V. A. Kuz'min

Transfer of a hydrogen atom to acridine in the triplet state

ConclusionsThe quenching of triplet acridine by substituted phenols, thiophenols, and anilines is accomplished with high rate constants by hydrogen atom transfer with the quantitative formation of the corresponding neutral radicals.The solvation of the reagents by ethanol molecules or a decrease in the electron donor capacity of the quencher is accompanied by a decrease in the reaction rate constants.

Effect of the external magnetic field strength on geminal recombination of triplet radical pairs in glycerin

Conclusions1.The dependences of the value of the magnetic effect on the field strength for geminal recombination of triplet radical pairs including the ketyl radical and phenoxyl, aminyl, or radical cation of aniline in glycerin are described within the framework of the HFI and relaxation mechanism of triplet-singlet transitions in radical pairs due to the anisotropy of the HFI and the dipole-dipole interaction between unpaired electrons.2.In recombination of triplet radical pairs in glycerin, the effect of an internal heavy atom is observed: on introduction of a Br atom in one of the radicals, recombination is accelerated, and the magnetic effect decreases. The intercombination transitions in contact states of radical pairs due to a spin—orbital interaction are important. The contribution of these transitions increases when heavy atoms are present in the system.

Cation-radical of pyridine N-oxide and its reactions with C−H bonds

Visible absorption spectrum for the cation-radical of pyridine N-oxide pyO+ obtained in the oxidation of the appropriate N-oxide by the photo-excited triplet state of chloranil, has a wide maximum at λ=430–460 nm. pyO+ reacts with cyclohexane, methanol or toluene with the formation of intermediate complexes. Equilibrium constants of their formation K×10−2 dm3/mol amount to 0.1, 1.0 and 2.0, respectively. The (pyO+...RH) complex readily reacts with O2, and in the absence of O2 its decomposition is independent of the hydrocarbon nature.AbstractКатион-радикал N-окиси пиридина (pyO+) получен окислением соответствующей N-окиси фотохимически генерируемым триплетным состоянием хлоранила. Спектр поглощения pyO+ в видимой области имеет широкий максимум, λ=430–460 нм. С циклогексаном, метанолом и толуолом pyO+ реагирует через промежуточное образование комплекса, константы равновесия которого, соответственно, равны (К×10−2 дм3/моль) 0,1, 1,0 и 2,0. Комплекс быстро реагирует с кислородом, в отсутствие которого распад комплекса нечувствителен к природе углеводорода.

Kinetics of geminal radical recombination in photoreduction of benzophenone and its derivatives in viscous solvents: Effect of a magnetic field

Conclusions1.The kinetics of geminal recombination of benzophenone ketyl radicals and donor radicals in viscous media were recorded by pulsed laser photolysis.2.On application of an external magentic field, the rate constant of geminal recombination decreases by up to two times, and the observed effects are comparable in value to the corresponding effects in microheterogeneous systems.3.The paramagnetic relaxation rate constants were determined based on a formal kinetic analysis of geminal recombination of radical pairs in a magnetic field and in the absence of a field and the corresponding experimental data.

Complexing of zinc and eosin porphyrins with viologens in aqueous solutions

Conclusions1.The complexing constants of eosin (ZD=−2) and zinc porphyrins (ZD=+4, −4) with charged viologens (ZA=0 to +4) with different ionic strengths were determined.2.It was shown that ΔG of complexing can be considered the additive sum of the ΔG of a donor-acceptor π interaction which is constant in the dye-viologens series and the ΔG of the electrostatic interaction which changes as a function of the charges of the reactants.

Kinetics of the geminal recombination of radical pairs in the photoreduction of uranyl in micelles

Conclusions1.The excited state of uranyl rapidly extracts an electron or hydrogen atom from amines and phenols in direct and inverted micelles.2.The kinetics of the geminal recombination of the radical-ion pair containing the UO22+ ion in inverted micelles was measured.

Effect of an external magnetic field on the yield of radicals during the photoreduction of xanthene dyes in viscous media

Conclusions1.The application of an external magnetic field (H=0.18 T) leads to an increase of 10–150% in the escape of the radicals of xanthene dyes from the cage in a viscous solvent.2.In erythrosine and ethyleosin, which contain heavy atoms, the magnetic effects (with η/T=3.3 cP/K) are smaller than in the case of fluorescein. This is due to the appearance of spin-orbital interaction in the radicals of the pair; the internal heavy atom effect appears in viscous media (η/T ≃ cP/K).3.The presence of an external heavy atom leads to a decrease of the magnetic effect during the photoreduction of fluorescein and erythrosine. The external heavy atom effect appears in the region of “moderate” fluidities (1 ≳η/T ≳ 7 cP/K, [KI]=0.1 M). This effect does not appear with large η/T values on account of the transition to the diffusion-controlled region.

Flash photolysis study of the kinetics of the destruction of mono- and disubstituted phenoxyl radicals

Conclusions1.The rate constants for phenoxyl radical recombination fall off with an increase in the size of the substituent and with the successive introduction of substituents into the 2 and 6 positions in the phenoxyl ring.2.The limiting step in recombination is translational diffusion in the case of the 4-phenoxyl radical, and translational and rotation diffusion in the case of the 2,6-di-tert-butylphenoxyl radical.3.The effect of the solvent on the rate constant for recombination is less pronounced in the 2,6-di-tert-butylphenoxyl radical than in the phenyl-substituted phenoxyl radicals.4.Enolization of the phenoxyl radical dimers is catalyzed by acids, the rate constant being weakly dependent on the viscosity and polarity of the solvent.

MAGNETIC FIELD AND MAGNETIC ISOTOPE EFFECTS ON THE RECOMBINATION KINETICS OF COVALENTLY-LINKED KETYL-PHENOXYL TRIPLET RADICAL PAIRS

The recombination kinetics of three photogenerated covalently-linked ketyl-phenoxyl triplet radical pairs,3[PhC.(OD)C6H4O(CH2)nOC6H4C6H4O.] (n=3, 6, and 10), and of the corresponding deuterated derivatives were examined by the laser flash technique under an external magnetic field (up to 0.2 T) in a CDCl3/CD3OD (2∶1) mixture. In zero magnetic field, radical pairs (RPs) with small exchange interactions (n=6 and 10) are characterized by high values of the magnetic isotope effect (MIE), which reach 3 for pairs withn=10. Under strong magnetic fields (up to 0.2 T), the values of MIE decrease to 1.2 to 1.1. The photochemical behavior of covalently-linked RPs is compared with that of similar unlinked RPs in micelles.

Role of steric and thermodynamic factors in the quenching of the luminescence of ruthenium trisdipyridyl complexes by aromatic electron donors and acceptors

ConclusionsAn increase in the length of the chain of aliphatic substituents in the ligands of ruthenium trisdipyridyl complexes is accompanied by a slight decrease in the rate quenching for the quenching of the luminescence of the complex by aromatic electron donors and acceptors due to steric effects.