Freon Matrices Research Articles

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.

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 γ-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.

ESR studies of the thietane and thiirane radical cations in freon matrixes. Evidence for ethylene molecule extrusion from a .sigma.* thiirane dimer radical cation [C2H4S-SC2H4.bul.+

ESR spectroscopy has been used to study the structures and reactions of the radical cations produced from the three- and four-membered sulfur-containing ring compounds, thiirane and thietane, the radical cations being generated by ..gamma.. irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K. With use of CFCl/sub 3/ as the matrix, the monomer radical cations have been identified and characterized as having /sup 2/B/sub 1/ ring-closed structures with the unpaired electron localized on the sulfur atom. The hyperfine coupling to the four equivalent ..beta..-hydrogens in the thietane cation (31.1 G) is normal, but the corresponding value for the thiirane cation (16.1 G) is lower by almost a factor of 2, suggesting that hyperconjugation to methylene groups is much reduced in three-membered rings. In the more mobile CFCl/sub 2/CF/sub 2/Cl and CF/sub 3/CCl/sub 3/ matrices, dimer radical cations of thiirane and thietane are produced by the combination of a monomer radical cation with a neutral molecule at low temperatures (< 100 K). ESR studies show that these dimer species are centrosymmetric, the binding between the molecules resulting from the formation of a 3-electron sigma/sup 2/sigma*/sup 1/ S-S bond. Both of the dimer cations are unstable abovemore » 105 K in the CFCl/sub 2/CF/sub 2/Cl matrix, the thietane dimer radical cation decomposing to give the 2-thietanyl radical as a result of hydrogen atom or proton transfer, whereas the thiirane dimer radical cation undergoes a novel reaction involving the extrusion of an ethylene molecule.« less

Temperature change of OD ESR and freon matrix ESR of the radical cations of 9,10-octalin and cyclohexene

The optical detection (OD) ESR and Freon matrix ESR spectra were observed for the radical cations of 9,10-octalin and cyclohexene from 77 to 300 K. The observed spectral change was attributed to ring inversion in both radical cations based on MO calculations and an analysis by the modified Bloch equation. The activation energy for the inversion of the radical cation of octalin was calculated as 18.8 kJ mol −1.

Chlorofluoroalkyl radical rearrangements mediated by electron donors. An e.s.r. study in freon matrices

E.s.r. studies reveal that the CF2ClĊFCl radical ordinarily produced in γ-irradiated CF2ClCFCl2 solid solutions can rearrange to CF3ĊCl2 in the presence of amines with low ionization potentials, suggesting that this unexpected radical isomerization proceeds through chlorofluoroalkyl carbanionic species formed reversibly by electron transfer.

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

ESR spectra of γ-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.

Ion-molecular reactions of primary radical cations of aliphatic amines in freon matrices

ESR spectra for γ-irradiated, at −196 °C, solution of Me2NH, Me3N, and EtNH2 in CFCl3 /0.05÷100% amine/ have been studied. Radical cations Me2NH+., Me3N+. and EtNH 2 +. were trapped in dilute solutions /less than 1% amine in CFCl3/. The yields of radical cations decrease and those of neutral radicals /Me2 N, CH2NMe2, Et NH/ correspondingly increase as the amine concentration increases. Radical cations Me2NH+. are transformed to Me2 N as well as Me3N+. to C H2NMe2 via proton transfer reaction, which is described by the “reaction volume” model.

ChemInform Abstract: Valence Isomerization of the Radical Cations of Bicyclopentanes in γ‐Irradiated Low‐ Temperature Freon Matrices.

AbstractThe radical cation of the spiropentane (I) is produced in a γ‐irradiated CCl3F matrix at 4 K. Analysis of the ESR spectrum leads to the conclusion that the radical cation is Jahn‐Teller distorted from D2d to C2V. The temp. dependence of the ESR spectrum is interpreted in terms of isomerization to the radical cation of methylene cyclobutane (II) at 77 Kand a nonclassical radical 12 cation (III) above 77 K. The radical cation of bicyclo[2.

ChemInform Abstract: An ESR Study of Lactone Radical Cations Formed in γ‐Irradiated Freon Matrices (of lactones (I)‐(IV) at 77 K).

ChemInform Abstract: An ESR Study of Lactone Radical Cations Formed in γ‐Irradiated Freon Matrices (of lactones (I)‐(IV) at 77 K).

Esr studies of the ring opening of cyclopropane radical cations in freon matrices

The radical cations of cyclopropane and several of its methyl derivatives have been characterized by ESR spectroscopy following their generation by γ irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K. In the CFCl 3, CF 3CCl 3, and CF 2ClCCl 3, matrices, only the ring-closed species is usually observed in the accessible temperature range up to ca 160 K. In the CFCl 2CF 2Cl matrix, however, the ring-closed radical cations initially formed at 77 K undergo ring opening between 83 and 110 K, the more highly substituted radical cations requiring a higher temperature for this transformation. The ring-closed radical cations are 2A 1 species for C 2v symmetry, the most substituted cyclopropane C-C bond being elongated with the spin density largely confined to the basal carbons in a face-to-face (90°, 90°) structure. In the ring-opened radical cations, the radical center is localized on the most substituted carbon atom following the breaking of the weakened C-C bond of the ring-closed species. The radical conformations of the ring-opened species have been determined, the RCH 2CH 2· center produced from cyclopropane having a bisected conformation while the RCH 2CMe 2· center obtained from 1,1,2,2-tetramethylcyclopropane is eclipsed, as expected for the presence of α-methyl substituents at the radical site. The nature of the putative carbocation center in the ring-opened radical cations is discussed with reference to recent proposals that this center is strongly coordinated to an electrophile (Cl - or RCl) thereby negating the requirement for an orthogonal structure. Consideration of the strong matrix dependence of the ring-opening reaction suggests a possible solvation effect, however, in which the CFCl 2CF 2Cl matrix assists the twisting of one of the CR 1R 2 groups at the most substituted bond, leading to the rupture of this one-electron σ bond. A strong solvation effect also explains why ring-opening can occur in a suitable polar solvent despite theoretical calculations of unfavourable energetics for a similar gas-phase reaction. Experiments are also described on spiro[2.5]octane, the cyclopropane ring undergoing scission at the CH 2-CH 2 bond of this radical cation to give an RCH 2· radical center. this radical then undergoes a H-atom abstraction with a neutral spiro[2.5]octane molecule in the CFCl 2CF 2Cl matrix at higher temperature to give the spiro[2.5]oct-6-yl radical.

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/.

Possible structures for trimethylene radical cations in freon matrices

It has recently been shown that the parent, cyclic structure for the cyclopropane radical cation undergoes ring opening in a CFCl 2CF 2Cl matrix. However, the ESR results show that the derived species has a SOMO localised on only one of the terminal CH 2 groups. This apparently requires a limiting orthogonal structure for the outer CH 2 groups rendering one similar to a normal alkyl radical and the other to a normal carbocation. Since CH hyperconjugation for the CH 2 + unit is minimised by the structure detected, it will be an extremely powerful electrophile. It is therefore suggested that bonding to one chlorine ligand of a solvent molecule occurs. However, if this is the species studied by ESR spectroscopy, there is no need to postulate that the uncomplexed cation has an orthogonal ground state, since the solvent adduct could have been formed from the linear radical cation in any conformation.

Reply to comment “possible structures for trimethylene radical cations in freon matrices”

The possible existence of an interaction between the ring-opened cyclopropane radical cations and the CFCl 2CF 2Cl solvent has been probed by ESR studies. No definitive evidence for this interaction has been obtained.

ChemInform Abstract: AN ESR INVESTIGATION OF ESTER Π‐CATION RADICALS IN A FREON MATRIX AT LOW TEMPERATURES: EVIDENCE FOR UNUSUAL BARRIERS TO METHYL GROUP ROTATION AND INTRAMOLECULAR BONDING

Chemischer InformationsdienstVolume 15, Issue 31 Physical Organic Chemistry ChemInform Abstract: AN ESR INVESTIGATION OF ESTER Π-CATION RADICALS IN A FREON MATRIX AT LOW TEMPERATURES: EVIDENCE FOR UNUSUAL BARRIERS TO METHYL GROUP ROTATION AND INTRAMOLECULAR BONDING M. D. SEVILLA, M. D. SEVILLASearch for more papers by this authorD. BECKER, D. BECKERSearch for more papers by this authorC. L. SEVILLA, C. L. SEVILLASearch for more papers by this authorS. SWARTS, S. SWARTSSearch for more papers by this author M. D. SEVILLA, M. D. SEVILLASearch for more papers by this authorD. BECKER, D. BECKERSearch for more papers by this authorC. L. SEVILLA, C. L. SEVILLASearch for more papers by this authorS. SWARTS, S. SWARTSSearch for more papers by this author First published: July 31, 1984 https://doi.org/10.1002/chin.198431059Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume15, Issue31July 31, 1984 RelatedInformation

Conformational change from chair to boat upon ionization of 1,4-dithiane as evidenced by optical spectra and MO calculations

1,4-dithiane has been ionized to the radical cation by γ-irradiation in a frozen freon matrix at 77 K. Comparison of the optical spectrum, photoelectron spectrum, and ab initio MO calculations indicates that the stable molecular conformation of the radical cation is a boat form. The conformational change accompanies a remarkable blue-shift of the charge-resonance band of the radical cation.

An ESR investigation of ester .pi.-cation radicals in a Freon matrix at low temperatures: evidence for unusual barriers to methyl group rotation and intramolecular bonding

..gamma..-Irradiation of dilute solutions of a number of esters in CFCl/sub 3/ at 77 K is shown by ESR spectroscopy to produce the ..pi..-cation radicals of these molecules. The ..pi..-cations formed are those of the methyl and ethyl formates, acetates, and propionates. Larger esters containing propyl and butyl side chains are suggested to be unstable toward deprotonation at 77 K. Synthesis of deuterium-labeled compounds was employed to assign proton hyperfine couplings to specific sites in the ..pi..-cation radicals. The ..pi..-cations of the ethyl esters show unusually large couplings to the terminal methyl groups. In addition, large barriers to methyl group rotation are found for both the methyl and ethyl ester cation radicals. The large barriers suggest intermolecular bonding between the alkyl group and the carbonyl oxygen in the ester functional group. Such a hypothesis is supported by the finding of a torsional motion in the terminal methyl group of ethyl formate ..pi..-cation which has a ca. 1.7 kcal/mol activation energy barrier. It is suggested that a new intramolecular ..pi..*-bond is responsible for the formation of cyclic rings in the methyl- and ethyl-substituted ester cation radicals.

Spin delocalization in radical cations of oxygen-containing organic compounds as revealed by long-range hyperfine interactions and solvent effects

E.s.r. evidence is presented for long-range 1H hyperfine interactions and thermally reversible solvent effects involving mainly the radical cations of aldehydes and ketones in Freon matrices. Long-range couplings to δ protons are found to be surprisingly large in the cations of acyclic, cyclic and polycyclic carbonyl compounds. The conformational requirements for these large couplings are discussed in terms of facile spin transmission via a trans arrangement of C—C and C—H σ bonds, the assumption of this trans effect providing a rigorous and consistent analysis of the experimental results for both non-rigid and rigid molecular geometries. The matrix perturbation observed at low temperatures is believed to arise from a weak complex formed between the radical cation and the solvent, the hyperfine interaction occurring mainly with a single chlorine nucleus. The reversible loss of the e.s.r. substructure associated with this complex at elevated temperatures is attributed to motional averaging of the chlorine hyperfine anisotropy rather than to dissociation of the complex.

Equilibrium geometries and hyperfine interactions in propane and cyclopropane cations

The equilibrium geometries of the propane cation in its three lowest electronic states, 2B1, 2B2 and 2A1(C2v symmetry assumed), have been calculated by the UHF method within the ab initio MO-LCAO-SCF approximation. The 2B1 state is predicted to be lowest in energy and the 2B2 and 2A1 states to be of almost the same energy, 12 kcal mol–1 above the 2B1 state. The isotropic hyperfine coupling constants have been calculated and compared with experimental data. The calculations confirm the e.s.r. identification of the 2B1 state, obtained in an SF6 matrix. The previous experimental assignment, 2B2, for the state obtained in Freon matrices was found to be ambiguous and an assignment to the 2A1 state can not be ruled out. Dipolar hyperfine coupling constants have been calculated for the 2B1, 2B2 and 2A1 states of the propane cation and for the 2A1 ground state of the cyclopropane cation. In the latter case, comparison with experimental data has been made with reasonable agreement. The equilibrium geometry agrees well with previous calculations. Complementary experimental data for the propane and cyclopropane cations are reported.

An ESR study of the acetaldehyde radical cation in freon matrices: Evidence for halogen superhyperfine interaction

The substructure associated with the doublet ESR spectrum of the acetaldehyde radical cation in Freon matrices below 100 K is shown to arise from a matrix interaction and not, as previously proposed, from coupling to the hydrogens of the methyl group. Since the reversible loss of this substructure at higher temperature is accompanied by almost no change in the large isotropic 1H coupling(136 G) to the aldehydic hydrogen, the matrix perturbation appears to have a negligible effect on the spin distribution in the radical cation and is according interpreted as a superhyperfine interaction

E.s.r. evidence for the static distortion of 2E1g benzene cations giving 2B2g with D 2h symmetry in low temperature matrices

We have obtained clear e.s.r. evidence that the orbital degeneracy of 2E1g benzene cations is removed at 4.2 K in Freon matrices and the unpaired electron occupies the b2g orbital with D2h symmetry, giving major spin densities on the C(1) and C(4) atoms; however the six carbon atoms become equivalent at >ca. 100 K by dynamic averaging.