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The analysis of single crystal electron magnetic resonance (EMR) data has traditionally been performed using software in programming languages that are difficult to update, are not easily available, or are obsolete. By using a modern script-language with tools for the analysis and graphical display of the data, three MatLab (R) codes were prepared to compute the g, zero-field splitting (zfs) and hyperfine coupling (hfc) tensors from roadmaps obtained by EPR or ENDOR measurements in three crystal planes. Schonlands original method was used to compute the g- and hfc-tensors by a least-squares fit to the experimental data in each plane. The modifications required for the analysis of the zfs of radical pairs with S = 1 were accounted for. A non-linear fit was employed in a second code to obtain the hfc-tensor from EPR measurements, taking the nuclear Zeeman interaction of an I = 1/2 nucleus into account. A previously developed method to calculate the g- and hfc -tensors by a simultaneous linear fit to all data was used in the third code. The validity of the methods was examined by comparison with results obtained experimentally, and by roadmaps computed by exact diagonalization. The probable errors were estimated using functions for regression analysis available in MatLab. The software will be published at https://doi.org/10.17632/ps24sw95gz.1, Input and output examples presented in this work can also be downloaded from https://old.liu.se/simarc/downloads?l=en. (C) 2021 The Author(s). Published by Elsevier Inc. Funding Agencies|Linkoping University; Ghent UniversityGhent University; University of Oslo

Key Points Question Does D-cycloserine potentiate the effect of concentrated exposure and response prevention in difficult-to-treat obsessive-compulsive disorder? Findings In this randomized clinical trial of 163 participants, D-cycloserine did not significantly affect treatment outcomes. Most patients responded to the concentrated exposure and response prevention treatment, and nearly 50% were recovered at 1-year follow-up. Meaning In this study, concentrated exposure and response prevention treatment was effective for patients with difficult-to-treat obsessive-compulsive disorder, but adding D-cycloserine did not potentiate the treatment., This randomized clinical trial evaluates whether D-cycloserine potentiates the effect of concentrated exposure and response prevention among patients with difficult-to-treat obsessive-compulsive disorder., Importance Evidence is lacking for viable treatment options for patients with difficult-to-treat obsessive-compulsive disorder (OCD). It has been suggested that D-cycloserine (DCS) could potentiate the effect of exposure and response prevention (ERP) treatment, but the hypothesis has not been tested among patients with difficult-to-treat OCD. Objective To evaluate whether DCS potentiates the effect of concentrated ERP among patients with difficult-to-treat OCD. Design, Setting, and Participants The study was a randomized placebo-controlled triple-masked study with a 12-month follow-up. Participants were adult outpatients with difficult-to-treat OCD. A total of 220 potential participants were referred, of whom 36 did not meet inclusion criteria and 21 declined to participate. Patients had either relapsed after (n = 100) or not responded to (n = 63) previous ERP treatment. A total of 9 specialized OCD teams within the public health care system in Norway participated, giving national coverage. An expert team of therapists from the coordinating site delivered treatment. Inclusion of patients started in January 2016 and ended in August 2017. Data analysis was conducted February to September 2019. Interventions All patients received individual, concentrated ERP treatment delivered during 4 consecutive days in a group setting (the Bergen 4-day treatment format) combined with 100 mg DCS, 250 mg DCS, or placebo. Main outcomes and Measures Change in symptoms of OCD and change in diagnostic status. Secondary outcomes measures included self-reported symptoms of OCD, anxiety, depression, and quality of life. Results The total sample of 163 patients had a mean (SD) age of 34.5 (10.9) years, and most were women (117 [71.8%]). They had experienced OCD for a mean (SD) of 16.2 (10.2) years. A total of 65 patients (39.9%) were randomized to receive 100 mg DCS, 67 (41.1%) to 250 mg of DCS, and 31 (19.0%) to placebo. Overall, 91 (56.5%) achieved remission at posttreatment, while 70 (47.9%) did so at the 12-month follow-up. There was no significant difference in remission rates among groups. There was a significant reduction in symptoms at 12 months, and within-group effect sizes ranged from 3.01 (95% CI, 2.38-3.63) for the group receiving 250 mg DCS to 3.49 (95% CI, 2.78-4.18) for the group receiving 100 mg DCS (all P

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 SO 3 − 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 -axis, coinciding with the direction of the S–S bonds of the two non-equivalent S 2 O 6 2 − ions in the crystal. A model for the radiation damage was proposed in which the SO 3 − 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 SO 2 − radical ions. An even weaker strongly anisotropic pair of lines was attributed to SO 3 − radical pairs separated by 0.93–0.95 nm along the trigonal axis.

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. Original Publication:A.V. Fionov, Anders Lund, Weimin Chen, N.N. Rozhkova, Irina Buyanova, G.I. Emelyanova, L.E. Gorlenko, E.V. Golubina, E.S. Lokteva, E. Osawa and V.V. Lunin, Paramagnetic centers in detonation nanodiamonds studied by CW and pulse EPR, 2010, Chemical Physics Letters, (493), 04-Jun, 319-322.http://dx.doi.org/10.1016/j.cplett.2010.05.050Copyright: Elsevier Science B.V., Amsterdam.http://www.elsevier.com/

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. Original Publication:Marek Danilczuk, Dariusz Pogocki and Anders Lund, Interaction of .(CH2OH) with silver cation in Ag-A/CH3OH zeolite: A DFT study, 2009, CHEMICAL PHYSICS LETTERS, (469), 1-3, 153-156.http://dx.doi.org/10.1016/j.cplett.2008.12.060Copyright: Elsevier Science B.V., Amsterdam.http://www.elsevier.com/

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 2nd 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 2nd 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. Original publication: A. Lund, P. Andersson, J. Eriksson, J. Hallin, T. Johansson, R. Jonsson, H. Löfgren, C. Paulin and A. Tell, Automatic fitting procedures for EPR spectra of disordered systems: matrix diagonalization and perturbation methods applied to fluorocarbon radicals, 2008, Spectrochimica Acta Part A, (69), 5, 1294-1300. http://dx.doi.org/10.1016/j.saa.2007.09.040. Copyright: Elsevier B.V., http://www.elsevier.com/

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. Original publication: M. Danilczuka, H. Gustafsson, M.D. Sastry, E. Lund and A. Lund, Ammonium Dithionate – a New Material for Highly Sensitive EPR Dosimetry, 2008, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, (69), 1, 18-21. http://dx.doi.org/ 10.1016/j.saa.2007.03.001. Copyright: Copyright: Elsevier B.V., http://www.elsevier.com/

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state)-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A)-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A)-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.

Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time TM, were carried out, in the 20-200 K temperature range, on an AsO44- paramagnetic probe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP), and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the 14N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP, and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number mI of 75As. The temperature dependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb0.5(NH4)0.5H2PO 4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample. © IOP Publishing Ltd.

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li 2 S 2 O 6 ·2H 2 O, 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 SO 3 − 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, T 1 , 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.

Copper (II) phthalocyanine (CuPc) molecules encapsulated into zeolite-Y with Si/Al ratios of 2.7 and 410 were prepared by an in situ synthesis and characterized by UV-Vis and electron paramagnetic resonance (EPR) spectroscopies. Resolved Cu-hyperfine and N-superhyperfine structures were observed in the EPR spectrum of CuPc encapsulated into zeolite-Y with a high Si/Al ratio. UV-Vis and EPR studies as well as theoretical calculations suggest that the encapsulated CuPc molecule was distorted in zeolite-Y with keeping of the square-planar symmetry around the center copper (II) ion. © 2005 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 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 an ...

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 theg-, 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 showingg and hyperfine anisotropic interactions and free radical systems featuring Δm1≠0 transitions due to the direct field effect or the presence of quadrupolar nuclei have been analyzed as well as NOx species on surfaces and radiation defects employed for EPR dosimetry. Analysis of systems withS>1/2, and magnetically coupled species has also been attempted.

CW-EPR studies of NO adsorbed on sodium ion-exchanged zeolites were focused on the geometrical structure of NO monoradical and (NO)2 biradical formed on zeolites. The EPR spectrum of NO monoradical adsorbed on zeolite can be characterized by the three different g-tensor components and the resolved y-component hyperfine coupling with the 14N nucleus. Among the g-tensor components, the value of g(zz) is very sensitive to the local environment of zeolite and becomes a measure of the electrostatic field in zeolite. The temperature dependence of the g-tensor demonstrated the presence of two states of the Na-NO adduct, in rigid and rotational states. The EPR spectra of NO adsorbed on alkaline metal ion-exchanged zeolite and their temperature dependency are essentially the same as that on sodium ion-exchanged zeolite. On the other hand, for NO adsorbed on copper ion-exchanged zeolite it is known that the magnetic interaction between NO molecule and paramagnetic copper ion are observable in the spectra recorded at low temperature. The signals assigned to (NO)2 biradical were detected for EPR spectrum of NO adsorbed on Na-LTA. CW-EPR spectra as well as their theoretical calculation suggested that the two NO molecules are aligned along their N-O bond axes. A new procedure for automatical EPR simulation is described which makes it possible to analyze EPR spectrum easily. In the last part of this paper, some instances when other nitrogen oxides were used as a probe molecule to characterize the zeolite structure, chemical properties of zeolites, and dynamics of small molecules were described on the basis of selected literature data reported recently.

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.

D/H isotope-substitution effects on the Jahn–Teller (J–T) distortion of cation radicals of partially deuteriated benzenes; benzene-d5, benzene-1,2,4,5-d4, benzene-1,4-d2, and benzene-1,3,5-d3, were investigated by electron spin resonance (EPR) using frozen haloalkanes and porous silicas as matrices. Quantum-chemical calculations using the density functional theory (DFT) method were also performed. The way of distortion of the cation radicals of selectively deuteriated benzenes is unique relative to the deuteriation sites in all the matrices used. The ground state of the distorted cation is 2B2g of D2h symmetry, regardless of the way of isotope labeling. The undeuteriated C–H bonds occupy the specific positions in the distorted form where the higher spin density appears, in agreement with the result of theoretical calculations. The role of the matrix to stabilize the distorted form is also discussed.

Single crystals and polycrystalline samples of lithium formate monohydrate (HCO2Li·H2O) were X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 200 or 295 K. Two different radical species were observed. The overall dominating species is the ˙CO2− radical trapped in the crystal matrix at an orientation not very different from that of the parent CO2 fragment in the unirradiated matrix. The g- and 13C hyperfine coupling tensors of the ˙CO2− radical were determined. The large linewidth (about 1.5 mT) of the polycrystalline EPR spectrum is due to extensive hyperfine couplings with lithium ions and protons in the environment. Four lithium couplings and four proton couplings associated with the ˙CO2− radical were measured, and all couplings were assigned to specific matrix nuclei. The spectra yield evidence for a second radical in low relative abundance. One small lithium hyperfine interaction detected was ascribed to this radical. Spectral simulations of the EPR and ENDOR spectra support the conclusions made.

Amine radical cations of the type R3N·+ and [R3NCH2]·+, R=CH3, 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 γ-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)3NCH2]·+ radical cation in several studied systems changed reversibly with temperature, indicating dynamical effects. Free rotation about the >N−CH2 bond of the [(CH3)3NCH2]·+ 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 more23Na nuclei, with approximative parameters A(23Na)=(4.6, 4.6, 8.2) MHz and Q(23Na)=(−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 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

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 α-coupling, and three β-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 θ ≈ 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.

Nitrogen monoxide (NO) introduced into partially and fully lithium ion (Li+)-exchanged A-type zeolite was studied by X-band EPR measurements. Two types of NO species, NO monoradical and NO–NO biradical, were detected for the partially ion-exchanged zeolite, while the latter was less detectable for the fully ion-exchanged one. The EPR parameters of the NO monoradical provided evidence that the electrostatic field associated with Li+ ions in A-type zeolites is weaker than that with Na+ ions. It was found that the molecular distance of NO biradical formed in partially Li+-exchanged zeolite was shorter than that in sodium ion-exchanged A-type zeolite.

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.

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.

To find an EPR dosimeter material that is sensitive enough for clinical use, the substance 2-methylalanine (2MA) with the chemical structure (CH(3))(2)C(NH(3)(+))COO(-) was tested for its sensitivity to ionizing radiation, dose response, and radical stability over time. At equal and moderate settings of microwave power and modulation amplitude, 2MA was found to be 70% more sensitive than L-alpha-alanine, which is the most common EPR dosimeter material today. The dose response is linear, at least in the dose range of interest (0.5-00 Gy), and the time-dependent variations in signal intensity are very small and may be corrected for easily. The energy dependence of the stopping power and energy absorption was calculated and was found to be similar to that of alanine. The dependence of the signal intensity on microwave power and modulation amplitude was investigated, and the optimal settings were found to be 25 mW (Bruker ER 4102ST) and 12 gauss, respectively. Single crystals of 2MA were analyzed using ENDOR and ENDOR-induced EPR to identify the radiation-induced radicals that formed. Only one radical, in which the amino group is detached from the original molecule, was identified. This radical is obviously dominating and is apparently the only one relevant for dosimetry purposes. The complete set of coupling parameters for three hyperfine couplings is reported. The power saturation properties and spectral line width are ruled by the relaxation times T(1) and T(2). To determine the relaxation times of 2MA, pulsed EPR experiments were performed on single crystals. Two different values of T(1) were obtained, one in the range 1-3 micros, shown to be of importance for the dosimetry properties, and another that is strongly anisotropic with a value between 10 and 35 micros that does not seem to affect the saturation behavior. T(2) was estimated to be of the order of 200-300 ns.

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.

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 1/7M 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 isotope13C substitution generates low-concentration radicals with relaxation properties different from the equivalent12C-substitued radicals. The different behavior is discussed.

An EPR study was carried out to investigate both the structure and molecular dynamics of triethylamine (Et3N) and tripropylamine (Pr3N) radical cations generated in γ-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 β-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 nature of quartet state nitrogen atoms formed in γ-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 14N signal pairs centered at aN/2 (half the 14N-hyperfine splitting) and at 3aN/2, strongly suggesting the existence of quartet state 14N atoms (4S3/2). At 160 K, the EPR spectrum exhibited a clear fine structure accompanied by a hyperfine coupling due to 14N. 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 14N 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.

An EPR study was carried out to elucidate the zeolite cage effect on the stability and molecular dynamics of [(CH3)3N]•+ and [(CH3)3NCH2]•+ radical cations generated in γ-irradiated Al-offretite, SAPO-37 and SAPO-42, which contained (CH3)4N+ ions as organic template. The [(CH3)3N]•+ 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 β-cages of SAPO-42. On the other hand, the [(CH3)3NCH2]•+ 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 α-cages of SAPO-42. Strongly temperature dependent EPR spectra were observed for the [(CH3)3NCH2]•+ 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)

Nitrogen monoxide (NO) introduced into sodium ion-exchanged A-type (Na-A) zeolite was studied by temperature- and NO pressure-dependent EPR measurements at Q-band in the temperature range between 5 and 140 K. The experimental spectra were successfully utilized to determine accurate values of the g and A(14N) tensors of NO monoradical and the g tensors and zero field splitting (ZFS) parameters, D and E, of the NO-NO biradical. Two different EPR spectra corresponding to low and high temperature forms of the NO monoradical were observed when the NO pressure was low (PNO=3.1 kPa). Based on the g and A tensors, the former was attributed to a rigid form and the latter to a rotational form of the monoradical. For higher NO pressure, the biradical became dominant. The structure of the biradical was discussed in terms of the D and g tensors combined with the spectral line shape simulation method.

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 ...

In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon X irradiation at room temperature, the (CD)-C-.(COOD)(2) radical is formed in an amount comparable to the (CH)-C-.(COOD): radical species. In the present work, EPR and ENDOR analyses of the a-deuteron hyperfine coupling (hfc) and nuclear quadrupolar coupling (nqc) tensors at room temperature have been performed. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the a-proton coupling of the (CH)-C-.(COOH)(2) radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 +/- 1 kHz, and the asymmetry factor eta = 0.092 +/- 0.020. It is known that, at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about +/-12 degrees. Assuming that the observed hfe and nqc tensors are the result of thermal avenging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nyc tensor, for which the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor eta = 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the alpha-deuteron caused by the charge distribution at the sp(2)-hybridized alpha-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been performed. Field gradients calculated by the DFT method significantly overestimate the quadrupolar tensors for both the alpha-deuteron of the radical and the methylene deuterons of the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest-neighbor atom of the Jeuteron are significant.

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 (aiso) 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‖∣

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N–CH2]+• radical formed in a γ-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 7.0×106 s−1 (140 K) to 4.1×108 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.

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 2B1g and 2B2g 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 2B2g-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.

Electron spin resonance (ESR) spectroscopy was used to study the effect of zeolite structure on the motional dynamics of NO 2 adsorbed on the zeolites. The temperature-dependent ESR spectral line shapes were quantitatively analyzed using the slow-motion ESR theory. It was observed that the motional dynamics of NO 2 is strongly dependent on the structure of the zeolite (Beta-type, ZSM-5, mordenite, L-type and ferrierite zeolites). The following important observations were made. (1) In zeolites with similar channel structures, the diffusion rate of NO 2 is proportional to the channel size, and the order of the diffusion rates is Beta-type>ZSM-5>ferrierite and L-type>mordenite. (2) The diffusion of NO 2 is faster in the zeolites with multi-dimensional channels (Beta-type, ZSM-5 and ferrierite) than that in those with uni-dimensional channels (L-type and mordenite).

A combination of EPR lineshapes, ENDOR and ESEEM was performed to investigate possible Jahn–Teller effects in mono- and perdeuterated benzene radical cation in a polycrystalline CFCl3 matrix. Replacement of one proton by a deuteron was utilized to exclude significantly fast quantum pseudorotation and overall rotary motion below 77 K. Clear evidence was obtained for static Jahn–Teller distortion at temperatures up to 30 K, with major spin densities on two para positions (1 and 4) in agreement with a b2g(χs+) localized orbital. From ENDOR measurements of C6H5D+, the isotropic and dipolar coupling constants for protons occupying both high and low spin density positions were accurately measured. Additional ESEEM experiments and simulations confirmed the results, indicating further that deuteron-isotope substitution does not disturb appreciably the fundamental dynamics of the benzene radical benzene. The hyperfine tensors of C6D6+ in the Jahn–Teller distorted configuration obtained by ESEEM were analogous to those of fully protonated compounds, but the components were scaled by the magnetic moment ratio of deuterons to protons.

The diffusion and bonding of NOx (x=1, 2) and Cu2+ species in zeolites are reviewed, based mainly on our own research. The molecular motion of adsorbed NO2 has been examined with EPR in several zeolite samples and analyzed using the slow-motional EPR theory. In X- and Y-type zeolites the broadening of the spectra with temperature could be analyzed by simulations using a rotational diffusion model in agreement with earlier results in Vycor glass and Cu-metal. For the diffusion of NO2 in Na-mordenite and Na-ZSM-5 the broadening of the spectra with increased temperature could be better simulated with the Heisenberg type of spin exchange model. The exchange was attributed to the interaction between NO2 molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na+, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of H2O molecules adsorbed on the zeolite because of weaker interaction between NO2 and the surface. A more detailed slow-motional analysis indicated that at each temperature a distribution of diffusion rates occurs. NO adsorbed on Na-A zeolite was found to be present as monomer at low pressure ( 100 mbar). The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NO–NO species depends on Na+ for its stability. The detailed structure of a Cu2+ complex with water molecules in Cu-ZSM-5 zeolite was characterised with ENDOR at 4 K. The complex has an axial distorted octahedral structure with two water molecules at a longer distance in axial position and four molecules in the equatorial positions. The Cu2+ complexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with 1H, 2H and 14N hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

ENDOR investigations of the benzene radical cation in a CFCl3 matrix at 30 K gave evidence for a static Jahn–Teller distortion with the unpaired electron density predominantly in the 2 b 2g orbital. In this distortion the equivalence of the six protons is lost, with the major spin densities on H1 and H4. From ENDOR measurements the isotropic and dipolar couplings were accurately measured (aiso=−25.31 MHz (H1 and H4) and aiso=−5.43 MHz (H2, H3, H5 and H6)) which agree with those reported earlier from EPR measurements and theoretical calculations. (C6H6)2+., formed by warming in CFCl3 at 140 K and initially present in CF3CCl3, was characterised with ENDOR and assigned to a π-electron complex. The intermolecular distance was estimated from the measured dipolar hyperfine coupling to be in the range 2–3 A.

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.

The temperature-dependent EPR line shapes from the methyl rotor of the X irradiation-induced CH3Ċ(COOH)2 radical in powder MMA (methyl malonic acid) and the X irradiation-induced radical CD3Ċ(COOH)2 in methyl-specifically deuterated powder MMA are studied experimentally for the temperature range 4.8 K (5 K) to 77 K (65 K). The hydrogenated system is simulated using a quantum inertial dynamical model with a hindering potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling frequency is negligible for the deuterated rotor, resulting in a stopped rotor low-temperature spectrum, while being sufficiently large for the hydrogenated system for this to exhibit tunneling. From the low-temperature deuterated analogue spectrum, the potential twist angle is estimated to δ = ±50° ± 2° (+n·60°, n ∈ Z). The site-exchange activation energy of the deuterium rotor is observed to b...

Single crystals of the amino acid analogue hippuric acid, PhCONHCH2COOH, have been X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 295 and 130 K. Two different radical species were observed and characterized. The dominant species is radical R1, PhCONH-•CH2, supposedly formed by net decarboxylation from a pristine oxidation product. The nitrogen hyperfine and quadrupolar interactions yield information on the electronic structure in the nitrogen valence orbitals. The second radical species, radical R2, is formed by a net hydrogen addition to the phenyl entity of hippuric acid. As a reduction product, it may be formed by protonation of the negatively charged anion of the phenyl group, but the alternative mechanism of direct hydrogen addition to the phenyl ring cannot be ruled out. Spectral simulations indicate that radical R1 contributes about 85% of the total EPR spectrum, while the remaining 15% is contributed by radical R2.

Electron spin resonance (ESR) was used to study the motional dynamics of NO 2 adsorbed on Na-ZSM-5 with different SiO 2 /Al 2 O 3 ratios at 3–160 K. The temperature dependence of the ESR spectral lineshape was quantitatively analyzed with slow-motion ESR theory. The computer analysis with the Heisenberg spin exchange model reveals that the spin exchange rate of NO 2 below 160 K increases quickly as the SiO 2 /Al 2 O 3 ratio is increased from 23.8 to 70 and to 200. It is concluded that the difference in the amount of Na + among the various zeolites is the main reason for the observed variation in spin exchange rates.

The naphthalene radical cation generated from the corresponding neutral compound by UV- or X-irradiation on H−ZSM-5 zeolite and in frozen CFCl3 solution has been studied by EPR, ENDOR, and ESEEM sp...

Electron spin resonance and electron nuclear double resonance were used to characterize the detailed structure of a copper complex with water molecules in Cu-ZSM-5 zeolites at 4 K. This Letter highlights the structure of the complex. It has an axial distorted octahedral symmetry. The distances between the copper ion and protons within the water molecules are 2.54 and 2.69 A. Accordingly, the water molecules can be divided into two groups. Two water molecules at the longer distance are in the axial position and four other water molecules at the shorter distance are in the equatorial position with respect to the Cu ion.

Two simultaneous dynamical processes are studied in solutions of varying viscosity by using the EPR linewidth data of a Cu(I) o-semiquinone complex with two triphenylphosphine ligands (PPh 3 ), i.e. the overall reorientation (tumbling) of the complex and intramolecular exchange of the PPh 3 ligands in the coordination sphere of the complex. It is shown that for fast motion the line broadening of the hyperfine components is due to both modulation of the anisotropic magnetic parameters by rotational diffusion and to modulation of the isotropic part of the 31 P hyperfine interaction by intramolecular exchange. The intramolecular dynamics of the coordination sphere appears to be dependent on the solvent dynamics.

The biphenyl radical cation absorbed on H-ZSM-5 zeolite and silica gel and isolated in frozen CFCl3 has been studied by EPR and ENDOR spectroscopy. The recorded spectra can be explained by anisotropy of proton hyperfine interactions in para and ortho position and of the g-tensor. The experimental results indicate that the cation has a nearly planar structure, with a twist angle of less than 10°. The ENDOR spectra are analysed by computer simulations. A simple theory to calculate ENDOR transition moments to first order in disordered systems is presented. The simulations show that the observed ENDOR line positions correspond to principal hyperfine tensor values of the para and ortho ring protons.

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.

Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo, (ESE) spectroscopy have been used to characterise radical cations of benzene (benzene-d6 and benzene-d1), generated by ionising radiation, and stabilised on silica gel and HY molecular sieve surfaces. The electronic structure and dynamic features of the benzene cation were found to be different from those in the case of stabilisation in a Freon matrix (CFCl3). The monomeric cation was found to undergo pseudorotation at 3.5 K with hyperfine structure (h.f.s.) constants due to six averaged hydrogen nuclei. Simulations of the time-domain deuterium electron spin echo envelope modulation (ESEM) (C6D+6 cation) gave the following h.f.s. constants: the perpendicular component of the axially symmetric tensor T⊥ = −0.8 MHz, and the isotropic component a = 2.17 MHz, consistent with ENDOR results at 105 K. It was concluded that the monodeuterated benzene cation ‘slowed down’ the rotation at 3.5 K, although not enough to allow an analysis in the rigid limit. ENDOR spectra of the protonated and deuterated monomeric and dimeric benzene radical cations on silica gel and HY molecular sieve surfaces are presented. On HY molecular sieve the ENDOR results revealed two types of dimer, one stabilised at low temperature (below 110 K) characterised by h.f.s. constants of 4.8 and 8.9 MHz, and the other stabilised at higher temperatures with an h.f.s. constant of 6.5 MHz. Only the latter type of dimer could be detected on silica gel. In addition to hyperfine splittings from ring protons, a smaller hyperfine splitting attributed to hydroxyl protons situated on the surface was determined from the ENDOR results.