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.     

Stabilization and reactions of tetramethylurea radical cations in Freon matrices at 77 K: Intramolecular hydrogen transfer,ion-molecular reactions,and fragmentation by electron spin resonance study

ESR spectra for -irradiated at 77 K solutions /0.02–16%/ of tetramethylurea /TMU/ in CFCl₃ and Freon-113 have been studied. TMU^+. radical cations radiolytically produced in dilute solutions have been shown to undergo intramolecular hydrogen transfer upon photobleaching resulting in CH₂N= type radical. Evidence for intermolecular proton transfer in TMU^+. radical cations after annealing to phase transition temperature /110–120 K/ in Freon-113 was obtained. Primary radical cations of TMU^+. at their ground state take part in ion-molecular reaction via proton transfer. Molecular cations in their excited states may undergo fragmentation producing Me₂N radicals, which were trapped in liquid phase by t-BuNO as a spin trap.     

Photochemical Reactions of Linear Alkane Radical Cations in Low-Temperature Matrices

The radical cations of linear alkanes (n-pentane, n-heptane) trapped in various matrices (Freon-11, Freon-113, Freon-113a, mixture of Freon-11 and Freon-114B2, and sulfur hexafluoride) were found to undergo the following types of photochemical reactions: (1) charge transfer to the matrix followed by neutralization, (2) isomerization and unimolecular decomposition, and (3) deprotonation. The absorption spectra of the radical cations were characterized, and the quantum yields of reactions occurring in different matrices at 77 K were determined. It was shown that the reaction pathway and efficiency of the photochemical processes observed for a given radical cation in different matrices with similar physical and chemical characteristics could considerably differ.     

Ion-molecule reactions and thermal isomerization of tricyclo[4.3.0.0]nona-4,8-diene radical cations to tricyclo[4.2.1.0]nona-2,7-diene radical cations in a γ-irradiated frozen Freon matrix

A four-step mechanism of isomerization of tricyclo[4.3.0.0^3,7]nona-4,8-diene radical cations to tricyclo[4.2.1.0^4,9]nona-2,7-diene radical cations in γ-irradiated frozen Freon-113 (CFCl₂CF₂Cl) matrix was suggested on the basis of ESR data. The rearrangement was found to occur via distonic form of the radical cations with spin and charge separation. Furthermore, it was shown that the primary radical cations abstracts hydrogen atom from methylene group of the parent molecule, whereas distonic radical cations reacts via attachment to the C=C bond at 110–119 K.     

Photochemistry of trimethylene oxide and trimethylene sulfide radical cations in freonic matrices at 77 K

It was shown that trimethylene oxide (oxetane) radical cations were converted at 77 K into either distonic radical cations ·CH₂CH₂CH=OH⁺ or 2-oxetanyl radicals, depending on the freonic matrix used, by the action of light at λ = 546 nm and trimethylene sulfide radical cations transformed into distonic radical cations CH₂CHSH⁺CH ₂ ^· under 436-nm irradiation. The quantum yields of the photochemical reactions were determined. Quantum-chemical calculations on the structure and HFC constants of the radical cations and possible paramagnetic products of their transformation were performed. The reasons behind the observed difference in reactivity between the radical cations under the action of light are discussed.     

Experimental and theoretical study on the structure and reactions of 1-methoxypropane radical cations

Structure and mechanism of thermal and photochemical reactions of radical cations of methyl n-propyl ether (MPE) were studied in irradiated freonic matrices CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃ at 77 K. The quantum chemical calculations of the structure of radical cations and products of their transformations were carried out with methods based on the density functional theory (DFT). Experimental and calculation results show that the MPE radical cations are characterized by substantial delocalization of spin density to the propyl group. The action of light on the MPE radical cations in a CF₃CCl₃ matrix at 77 K results in intramolecular rearrangement yielding the distonic radical cation ^.CH₂CH₂CH₂(OH⁺)CH₃. It was found that the primary MPE radical cations underwent irreversible transformation to CH₃CH₂CH₂OCH ₂ ^. radical as a result of an ion-molecule reaction that occurred in a CF₂ClCFCl₂ matrix upon heating the sample to 110–120 K or in a CFCl₃ matrix upon increasing the solute concentration.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 105–113.Original Russian Text Copyright © 2005 by Belevskii, Feldman, Tyurin.This revised version was published online in April 2005 with a corrected cover date.     

Phototransformations of methylsubstituted oxiranes’ radical cations

It has been established that reversible photoinduced transformations of 2,3-dimethyloxirane and methyloxirane radical cations (RCs), observed in freonic matrices at 77 K, are related to the conversion between the open and cyclic forms of the RCs. For the trimethyloxirane RC the action of light on the trans-isomer of the open form results in its photoinduced transformation into a C-centered radical with low quantum efficiency (≈4 × 10⁻³). Upon the X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77 K, a cyclic form of the RC is stabilized (presumably, as part of a complex with matrix molecules) which transforms into a distonic C-centered RC under the action of light with the quantum yield of ≈10⁻³. Tetramethyloxirane RC, stabilized in its open form, is resistant to the action of light. Probable causes of the observed effects are discussed.     

Phototransformations of azetidine radical cations stabilized in freonic matrices

It has been established that transformations of azetidine radical cations observed in freonic matrices under the action of light with λ = 436 nm (T = 77 K) are associated with C-N bond cleavage which corresponds to the cyclic form yielding a mixture of open distonic C-centered radical cations of the following structure: ·CH₂CH₂CH=NH ₂ ⁺      

Phototransformations of methylsubstituted oxiranes’ radical cations in freonic matrices at 77 K

It has been established that, upon X-ray irradiation of various methyloxiranes in freonic matrices at 77 K, both open and cyclic (with the elongated C-C bond) forms of radical cations are stabilized. It has been shown that observed reversible photoinduced transformations of 2,3-dimethyloxirane and methyloxirane radical cations are related to the conversion between the open and cyclic forms of the radical cations with high quantum yields (0.02?C0.39, depending on the oxirane and the matrix). For the trimethyloxirane radical cation the action of light on the trans-isomer of the open form results in its photoinduced transformation into a C-centered radical with low quantum efficiency (??4 × 10^?3). Tetramethyloxirane radical cations, stabilized in their open form, are resistant to the action of light. Probable causes of the observed effects are discussed. Upon the X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77 K, a cyclic form of the radical cation is stabilized (presumably, as part of a complex with matrix molecules) which transforms into a distonic C-centered radical cation under the action of light with the quantum yield of ??10^?3.     

Photochemistry of Ethylbenzene Radical Cations in Low-Temperature Freonic Matrices

Two different conformers of ethylbenzene radical cations (or a mixture of both conformers) can be stabilized in various freonic matrices. The first conformer retains the geometry of the parent molecule, whereas the second one corresponds to minimum energy. It was shown that the photochemical reactions of the radical cations in various freons at 77 K were not accompanied by a change in their conformational state. The spectral characteristics of ethylbenzene radical cations and the quantum yields of photoinduced charge transfer reactions were determined. The reasons for stabilization of different conformers of the radical cations in different freonic matrices are discussed.     

Photochemistry of 1,3-Dioxolane Radical Cations in Sulfur Hexafluoride and Freonic Matrices at 77 K

The elementary stages and efficiency of the photochemical reactions of 1,3-dioxolane radical cations in various low-temperature matrices (sulfur hexafluoride, Freon-11, Freon-113) were determined. The matrix effects in the photochemical processes were observed experimentally. Possible reasons for these effects are discussed.     

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.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

Radical products of γ-radiolysis of 12-crown-4 at 77 K

Paramagnetic products of γ-radiolysis of 12-crown-4 and its solutions in CFCl₃ and CFCl₂CF₂Cl at 77 K were studied by ESR spectroscopy. It was found that the ESR spectra of 12-crown-4 irradiated with γ-rays at 77 K contained superimposing signals of at least two species, the radicals resulting from macrocycle opening -?H-C(H) = O and macrocyclic radicals -O-?H-CH₂-, which are formed with nearly equal yields. It was shown that -O-?H-CH₂-radicals rapidly decayed at temperatures above 140 K. However, the -?H-C(H)=O radicals are stable almost up to the matrix softening temperature. The radical cations of 12-crown-4 are not stabilized in the matrices of Freon 11 and Freon 113, since they undergo transformation to macrocyclic radicals -O-?H-CH₂-via the deprotonation reaction.     

Distinguishing conventional and distonic radical cations by using dimethyl diselenide

Dimethyl diselenide is demonstrated to be among the most powerful reagents used to identify distonic radical cations. Most such ions readily abstract CH₃Se^. from dimethyl diselenide. The reaction is faster and more exclusive than CH₃S^. abstraction from dimethyl disulfide, a reaction used successfully in the past to identify numerous distonic ions. Very acidic distonic ions, such as HC⁺(OH)OCH^.₂, do not undergo CH₃Se^. abstraction, but instead protonate dimethyl diselenide. In sharp contrast to the reactivity of distonic ions, most conventional radical cations were found either to react by exclusive electron transfer or to be unreactive toward dimethyl diselenide. Hence, this reagent allows distinction of distonic and conventional isomers, which was demonstrated directly by examining two such isomer pairs. To be able to predict whether electron transfer is exothermic (and hence likely to occur), the ionization energy of dimethyl diselenide was determined by bracketing experiments. The low value obtained (7.9 ± 0.1 eV) indicates that dimethyl diselenide will react with many conventional carbon-, sulfur-, and oxygen-containing radical cations by electron transfer. Nitrogen-containing conventional radical cations were found either to react with dimethyl diselenide by electron transfer or to be unreactive.     

Mechanism of radical reactions in cyclic and linear methylsiloxanes

The radical reactions of linear and cyclic dimethylsiloxanes were studied. It was found that methyl radicals decayed in linear and branched polymethylsiloxanes and dimethylsilanediol at 77 K as a result of radical center transfer. It was established that distonic radical cations were formed and the siloxane ring was opened in cyclic siloxanes D₃ and D₄ upon irradiation in the bulk at 77 K.     

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of <Emphasis Type="Italic">n</Emphasis>-alkanes: An EPR study

Radiation-chemical yields the liquid-phase radiolysis of C₅–C₁₂ n-alkanes were measured using the spin trap technique. The yields of n-alkyl radicals depended only slightly on the chain length in C₅–C₉ alkanes and amounted up to 30% of the total yield of trapped radicals; they were inhibited by the addition of charge scavengers. An analysis of the experimental results together with data on radicals in irradiated crystalline alkanes and radical cations in freon matrices showed that n-alkyl radicals results from the ion-molecule reactions of primary radical cations, whereas the protonated ions RH₂⁺ as products of these reactions are a source of sec-alkyl radicals. At least 60% of primary radical cations are consumed via these reaction pathways. A part of sec-alkyl radicals is due to gauche-conformers. The relative amount of primary alkyl radicals formed in the degradation of excited states and the subsequent charge neutralization processes should be insignificant.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 5–14.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.     

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.     

Variable-temperature ion mobility time-of-flight mass spectrometry studies of electronic isomers of Kr2+ and CH3OH*+ radical cations

Preliminary results from a liquid nitrogen-cooled ion mobility (IM) orthogonal-time-of-flight (o-ToF) mass spectrometer applied to the separation of electronic isomers of Kr2+ and methanol radical cations (conventional and distonic) are presented. Ab initio calculations were used to estimate the energies and energy barriers to interconversion between conventional (CH3OH*+) and distonic (CH2*OH2+) radical cations. In addition, computations and experiments are used to compare ion-neutral collision cross-sections for CH3OH*+ and CH2*OH2+ radical cations and suggest that the mobility separation is achieved by ion-neutral interactions between ions and neutral buffer gas.     

Spectral Characteristics and Transformations of Intermediates in Irradiated Freon 11, Freon 113, and Freon 113a

Optical absorption bands observable in Freon 11, Freon 113, and Freon 113a irradiated at 77 K were assigned to various intermediates (radical cations, radical ion pairs, and complexes of radicals with ions). The transformations of these species in thermal and photochemical reactions occurring at 77 K were studied. On the basis of experimental results, it was suggested that the radical anions of Freon 11 and Freon 113 are unstable at 77 K and the spatial distribution of the intermediates produced is inhomogeneous.