Determination of the g-, hyperfine coupling- and zero-field splittingtensors in EPR and ENDOR using extended Matlab codes

The analysis of single crystal electron magnetic resonance (EMR) data has traditionally been performedusing software in programming languages that are difficult to update, are not easily available, or areobsolete. By using a modern script-language with tools for the analysis and graphical display of the data,three MatLabÒcodes 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.Schonland’s original method was used to compute the g- andhfc-tensors by a least-squares fit to theexperimental data in each plane. The modifications required for the analysis of thezfsof radical pairs withS = 1 were accounted for. A non-linear fit was employed in a second code to obtain thehfc-tensor fromEPR measurements, taking the nuclear Zeeman interaction of an I = ½ nucleus into account. A previouslydeveloped method to calculate the g- andhfc-tensors by a simultaneous linear fit to all data was used inthe third code. The validity of the methods was examined by comparison with results obtained experi-mentally, and by roadmaps computed by exact diagonalization. The probable errors were estimated usingfunctions for regression analysis available in MatLab. The software will be published athttps://doi.org/10.17632/ps24sw95gz.1, Input and output examples presented in this work can also be downloaded fromhttps://old.liu.se/simarc/downloads?l=en.Ó2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/)

On the identity of the last known stable radical in X-irradiated sucrose

Identification of radiation-induced radicals in relatively simple molecules is a prerequisite for the understanding of reaction pathways of the radiation chemistry of complex systems. Sucrose presents an additional practical interest as a versatile radiation dosimetric system. In this work, we present a periodic density functional theory study aimed to identify the fourth stable radical species in this carbohydrate. The proposed model is a fragment suspended in the lattice by hydrogen bonds with an unpaired electron at the original C5’ carbon of the fructose unit. It requires a double scission of the ring accompanied by substantial chemical and geometric reorganization

Relaxation-Time Determination from Continuous-Microwave Saturation of EPR Spectra

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

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

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggest that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5’ and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH2OH hydroxymethyl group is a better radical trapping site than other positions

Potassium sodium (2R,3R)-tartrate tetra­hydrate: the paraelectric phase of Rochelle salt at 105 K

Rochelle salt, K+·Na+·C4H4O6 2−·4H2O, is known for its remarkable ferroelectric state between 255 and 297 K. The current investigation, based on data collected at 105 K, provides very accurate structural information for the low-temperature paraelectric form. Unlike the ferroelectric form, there is only one tartrate molecule in the asymmetric unit, and the structure displays no disorder to large anisotropic atomic displacements

Solved?: the reductive radiation chemistry of alanine

The structural changes throughout the entire reductive radiation-induced pathway of L-alpha-alanine are solved on an atomistic level with the aid of periodic DFT and nudged elastic band (NEB) simulations. This yields unprecedented information on the conformational changes taking place, including the protonation state of the carboxyl group in the "unstable'' and "stable'' alanine radicals and the internal transformation converting these two radical variants at temperatures above 220 K. The structures of all stable radicals were verified by calculating EPR properties and comparing those with experimental data. The variation of the energy throughout the full radiochemical process provides crucial insight into the reason why these structural changes and rearrangements occur. Starting from electron capture, the excess electron quickly localizes on the carbon of a carboxyl group, which pyramidalizes and receives a proton from the amino group of a neighboring alanine molecule, forming a first stable radical species (up to 150 K). In the temperature interval 150-220 K, this radical deaminates and deprotonates at the carboxyl group, the detached amino group undergoes inversion and its methyl group sustains an internal rotation. This yields the so-called "unstable alanine radical''. Above 220 K, triggered by the attachment of an additional proton on the detached amino group, the radical then undergoes an internal rotation in the reverse direction, giving rise to the "stable alanine radical'', which is the final stage in the reductive radiation-induced decay of alanine