Victor A. Bagryansky

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

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

Interaction of 1,3,2,4-Benzodithiadiazines and Their 1-Se Congeners with Ph3P and Some Properties of the Iminophosphorane Products

Interaction between Ph(3)P and 1,3,2,4-benzodithiadiazine (1); its 6,7-difluoro (2), 5,6,8-trifluoro (3) and 5,6,7,8-tetrafluoro (4) derivatives; and 5,6,8-trifluoro-3,1,2,4-benzothiaselenadiazine (5) proceeded via a 1:1 condensation to give Ph(3)P═N-R iminophosphoranes (1a-5a, R = corresponding 1,2,3-benzodichalcogenazol-2-yls), which are inaccessible by general approaches based on the Staudinger and Kirsanov reactions. In contrast, neither Ph(3)As nor Ph(3)Sb reacted with 1 and 4. Molecular structures of 1a-5a and 5 were confirmed by X-ray diffraction (XRD). The crystals formed by chiral molecules of 2a-5a were racemic, whereas the crystal of 1a was formed by a single enantiomer. In all of the Ph(3)P═N-R derivatives, one of the Ph rings is oriented face-to-face to the hetero ring, R. Upon heating to ∼120 °C in squalane (1a, 3a, 4a) or dissolving in chloroform at ambient temperatures (1a, 2a, 4a), the Ph(3)P═N-R derivatives generated the 1,2,3-benzodithiazolyls (1b-4b, respectively) whose identity was confirmed by electron paramagnetic resonance (EPR). 2,1,3-Benzothiaselenazolyls 5b and 6b were detected by EPR as the main paramagnetic products of solution thermolysis of 5 and its 5,6,7,8-tetrafluoro congener (6), respectively. Passing a chloroform solution of 4a through silica column unexpectedly gave 5-6-6-6 tetracyclic (9) and 6-10-6 tricyclic (10) sulfur-nitrogen compounds, which were characterized by XRD.

Radical cations of branched alkanes in solutions: time-resolved magnetic field effect and quantum chemical studies.

Radical cations of heptane and octane isomers, as well as several longer branched alkanes, were detected in irradiated n-hexane solutions at room temperature by the method of time-resolved magnetic field effect (TR MFE). To identify radical cations, the hyperfine coupling constants as determined by simulation of the TR MFE curves were compared to the constants calculated using the density functional theory (DFT) approach. The g-values of the observed radical cations were close to that of the 2,2,3,3-tetramethylbutane radical cation studied earlier by optically detected electron spin resonance (ESR) and TR MFE techniques. No evidence of the decay of the radical cations of branched alkanes to produce olefin radical cations was found, which was further supported by the observation of positive charge transfer from the observed radical cations to cycloalkane molecules. The lifetimes of the radical cations of the branched alkanes were found to be longer than tens of nanoseconds.

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.

Solvent Radical Anions in Irradiated Aliphatic Ketones and Esters as Observed Using Time-Resolved Magnetic Field Effects in the Recombination Fluorescence

Abstract It has been found that addition of alcohols (~0.1 M) to some liquid ketones and esters results in well-pronounced oscillations in the decay of the delayed fluorescence intensity from irradiated solutions. The analysis of the time-resolved magnetic field effects (TR MFEs) in the recombination fluorescence has shown that these oscillations are a manifestation of singlet-triplet transitions in spin-correlated radical ion pairs (RIPs) created by irradiation. Comparison with literature data indicates that the transitions are due to hyperfine couplings (HFCs) in the solvent radical anion (RA), stabilized due to the presence of alcohol molecules. In acetone, this stabilization effect has been observed for methanol, ethanol, 2- propanol, and, to a smaller extent, for tert-butanol. Similar effects have also been observed in diethyl ketone, ethyl acetate, and methyl propionate but not in methyl tert-butyl ketone and ethyl trimethylacetate. The results obtained indicate that the interaction between the radical anions (RAs) of carbonyl compounds and alcohol molecules is of importance in pulse radiolysis studies of organic liquids and their mixtures.