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.

The structures of the free radicals formed by the irradiation of potassium dithionate (K 2S2O6) with 60Co g -rays and 14N7þ 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 At ¼ 12.49, Ajj ¼ 15.60 mT, the other (S2) A t ¼ 11.29, Ajj ¼ 13.92 mT. The g-factors were nearly isotropic, gt ¼ 2.0010, gjj ¼ 2.0003; and g t ¼ 2.0026, gjj ¼ 2.0008, respectively. The 33S hyperfine coupling tensors and g-tensors were axially symmetric about the trigonal <c>-axis, coinciding with the direction of the SeS bonds of the two nonequivalent S 2O2 6 ions in the crystal. A model for the radiation damage was proposed in which the SO 3 radical ions retain the orientation of the SO 3 groups, aligned along the trigonal axis. The structure of a third main radical species (S3) with gt ¼ 2.0026, gjj ¼ 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 g-rays and 0.47: 0.38: 0.15 for 14N7þ 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 e0.95 nm along the trigonal axis.

The catalytic activity of detonation nanodiamond and its modifications obtained through treatment with hydrogen or air at elevated temperatures is studied in the conversion of C(2)-C(3) alcohols. The catalysts were characterized by means of electron microscopy, optical (FTIR) spectroscopy, elemental analysis and pulse microcatalytic method. It has been established that nanodiamond exhibits high catalytic activity in the conversion of alcohols. The oxidizing and reducing treatment of nanodiamond changes its activity and selectivity, and the activity of oxidized nanodiamond is considerably higher than that of reduced nanodiamond.

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) 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.

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.

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.

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.

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.

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.