Reaction of Aromatic Peroxyl Radicals with Alkynes: A Mass Spectrometric and Computational Study Using the Distonic Radical Ion Approach

The reaction of the aromatic distonic peroxyl radical cations N‐methyl pyridinium‐4‐peroxyl (PyrOO^.+) and 4‐(N,N,N‐trimethyl ammonium)‐phenyl peroxyl (AnOO^.+), with symmetrical dialkyl alkynes 10a – c was studied in the gas phase by mass spectrometry. PyrOO^.+ and AnOO^.+ were produced through reaction of the respective distonic aryl radical cations Pyr^.+ and An^.+ with oxygen, O₂. For the reaction of Pyr^.+ with O₂ an absolute rate coefficient of k₁=7.1×10^?12 cm³ molecule^?1 s^?1 and a collision efficiency of 1.2 % was determined at 298 K. The strongly electrophilic PyrOO^.+ reacts with 3‐hexyne and 4‐octyne with absolute rate coefficients of k_hexyne=1.5×10^?10 cm³ molecule^?1 s^?1 and k_octyne=2.8×10^?10 cm³ molecule^?1 s^?1, respectively, at 298 K. The reaction of both PyrOO^.+ and AnOO^.+ proceeds by radical addition to the alkyne, whereas propargylic hydrogen abstraction was observed as a very minor pathway only in the reactions involving PyrOO^.+. A major reaction pathway of the vinyl radicals 11 formed upon PyrOO^.+ addition to the alkynes involves γ‐fragmentation of the peroxy O? O bond and formation of PyrO^.+. The PyrO^.+ is rapidly trapped by intermolecular hydrogen abstraction, presumably from a propargylic methylene group in the alkyne. The reaction of the less electrophilic AnOO^.+ with alkynes is considerably slower and resulted in formation of AnO^.+ as the only charged product. These findings suggest that electrophilic aromatic peroxyl radicals act as oxygen atom donors, which can be used to generate α‐oxo carbenes 13 (or isomeric species) from alkynes in a single step. Besides γ‐fragmentation, a number of competing unimolecular dissociative reactions also occur in vinyl radicals 11 . The potential energy diagrams of these reactions were explored with density functional theory and ab initio methods, which enabled identification of the chemical structures of the most important products.     

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

Metalloradical species [Co₂Fv(CO)₄]^.+ ( 1 ^.+, Fv=fulvalenediyl) and [Co₂Cp₂(CO)₄]^.+ ( 2 ^.+, Cp=η⁵‐C₅H₅), formed by one‐electron oxidations of piano‐stool cobalt carbonyl complexes, can be stabilized with weakly coordinating polyfluoroaluminate anions in the solid state. They feature a supported and an unsupported (i.e. unbridged) cobalt–cobalt three‐electron σ bond, respectively, each with a formal bond order of 0.5 (hemi‐bond). When Cp is replaced by bulkier Cp* (Cp*=η⁵‐C₅Me₅), an interchange between an unsupported radical [Co₂Cp*₂(CO)₄]^.+ (anti‐ 3 ^.+) and a supported radical [Co₂Cp*₂(μ‐CO)₂(CO)₂]^.+ (trans‐ 3 ^.+) is observed in solution, which cocrystallize and exist in the crystal phase. 2 ^.+ and anti‐ 3 ^.+ are the first stable thus isolable examples that feature an unsupported metal–metal hemi‐bond, and the coexistence of anti‐ 3 ^.+ and trans‐ 3 ^.+ in one crystal is unprecedented in the field of dinuclear metalloradical chemistry. The work suggests that more stable metalloradicals of metal–metal hemi‐bonds may be accessible by using metal carbonyls together with large and weakly coordinating polyfluoroaluminate anions.     

Charge transfer complexes of N-ethylcarbazole and poly-N-vinylcarbazole

Poly-N-vinylcarbazole and its monomeric analog, i.e., N-ethylcarbazole, act as donors in charge transfer complexes with such electron acceptors as iodine, tetracyanoethylene, and tetracyanoquinodimethane. Poly-N-vinylcarbazole and N-ethylcarbazole are also capable of accepting electrons from an alkali metal such as sodium. A study of the spectral properties of both types of complexes showed that the monomer can both accept and donate electrons to a greater extent than the corresponding unit segment of the polymer chain. Equilibrium constants are presented for both cases, and estimates of the ionization potential of N-ethylcarbazole and related compounds are made from the characteristic charge transfer frequencies. Good agreement between these values and those calculated from molecular orbital theory is outlined.     

Influence of Ring Annelation on the Mode Selectivity of the Ionized Diazabicycloheptenes and Corresponding Housanes: An ESR and ENDOR Study

The tricyclic azoalkanes, (1α,4α,4aα,7aα)‐4,4a,5,6,7,7a‐hexahydro‐1,4,8,8‐tetramethyl‐1,4‐methano‐1H‐cyclopenta[d]pyridazine ( 1c ), (1α,4α,4aα,6aα)‐4,4a,5,6,6a‐pentahydro‐1,4,7,7‐tetramethyl‐1,4‐methano‐1H‐cyclobuta[d]pyridazine ( 1d ), (1α,4α,4aα,6aα)‐4,4a,6a‐trihydro‐1,4,7,7‐tetramethyl‐1,4‐methano‐1H‐cyclobuta[d]pyridazine ( 1e ), and (1α,4α,4aα,5aα)‐4,4a,5,5a‐tetrahydro‐1,4,6,6‐tetramethyl‐1,4‐methano‐1H‐cyclopropa[d]pyridazine ( 1f ), as well as the corresponding housanes, the 2,3,3,4‐tetramethyl‐substituted tricyclo[3.3.0.0^2,4]octane ( 2c ), tricyclo[3.2.0.0^2,4]heptane ( 2d ), and tricyclo[3.2.0.0^2,4]hept‐6‐ene ( 2e ), were subjected to γ‐irradiation in Freon matrices. The reaction products were identified with the use of ESR and, in part, ENDOR spectroscopy. As expected, the strain on the C‐framework increases on going from the cyclopentane‐annelated azoalkanes and housanes ( 1c and 2c ) to those annelated by cyclobutane ( 1d and 2d ), by cyclobutene ( 1e and 2e ), and by cyclopropane ( 1f ). Accordingly, the products obtained from 1c and 2c in all three Freons used, CFCl₃, CF₃CCl₃, and CF₂ClCFCl₂, were the radical cations 3c ^.+ and 2c ^.+ of 2,3,4,4‐tetramethylbicyclo[3.3.0]oct‐2‐ene and 2,3,3,4‐tetramethylbicyclo[3.3.0]octane‐2,4‐diyl, respectively. In CFCl₃ and CF₃CCl₃ matrices, 1d and 2d yielded analogous products, namely the radical cations 3d ^.+ and 2d ^.+ of 2,3,4,4‐tetramethylbicyclo[3.2.0]hept‐2‐ene and 2,3,3,4‐tetramethylbicyclo[3.2.0]heptane‐2,4‐diyl. The radical cations 3c ^.+ and 3d ^.+ and 2c ^.+ and 2d ^.+ correspond to their non‐annelated counterparts 3a ^.+ and 3b ^.+, and 2a ^.+ and 2b ^.+ generated previously under the same conditions from 2,3‐diazabicyclo[2.2.1]hept‐2‐ene ( 1a ) and bicyclo[2.1.0]pentane ( 2a ), as well as from their 1,4‐dimethyl derivatives ( 1b and 2b ). However, in a CF₂ClCFCl₂ matrix, both 1d and 2d gave the radical cation 4d ^.+ of 2,3,3,4‐tetramethylcyclohepta‐1,4‐diene. Starting from 1e and 2e , the radical cations 4e ^.+ and 4e′ ^.+ of the isomeric 1,2,7,7‐ and 1,6,7,7‐tetramethylcyclohepta‐1,3,5‐trienes appeared as the corresponding products, while 1f was converted into the radical cation 4f ^.+ of 1,5,6,6‐tetramethylcyclohexa‐1,4‐diene which readily lost a proton to yield the corresponding cyclohexadienyl radical 4f ^.. Reaction mechanisms leading to the pertinent radical cations are discussed.     

Synthesis and properties of <Emphasis Type="Italic">N</Emphasis>-nitro-<Emphasis Type="Italic">O</Emphasis>-(4-nitrophenyl)hydroxylamine

Salts of N-nitro-O-(4-nitrophenyl)hydroxylamine were synthesized by a new method of oxidative nitration, involving the reaction of O-(4-nitrophenyl)hydroxylamine with KNO₂ or NaNO₂ in the presence of PhI(OAc)₂ or PhIO as oxidants. When using Na¹⁵NO₂, the samples containing the nitro group labeled with the ¹⁵N isotope were obtained. Acidification of the appropriate salt gave N-nitro-O-(4-nitrophenyl)hydroxylamine. It is the first N-nitrohydroxylamine isolated in the H-form. Its thermal stability was investigated and the probable mechanism of decomposition was suggested. From a comparison of the ¹⁴N and ¹⁵N NMR spectra of N-nitro-O-(4-nitrophenyl)hydroxylamine with those of its O- and N-methylated derivatives, its equilibrium with the aci-form (N=NOOH) was inferred.     

Cation radicals. XXVIII. Isolation and some reactions of dibenzodioxin cation radical perchlorate

Preparation and isolation of dibenzodioxin cation radical perchlorate ( 2 ) by oxidation of dibenzodioxin in ethyl acetate-lithium perchlorate at a platinum anode has been achieved. Reasonably pure 2 in amounts of 150–200 mg. were obtained reliably and reproducibly. Reaction of 2 with both nitrite and nitrate ions gave 2-nitrodibenzodioxin ( 3 ). Reaction of 2 with pyridine gave N-(2-dibenzodioxinyl)pyridinium perchlorate ( 4 ). Reaction with water gave, as anticipated, the stoichiometric amount of dibenzodioxin. Reaction with ammonia, propylamine, t-butylamine, and cyanide ion also gave dibenzodioxin with no evidence that nucleophilic substitution had occurred. It is believed that the formation of 3 and 4 represent the first examples of nucleophilic substitution into dibenzodioxin via its cation radical.     

Rearrangement reactions of aziridinylbenzaldoximes

Reactions of 2,2-dimethylaziridine with benzohydroximoyl chlorides [ArC(Cl)?NOH] give aziridinylbenzaldoximes 1 . It has been found that the aziridine ring in these compounds undergoes ring opening in hydrogen chloride-dioxane solution to give (Z)-N-hydroxy-N′-(2-chloro-2-methylpropyl)benzenecarboximidamides [ArC(NHCH₂CR¹R²Cl)?NOH, 4 ]. Treatment of 1 with hydrochloric acid followed by neutralization with aqueous sodium hydroxide gave 6,6-dimethyl-3-aryl-1,2,4-oxadiazines 2. Reaction of 4 with sodium hydride in dioxane gave 5-isopropyl-3-aryl-4,5-dihydro-1,2,4-oxadiazoles 5. Reaction of the 4,5-dihydro-1,2,4-oxadiazoles 5 with N-chlorosuccinimide gave the heteroaromatic 1,2,4-oxadiazoles 6 . It is suggested that reactions of 4 with sodium hydride in dioxane solution involve the conjugate base of 4 which undergoes a 1,2-hydride shift that is concerted with loss of chloride ion. In aqueous sodium hydroxide solution it is suggested that the conjugate base of 4 undergoes ionization of the chlorine atom followed by nucleophilic attack by the oximate anion.     

Synthesis of 3-acetyl-6-(<Emphasis Type="Italic">o</Emphasis>-methyl benzoyl)-<Emphasis Type="Italic">N</Emphasis>-ethylcarbazole

3-Acetyl-6-(o-methyl benzoyl)-N-ethylcarbazole was prepared through successive o-methyl benzoylation and acetylation of N-ethylcarbazole in one pot. The overall yield was 85.6% and the structure was confirmed by ¹H-NMR and ¹³C-NMR. A preliminary investigation had also been carried out on the mechanism of the o-methyl benzoylation of N-ethylcarbazole.     

Structural studies of (prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine hetero‐scorpionate copper complexes

The structures of five compounds consisting of (prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine complexed with copper in both the Cu^I and Cu^II oxidation states are presented, namely chlorido{(prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine‐κ³N,N′,N′′}copper(I) 0.18‐hydrate, [CuCl(C₁₅H₁₇N₃)]·0.18H₂O, (1), catena‐poly[[copper(I)‐μ₂‐(prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine‐κ⁵N,N′,N′′:C²,C³] perchlorate acetonitrile monosolvate], {[Cu(C₁₅H₁₇N₃)]ClO₄·CH₃CN}_n, (2), dichlorido{(prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine‐κ³N,N′,N′′}copper(II) dichloromethane monosolvate, [CuCl₂(C₁₅H₁₇N₃)]·CH₂Cl₂, (3), chlorido{(prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine‐κ³N,N′,N′′}copper(II) perchlorate, [CuCl(C₁₅H₁₇N₃)]ClO₄, (4), and di‐μ‐chlorido‐bis({(prop‐2‐en‐1‐yl)bis[(pyridin‐2‐yl)methylidene]amine‐κ³N,N′,N′′}copper(II)) bis(tetraphenylborate), [Cu₂Cl₂(C₁₅H₁₇N₃)₂][(C₆H₅)₄B]₂, (5). Systematic variation of the anion from a coordinating chloride to a noncoordinating perchlorate for two Cu^I complexes results in either a discrete molecular species, as in (1), or a one‐dimensional chain structure, as in (2). In complex (1), there are two crystallographically independent molecules in the asymmetric unit. Complex (2) consists of the Cu^I atom coordinated by the amine and pyridyl N atoms of one ligand and by the vinyl moiety of another unit related by the crystallographic screw axis, yielding a one‐dimensional chain parallel to the crystallographic b axis. Three complexes with Cu^II show that varying the anion composition from two chlorides, to a chloride and a perchlorate to a chloride and a tetraphenylborate results in discrete molecular species, as in (3) and (4), or a bridged bis‐μ‐chlorido complex, as in (5). Complex (3) shows two strongly bound Cl atoms, while complex (4) has one strongly bound Cl atom and a weaker coordination by one perchlorate O atom. The large noncoordinating tetraphenylborate anion in complex (5) results in the core‐bridged Cu₂Cl₂ moiety.     

1,3-dipolar cycloadditions induced by cation radicals. Formation of 1,2,4-triazoles from oxidative addition of 1,4-diphenylazomethane and aryl aldehyde phenylhydrazones to nitriles

Reaction of thianthrene cation radical perchlorate (Th^.+ClO₄^?) with 1,4-diphenylazomethane (DPAM) in MeCN and EtCN led to the formation of 1,2,4-triazoles. Triazoles formation is attributed to oxidative cycloaddition of benzaldehyde benzylhydrazone, the tautomer of DPAM, to the solvent nitriles. In confirmation, analogous cycloadditions were achieved by reaction of Th^.+ClO₄^? with some benzaldehyde phenylhydrazones in the same solvents.     

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)-C ₅H₈ ^.+, is established in frozen Freon matrices and the VCP host matrix; the structure of dist(90,0)****-C ₅H₈ ^.+ is stabilized by a molecule ofanti-VCP. In CFC₃, along with dist(90,0)-C₅H₈ ^.+,-dimeric resonance [anti-VCP]₂ ^.+ complex was detected. A general scheme of the transformations of VCP^.+ in the solid phase has been proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 11–21, January, 1994.     

Evidence for a Proton-Coupled Electron Transfer Mechanism in a Biomimetic System for Monoamine Oxidase B Catalysis

Mechanistic studies with 5-ethyl-3-methyllumiflavinium (Fl⁺) perchlorate, a biomimetic model for flavoenzyme monoamine oxidase B (MAO-B) catalysis, and the tertiary, allyl amine 1-methyl-4-(1-methyl-1 H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP) reveal that proton-coupled electron transfer (PCET) may be an important pathway for MAO catalysis. The first step involves a single-electron transfer (SET) leading to the free radicals Fl^. and MMTP^., the latter produced by deprotonation of the initially formed and highly acidic MMTP^.+. Molecular oxygen (O₂) is found to play a hitherto unrecognized role in the early steps of the oxidation. MMTP and several structurally similar tertiary amines are the only tertiary amines oxidized by MAO, and their structural/electronic properties provide the key to understanding this behavior. A general hypothesis about the role of SET in MAO catalysis, and the recognition that PCET occurs with appropriately substituted substrates is presented.     

Extractive spectrophotometric determination of trace amounts of nitrogen dioxide,nitrite, and nitrate

A sensitive extractive spectrophotometric method for the determination of nitrogen dioxide in air and nitrite and nitrate in water, soil and blood serum is described. Nitrogen dioxide in air is fixed as nitrite in a suitable trapping solution. The method is based on the diazo coupling reaction betweenp-nitroaniline andN-(1-naphthyl)ethylenediamme dihydrochloride [NEDA]. The azo dye formed under aqueous conditions is extracted with isobutyl methyl ketone [IBMK]. The system obeys Beer's law over the range 0–3 g of nitrite at 545 nm and the colour is stable for 3h. The molar absorptivity of the colour system is 5.7 × 10⁴ L mol^–1 cm^–1. The relative standard deviation is 1.3% for ten determinations at 2 ug of nitrite. Nitrate is determined as nitrite after reduction on a cadmium column. Negative interferences from SO₂, H₂S, Cu²⁺ and Cr³⁺ and positive interference from Fe²⁺ and Fe³⁺ can be simply masked.     

In situ preparation of ultrafine Ru nanocatalyst supported on nitrogen-doped layered double hydroxide by nitrogen glow discharge plasma for catalytic hydrogenation of N-ethylcarbazole

Ultrafine Ru nanoparticles (RuNPs) supported on nitrogen-doped layered double hydroxide (Ru/LDH) were in situ prepared by nitrogen glow discharge plasma (nGDP) without adding any chemical reducing agents or stabilizers. The as-synthesized Ru/LDH catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. During treatment with nGDP, the reduction of Ru³⁺ and nitrogen doping were carried out simultaneously. The resulting RuNPs has a narrow particle size distribution of 1.41–2.61 nm, an ultrafine average particle size of 1.86 nm, and were uniformly dispersed on nitrogen-doped LDH. The complexation of RuNPs and O/N-containing functional groups on LDH improve the catalytic activity and stability of Ru/LDH. The catalyst exhibited excellent properties for the hydrogenation reaction of N-ethylcarbazole (NEC). The conversion of NEC and the selectivity of 12H-NEC were 100% and 99.06% for 1 hr at 120°C and 6 MPa H₂, respectively. The mass hydrogen storage capacity was 5.78 wt%. The apparent activation energy was 35.78 kJ/mol.     

Synthesis,Crystal Structure,and Physical Property of Sterically Unprotected Thiophene/Phenylene Co‐Oligomer Radical Cations: A Conductive π–π Bonded Supermolecular meso‐Helix

Sterically unprotected thiophene/phenylene co‐oligomer radical cation salts BPnT^.+[Al(OR_F)₄]^? (OR_F=OC(CF₃)₃, n=1–3) have been successfully synthesized. These newly synthesized salts have been characterized by UV/Vis‐NIR absorption and EPR spectroscopy, and single‐crystal X‐ray diffraction analysis. Their conductivity increases with chain length. The formed meso‐helical stacking by cross‐overlapping radical cations of BP2T^.+ is distinct from previously reported face‐to‐face overlaps of sterically protected (co‐)oligomer radical cations.     

Spectroelectrochemical and spectrophotochemical properties of N-tetradecyl-N '-ethyl-viologen: Micellar effects

The results of Spectroelectrochemical studies in homogenous solutions have shown that below the cmc value the cation radical of N-tetradecyl-N '-ethyl viologen (TDEV) dimerizes. The TDEV and tetradecyltriethyl-ammonium bromide (TDEA) micelles were found to stabilize the cation radical TDEV^.+ and increase the rate constant for the reaction TDEV+TDEV²⁺ = TDEV^.+ as compared with the results obtained at concentrations below cmc.Based on the spectrophotochemical measurements for TDEV it was found that the quantum yield (Φ) of photoreduction in micellar evironment of TDEA was twice as large as Φ for reactions performed in homogenous solution. Moreover, in micellar solutions photoreduction of TDEV leads to a cation radical of reduced TDEV (TDEV⁺), but in homogenous solution to the dimer of TDEV [TDEV]₂. Therefore, the process of dimerization of TDEV^.+ cation radical is inhibited by micellar catalysis.     

Single‐Electron Self‐Exchange between Cage Hydrocarbons and Their Radical Cations in the Gas Phase

We show that the radical cations of adamantane (C₁₀H₁₆^.+, 1 H^.+) and perdeuteroadamantane (C₁₀D₁₆^.+, 1 D^.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C₂₀H₂₈^.+, 2 H^.+) is also stable (as in solution). By using the natural ¹³C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H^.+/ 1 H, 1 D^.+/ 1 D and 2 H^.+/ 2 H. Rate constants for the reaction 1 H^.++ 1 D? 1 H+ 1 D^.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (H_DA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.     

Tuning Electrical- and Photo-Conductivity by Cation Exchange within a Redox-Active Tetrathiafulvalene-Based Metal–Organic Framework

To activate electronic and optical functions of the redox-active metal–organic framework, (Me₂NH₂)[In^III(TTFTB)]⋅0.7 C₂H₅OH⋅DMF (Me₂NH₂@ 1 , TTFTB=tetrathiafulvalene-tetrabenzoate, DMF=N,N-dimethylformamide), has been exchanged by tetrathiafulvalenium (TTF^.+) and N,N′-dimethyl-4,4′-bipyridinium (MV²⁺). These cations provide electron carriers and photosensitivity. The exchange retains the crystallinity allowing single-crystal to single-crystal post-synthetic transformation to TTF@ 1 and MV@ 1 . Both TTF^.+ and MV²⁺ enhance the electrical conductivity by a factor of 10² and the visible light induced photocurrent by 4 and 28 times, respectively. EPR evidences synergetic effect involving charge transfer between the framework redox-active TTFTB bridges and MV²⁺. The results demonstrate that functionalization of MOF by cation exchange without perturbing the crystallinity extends possibilities to achieve switchable materials.     

Metal‐Free,Room‐Temperature Oxygen‐Atom Transfer in the N2O/CO Redox Couple as Catalyzed by [Si2Ox].+ (x=2–5)

The thermal reduction of N₂O by CO mediated by the metal‐free cluster cations [Si₂O_x]^.+ (x =2–5) has been examined in the gas phase using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry in conjunction with quantum chemical calculations. Three successive oxidation/reduction steps occur starting from [Si₂O₂]^.+ and N₂O to form eventually [Si₂O₅]^.+; the latter as well as the intermediate oxide cluster ions react sequentially with CO molecules to regenerate [Si₂O₂]^.+. Thus, full catalytic cycles occur at ambient conditions in the gas phase. Mechanistic aspects of these sequential redox processes have been addressed to reveal the electronic origins of these unparalleled reactions.     

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 (C ₁,C _s) 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.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 212–235, February, 1995.This work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-04075).