The (NO)2-Na+ triplet state complex stabilised in Na-A zeolites at low temperature was studied by pulsed EPR. The 3-pulse ESEEM spectra at 5 K were analysed by simulations in the time-domain, followed by Fourier transformation. The ESEEM was attributed to the interaction with Na+. The hyperfine coupling A(23Na)=(4.6, 4.6, 8.2) MHz and nuclear quadrupole Q(23Na)=(0.3, 0,3, -0.6) MHz tensors are of comparable magnitude as in the NO-Na+ complex determined earlier. The complexes are proposed to be either pairs of NO-Na+ or ON-Na+-NO.

Echo-detected electron paramagnetic resonance (echoEPR) profiles for irradiated deuterated ammonium tartrate single crystals depend strongly on the delays between pulses of the echo sequence. This is mainly due to instantaneous and spectral diffusion that plays a crucial role in determining the decay of the echo at every field position: the dephasing rate 1/T-M depends on the number of spins excited by the pulses and on the total number of interacting spins. A rigorous simulation of the echoEPR profiles at different delays requires the evaluation of the modulation pattern (ESEEM) and of the dephasing processes at every field position. From the simulations, information on the microscopic radical concentration, and on the electron-electron flip-flop rates of the single radical species can be obtained. Natural isotope C-13 substitution generates low-concentration radicals with relaxation properties different from the equivalent C-12-substitued radicals. The different behavior is discussed.

Polycrystalline ammonium dithionate has been examined for its radiation response in the low dose range (< 5 Gy) using EPR technique. The •SO3- radical ion was detected as a single EPR line with a peak-to-peak derivative width of ca. 0.44 mT in irradiated samples and its intensity was found to vary linearly with dose. At equal and moderate settings of microwave power and modulation amplitude ammonium dithionate was at least 7 times more sensitive than L-alanine which is the most common EPR dosimeter standard. Pulse experiments were performed on the powder samples to obtain the longitudinal relaxation time. These and microwave saturation experiments served to indicate the optimal microwave power to be applied during measurements as an EPR dosimeter for best sensitivity of this material. It is thus claimed that ammonium dithionate has excellent potential to become an EPR dosimeter with a low limit of the measurable dose for cases where tissue equivalence is not required or can be corrected for.

Electron spin resonance (ESR) spectroscopy has been used to study the conduction electron spin resonance (CESR) of small silver particles stabilized in dehydrated Ag-rho zeolite. Silver particles were produced by hydrogen reduction at elevated temperatures and diameter of the stabilized particles was calculated based on the linewidth of CESR signal using Kawabata theory. © 2005 Elsevier B.V. All rights reserved.

Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and ?-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to 13CH4 the multiplet observed at 110 K was assigned to a ?CH3?Na+ complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that ?CH3 radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the ?CH3?Na+ adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the ?CH3?Na+ adsorption complex for both applied models show very good agreement with those obtained experimentally. © 2006 American Chemical Society.

The formation of silver-ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after gamma-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)(2) stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The 'gas-phase' geometries of Ag(C2H4) and Ag(C2H4)(2) complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)(2) complex of D-2h symmetry are in very good agreement with those obtained experimentally.

Density functional theory (DFT) calculations were applied for the modeling of the adsorption sites of nitric oxide in Li+ exchanged zeolite A previously studied experimentally. Two model clusters, a 6T six-membered ring, and a 3T fragment of an octagonal structure were examined. The obtained results showed that the geometry of the formed [Li-NO](+) complex depended on the coordination of the exchangeable cation with the oxygen atoms of the zeolite framework. Calculated anisotropy of the g-factor and the magnetic parameters of N-14 and Li-7 are in the range of experimental values observed for those types of complexes.

Density functional theory (DFT) has been applied to model the structure of (Ag•CH₂OH/A)⁺ complexes previously experimentally characterized by electron paramagnetic resonance (EPR) in zeolite matrices. The magnetic parameters of (Ag•CH₂OH/A)⁺ were found to depend on the local structure of the zeolite represented by clusters referred to as 3T and 6T, and also on the applied computational method. A spin distribution analysis confirms the one-electron silver-carbon bonding, showing delocalization of an unpaired electron density distributed between the sliver and carbon atoms along the one-electron silver-carbon bond. The results are of relevance for a deeper understanding of the electronic and catalytic properties of zeolites containing silver atoms and clusters.

Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose to obtain information about possible ring opening and other fragmentation processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDOR and EIE spectroscopy, two H-abstraction radicals present in a 3:1 relative ratio in the irradiated crystals were identified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra the relative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III, respectively. Radical III is proposed to be of the allyl radical type -CH=CH-1CH- formed formally by a concerted H2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal and structure is at variance with observations from aqueous solution studies of succinimide, where the CO-CH2 bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT) calculations, predicting equivalent hyperfine interactions to the two a-type protons at C2 and C4 in excellent. agreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that both couplings originate from the same radical species. Possible mechanisms for the formation of radical III are discussed.

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B(1g) and 2B(2g) states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B(2g)-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

Dispersed detonation nanodiamonds have been studied by continuous-wave (CW) and pulse EPR techniques. The spectrum of bulk radicals (g = 2.0025 +/- 0.0002, a Lorentz line shape with Delta H-pp = 0.95 +/- 0.05 mT) dominated in CW EPR and prevented to record spectra from other paramagnetic species. The pulse EPR-spectrum was the superposition of the distorted P1-center spectrum with parameters (g = 2.0025, A(xx) = 2.57 mT, A(yy) = 3.08 mT, A(zz) = 4.07 mT), the H1-center spectrum (g = 2.0028) and the single line (g = 2.0025, DHpp = 0.40 +/- 0.05 mT) from other centers which may be assigned to surface radicals. The concentration of P1-centers has been estimated by CW EPR as 2 +/- 1 ppm N.

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li₂S₂O₆·2H₂O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO₃⁻ anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T₁, which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.

Head and neck squamous cell carcinoma (HNSCC) tumours are associated with high mortality despite advances in therapy. The monoclonal antibody cetuximab (Erbitux^®) has been approved for the treatment of advanced HNSCC. However, only a subset of HNSC patients receiving cetuximab actually responds to treatment, underlining the need for a means to tailor treatments of individual patients. The aim of the present study was to investigate the effect of cetuximab treatment on tumour growth, on tumour partial oxygen pressure as measured by LiPc electron paramagnetic resonance oximetry and on the expression of proteins involved in tumour growth, metabolism and hypoxia. Two HNSCC cell lines, UT-SCC-2 and UT-SCC-14, were used to generate xenografts on female BALB/c (nu/nu) nude mice. Mice with xenografts were given three injections of intraperitoneal cetuximab or phosphate-buffered saline, and the tumour volume was recorded continuously. After treatment the tumour partial oxygen pressure was measured by LiPc electron paramagnetic resonance oximetry and the expression of epidermal growth factor receptor (EGFR), phosphorylated EGFR, Ki-67, MCT1, MCT4, GLUT1, CAIX and HIF-1α were investigated by immunohistochemistry. In xenografts from both cell lines (UT-SCC-2 and UT-SCC-14) cetuximab had effect on the tumour volume but the effect was more pronounced on UT-SCC-14 xenografts. A higher tumour oxygenation was measured in cetuximab-treated tumours from both cell lines compared to untreated controls. Immunocytochemical staining after cetuximab treatment shows a significantly decreased expression of EGFR, pEGFR, Ki67, CAIX and nuclear HIF-1α in UT-SCC-14 tumours compared to untreated controls. MCT1 and GLUT1 were significantly decreased in tumours from both cell lines but more pronounced in UT-SCC-14 tumours. Taken together, our results show that cetuximab treatment decreases the tumour growth and increases the tumour partial oxygen pressure of HNSCC xenografts. Furthermore we found a potential connection between the partial oxygen pressure of the tumours and the expression of proteins involved in tumour growth, metabolism and hypoxia.

With an increasing interest in using protons and light ions for radiation therapy there is a need for possibilities to simultaneously determine both absorbed dose (D) and linear energy transfer, LET, (L_Δ). Potassium dithionate (K₂S₂O₆) tablets were irradiated in a conventional 6 MV linear accelerator photon beam and a N⁷⁺ beam (E = 33.5 MeV/u) respectively. The EPR spectrum of irradiated potassium dithionate is a narrow doublet consisting of two signals, R₁ and R₂, with different microwave power saturation properties. On the basis of identification in related substances by EPR and ENDOR, these two signals are assigned to two non-equivalent SO₃⁻ – radicals. Our experiments showed that the ratios of these two lines (R₁/R₂) were clearly connected to beam LET. Irrespective of the mechanistic details this investigation suggests a new method for measurement of absorbed dose and beam LET by using potassium dithionate EPR dosimetry.

Alanine EPR dosimetry has been applied successfully when measuring intermediate and high radiation doses. Although the performance of alanine dosimetry is being improved, the sensitivity of the material is too low for a fast and simple low- dose determination. Here we present the results using ammonium formate as an EPR dosimeter material. Ammonium formate is seven times more sensitive than alanine, using spectrometer settings optimized for the latter. Deuterated ammonium formate is found to be more than eight times more sensitive than alanine. Analysis of signal stability with time shows that the ammonium formate signal is stable by 5 min after irradiation and that no change in signal intensity is found during 8 days. The atomic composition of ammonium formate is closer to that of tissue than alanine, and thus the energy dependence is smaller than that of alanine at photon energies below 200 keV. Power saturation studies indicate that the energy transfer between the spins and the lattice is fast in ammonium formate, which gives the possibility of using high microwave power without saturation to further increase the sensitivity. These results suggest that ammonium formate has some important properties required of an EPR dosimeter for applications in dosimetry in the dose range typical for radiation therapy.

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one a-coupling, and three ß-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1-H bond with respect to the N-C1-C2 plane is estimated to be 13-15° (bending angle 0˜ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions. © The Owner Societies 2002.

The structure of dimeric cations of benzene and toluene formed in X- irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (a(iso)) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with A(is parallel with) < A(is perpendicular to) were obtained in CF3CCl3 even at 50 K. The rhombic dipolar coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4') protons were observed. The isotropic hf coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one- third of the value of the monomer. The hf coupling of the H(4,4') protons was almost half the coupling of the monomer. It was suggested that the anomalous hf couplings of the CH3 protons were due to the interaction between the two rings through the s-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf coupling in the (CH3C6H5)2+ were successfully evaluated.

X-irradiated Li(N2H5)SO4 single crystals were investigated using ESR and ENDOR spectroscopy at several temperatures. The hydrazine radical cation N2H4.+ was selectively produced by irradiation at room temperature. From the analysis of the orientation dependent ENDOR spectra, the 1H-hfc tensors of the cation radical were precisely obtained and the radical structure was supported by theoretical calculations. It is suggested that the cation radical has a planar p* structure D(2h) (2B(2g)) in the crystal down to 230 K. By using the evaluated 1H-hf tensor the powder ESR line shape was successfully simulated. Concomitant with the radical formation, the N-N bond of N2H4.+ is suggested to reorient so as to optimize hydrogen bond interactions. 1H-ENDOR line splitting for the N2H4.+ radical was observed at temperatures below 230 K. Apparently this splitting is due to a reversible structural change where one of the NH2 moleties in N2H4.+ becomes slightly bent out of the molecular plane, whereas the other one remains planar. This deformation evidently arises from interactions between the cation radical, adjacent H2N-NH3+ molecules and the SO4-LiO4 framework. Interacting N2H5+···N2H4.+···N2H5+ molecules along the c-axis are proposed to explain the deformation mechanism.

The nature of quartet state nitrogen atoms formed in gamma -irradiated NaN3 powder was investigated by EPR, ENDOR and ENDOR-induced EPR (EIE). The ENDOR spectrum observed at 110 K clearly showed two sets of N-14 signal pairs centered at a(N)/2 (half the N-14-hyperfine splitting) and at 3a(N)/2, strongly suggesting the existence of quartet state N-14 atoms (S-4(3/2)). At 160 K, the EPR spectrum exhibited a clear fine structure. accompanied by a hyperfine coupling due to N-14. The zero-field splitting, D, gradually increased with decreasing temperature from 170 to 40 K, and then became nearly constant. The temperature dependence of D was explained in terms of a local lattice vibration coupled with spin states of N-14 atoms. At below 95 K, a new fine structure with larger D was observed. The observed two D-values were attributed to 14N atoms located at two different interstitial sites in the monoclinic NaN3 structure.

X-irradiated Li(N2D5)SO4 single crystals were investigated using EPR and ENDOR spectroscopy. The N-14-ENDOR spectra of the deuterated hydrazine radical cation N2D4.+ were clearly observed at 240 K. N-14 hyperfine (life) and nuclear quadrupole (nqc) tensors of the N2D4.+ cation were determined from angular variation ENDOR measurements in the three orthogonal planes of the crystal. The life tensor obtained for the two equivalent N-14 atoms is very close to that reported in a N2H5HC2O4 single crystal, and the present results support the previous conclusion that the cation has a planar structure at 240 K. The N-14 nqc tensor was estimated using semiempirical methods and also calculated from the field gradients in the LiHzS crystal as evaluated by density functional theory methods. The general agreement with experimental observations further supported the suggested geometrical structure of the N2H4.+ radical. The H-1 and N-14-ENDOR enhancements observed may be due to weakened dipolar interactions between N-14-D-2 and H-1-D-2 leading to increased T-ln relaxations of the N-14 and residual H-1 nuclei of the N2D4.+ cation (and/or other isotopomers) in the deuterated crystals as compared to those in the Li(N2H5)SO4 crystals.

The zinc and copper(II) morpholineditiohcarbamate complexes [M-2(MfDtc)(4)] and their adducts with morpholine [M(Mf)(MfDtc)(2)], including the solvated forms [M(Mf)(MfDtc)(2)] . Mf (M = Zn, Cu-63, Cu-65), have been synthesized and characterized by thermal analysis, EPR, and CP/MAS C-13 NMR data. Upon thermal destruction, the initial complexes, their adducts, and solvated forms lose weight in one, two, and three steps, respectively. The ultimate products of the thermal destruction of these complexes are zinc and copper sulfides. The range of the thermal destruction of the "dithiocarbamate part" is progressively shifted to lower temperatures in going from the initial binuclear complexes to their adducts and solvated forms. EPR shows that the geometry of the [CuS4N] chromophore in the [Cu(Mf)(MfDtc)(2)] adduct is close to square-pyramidal, the unpaired electron being predominantly localized in the metal 3d(x2-y2) AO. The outer-sphere solvation of the adduct is accompanied by structural reorganization at the molecular level. This reorganization involves a distortion of the [CuS4] base of the square-pyramidal polyhedron [CuS4N] toward a trigonal bipyramid. C-13 NMR shows that all of the zinc complexes under consideration are characterized by intramolecular nonequivalence of dithiocarbamate ligands. The N-15 NMR spectra adequately reflect the difference in the structural functions of pairs of dithiocarbamate ligands in [Zn-2(MfDtc)(4)].

Photoinduced reactions of 1-(dimethylethyl)-2,2-dimethylpropyl and cyclohexyl radicals in 77 K solids were studied by the ESR method. 1-(Dimethylethyl)-2,2-dimethylpropyl is converted to 2,2,4,4-tetramethylpentyl radical with photoirradiation of 254 nm light. A hydrogen atom of methyl groups can hence directly transfer to the radical site at a carbon atom other than an adjacent one in the photoinduced reactions of alkyl radicals. Cyclohexyl radical is converted to cyclopentylmethyl radical with irradiation of 254 nm light. The photolysis of cyclohexyl radical causes C-C bond scission, and results in the formation of 5-hexenyl radical. It is then converted to cyclopentylmethyl radical through intramolecular rearrangement.

Lithium formate (6LiOOCH·H2O), 95% 6Li enrichment, combined with an exchange of crystallization water with D2O was investigated. The ESR spectrum of the radiation induced free radicals stable at room temperature consists of a singlet with a narrow line width, 0.92 mT. 6Li has smaller magnetic moment and nuclear spin, which resulted in the narrower line width accompanied with an increase in peak amplitude. In comparison with lithium formate with natural isotopic composition, 6Li (7.5%, I = 1) and 7Li (92.5%, I = 3/2), the sensitivity was increased by a factor of two. With optimised spectrometer settings 6Li formate had seven times higher sensitivity compared to alanine. Therefore this material is proposed as a dosimeter material in a dose range down to 0.1 Gy. The g and the 13C-hyperfine (hf) tensors of the CO2- radical anion, major paramagnetic products, were evaluated to be g = (2.0037, 1.9975, 2.0017), and A(13C) = (465.5, 447.5, 581.3) MHz for polycrystalline samples at room temperature. Furthermore, the 1H-hf and 6Li-hf tensors observed for the surroundings of CO2- by ENDOR technique were in fairly good agreement with DFT calculations. The CO2- radicals are found to be so stable that the formate is applicable to the ESR dosimetry, because of fully relaxing in a fully relaxed geometrical structure of the CO2- component and remaining tight binding with the surroundings after the H atom detachment from HCO2-. © 2006 Elsevier B.V. All rights reserved.

Highly resolved ESR spectra of monomer, dimer and trimer radical cations of coronene (C24H12) were observed at room temperature for a solution of 1,1,1,3,3,3-hexafluoro-2-propan-2-ol (HFP) containing thallium(III) trifluoroacetate as oxidant. The spectra consisting of multiple lines with isotropic 1H-hyperfine splitting (hfs) constants of 0. 0766 mT (24H) and 0.013 mT (6H) were attributable to a mixture of the dimer with the trimer radical cations, (C24H12)2+ and (C24H12)3+. For (C24H12)2+, the 1H-hfs constant agreed well with the reported value, 0.077 mT. However, for (C 24H12)3+, the values were significantly different from the reported ones, 0.117 mT (12H) and 0.020 mT (24H), by Ohya Nishiguchi et al. [H. Ohya-Nishiguchi, H. Ide, N. Hirota, Chem. Phys. Lett. 66 (1979) 581], but rather similar to those reported by Willigen et al. [H. van Willigen, E. De Boer, J.T. Cooper, W.F. Forbes, J. Chem . Phys. 49 (1968) 1190]. In conflict with Willigen's report, however, no ESR line broadening which has been ascribed to a low stationary concentration of (C 24H12)3+ was detected. Based on ab initio MO calculations for benzene as a compact model of C24H 12, the structure of (C24H12)3+ was investigated in terms of the observed 1H-hfs constants. A staggered sandwich C2v structure was suggested being at the "global" minimum for the benzene trimer cation. In the structure, the unpaired electron spin is predominantly localized to the central ring, which is qualitatively in agreement with the previous ESR results of (C 24H12)3+ by Ohya-Nishiguchi et al. In addition, as a "local" minimum, the benzene trimer was indicated to have a slipped sandwich Cs structure, which is less stable by ca. 19 kJ mol-1 than the "global" minimum. In this structure, the unpaired electron spin was nearly equally distributed on both the central and one of the two side C24H12 molecules. The observed 1H-hfs constants were possibly attributable to the (C 24H12)3+ cation with the analogous slipped sandwich Cs structure.

Local structures and motions of radical reactants in highly selective tunneling abstraction reaction in neopentane-ethane mixture are investigated with CW-ESR, ENDOR, and ESEEM spectroscopy at different temperatures. H atoms produced by the radiolysis of neo-C5H12 in the neo-C5H12-C2H6 (2 mol %) mixtures above 40 K react selectively with the solute C2H6 to give ethyl radicals. The selective reaction is suppressed below 30 K. The local structures of neopentane matrix around neopentyl and ethyl radicals are approximately the same in the temperature range from 4.4 to 45 K. Although neopentyl radicals are rigidly trapped below 100 K, ethyl radicals in neopentane matrix begin to librate even at 6 K and increase the degree of motion up to 30 K, Finally, the librating motion of the ethyl radicals in neopentane matrix is vigorous above 40 K. The ethane molecule can take more favorable orientation for the hydrogen abstraction reaction by H atoms than rigid neopentane molecules by the vigorous libration motion, resulting in the selective reaction with H atoms.

Photo-physical properties relevant for optical power limiting in the near-visible and visible regions of the spectrum are reported for a series of substituted diarylalkynyl chalcogenophenes (furans, thiophenes, selenophenes, and tellurophenes). The linear ground and excited state absorption as well as the nonlinear (two-photon) absorption were determined at the time-dependent density functional theory level with use of the hybrid exchange-correlation functionals B3LYP and CAM-B3LYP. A selected number of the theoretically studied molecules were synthesized and characterized experimentally with the use of absorption and luminescence spectroscopy. The photo-physical data are compared to the results from optical power limiting measurements performed in THF solution at a wavelength of 532 nm, with a laser pulse length of 5 ns and pulse energies up to 150 J. The best compounds in the present investigation display an energy damping by approximately a factor of 10 at a concentration of 0.010 M.

Amine radical cations of the type R3N.+ and [R3NCH2](.+), R = CH3, C2H5, C3H7, and nitric oxide, NO, have been used to probe the bonding to the surface and the dynamics of the radicals trapped in the confined space of cages or channels in the zeolite. Regular continuous-wave electron spin resonance (ESR) was employed to study the internal motion of the cation radicals formed by gamma-irradiation of amines and related ammonium ions, introduced during the synthesis of the zeolites Al-offretite, SAPO-37, SAPO-42 and AlPO4-5. The ESR spectra of [(CH3)(3)NCH2](.+) radical cation in several studied systems changed reversibly with temperature, indicating dynamical effects. Free rotation about the >N-CH2 bond of the [(CH3)(3)NCH2](.+) species was found to occur in the temperature range of 110 to 300 K, while the rotation about the >N-CH3 bonds was hindered. The observations confirm the theoretical prediction on the basis of density functional theory calculations, which indicate that the corresponding barriers are of the order of 0.3 and 7 kJ/mol, respectively. The radical cations of the type R3N.+ with R = C2H5, C3H7 were found to undergo a different type of dynamics, involving a two-jump process of the methylene hydrogens next to the nitrogen. A cage or channel size effect on the stability and molecular dynamics was inferred in some cases. Pulsed ESR was employed to study the (NO)(2) triplet-state dimers in Na-A type zeolite, with the purpose to resolve the interaction with surface groups, and to elucidate the role of the zeolite on stabilizing the triplet rather than the usual singlet state. Measurements performed at 5 K gave rise to Fourier transform spectra that were assigned to the dimer species interacting with one or more Na-23 nuclei, with approximative parameters A(Na-23) = (4.6, 4.6, 8.2) MHz and Q(Na-23) = (-0.3, -0.3, 0.6) MHz for the hyperfine and nuclear quadrupole coupling tensors, respectively. The values are of similar magnitude as those determined for the NO---Na+ complex. The stability of the triplet-state structure was attributed to an unusual geometric structure imposed by the zeolite matrix, with the N-O bonds along a line as in [O-N---Na+---N-O], which according to UHF ab initio calculations has a triplet ground state.

An EPR study was carried out to elucidate the zeolite cage effect on the stability and molecular dynamics of [(CH3)(3)N](.+) and [(CH3)(3)NCH2](.+) radical cations generated in gamma -irradiated Al-offretite, SAPO-37 and SAPO-42, which contained (CH3)(4)N+ ions as organic template. The [(CH3)(3)N](.+) radical cation was stable at room temperature when it was generated in relatively small size cages such as the sodalite cages of SAPO-37 and the beta -cages of SAPO-42. On the other hand, the [(CH3)(3)NCH2](.+) radical cation was stable not only in the sodalite cages of SAPO-37, but also in relatively large cages such as gmelinite cages or the main channels of Al-offretite and the alpha -cages of SAPO-42. Strongly temperature dependent EPR spectra were observed for the [(CH3)(3)NCH2](.+) radical cation stabilized in Al-offretite and SAPO-42. They were successfully analyzed by a spectral simulation method assuming a three-site exchange for the methyl protons. The evaluated exchange rates were in the order SAPO-37 (sodalite cages) < Al-offretite (gmelinite cages or main channels) < SAPO-42 (alpha -cages) in the temperature range 110-300 K.

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N-CH2]+· radical formed in a ?-irradiated Al- offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed 1H hyperfine (hf) splitting was a1 = 0.46 mT for three magnetically equivalent hydrogens, one of each methyl group at 110 K and a1 = 0.15 mT for three of each methyl group at 300 K, the hf splittings due to two equivalent CH2 hydrogens and the central 14N were a2 = 2.24 and a3 = 0.35 mT, respectively, independent of the temperature. This result suggests that rotation of the methyl group around the N-CH3 bond is responsible for the temperature dependent line shapes. Assuming a three-site jump model in which three methyl protons interchange their positions with each other, the EPR line shapes were successfully analyzed by a simulation method using the jump rate as a variable parameter in the temperature range from 140 to 270 K. The rate was found to increase from 7.0 x 106 s-1 (140 K) to 4.1 x 108 s-1 (270 K) with temperature. From an Arrhenius plot of the rate constants, an activation energy of 8.1 kJ mol-1 was evaluated for the methyl group rotation of [(CH3)3N-CH2]+· in Al-offretite.

An EPR study was carried out to investigate both the structure and molecular dynamics of triethylamine (Et3N) and tripropylamine (Pr3N) radical cations generated in gamma -irradiated AlPO4-5 from neutral mother molecules incorporated as an organic template. Both Et3N+. and Pr3N+. radical cations were stable at 300 K and showed strongly temperature-dependent EPR spectral line-shapes in the temperature range from 77 to 300 K. The spectral change was successfully explained by assuming a two-site exchange model in which two inequivalent beta -hydrogens interchange their positions with each other. The experimental spectra were quantitatively analyzed by a simulation method using the exchange rate as a variable parameter. From an Arrhenius plot of the rate constants, activation energies of 9.1 and 11.4 kJ mol(-1) were evaluated for Et3N+. and Pr3N+. in AlPO4-5, respectively.

The geometric and electronic structure of the diphenylamine radical cation Ph2(H)N+ obtained by X-irradiation of diphenylamine in CFCl3 at low temperature have been studied by the methods of EPR and ENDOR combined with theoretical calculations. The 110 K ENDOR spectrum of Ph2(H)N+ is mainly attributed to the ortho and meta phenyl ring protons. The accurate assignment of spectroscopic parameters is based on spectral simulations and theoretical calculations. Close agreement was obtained between the experimental and calculated hyperfine tensors of ortho and meta protons. © 2004 Elsevier B.V. All rights reserved.

Density functional theory was employed to investigate the adsorption site and hyperfine interactions of nitric oxide adsorbed in Na-LTA (previous name NaA) zeolite. Three different cluster models of increasing complexity were used to represent the zeolite network: (1) a six-membered ring terminated by hydrogen atoms with one sodium ion above the ring, (2) as model 1 with the addition of three sodium ions located at the centers of three imagined four-membered rings adjacent to the six-membered ring, and (3) as model 2 with the addition of the three four-membered rings adjacent to the six-membered ring. Calculations on the largest system (model 3) showed very good agreement with measured electronic Zeeman interaction couplings, 14N hyperfine coupling tensors, and 23Na hyperfine and nuclear quadruple coupling tensors of the S = 1/2 Na+⋯N-O adsorption complex when the position of the sodium ion was relaxed. The optimized geometry of the complex agreed nicely with that estimated experimentally, except for the Na-N distance, where the present results indicate that the distance deduced from previous ENDOR experiments may be underestimated by as much as 0.5 Å. © 2005 American Chemical Society.

An automatic fitting procedure has been employed to analyze experimentally studied paramagnetic complexes in powder form by electron paramagnetic resonance (EPR). A least-squares fitting procedure utilizing analytical derivatives of the theoretically calculated spectrum with respect to the g-, zero-field, nuclear quadrupole, and hyperfine tensors was used to refine those parameters. An anisotropic line width could also be fitted. The theoretically calculated spectra were obtained by matrix diagonalization of a general spin Hamiltonian allowing also magnetically coupled systems to be analyzed. A VO2+ S = 1/2 complex showing g and hyperfine anisotropic interactions and free radical systems featuring Deltam(1) not equal 0 transitions due to the direct field effect or the presence of quadrupolar nuclei have been analyzed as well as NO, species on surfaces and radiation defects employed for EPR dosimetry. Analysis of systems with S > 1/2 and magnetically coupled species has also been attempted.

Two types of automatic fitting procedures for EPR spectra of disordered systems have been developed, one based on matrix diagonalisation of a general spin Hamiltonian, the other on 2^nd order perturbation theory. The first program is based on a previous Fortran code complemented with a newly written interface in Java to provide user-friendly in- and output. The second is intended for the special case of free radicals with several relatively weakly interacting nuclei, in which case the general method becomes slow. A least squares’ fitting procedure utilizing analytical or numerical derivatives of the theoretically calculated spectrum with respect to the g-and hyperfine structure (hfs) tensors was used to refine those parameters in both cases. ‘Rigid limit’ ESR spectra from radicals in organic matrices and in polymers, previously studied experimentally at low temperature, were analysed by both methods. Fluoro-carbon anion radicals could be simulated, quite accurately with the exact method, whereas automatic fitting on e.g. the c-C₄F₈^- anion radical is only feasible with the 2^nd order approximative treatment. Initial values for the ¹⁹F hfs tensors estimated by DFT calculations were quite close to the final. For neutral radicals of the type XCF₂CF₂• the refinement of the hfs tensors by the exact method worked better than the approximate. The reasons are discussed. The ability of the fitting procedures to recover the correct magnetic parameters of disordered systems was investigated by fittings to synthetic spectra with known hfs tensors. The exact and the approximate methods are concluded to be complementary, one being general, but limited to relatively small systems, the other being a special treatment, suited for S=½ systems with several moderately large hfs.

Electron paramagnetic resonance (EPR) spectroscopy was applied to study the paramagnetic species formed from styrene, 1,1-diphenylethylene, alpha-methylstyrene, beta-methylstyrene and methylmethacrylate adsorbed on amorphous silica gel after gamma-irradiation at 77 K. Radicals formed by the hydrogen atom addition at the vinyl group of the monomers were observed in all samples. The hydrogen atoms were shown to originate to a large extent from the adsorbent by using silica gel with deuterated silanol groups. An EPR spectrum assigned to a propagating radical was observed at increased temperature for samples containing methylmethacrylate (MMA). The structures of the adsorption complexes, the respective hyperfine splitting constants and the adsorption energies were calculated by applying OFF quantum chemical methods. The reaction between an MMA molecule and the MMA radical and the structure of the propagating radical was modeled. The calculated hyperfine splitting constants for all radicals confirmed the assignment of the experimental spectra.

A previous automatic fitting procedure of EPR spectra has been extended with the purpose to characterise coupled paramagnetic complexes in powders and frozen solutions. The theoretical EPR spectra were obtained by matrix diagonalization of a general spin Hamiltonian. A least-squares fitting procedure using analytical derivatives of the calculated spectrum with respect to the spectroscopic, fine structure, nuclear quadrupole, electron-electron, and hyperfine coupling tensors was used to refine those parameters. The powder spectra of matrix isolated CF3 and RCF2CF2 radicals, previously measured at low temperature, were reanalysed with this method. A theoretically modeled complex consisting of a Cu2+ ion, featuring an axially symmetric g-tensor and 63Cu hyperfine structure anisotropy, and a free radical located at different orientations, with respect to the symmetry axis of the Cu2+ ion, was examined in order to investigate the possibility to recover the magnetic parameters of the separate units and the magnetic couplings between them. © 2005 Elsevier B.V. All right reserved.

The ESR spectrum of the chain-end radical RCF2CF 2. detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F a and two Fß nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two Fa nuclei, indicating a nonplanar structure about the Ca radical center. The fitted isotropic hyperfine splittings for the two Fß nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF 2CF2CF2. or CF3OCF 2CF2. radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two Fß nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2., which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2. in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions. © 2007 American Chemical Society.

New materials for electron spin resonance (ESR) dosimetry have been investigated with the aim to find systems more sensitive than L-alanine accepted as a standard for high dose determinations. Among the investigated systems ammonium tartrate, 2-methylalanine, salts of formic acids and dithionates have been found to be more sensitive than alanine by a factor 2-10. The lower limit applies to tissue equivalent materials, while much higher sensitivities were obtained with formates and dithionates containing heavier atoms. The increased sensitivity was mainly attributed to suitable ESR properties of the room temperature stable radicals as regards spectral shape (narrow lines, little or no hyperfine structure) and microwave saturation properties (short relaxation times). The radical structures have when necessary been clarified by ENDOR spectroscopy, while the saturation properties have been screened by pulsed ESR measurements.

Lund, A., Sagstuen, E., Sanderud, A. and Maruani, J. Relaxation-Time Determination from Continuous-Microwave Saturation of EPR Spectra. Radiat. Res. 172, 753-760 (2009). Based on the theories of Portis and of Castner 50 years ago, different continuous-wave measurement procedures for analyzing the microwave saturation power dependence of inhomogeneously broadened EPR lines were developed. Although these procedures have been refined, they still use only a few selected points on the saturation curve. A non-linear least-squares procedure for analyzing the microwave-power dependence of inhomogeneously broadened lines using all data points on a saturation curve has been developed. This procedure provides a simple alternative method to obtain magnetic relaxation data when the more direct pulse-saturation techniques are not available or are less suitable. The latter includes applications of quantitative EPR such as dosimetry. Then microwave saturation data should be obtained under conditions similar to those used in the quantitative measurements, which are usually made on first derivative spectra recorded using continuous-wave spectrometers. Selected applications to benchmark literature data and within the field of EPR dosimetry are discussed. The results obtained illustrate that relaxation times comparable to those yielded by various pulse-saturation EPR techniques can be obtained. It appears as a systematic feature that, whenever the pulse EPR data are fitted using bi-exponential functions, the shortest relaxation times obtained are those that correspond best to those measured using the current continuous-wave saturation method.

Applications of EPR and ENDOR simulations of relevance in radiation research involving free radicals, radical pairs, triplet states and to less extent metal complexes are treated. Early fundamental work involving in situ radiolysis of liquids and stickplot analysis of spectra is reviewed, while single crystal analysis is only briefly discussed. The analysis of data obtained with continuous wave methods of species trapped in disordered solid s, “powders” is emphasized. Simulations based on first and second order and exact theory are described and exemplified. Methods to obtain parameters for the dynamics of radicals in irradiated solids and for the simulation of spectra at microwave saturation are discussed. Procedures for the simulation of powder ENDOR spectra of radicals are described in detail, with special emphasis on the influence of nuclear quadrupole couplings due to nuclei with I ≥ 1. EPR and ENDOR simulation programs known to us are presented in an Appendix, including addresses for downloading when available.

Formates and dithionates of Li-6, enriched and Li-7 in natural composition of Li offer a possibility to measure the absorbed dose from photons and thermal neutrons in a mixed radiation field for instance at a boron neutron capture therapy (BNCT) facility. Tests with formates and dithionates of enriched 6Li and lithium compounds with natural composition have been performed at the BNCT facility at Studsvik, Sweden. Irradiations have been performed at 3 cm depth in a Perspex phantom in a fluence rate of thermal neutrons 1.8 x 10(9) n cm(-2) s(-1). The compounds were also irradiated in a pure X-ray field from a 4 MV linear accelerator at 5 cm depth in a phantom with accurately determined absorbed doses. The signal intensity and shape was investigated within 3 h after the irradiation. A single line spectrum attributed to the CO2- radical was observed after irradiation of lithium formate. An increase in line width occurring after neutron irradiation in comparison with photon irradiation of the Li-6 sample was attributed to dipolar broadening between CO2- radicals trapped in the tracks of the alpha particles. A spectrum due to the SO3- radical anion was observed after irradiation of lithium dithionate. The signal amplitude increased using the Li-6 in place of the Li with natural composition of isotopes, in studies with low energy X-ray irradiation. Due to the decreased line width, caused by the difference in g(N) and I between the isotopes, the sensitivity with Li-6 dithionate may be enhanced by an order of magnitude compared to alanine dosimetry. After comprehensive examination of the different combinations of compounds with different amounts of Li-6 and Li-7 regarding dosimetry, radiation chemistry and EPR properties these dosimeter material might be used for dose determinations at BNCT treatments and for biomedical experiments. Interesting properties of the radical formation might be visible due to the large difference in ionization density of neutrons compared to photons. (C) 2003 Elsevier B.V. All rights reserved.

Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.

The radical cations of piperazine, morpholine, thiomorpholine, and thioxane were investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy in a solid Freon matrix. Optimized geometry and magnetic parameters of the radical cations were calculated using a density functional theory (DFT)/Perdew-Burke-Ernzerhof (PBE) method. Both experimental and theoretical results suggest that all the studied species adopt chair (or distorted chair) conformations. No evidence for the boat conformers with intramolecular σ*-bonding between heteroatoms were obtained. In the cases of morpholine and thioxane, the oxygen atoms are characterized by relatively small spin populations, whereas a major part of spin density is located at N and S atoms, respectively. The thiomorpholine radical cation exhibits nearly equal spin population of N and S atoms. In most cases (except for thioxane), the calculated magnetic parameters agree with the experimental data reasonably well. © 2005 American Chemical Society.

ESR dosimetry with L-α-alanine as a dosimeter material is a well known and frequently used method for measuring high absorbed doses, for example at sterilisation and food irradiation. The increased sensitivity of modem spectrometers and an increased knowledge of the radiochemical properties of alanine have lowered the detection limit of ESR/alanine dosimetry to the dose range relevant for radiation therapy. The aim of this thesis is to contribute to an extended use of ESR dosimetry in radiation therapy, including development of an alanine-based gel dosimeter and investigation of alternative dosimeter materials.

The dosimeter material is often a polycrystalline powder, which can be mixed with a binder and formed to solid dosimeters in the shape of tablets, rods or films. It can also be distributed in a gel, to serve as both dosimeter material and phantom material. Thus, problems caused by the detector displacing the medium are avoided. The gel developed in this thesis is based on polycrystalline alanine, distributed in an agarose gel. It was tested for calibrated measurements in a brachytherapy situation. With further development, the alanine/agarose gel was found to have potential as a tool for verification of treatment plans, also at complicated dose distributions. One problem to be solved before the gel can be used at an arbitrary irradiation geometry is the low sensitivity of the gel. At present, the sensitivity puts a limit on the spatial resolution at low doses because of the high demands on measurement precision in radiation therapy.

The low sensitivity of the alanine/agarose gel, and also the need for a high precision at low doses together with a small dosimeter size when using traditional solid dosimeters, have raised the need for alternative, more sensitive, dosimeter materials than alanine. Such a material should form only one radiation induced radical, giving an ESR spectrum with few and narrow lines for easy evaluation. It should also be possible to increase the microwave power and the field modulation amplitude without severe distortion of the spectrum shape. Other important criteria are a low effective atomic number, a signal that is stable over time and, preferably, a linear dose response.

Two compounds are presented; ammonium tartrate and 2-methylalanine. Both  are more sensitive than alanine at low settings of microwave power and modulation amplitude, and fulfil the criteria of a low effective atomic number and a linear dose response. Ammonium tartrate is saturated already at low microwave power levels whereas the modulation amplitude can be increased to very high levels without saturation. However, the spectrum shape is then distorted. The signal-changes over time are rapid during the first hours after irradiation, but are then stabilised and can be corrected for.

The other investigated substance, 2-methylalanine, is more similar to alanine regarding signal stability over time and dependency of microwave power and field modulation amplitude. However, irradiation of 2-methylalanine results in only one detectable radical, where the alanine spectrum is composed of at least two radicals. The less complicated spectrum of 2-methylalanine makes it more safe to increase the microwave power and the modulation amplitude to their saturation levels.

This study is one step in the search for an ESR dosimeter material with a higher signal intensity than the commonly used l-α-alanine, to be useful in the clinical dose range (approximately 0.1–20 Gy). The substance ammonium tartrate was found and investigated regarding signal intensity, radical stability, dose response and dose resolution. The ESR signal intensity of ammonium tartrate was shown to be more than twice the intensity of the alanine signal. The data indicate that an unstable radiation induced radical contributes to the ESR signal initially; after a couple of hours it has converted to a secondary radical which has a decay slow enough to be considered stable during the first two weeks after irradiation. Ammonium tartrate has a linear dose response in the investigated range of 0.5–4000 Gy and a dose resolution of 0.1 Gy at the 0.5 Gy level where, as a comparison, the corresponding value for alanine is 0.3 Gy. We thus find the substance suitable for clinical dosimetry.

The crystalline substance ammonium tartrate is investigated with respect to its suitability as a clinical dosimetry material. The properties investigated are: the radical stability for absorbed doses relevant for clinical use, the improvement in sensitivity when the crystals are deuterated and the linear electron tranfer (LET)-dependence. After photon irradiation to an absorbed dose of 20 Gy, the signal increases rapidly during the first 6 h. After this period, the changes are more moderate and can be corrected for. The signal-to-noise ratio of irradiated ammonium tartrate is twice the corresponding value of alanine. By deuterating the crystals, the sensitivity can be further improved by a factor of 1.4. As expected, the signal decreases with increasing LET of the radiation, but no changes in the spectrum shape was observed.

The alpha-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical (CH)-C-.(COOH)(2) in irradiated crystals of malonic acid, CH2(COOH)(2), has served as a standard against which hundreds of observations of similar couplings have been held and scaled. The major doublet of the malonic acid radical is accompanied by less intense "forbidden" (f) alpha-proton transitions and "spin-flip" (s) transitions due to weakly interacting protons. Both s and f transition lines exhibit microwave power saturation behaviors different from that of the major doublet. At high microwave power, the prominence of these s and f lines may be misinterpreted as originating from different radical species. Computer simulations could help distinguish between the different cases, but no computer simulation programs taking into account the microwave power saturation case are commonly available. On the basis of classical line-shape theory, an algorithm describing the microwave power dependence of an EPR line shape has been developed and implemented in an existing simulation program. To test this new program, malonic acid was selected because of the simplicity of its EPR spectra. However, sufficiently detailed information about the hyperfine coupling parameters for a satisfactory simulation of the room-temperature data (including s and f lines) was not available in the literature. Therefore, a detailed room-temperature EPR/ENDOR study on a single crystal of malonic acid was performed. In addition to the major cc-proton coupling, seven weaker proton interactions have been characterized and partly identified. Simulations under nonsaturating conditions reproduce very well all features of the experimental EPR spectra. Simulations under saturating conditions similarly reproduce the power-dependent EPR spectra and yield information about the relaxation behavior of the radical system, which is amenable to verification using other spin-resonance methods.

Single crystals of Rochelle salt, [(-)OOC-CHOH-CHOH-COO(-), Na(+) K(+)]center dot 4H(2)O, X-irradiated at 10 K, have been examined using EPR, ENDOR and EIE spectroscopic techniques to characterize the radiation induced radicals stable at that temperature and their reactions upon warming. The one-electron gain product was observed and from the hyperfine interaction with a beta-proton it was unambiguously centered at the C(4) position of the tartrate moiety. An additional nearly isotropic hyperfine structure of about 21 MHz was tentatively assigned to interaction with a sodium ion exhibiting a close contact to O(3) in the crystal. Evidence was obtained that the one-electron reduced radical had become protonated at one of the C(4) bonded carboxyl oxygens, most probably O(4). No evidence for the corresponding C(1)-centered reduction product was found. Two resonance lines (R2, A1) were shown by EIE to belong to a species formed by decarboxylation at C(3), a secondary oxidation product. Two other resonance lines (K1, K2) were assigned to two varieties of another decarboxylation radical, centered at C(2), distinguished by differences in the potassium ion coordination. Furthermore, one other resonance line (A2) was tentatively ascribed to a third decarboxylation radical, centered at the opposite end of the tartrate moiety. The precursor of these products, that is, the one-electron loss product, was not observed after X-irradiation at 10 K. Thermally induced free radical reactions followed by EPR in the temperature range of 12-119K indicate that a water molecule or a hydroxyl ion is eliminated from the one-electron reduction product radical and that a C(3)-centered radical is formed. The reduction and oxidation reaction pathways of hydroxy acid derivatives are discussed.