Yuri N. Molin

1,2,3-benzodithiazolyl radicals formed by thermolysis and photolysis of 1,3,2,4-benzodithiadiazines

Mild thermolysis (at 110–150°C) of 1,3,2,4-benzodithiadiazine 1 and its carbocyclic substituted derivatives 2–15 in hydrocarbon solvents quantitatively yields stable 1,2,3-benzodithiazolyl π-radicals 1•–15•. Kinetics of this reaction can be described as a first-order process. Arrhenius parameters of the effective rate constant are Ea = 80 ± 8 kJ mol−1, k0 = 106.4 ± 1.1 s−1 for 1 in squalane. Room-temperature photolysis of 1 in hydrocarbon solvents also affords radical 1• in nearly quantitative yield. Quantum yield of photolysis is wavelength dependent and is equal to 0.08 ± 0.01 at 313 nm in benzene. Experimental hyperfine coupling (hfc) constants in the ESR spectra of 1•–15• agree fairly well with those calculated at the B3LYP/CC-pVDZ level of theory. Spin density distribution in 1•–15• is in striking contrast to that of isomeric 1,3,2-benzodithiazolyls but resembles the distribution in correspondingly substituted benzyl radicals. ESR linewidths of radicals 1•-15• display some features likely related to spin-rotational relaxation.

Spectroscopic capabilities of the time-resolved magnetic field effect technique as illustrated in the study of hexamethylethane radical cation in liquid hexane

The method of the time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs can provide information on the parameters of the EPR spectra of short-lived radical ions. The present paper demonstrates these spectroscopic potentialities using, as an example, hexamethylethane radical cation whose EPR spectrum in solution is determined by isotropic hfc with 18 equivalent protons. The hfc constant is determined from the position of peaks of the time-resolved magnetic field effect of the hexamethylethane and perdeuterated p-terphenyl solution in n-hexane at ambient temperature. The difference in the g-values of (hexamethylethane)˙+ and (p-terphenyl-d14)˙− radical ions is found by analyzing the curve shape of magnetic field effect for various magnetic field strength. Both the hfc constant and the difference in the g-values coincide with the data obtained by the OD EPR method. Independence of the curve shape on hexamethylethane concentration testifies to the low rate of ion-molecular charge transfer. The method of time resolved magnetic field effect is used to determine the times of longitudinal and transverse paramagnetic relaxation. The dependency of the relaxation rates on magnetic field strength is derived and feasible relaxation mechanisms are discussed.

Spin-selective reaction with a third radical destroys spin correlation in the surviving radical pairs.

The goal of this work is to reveal the effect that an irreversible spin-selective reaction of a partner of the spin-correlated radical pair (SCRP) with a third paramagnetic particle has on the spin state of the surviving SCRPs in the absence of spin exchange interaction. As studied SCRPs, we used the geminate (excess electron/radical cation) pairs generated by ionizing irradiation of tetramethyl-para-phenylenediamine solutions in n-dodecane. As a spin-selective reaction, the scavenging of electrons by nitroxide radicals from the bulk of the solution was used. Both the electron scavenging reaction and the spin correlation in the surviving SCRPs were monitored by measuring the recombination fluorescence decays of the irradiated solutions under the same experimental conditions. It was found that the spin-selective electron scavenging results in the acceleration of spin correlation decay in the remaining unreacted SCRPs. In accordance with the suggested theoretical model, the rate of this additional spin correlation decay is revealed to be equal to the scavenging rate.

Stimulation of luminescence of mycelium of luminous fungus Neonothopanus nambi by ionizing radiation

The luminescent system of higher luminous fungi is not fully understood and the enzyme/substrate pair of the light emission reaction has not been isolated. It was suggested that luminescence of fungi involves oxidase-type enzymes, and reactive oxygen species are important for fungal light production. Generation of reactive oxygen species can be stimulated by ionizing irradiation, which has not been studied for luminous fungi. We report the effect of X-irradiation on the luminescence of fungus Neonothopanus nambi. Experiments were performed with mycelium on a home-built setup based on an X-ray tube and monochromator/photomultiplier tube. Application of X-rays does not change the emission spectrum, but after approximately 20 min of continuous irradiation, light production from unsupported mycelium starts growing and increases up to approximately five times. After peaking, its level decreases irrespective of the presence of X-irradiation. After staying at a certain level, light production collapses to zero, which is not related to the drying of the mycelium or thermal impact of radiation. The observed shape of kinetics is characteristic of a multistage and/or chain reaction. The time profile of light production must reflect the current levels of radicals present in the system and/or the activity of enzyme complexes involved in light production.

Ab initio G2 and DFT calculations on electron affinity of cyclopentadiene, silole, germole and their 2,3,4,5-tetraphenyl substituted analogs: structure, stability and EPR parameters of the radical anions

Energies, geometries and electronic structures of cyclopentadiene (1), silole (2), germole (3) and their radical anions (RAs) 1a–3a have been studied by quantum-chemical methods at the G2(MP2, SVP) and DFT B3LYP/6-31G(d,p) levels. According to G2(MP2, SVP) calculations 1–3 have negative electron affinities (EAs) equal to −1.09, −0.06 and −0.15 eV, respectively, and hence their RAs are unstable in the gas phase. 1,2,3,4-Tetraphenylcyclopenta-1,3-diene (4) and 2,3,4,5-tetraphenylsubstituted-2 (5) and 3 (6) and their radical anions (4a–6a) were studied at the B3LYP/6-31G(d)//AM1 level. Substitution of cyclodienes results in significant stabilization of RAs 4–6 because of the effective delocalization of an extra electron within phenyl groups, especially those in positions 2 and 5, so 4–6 all have a positive EA. Our best estimates give for 4, 5 and 6 EA values 0.98, 1.30 and 1.13 eV respectively. The calculated hyperfine coupling (hfc) constants a(H) for radical anions 4a–6a are in a good agreement with experimental values. Variation of group 14 element (C, Si, Ge) in the series 1–3 and 4–6 do not produce a monotonous change of EA and hfc values.