mo, W): DFT and single crystal EPR investigations

[M(CO) 4PPh 3] − (M = Mo, W) were trapped at 77 K in X-irradiated single crystals of M(CO) 5PPh 3 and studied by EPR. Structures of [M(CO) 4PPh 3] − (M = Cr, Mo, W) were optimized by DFT; predicted g and 31P-hyperfine tensors agree with experiments for M = Mo, W. The anions adopt a slightly distorted pyramidal structure with PPh 3 in basal position and the spin mostly delocalized in a metal- d z 2 orbital and carbon-p z orbitals of carbonyls. The EPR tensors are slightly modified by annealing, they suggest that new constraints in the matrix distort the structure of [M(CO) 4PPh 3] − (M = Cr, Mo, W).

EPR and ENDOR Study of Radiation-Induced Radical Formation in Purines: Sodium Inosine Crystals X-irradiated at 10 K

X-irradiated single crystals of sodium inosine (Na(+)*Inosine(-)*2.5H(2)O), in which the hypoxanthine base is present as the N1-deprotonated anion, were investigated using K-band (24 GHz) electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques at 10 K. At least five different radicals were present immediately after irradiation at 10 K. R1, which decayed upon warming the crystals to 50 K, was identified as the electron-loss product of the parent N1-deprotonated hypoxanthine base. Hyperfine couplings to HC8 and HC2 were fully characterized with ENDOR spectroscopy, and the identification was supported by DFT calculations. R2, which also decayed on warming to 50 K, exhibited nearly equal couplings to HC2 and HC8. Taken in combination with an extensive set of DFT calculations, the experimental results indicate that R2 is the (doubly negative) product of electron-gain by the initially anionic N1-deprotonated hypoxanthine parent. R3, which exhibited hyperfine coupling only to HC8 could not be identified. R4, which persisted on annealing to 260 K, exhibited one large alpha-proton hyperfine coupling which was fully characterized by ENDOR. Based on DFT calculations and the experimental data, R4 was identified as the product of net H-abstraction from C5'. The remaining HC5' was the source of the measured alpha-proton coupling. R5, present at low temperature and the only observable radical after warming the crystals to room temperature, was identified as the C8-H addition radical. The alpha-coupling to HC2 and beta-couplings to the pair of C8 methlyene protons were fully characterized by ENDOR.

High-field/ high-frequency EPR study on stable free radicals formed in sucrose by gamma-irradiation

The EPR spectrum of sucrose irradiated by high-energy radiation is complex due to the presence of more than one radical species. In order to decompose the spectrum and elucidate the radical magnetic parameters a high-field (HF(-)EPR) study on stable free radicals in γ-irradiated polycrystalline sucrose (table sugar) was performed at three different high frequencies—94, 190 and 285 GHz as well as at the conventional X-band. We suggest a presence of three stable radicals R1, R2 and R3 as the main radical species. Due to the increase of g-factor resolution at high fields the g-tensors of these radicals could be extracted by accurate simulations. The moderate g-anisotropy suggests that all three radicals are carbon-centred. Results from an earlier ENDOR study on X-irradiated sucrose single crystals (Vanhaelewyn et al., Appl Radiat Isot, 52, 1221 (2000)) were used for analyzing of the spectra in more details. It was confirmed that the strongest hyperfine interaction has a relatively small anisotropy, which indicates either the absence of α-protons or a strongly distorted geometry of the radicals.

The Role of Oxygen Containing Impurities in Defects Formation in Cesium Halide Crystals

The dependence of defect formation efficiency in CsI single crystals both on the type of oxygen containing impurities and the value of the absorbed irradiation dose was studied. Correlative results were obtained under investigation by methods of ionic thermocurrent (ITC), thermostimulated exoemission (TSEE), electrical conductivity and optical spectroscopy. The peculiarities of defect formation in γ-irradiated CsI-CO3(SO4, OH) and X-irradiated CsI-OH single crystals are discussed.

Intramolecular motion in dibenzobarrelenephosphinyl radical: a single crystal EPR study at variable temperature

The g, 31 P and 1 H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ≅0.5 kcal mol −1) which correspond to the conformation of the PH 2 moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.

EPR/ENDOR study of an X-irradiated single crystal of

As shown from the crystal structure, the oxygen atom of Ph3P1CH0C(O)CH3 forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyperfine couplings with those obtained from ab initio calculations shows that the radical cation Ph3P + 0CH1C(OH)CH2 is formed under radiolysis. The principal directions of the hyperfine tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph3P0C bond. # 1999 Elsevier Science Ltd. All rights reserved.

EPR and ENDOR Studies of X-Irradiated Single Crystals of Deoxycytidine 5‘-Phosphate Monohydrate at 10 and 77 K

The EPR spectra of single crystals of deoxycytidine 5‘-phosphate monohydrate (5‘dCMP), X-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6−Hα proton, and an additional small exchangeable hyperfine coupling from the N3−H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5−Hα and C1‘−Hβ, and two small exchangeable couplings from the C4−NH2 protons. Since no hyperfine coupling to N3−H was observed, the oxidized species is believed to be the N3−H deprotonated cation. At high X-ray dose there is also evidence for both C5 and C6 H-addition radicals...

Radiation damage in Pt(II) complexes: EPR study of an X-irradiated single crystal of Pt(1,3-dimethyl-imidazoline-2-thione) 4Cl 2.4H 2O

Several radiation defects have been detected by EPR in a single-crystal of Pt(dmimt) 4Cl 2.4H 2O (dmit = 1,3-dimethyl-imidazoline-2-thione). In order to identify these rediogenic species, the structure of the crystal has been resolved and the angular dependence of the EPR signals has been analysed. The resulting g tensors and 195Pt hyperfine tensors have been determined and the orientations of their principal axes have been compared to those of the bond directions of the precursor. It is shown that both Pt(I) and Pt(III) complexes are trapped, whereas Pd(dmimt) 4 2+ present as an impurity, leads only to the Pd(I) species. The temperature dependende of the EPR spectra gives information about the relative stability of the paramagnetic species and shows that the formation of some species, especially the Pt(III) complexes, requires drastic modifications of the parent Pt(II) cation.

Conversion of free radicals upon annealing of x-irradiated single crystal of cholest-4-EN-3-one

Single crystals of the steroid cholest-4-en-3-one were irradiated at 68K and studied at 68 and 300 K using X-band ESR and ENDOR. At 68 K, two A- and B-types of radicals were detected. The A-type radical was identified as the one formed by attachment of an electron to the O3 oxygen atom. The B-type was probably formed on the aliphatic part of the molecule. Upon heating in the temperatures ranging from 90–100 K both types of radicals undergo a conversion. The radical A converts into neutral one and becomes like a radical formed in single crystal of cholest-4-en-3-one. Explanation of the radical B conversion would need more research. From ESR and ENDOR data collected for three planes of single

EPR and piezooptic studies in doped and irradiated tartrates

Abstract Stress optical and EPR measurements are made on doped and X-irradiated single crystals of Rochelle salt in the paraelectric phase. With the nickel impurity no EPR signal is detected upto liquid nitrogen temperature. Manganese impurity forms two distinct types of paramagnetic centres. The spin Hamiltonian parameters are calculated and discussed for the Mn2+ defect centres. The stress optical constant is found to change with the introduction of impurities and on X-irradiation of the crystals.

Paramagnetic Centers in X-Irradiated LiH2PO4 Single Crystals

L'irradiation X est realisee a temperature ambiante et a la temperature de l'azote liquide. On enregistre les spectres de resonance paramagnetique electronique a ces deux temperatures

Electron capture by trivalent phosphorus compounds: A single crystal ESR study of the chlorodithiaphospholane anion

The 2-chloro-1,3,2-dithiaphospholane anion has been trapped in an X-irradiated single crystal of 1. The anion has been identified by comparing its 31P and 35Cl hyperfine tensors with the corresponding couplings obtained from ab initio calculations on H 2PCl −. and PCl 3 −.. The structure of R 2PCl − is discussed. Increasing the temperature causes the dissociation reaction to occur, and the phosphinyl radical has been observed after annealing at 220 K.

Intermolecular effects on the radiogenic formation of electron-capture phosphorus-centered radicals. A single-crystal ESR study of diastereoisomeric precursors

ESR experiments on X-irradiated single crystals of the 2R,4S,5R and 2S,4S,5R diastereoisomers of 2-chloro- 3,4-dimethyI-5-phenyl- 1,3,2-oxazaphospholidine 2-sulfide reveal that the yield of radiogenic electron-capture reactions in the solid state strongly depends on intermolecular interactions in the crystal. In the present case a high yield of P-CI three-electron-bond phosphoranyl radical anions is found in crystals of the 2R,4S,5R isomer, whereas no radical formation can be detected for the 2S,4S,5R isomer. An analysis of nonbonded interactions with neighboring molecules reveals that the geometry relaxation necessary for the radical stabilization is easily accommodated in crystals of the 2R,4S.5R isomer but not in the 2S,4S,5R isomer, explaining the observed difference in electron-capture efficiency. Experiments on radical formation in a MeTHF host matrix give further insight into the importance of the environment on radiogenic radical formation. The possible concurrent effect of the matrix on the electronic configuration and spin density distribution of the resulting phosphoranyl radical is discussed. The radiogenic formation of phosphorus-centered radicals has been extensively studied over the last decades.' The continuing interest in phosphorus-centered radicals originates partly from their possible role in the radiation chemistry of nucleic acids and is partly a consequence of their interesting electronic, structural, and dynamical properties. It has been established that the radical configuration, formed as a result of the electron-addition process, strongly depends on the nature of the substituents linked to the central phosphorus atom.2 Recently, however, it has become clear that other effects, which are not generally taken into account, can also be decisive for the nature of the observed radiation products. In particular we reported on the pronounced differences observed in electron capture of rac- and meso- 1,2-dimethyl- 1,2-di- phenyldiphosphine disulfide, demonstrating the importance of stereochemical effect^.^ Simultaneously, Cattani-Lorente et al. showed, by studying the electron capture of 1,2-phenylene phosphorochloridate in the pure compound and in frozen solutions, that the environment of the precursor molecule influences the resulting radical configuration after X-irradiati~n.~ In this paper we examine the influence of the surrounding matrix by studying radical formation in single crystals of two diastereoisomeric compounds, (2R,4S,5R)- and (2S,4S,5R)-2- chloro-3,4-dimethyl-5-phenyl-l,3,2-oxazaphospholidine 2-sulfide

Structure of the radical resulting from the addition of H on a P=P bond: A theoretical and single crystal electron spin resonance study

Room temperature ESR spectra of x-irradiated single crystals of bis (2,4,6-tri-tert-butylphenyl) diphosphene provide evidence for the trapping of a R(H)PṖR radical produced by hydrogen addition on the P=P bond. The various experimental hyperfine tensors are compared with those calculated for H2PṖH and CH3(H)PṖCH3 after optimization of the structure of these model radicals by ab initio methods. The barrier to internal rotation about the P–P bond is calculated to be equal to 2.8 kcal/mol for H2PṖH.

A room-temperature ENDOR study of X-irradiated pyridoxine hydrochloride single crystal

Single crystals of pyridoxine hydrochloride have been irradiated and investigated at room temperature. Two radicals of different stability have been observed. Hyperfine couplings to six protons were fully characterised for each radical. The trapping sites for both radicals were found by comparison of experimental results with crystal data. The stable radical is formed by loss of hydrogen at C6, as observed earlier in γ-irradiated pyridoxine hydrochloride. The unstable radical is formed by loss of hydrogen at C8.

Radiation damage in vitamin B 1: An endor study of an x-irradiated single crystal of thiamine

A single crystal of thiamine chloride hydrochloride has been x-irradiated at room temperature and studied by 1H-ENDOR spectroscopy at 110 K. It is shown that at least two radical species are trapped in the crystal. Several 1H-hyperfine tensors have been determined for each radical; they indicate that one species is due to cleavage of the thiamine molecule into its pyrimidine and thiazole moieties while the other species is due to hydrogen addition onto the pyrimidine ring.