V. N. Belevskii

Reactions of radical cations of acetals: Evidence for unimolecular decomposition

The thermal and photochemical reactions of the methylal radical cation /I/ in freon matrices were studied using selective deuteration for elucidating the structure of the resulting species. /I/ has been shown to decay by unimolecular reaction upon heating to 140 K as well as upon photolysis in CFCl3 matrix and the product of decay has been assumed to be the complex of formaldehyde radical cation with CFCl3. Such decay reaction has been demonstrated for 1,3-dioxolan radical cation as well.

ESR AND QUANTUM CHEMICAL STUDIES OF THE STRUCTURES AND THERMAL TRANSFORMATIONS OF THE RADICAL CATIONS OF VINYLCYCLOPROPANE IN IRRADIATED FROZEN FREON MATRICES. SIMULATION OF RADICAL PROCESSES IN SOLIDS

Thermal transformations of vinylcyclopropane radical cations (VCP.+) in X-ray-irradiated frozen Freon matrices (CFCl2CF2Cl and CFCl3) were studied by ESR; radical processes involving VCP.+ in solid VCP were simulated.Gauche- andanti-VCP.+ were found to be the primary radical cations, however, the former, unlike the latter, is stable only under “gas-phase” conditions. The thermodynamic equilibrium betweenanti-VCP.+ and its less stable distonic form,dist(90,0)-C5H8.+, is established in frozen Freon matrices and the VCP host matrix; the structure of dist(90,0)****-C5H8.+ is stabilized by a molecule ofanti-VCP. In CFC3, along with dist(90,0)-C5H8.+,π-dimeric resonance [anti-VCP]2.+ complex was detected. A general scheme of the transformations of VCP.+ in the solid phase has been proposed.

Photochemical reactions of radical cations of dimethylformamide in freon matrices at 77 K

Spectral characteristics of the radical cations (RC) of DMF (λmax = 415 nm, εmax = (2.6±0.8)·103 L mol−2 cm−2) stabilized in an irradiated glassy freon mixture (CFCl3 and CF2BrCF2Br) at 77 K were determined. Amide type radicals and RC of the matrix were shown to be formed by irradiation (λ=365–436 nm) of the radical cations of DMF in freon matrices using ESR and UV spectroscopy. The quantum yields of photoconversion of the DMF radical cations are independent of the wavelength of exciting light. It was found that the matrix structure affects the processes stabilizing the products of photoconversion of the DMF radical cations.

Reactions of the radical cations of aliphatic aldehydes in freon matrices

ESR spectra of γ-irradiated, at −196 °C, solutions of acetic and propionic aldehydes in freon-11 and freon-113 affected by aldehyde concentration, temperature, and the action of light have been studied. It has been shown that the radical cations CH3CHO+ are converted into neutral radicals CH3ĊO and CH3ĊHOH and the cations CH3CH2CHO+. are converted to RĊO and CH3ĊHCHO due to ion-molecular reactions of proton transfer /CH3ĊO and CH3ĊHCHO/ of hydrogen atom transfer /CH3ĊHOH/.

ESR study of the mechanism of radical formation during liquid-and solid-phase radiolyses of ethylamines

The formation of radicals during the liquid-phase radiolysis of ethylamine, diethylamine, and triethylamine was studied by means of the spin trapping technique. The radicals produced in ion-molecule reactions and in the rearrangement and fragmentation reactions of the primary radical cations of the amines were identified. The structure and reactions of the primary radical cations were studied in a low-temperature CFCl3 freonic matrix in which amine radical cations were generated via charge transfer from matrix radical cations to amines during freon irradiation. The results of experiments in the liquid and solid phases are consistent with one another. The structure of neutral radicals and radical cations of the ethylamines was corroborated by quantum-chemical calculations.

ESR and quantum-chemical studies of the structure and thermal transformations of vinylcyclopropane radical cations in irradiated frozen Freon matrices. Simulation of radical processes in the gas phase

Thermal trasfomations of vinylcyclopropane (VCP) radical cations (RC) in X-ray irradiated frozen Freon matrices, CFCl2CF2Cl and CFCl3, were studed by ESR. Radical processes involving VCP.+ in very rarefied and moderately thickened gaseous VCP were simulated. Monomolecular cleavage of the cyclopropane ring ofgauche-VCP.+ (1) occurs to give the more thermally stable distonic radical cationdist(0.90)-C5H8.+ (3). As the density of VCP increases RC3 adds at the double bond ofanti-VCP to give the distonic RC,.CH2CH2CHCH(CH2)3CHCHCH2+ (5). Under the same conditions, the less thermally stableanti-VCP.+(2) undergoes monomolecular isomerization into RC1 or reacts withanti-VCP with the rearrangement (as in the condensed phase) to give its distonic form,dist(90.0)-C5H8.+ (4). The MNDO-UHF method was adapted for quantum-chemical analysis of the constants of isotropic hyperfine coupling with1H and13C nuclei in neutral and charged hydrocabon radicals, since the standard version of this method inadequately reproduces the structural parameters of low-symmetry (C1,Cs) paramagnetic species. A quantum-chemical analysis of the radiospectroscopic information and of the stereoelectronic control of thermal transformations of conformers of RC1 and2 into their structurally nonequivalent distonic forms3 and4, respectively, was carried out.

Reactions of the radial cattions of acetic acid and acetic anhydride in CFCl3

The ESR spectra of γ-irradiated, at −196 °C, solutions of acetic acid and acetic anhydride were studied depending on their concentrations in CFCl3. The structure of thus produced radical cations is confirmed with the deuterium substituted analogues. It has been shown that the ion-molecular reaction of the radical cation CH2COOH+ in the isolated dimer takes place for the dilute solutions of acetic acid in CFCl3 resulting in the formation of CH3COO follwed by its decomposition to CH3+CO2 while the radicals CH2COOH are formed via secondary processes. The reactions of radical cations of acetic oxide have been also studied.

Photochemical transformations of 1,3,5-trioxane radical cations in freonic matrices at 77 K

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.

Photochemical transformations of cyclic acetal radical cations in Freon matrices at 77 K

The efficiency of photochemical reactions of radical cations of cyclic acetals (1,3-dioxolane, 1,3-dioxane) is measured in different Freon matrices at 77 K and the influence of the latter on the reaction path is discovered. The possible nature of the paramagnetic complexes that form in photochemical reactions of cyclic acetal radical cations in Freon-11 is suggested.