D. V. Stass

Ion-molecular charge transfer reactions of hexafluorobenzene and cis-decalin in nonpolar solutions studied by linewidth broadening in MARY spectra

Abstract The effect of an external magnetic field on the radiofluorescence arising from the recombination of spin-correlated radical ion pairs (MARY spectrum) has been studied for dilute nonpolar solutions of hexafluorobenzene and cis-decalin. MARY spectra of these systems show additional local fluorescence intensity maxima at zero field and in the field equal to triple the hfi constant. The breaking down of spin coherence in the course of the ion-molecular charge transfer reaction leads to broadening of the maxima. Relations between the line broadening and the rate of charge transfer reaction have been derived. Rate constants of charge transfer reactions for hexafluorobenzene radical anion in squalene and cis-decalin radical cation in hexane have been obtained experimentally.

Estimation of lifetimes of solvent radical cations in liquid alkanes using the level crossing spectroscopy technique

Abstract The technique of radical ion pair level crossing spectroscopy was employed to obtain the electron spin resonance spectrum parameters of solvent radical cations for a series of linear nonviscous alkanes (pentane to hexadecane) at room temperature. The spectra consist of a single homogeneous line that becomes broader as the carbon chain of the alkane molecule becomes shorter. Effective hyperfine interactions in radical cations are negligible. The lower estimates for the lifetimes of radical cations were obtained. Assuming that the contribution of spin relaxation to the line width is not important, the lower estimates increase from 1 to 33 ns in the sequence C 5 –C 16 .

Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in “Breathing Crystals”

Breathing crystals based on polymer-chain complexes of Cu(hfac)(2) with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu(hfac)(2) complexes with tert-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets.

Study of interaction of aliphatic alcohols with primary radical cations of n-alkanes using MARY spectroscopy

MARY spectroscopy (the radical ion pair level crossing technique) was employed to probe the reaction of proton transfer from primary radical cations of n-alkanes to alcohol molecules in liquid solution. Alcohols were demonstrated to react with the primary radical cation of the solvent, leaving the counterion of the radical ion pair unaffected. The broadening of the zero field MARY line, tentatively attributed to the proton transfer reaction, was found to be independent of the proton affinity of the species in the studied systems, estimated from gas-phase data. The rate constant of the reaction is close to the diffusion controlled limit within the experimental accuracy for all the studied alcohol/solvent combinations.

Kinetic magnetic-field effect involving the small biologically relevant inorganic radicals NO and O2(·-).

Oxidation of dihydrorhodamine 123 (DHR) to rhodamine 123 (RH) by oxoperoxonitrite (ONOO(-)), formed through recombination of NO and O(2)(·-) radicals resulting from thermal decomposition of 3-morpholinosydnonimine (SIN-1) in buffered aerated aqueous solution at pH 7.6, represents a kinetic model system of the reactivity of NO and O(2)(·-) in biochemical systems. A magnetic-field effect (MFE) on the yield of RH detected in this system is explored in the full range of fields between 0 and 18 T. It is found to increase in a nearly linear fashion up to a value of 5.5±1.6 % at 18 T and 23 °C (3.1±0.7 % at 40 °C). A theoretical framework to analyze the MFE in terms of the magnetic-field-enhanced recombination rate constant k(rec) of NO and O(2)(·-) due to magnetic mixing of T(0) and S spin states of the radical pair by the Δg mechanism is developed, including estimation of magnetic properties (g tensor and spin relaxation times) of NO and O(2)(·-) in aqueous solution, and calculation of the MFE on k(rec) using the theoretical formalism of Gorelik at al. The factor with which the MFE on k(rec) is translated to the MFE on the yield of ONOO(-) and RH is derived for various kinetic scenarios representing possible sink channels for NO and O(2)(·-). With reasonable assumptions for the values of some unknown kinetic parameters, the theoretical predictions account well for the observed MFE.

MARY-detected ESR spectrum of solvent radical cations (holes) in squalane

Abstract The technique of magnetic field modulation of reaction yield (MARY) spectroscopy was employed to obtain the ESR spectrum of squalane holes that are formed under the action of ionizing radiation at room temperature. The spectrum is shown to consist of a single line which is homogenously narrowed due to resonance charge transfer over the solvent molecules with the rate constant exceeding the diffusion-controlled one.

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

Abstract 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.

Synthesis of nitroxyl radical by direct nucleophilic functionalization of a C-H bond in the azadiene systems

Cyclic dinitrones underwent nucleophilic substitution of the hydrogen atom in the reaction with a paramagnetic carbanion, the lithium derivative of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, to give polyfunctional nitronyl nitroxyls.

The influence of coordination compounds on spin chemical processes in irradiated hydrocarbons

12 1 The methods of spin chemistry (chemically induced dynamic electron and nuclear polarization, optically detected ESR, quantum beats, MARY spec troscopy, magnetic isotope effect, etc.) are widely used to study short lived paramagnetic intermediates and mechanisms of reactions involving organic com pounds [1–3]. They also proved very useful for the studies of organoelement and organometallic systems [4]. Coordination compounds studied by spin chemis try methods are rather scarce [5–7], although this approach could provide unique information for inor ganic chemistry. It would allow detecting short lived radical ionic intermediates of coordination com pounds, reveal such intermediates with non standard metal oxidation degrees, establish the mechanisms of their reactions. Such an extension of the range of available systems is also important for spin chemistry, since it would provide species with nuclear spins exceeding the conventional values of 1/2 and 1 for organic radicals. Furthermore, a change of the central ion in the coordination compound opens the possibil ity to obtain a set of systems with close structure but varied magnetic and redox properties.

Effect of ionizing radiation on the luminescence of mycelium of luminous fungus Neonothopanus nambi

30 Currently, the luminescent systems and the mechh anisms of luminescence of many living organisms are well studied: the enzymes (luciferases) catalyzing the lighttemission reactions and their substrates (luciferins) of these organisms were isolated and charr acterized [1]. However, this problem for higher lumii nous fungi remains unsolved, and the molecular orgaa nization of their luminescent system is still poorly understood. First of all, it remains unclear which enzyme (or enzyme complex) performs the function of luciferase in fungi and what is the structure of luciferin, the substrate of lighttemitting reaction. In the early 1990s, it was assumed that the mechanism of fungal luminescence involve reactive oxygen species (ROS) and enzymes with oxidase function [1, 2]. The results of our recent studies of the luminous fungus Neonothopanus nambi also indicate that ROS and oxidases are involved in the mechanism of its luminescence and indicate the relationship of the fungal luminescent system with membrane structures [3–5]. On the basis of these data, it was assumed that the foll lowing membraneebound enzyme systems can be involved in the lighttemitting reaction: oxidases (including peroxidases) of the ligninndegrading comm plex, cytochrome PP450 system, and enzymes of the mitochondrial respiratory chain. These enzyme syss tems can produce ROS, and two of them (ligninolytic oxidase complex and cytochrome PP450 system) can catalyze the oxidation of organic substrates (including luciferin) with the involvement of ROS. Thus, the study of the relationship between the forr mation and transformation of reactive oxygen radicals and the luminescence of higher fungi is important for understanding the mechanisms of luminescence. It is well known [6, 7] that the production of ROS in bioo logical objects can be stimulated by exposure to physs ical, chemical, and biological factors. In particular, the activation of ROS generation under the influence of ionizing radiation and, as a result, the stimulation of superweak chemiluminescence of plant and animal cells was shown [8, 9]. However, in available literature we found no similar papers describing the stimulation of luminescence of luminous fungi. In the present study, we investigated the effect of ionizing radiation on the luminescence of the fungus N. nambi. Experiments were performed with the mycelium of N. nambi inhabiting the tropical forests of South Viett nam [10]. The culture of the fungus for research was kindly provided by Vietnamese researcher Dao Thi Van (private collection of strains of the BIOOLUMI Co., Ltd. company, Vietnam). Samples of luminous …