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.     

Photoinduced electron transfer and geminate recombination for photoexcited acceptors in a pure donor solvent

Photoinduced electron transfer and geminate recombination are studied for the systems rhodamine 3B (R3B(+)) and rhodamine 6G (R6G(+)), which are cations, in neat neutral N,N-dimethylaniline (DMA). Following photoexcitation of R3B(+) or R6G(+) (abbreviated as R(+)), an electron is transferred from DMA to give the neutral radical R and the cation DMA(+). Because the DMA hole acceptor is the neat solvent, the forward transfer rate is very large, approximately 5x10(12) s(-1). The forward transfer is followed by geminate recombination, which displays a long-lived component suggesting several percent of the radicals escape geminate recombination. Spectrally resolved pump-probe experiments are used in which the probe is a "white" light continuum, and the full time-dependent spectrum is recorded with a spectrometer/charge-coupled device. Observations of stimulated emission (excited state decay-forward electron transfer), the R neutral radical spectrum, and the DMA(+) radical cation spectrum as well as the ground-state bleach recovery (geminate recombination) make it possible to unambiguously follow the electron transfer kinetics. Theoretical modeling shows that the long-lived component can be explained without invoking hole hopping or spin-forbidden transitions.     

Ion—molecular charge transfer as studied by the method of optically detected ESR of radical pairs

Effects of ion—molecular charge transfer upon optically detected (OD) ESR spectra of radical pairs have been studied experimentally and theoretically. Theoretical analysis has shown that under certain conditions (often realized in experiments) OD ESR spectra are analogous to standard ESR ones. Broadening and collapse of the hyperfine structure (exchange broadening) have been observed experimentally depending on the concentration of neutral acceptors. Ion—molecular charge transfer rate constants have been calculated for some radical-anions from the experimental data.     

ESR Spectra and structure of radical-anions of fluorinated benzonitrile derivatives

Isotropic ESR spectra have been taken from radical-anions of fluorinated benzonitrile derivatives. Quantum-chemical analysis of the hfi constants has demonstrated that 3-fluoro- and 3,5-difluoro-benzonitrile radical-anions are planar π radical,s while the radical-anions with fluorine atoms in the positions 2 and/or 4 relative to the nitrile group are characterized by non-planar deformation resulting from the interaction between the ground π and the lower excited σ states within the pseudo-Jahn-Teller effect.     

Thermally induced opening of the diaziridine ring in 6-aryl-2-methyl-1,5-diazabicyclo[3.1.0]hexanes

Thermally induced opening of the diaziridine ring in 6-aryl-2-methyl-1,5-diazabicyclo[3.1.0]-hexanes at the carbon-nitrogen bond is characterized by low regioselectivity; isomerization of unstable intermediate azomethine imines leads to mixtures of the corresponding 1-arylmethyl-5-methyl-4,5-dihydro-1H-pyrazoles and 1-arylmethyl-3-methyl-4,5-dihydro-1H-pyrazoles at a ratio of ～6:5. Analogous regioselectivity in opening of the three-membered ring is observed in the presence of phenyl isocyanate. In this case, adducts with cis arrangement of the aryl and methyl groups are formed as the major products (cis/trans ratio ～3:1).     

Stable azomethine imines having a 3,4-dihydroisoquinoline fragment and their cycloaddition to <Emphasis Type="Italic">N</Emphasis>-arylmaleimides

Aroylation of 5,6,8,8a,13,14,16,16a-octahydro[1,2,4,5]tetrazino[6,1-a:3,4-a′]diisoquinoline or 1,3,4,8b-tetrahydro[1,2]diazireno[3,1-a]isoquinoline, as well as reactions of 2-(2-bromoethyl)benzaldehyde with aroylhydrazines followed by treatment with triethylamine, led to the formation of stable azomethine imines, aroyl(3,4-dihydroisoquinolinium-2-yl)azanides. 1,3-Dipolar cycloaddition of the latter to N-mesitylmaleimide was stereoselective: the ratio of the trans- and cis-adducts was ∼(3–7): 1, the former prevailing. The reactions with N-arylmaleimides having no ortho-substituents in the aryl group gave the corresponding cis-adducts as the major products [trans/cis ratio ∼1: (2.5–10)].     

Generation of azomethine imines via opening of the diaziridine ring in unsymmetrically substituted 6-aryl-1,5-diazabicyclo-[3.1.0]hexanes and their transformations

Thermally induced opening of the diaziridine ring at the carbon-nitrogen bond in unsymmetrically substituted 6-aryl-1,5-diazabicyclo[3.1.0]hexanes is characterized by low regioselectivity which is likely to be determined by the inductive effect of substituents in the trimethylene bridge. 1,3-Dipolar cycloaddition of the resulting azomethine imines to phenyl isocyanate is regioselective: it occurs at the double carbon-nitrogen bond with predominant formation of the corresponding cis adducts due to dipolarophile approach to Z-azomethine imine at the sterically less hindered side. Analogous approach of dipolarophile is also observed in the reaction with N-arylmaleimides.     

Thermal opening of the diaziridine fragment in 1-methyl-and 1,3,3-trimethyl-1,3,4,8b-tetrahydro-[1,2]diazirino[3,1-<Emphasis Type="Italic">a</Emphasis>]isoquinolines in the presence of n-arylmaleimides

The thermal opening of the diaziridine ring in 1-methyl-and 1,3,3-trimethyl-1,3,4,8b-tetra-hydro[1,2]diazirino[3,1-a]isoquinolines in the presence of N-arylmaleimides leads to the predominant or exclusive formation of the trans isomers of the products of 1,3-dipolar cycloaddition. In the absence of dipolarophile, the conversion of the starting diaziridines is incomplete over the same time period, while the thermolysis products are N-[3,4-dihydro-2(1H)-isoquinolyl]-and N-[3,3-dimethyl-3,4-di-hydro-2(1H)-isoquinolyl]-N-methyleneamines formed as the result of isomerization of intermediate labile azomethineimines. Dedicated to the memory of A. A. Potekhin. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1071–1079, July, 2008.