Has the trimethylene radical cation been observed?: An Ab initio molecular orbital study

Ab initio calculations have been performed on the cyclopropane radical cation and the trimethylene radical cation. The former radical cation has been claimed to undergo irreversible ring opening to the latter in CFCl₂CF₂Cl matrices at 80 K. However, the computational results reported here show that the energy of the cyclopropane radical cation is much lower than that of the (0,0) trimethylene radical cation, thus casting doubt on the possibility of irreversible ring opening of the former to the latter. It is suggested that the ring-opened species that is observed in the matrix by EPR has a nucleophile strongly coordinated to the carbocationic center.     

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.     

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

Spectral characteristics of the radical cations (RC) of DMF (λ_max = 415 nm, ε_max = (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.     

Photoinduced and thermal reactions of molecular σ radical cations of some branched alkanes in low-temperature matrices: An ESR study

The photoinduced and thermal reactions of σ radical cations of methyl-substituted propanes and butanes in the condensed phases have been characterized by ESR. Photoelimination of CH₄ giving olefinic π radical cations commonly takes place except for H₂ elimination from isobutane radical cations. The thermal reactions of methyl-substituted butane radical cations in CFCl₂CF₂Cl were found to be deprotonation from the CH₃ group to give primary alkyl radicals.     

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.     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF2ClSH

Chemically activated CF₃SH, CFCl₂SH, and CF₂ClSH were formed through combination of SH and CF₃, CFCl₂, and CF₂Cl radicals, respectively. The SH radical was prepared by abstraction of an H‐atom from H₂S by the halocarbon radical produced during photolysis of (CF₃)₂C=O, (CFCl₂)₂C=O, or (CF₂Cl)₂C=O. 1,2‐HX (X = F, Cl) elimination reactions were observed from CF₃SH, CFCl₂SH, and CF₂ClSH with products detected by GC‐MS. The combination reaction of CF₂Cl radicals with SH radicals prepared CF₂ClSH molecules with approximately 318 kJ/mol of internal energy. The experimental rate constants for elimination of HCl and HF from CF₂ClSH were 3 ± 3 × 10¹⁰ and 2 ± 1 × 10⁹ s^?1, respectively. Comparison to Rice–Ramsperger–Kassel–Marcus (RRKM) calculated rate constants assigned the threshold energies as 171 ± 12 and 205 ± 12 kJ/mol for the unimolecular elimination of HCl and HF, respectively. Theoretical calculations using the B3PW91, MP2, and M062X methods with the 6311+G(2d,p) and 6‐31G(d',p') basis sets established that for a specific method the threshold energies differ by only 4 kJ/mol between the two different basis sets. There was wide variation among the three methods, but the M062X approach appeared to give threshold energies closest to the experimental values. Chemically activated CF₃SH and CFCl₂SH were also prepared with about 318 kcal mol^?1 of internal energy, and the HX (X = F, Cl) elimination reactions were observed. Only HCl loss was detected from CFCl₂SH, but the rate was too fast to measure with our kinetic method; however, based on our detection limit the HF elimination channel is at least 50 times slower.     

Hydrogen abstraction reactions of OH radicals with CF<Subscript>2</Subscript>ClCClXH (X = F,Cl) and CFCl<Subscript>2</Subscript>CClXH (X = F,Cl): a mechanistic and kinetic study

The reactions of OH (OD) radicals with CF₂ClCClFH (R1), CF₂ClCCl₂H (R2), CFCl₂CClFH (R3), and CFCl₂CCl₂H (R4) have been investigated theoretically by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MPW1K/6-311+G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single-point energy calculation is made by the MC-QCISD method. The enthalpies of formation of the species CF₂ClCClFH, CF₂ClCCl₂H, CFCl₂CClFH, CFCl₂CCl₂H, CF₂ClCClF, CF₂ClCCl₂, CFCl₂CClF, and CFCl₂CCl₂ are evaluated by two sets of isodesmic reactions. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants of OH and OD radicals with CF₂ClCClXH (X = F, Cl) and CFCl₂CClXH (X = F, Cl) are evaluated over a wide temperature range of 100–2,000 K at the MC-QCISD//MPW1K/6-311+G(d,p) level. The calculated CVT/SCT rate constants are consistent with available experimental data. The results show that the tunneling correction has an important contribution in the calculation of rate constants at lower temperatures. For the above-mentioned four reactions, the kinetic isotope effects are also calculated. Finally, the effect of fluorine or chlorine substitution on reactivity of the C–H bond is discussed.     

A 4 K matrix ESR study of the rearrangement and fragmentation of molecular radical cations of alkyl formates and acetates: Direct evidence for a McLafferty rearrangement

A 4 K matrix ESR study shows that the molecular radical cations of isopropyl formate and acetate, produced radiolytically in halocarbon matrices at 4.2 K, undergo spontaneous rearrangement due to a selective intramolecular hydrogen shift from the tertiary CH bond in the isopropyl group to the carbonyl oxygen atom giving RC⁺(OH)OC(CH₃)₂, where R = H or CH₃. The radical cation of tert-butyl acetate undergoes further fragmentation at the ester CO bond following a similar rearrangement to give an isobutene radical cation in CFCl₃.     

Structure and Reactivity of the Radical Cations of Methyl tert-Butyl Ether in Condensed Phase: An ESR and Quantum Chemical Study

The structure of the methyl tert-butyl ether (MTBE) radical cation and mechanism of its thermal and photochemical reactions in irradiated freons (CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃) were studied. Radical products of MTBE radiolysis in the liquid phase were investigated by the spin trapping technique. The quantum-chemical calculations of the structure of MTBE radical cations and products of their transformations were carried out by density functional theory (DFT) and ab initioMP2 methods. The primary MTBE radical cations are stabilized in dilute solutions in CFCl₃and CF₃CCl₃. The ion–molecule reaction (proton transfer from the radical cation) was found to occur in concentrated solutions in CFCl₃immediately during irradiation. The action of light ( = 436 to 546 nm) at 77 K on the MTBE radical cation in CFCl₃and CF₃CCl₃matrices results in intramolecular migration of the methyl group to yield the distonic radical cation (CH₃)₂ ^.CO⁺(CH₃)₂. The primary MTBE radical cations undergo an irreversible transformation with methane elimination resulting in formation of the 2-methoxypropene radical cation ^.CH₂=⁺(₃)₃in CFCl₃and CF₃CCl₃matrices in the temperature range 110–130 K. In the case of CF₂ClCFCl₂matrix, such a reaction occurs during irradiation at 77 K. Using the spin trapping technique, it was shown that the liquid-phase radiolysis of the neat ether resulted in the formation of fragmentation products (^.CH₃,CH₃^., and t-BuO^. radicals) from the primary radical cations, as well as the products of their rearrangements and ion–molecule reactions.     

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

The recombination of CF₂Cl with CH₂Cl and CFCl₂ with CH₂F were employed to generate CF₂ClCH₂Cl* and CFCl₂CH₂F* molecules with 381 and 368 kJ mol^?1, respectively, of vibrational energy in a room‐temperature bath gas. The unimolecular reactions of these molecules, which include HCl elimination, HF elimination, and isomerisation by interchange of chlorine and fluorine atoms, were characterized. The three rate constants for CFCl₂CH₂F were 2.9×10⁷, 0.87×10⁷ and 0.04×10⁷ s^?1 for HCl elimination, isomerisation and HF elimination, respectively. The isomerisation reaction must be included to have a complete characterization of the unimolecular kinetics of CFCl₂CH₂F. The rate constants for HCl elimination and HF elimination from CF₂ClCH₂Cl were 14×10⁷and 0.37×10⁷ s^?1, respectively. Isomerisation that has a rate constant less than 0.08×10⁷ s^?1 is not important. These experimental rate constants were matched to calculated statistical rate constants to assign threshold energies, which are 264, 268, and 297 kJ mol^?1, respectively, for isomerisation, HCl elimination, and HF elimination for CFCl₂CH₂F and 314, 251, and 289 kJ mol^?1 in the same order for CF₂ClCH₂Cl. Density functional theory was used to evaluate the models that were needed for the statistical rate constants; the computational method was B3PW91/6‐31G(d′,p′). Threshold energies for the unimolecular reactions of CF₂ClCH₂Cl and CFCl₂CH₂F are compared to those for CF₂ClCH₃ and CFCl₂CH₃ to illustrate the elevation of threshold energies by F‐ or Cl‐atom substitution at the beta carbon atom (identified by C_H). The DFT calculations systematically underestimate the threshold energy for HCl elimination.     

Kinetics of Cl(2P) reactions with CF3CHCl2, CF3CHFCl,and CH3CFCl2

The rate coefficients for the removal of Cl atoms by reaction with three HCFCs, CF₃CHCl₂ (HCFC-123), CF₃CHFCl (HCFC-124), and CH₃CFCl₂ (HCFC 141b), were measured as a function of temperature between 276 and 397 K. CH₃CF₂Cl (HCFC-142b) was studied only at 298 K. The Arrhenius expressions obtained are: k₁ = (3.94 ± 0.84)× 10^?12 exp[?(1740 ± 100)/T] cm³ molecule^?1 s^?1 for CF₃CHCl₂ (HCFC 123); k₂ = (1.16 ± 0.41) × 10^?12 exp[?(1800 ± 150)/T] cm³ molecule^?1 s^?1 for CF₃CHFCl (HCFC 124); and k₃ = (1.6 ± 1.1) × 10^?12 exp[?(1800 ± 500)/T] cm³ molecule^?1 s^?1 for CH₃CFCl₂ (HCFC 141b). In case of HCFC 141b, non-Arrhenius behavior was observed at temperatures above ca. 350 K and is attributed to the thermal decomposition of CH₂CFCl₂ product into Cl + CH₂CFCl. In case of HCFC-142b, only an upper limit for the 298 K value of the rate coefficient was obtained. The atmospheric significance of these results are discussed. © 1993 John Wiley & Sons, Inc.     

Radiation chemistry of CF2 Cl2 in the gas and liquid phases: Effect of doses rate

The dose rate dependence of CF₂ Cl₂ decomposition was studied in both, gas and liquid phase, for the range of 2.95·10^{1 7}–1.9·10^{1 9} eV·g^–1·h^–1. The major products were found to be CF₃Cl, CFCl₃, CF₂Cl–CF₂Cl and CF₂Cl–CFCl₂. The decomposition of CF₂Cl₂ was found to decrease with decreasing dose rate for the liquid phase, while an opposite trend was found for gas phase radiolysis. A new mechanism which explains these contrasting findings was suggested. The contribution of radical and of ionic (or molecular) processes to the yields of the various products was estimated.     

Rate constants for the gas-phase reactions of Cl atoms with a series of hydrofluorocarbons and hydrochlorofluorocarbons at 298 ± 2 K

A relative rate method has been used to determine rate constants for the gas-phase reactions of a series of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) at 298 ± 2 K and atmospheric pressure of air. Based on a rate constant for the reaction of the Cl atom with CH₄ of (1.0 ± 0.2) ? 10^?13 cm³ molecule^?1 s^?1 at 298 K, the following Cl atom reaction rate constants (in units of 10^?15 cm³ molecule^?1 s^?1) were obtained: CH₃F, 340 ± 70; CH₃CHF₂, 240 ± 50; CH₂FCl, 110 ± 25; CHFCl₂, 21 ± 4; CHCl₂CF₃, 14 ± 3; CHFClCF₃, 2.7 ± 0.6; CH₃CFCl₂, 2.4 ± 0.5; CHF₂Cl, 2.0 ± 0.4; CH₂FCF₃, 1.6 ± 0.3; CH₃CF₂Cl, 0.37 ± 0.08; and CHF₂CF₃, 0.24 ± 0.05. These Cl atom reaction rate constants are compared with literature data and with the corresponding OH radical reaction rate constants. © John Wiley & Sons, Inc.     

A 4 k matrix esr study of the radical cations of methyl formate: The primary radical cation and its rearrangement

It is found that an oxygen-centred n_π radical of HCOOCH₃ is produced radiolytically in CFCl₃ at 4.2 K without forming a σ* complex with a matrix molecule. This cation converts into the carbon-centred radical cation HC⁺(OH)OCH₂ by an intramolecular hydrogen-atom transfer upon warming to 77 K. This is clear experimental evidence for a McLafferty rearrangement of ester radical cations.     

The CO_2 laser-induced radical-chain reactions between CH_3CF_2H and chlorine-atom donors CF_2ClCFCl_2, CF_3CCl_3 CFCl_3 or CF_2Cl_2

The CO_2 laser induced room temperature reactions of CH_3CF_2H or another protium-donorCH_3CHClCH_3 with chlorine-atom donors (Z--Cl) CFCl_2CF_2Cl, CF_3CCl_3, CFCl_3 or CF_2Cl_2, havebeen investigated. Some of these reactions can yield two important monomers (CF_2=CH_2 andCF_2=CFCl) for fluoropolymers simultaneously. The yield dependence of these two alkenes on experi-mental conditions has been studied. A laser-initiated chain process is supported by identifica-tion of Z--H intermediates in these reactions.     

Negative ion formation in CF2Cl2, CF3Cl and CFCl3 following low energy (0—10 eV) impact with near monoenergetic electrons

Negative ion formation in CF₂Cl₂, CF₃Cl and CFCl₃ under low-energy electron impact has been investigated using a trochoidal monochromat The ions observed are F^?, Cl^?, FCl^?, Cl₂^?, CFCl₂^? from CF₂Cl₂; F^?, Cl^?, FCl^?, CF₂Cl Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance The extremely high yield of Cl^? in CFCl₃, which is observed at ε = 0.0 eV, will be discussed with regard to the lifetime of this molecule i     

Weak hydrogen and halogen bonding in 4‐[(2,2‐difluoroethoxy)methyl]pyridinium iodide and 4‐[(3‐chloro‐2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium iodide

To enable a comparison between a C—H…X hydrogen bond and a halogen bond, the structures of two fluorous‐substituted pyridinium iodide salts have been determined. 4‐[(2,2‐Difluoroethoxy)methyl]pyridinium iodide, C₈H₁₀F₂NO⁺·I⁻, (1), has a –CH₂OCH₂CF₂H substituent at the para position of the pyridinium ring and 4‐[(3‐chloro‐2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium iodide, C₉H₉ClF₄NO⁺·I⁻, (2), has a –CH₂OCH₂CF₂CF₂Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N—H…I and three C—H…I hydrogen bonds, which, together with C—H…F hydrogen bonds, link the cations and anions into a three‐dimensional network. For salt (2), the iodide anion is involved in one N—H…I hydrogen bond, two C—H…I hydrogen bonds and one C—Cl…I halogen bond; additional C—H…F and C—F…F interactions link the cations and anions into a three‐dimensional arrangement.     

Formation and reactions of monomeric and dimeric radical cations of aliphatic ketones in freon matrices

ESR spectra of -irradiated, at 77 K, acetone and CH₃COEt solutions /0.1–22%/ in CFCl₃ were studied. The yields of monomer and dimer radical cations of ketones and RCHCOCH₃ radicals depend on ketone concentration in CFCl₃. When exposed to light the dimeric radical cations are transformed into RCHCOMe, while the monomeric radical cations disappear without further radical production. Different types of solid phase ion-molecular reaction for monomer and dimer radical cations are discussed.     

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

The ESR spectra of -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.