CIDNP in photolysis of acetone solutions in plastic crystals of cyclohexane-d12

The emissive CIDNP was observed during photolysis of acetone solutions in solid cyclohexane-d₁₂. The polarization is assumed to arise in contact triplet radical pairs as a result of their T_-S, conversion and their recombination.     

A novel version of the method of radiofrequency labeling in chemical reactions

A novel version of the radiofrequency labeling method is described, which possesses essential advantages over the recently proposed method of “saturation transfer”. The conditions necessary, for realization of this method are discussed. The use of the method is exemplified by an investigation of a reaction mechanism.     

On optically detected ESR spectra from radical cations of liquid hydrocarbon solvents

The optically detected electron spin resonance (OD ESR) method has been employed to study the origin of radical-cation ESR signals in some saturated hydrocarbons with small amounts of 2.5-diphenyloxazol or p-terphenyl under radiolysis. In cyclohexane, the ESR, signal with resolved hyperfine structure was ascribed to c-C₆H₁₀⁺/PPO^? radical-ion pairs produced from primary c-C₆H₁₂⁺/PPO^? ones by monomolecular decay, of cyclohexane radical cations to cyclohexene radical cations. Cis- and trans-decalin under radiolysis accumulate 9,10-octalin which captures solvent holes and form 9,10-octalin radical cations giving a resolved OD ESR spectrum. 9,10-octalin is present in non-irradiated commercial decalin as an impurity. The OD ESR technique has been shown to be very sensitive to some impurities in hydrocarbon solvents.     

Correcting measurement errors using reference materials in method validation

Quality assurance and method validation are needed to reduce false decisions due to measurement errors. In this context accuracy and standard uncertainty for the analytical method need to be considered to ensure that the performance characteristics of the method are understood. Therefore, analytical methods ought to be validated before implementation and controlled on a regular basis during usage. For this purpose reference materials (RMs) are useful to determine the performance characteristics of methods under development. These performance parameters may be documented in the light of a method evaluation study and the documentation related to international standards and guidelines. In a method evaluation study of Pb in blood using reference samples from the Laboratoire Toxicologie du Quèbec, Canada, a difference between the systematic errors was observed using a Perkin-Elmer Model 5100 atomic absorption spectrometer and a Perkin-Elmer Model 4100 atomic absorption spectrometer, both with Zeeman background correction. For measurement of blood samples, the performance parameters obtained in the method evaluation studies, i.e. slopes and intercepts of the method evaluation function (MEF), were intended to be used for correcting the systematic errors. However, the number of MEF samples was insufficient to produce an acceptable SD for the MEF slopes to be used for correction. In a method evaluation study on valproate in plasma using the SYVA's EMIT assay on COBAS MIRA S a significant systematic error above the concentration 300 mmol dm^–3 was demonstrated (slope 0.9541) and consequently the slope was used for correction of results. For analytes, where certified RMs (CRMs) exist, a systematic error of measurements can be reduced by correcting errors by assessment of the trueness as recommended in international guidelines issued by ISO or the National Institute of Standards and Technology (NIST). When possible, the analysis of several RMs, covering the concentration range of interest, is the most useful way to investigate measurement bias. Unfortunately, until recently only few RMs existed and only few had been produced and certified by specialized organizations such as NIST or the Standards, Measurements and Testing (SMT, previously BCR) programme. Due to the lack of such RMs, network organizations are nowadays established with the aim of supporting the correct use and production of high-quality CRMs.     

MARY-detected ESR spectra of radical ions in liquid solutions for systems with crossing Zeeman levels

The effect of Zeeman levels crossing in spin-correlated radical ion pairs (naphthalene)⁺/(hexafluorobenzene)⁻ was monitored as the influence of an external magnetic field on the solution fluorescence under X-irradiation (MARY spectrum). The spectra obtained exhibit a line in the field, equal to triple the hfi coupling constant of hexafluorobenzene radical anion, its linewidth being determined by the unresolved hyperfine structure of the naphthalene cation. The theory predicts that the presence of weak hfi in the partner cation splits this line by the projections of the cation total nuclear momentum M_Z according to nuclear statistics. Thus, a direct correspondence between the MARY and ESR spectra of radical cations allows registration of the ESR spectrum without microwave pumping.     

Intramolecular dynamics of 1,2,3-trifluorobenzene radical anions as studied by OD ESR and quantum-chemical methods

Ab initio UMP2, RMP2, DFT/UB3LYP, and CBS-QB3 calculations have shown that the adiabatic potential energy surface (PES) of the 1,2,3-trifluorobenzene radical anion is a pseudorotation surface formed by nonplanar stationary structures. The low (approximately 2-4 kcal/mol) energy barriers in the path of pseudorotation imply manifestations of spectral exchange in the ESR spectra of this radical anion. The optically detected ESR of radical ion pairs was used to obtain the ESR spectrum of 1,2,3-trifluorobenzene radical anion in liquid squalane solution and to study temperature variations in the spectrum over the range of 243-325 K. The spectrum is a doublet of triplets with hfc constants of a(F(2)) = 29 mT and a(2F(1,3)) = 7.6 mT at T = 243 K. The experimental hfc constants are temperature-dependent. Calculations of the temperature dependence of hfc constants in the framework of the model of classical nuclei motion along the pseudorotation coordinate reproduce well the experimental data.     

Optical detection of the ESR spectrum of hexafluorobenzene anion-radicals in squalane at room temperature

The technique of optical detection of ESR spectra from recombining radical pairs has been employed to observe the hyperfine structure of C₆F₆^? ion-radicals formed in squalane under ionizing irradiation. Anthracene introduced into the solution enhances the ESR intensity. The spectra are well described by second-order perturbation theory with a hyperfine interaction constant 133.6 G and g-value 2.0014.