Electron spin resonance study of adsorption stabilization of photolytic trifluoromethyl radicals on zeolites

Svejda, P.

doi:10.1021/j100663a009

Cited by 10 publications

(4 citation statements)

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“…(5) I (CF3)2COCF3 + cf3 -* (CF3)3COCF3 (6) harmony with the results of a recent investigation which shows that this perfluoro ether is in fact the major product (50-70% yield) obtained after prolonged exposure of liquid HFA to ultraviolet light. 8 The role and stability of trifluoroacetyl radicals during photolysis of HFA is also controversial.…”

Section: Introductionsupporting

confidence: 86%

“…Chemical studies indicate that the ultraviolet irradiation of hexafluoroacetone (HFA) in the gas phase affords principally carbon monoxide and porfluoroethane as the recombination product of two triflu-CF3COCF3 ^[CFjCO CF3] -2CF3 + CO (4) oromethyl radicals.5 The latter have been detected by esr as stabilized adsorbed species following the photolysis of HFA on zeolites at 77°K. 6 In one gas-phase study,7 however, significant amounts of perfluoro-£er£-butyl methyl ether were also found, particularly at higher vapor pressures, in addition to carbon monoxide and perfluoroethane. A reversible addition of the trifluoromethyl radicals to the oxygen atom of an HFA molecule was postulated followed by the coupling of the resulting -trifluoromethoxy radical…”

Section: Introductionmentioning

confidence: 98%

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Electron spin resonance studies of fluoroalkyl radicals in solution. III. Photolysis of perfluoroketones and adduct formation

Krusic¹,

Chen²,

Meakin³

et al. 1974

J. Phys. Chem.

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The esr spectra of paramagnetic species formed during the ultraviolet photolysis of various fluorinated ketones are examined directly in the cavity of the spectrometer. The production of fluoroalkyl radicals by Norrish type I photocleavage is observed in inert solvents. The fluoroalkyl and fluoroacyl radicals produced in the primary photochemical process add to the oxygen atom of the fluoroketones forming a-alkoxy-and -acyloxyfluoroalkyl radicals. Fluoroacyl radicals have been detected by hydrogen atom abstraction from fluoroaldehydes. In cyclopropane solutions, -hydroxyfluoroalkyl radicals resulting from hydrogen transfer from the solvent to the photoexcited ketone are observed, and their identity is verified by generating them by an independent procedure from the corresponding fluoroalcohol. The susceptibility of fluoroketones to addition of other radicals is also probed by the examination of the esr spectra of various adducts with silyl radicals. Addition of silyl radicals to sterically crowded perfluoroketones produces fluoroalkyl radicals of unusual stability. The esr spectrum of the carbonyl fluoride adduct EtsSiOÓFa like other , -difluoroalkyl radicals shows a pronounced line width effect due to dipolar broadening, which is simulated by the relaxation matrix method assuming a correlation time in the region of 10-11 sec for the tumbling motion of the free radical in solution.

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Section: Introductionsupporting

confidence: 86%

Section: Introductionmentioning

confidence: 98%

Electron spin resonance studies of fluoroalkyl radicals in solution. III. Photolysis of perfluoroketones and adduct formation

Krusic¹,

Chen²,

Meakin³

et al. 1974

J. Phys. Chem.

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“…Finally, we would like to discuss the sample preparation of both the CH 3 vs the CF 3 radicals, related to the above differences in the EPR lineshape of CF 3 reported by different groups. All groups used certain kind of irradiation: X-irradiation of trifluoroacetamide crystal by Kalyanaraman and Kispert; UV-irradiation of CF 3 I in rare gases, Maruani et al; 1-MeV electron bombardment of trifluoroacetamid crystal, Rogers and Kispert; photolysis of [(CF 3 ) 2 CF] 2 C 2 F 5 radicals in a hexafluoropropylene trimer matrix in Allayarov et al; photolysis of hexafluoroacetone adsorbed on 13X zeolite in Svejda; and γ-irradiation of CF 4 in solid Xe, Florin et al In all experiments, except that by Florin et al, the CF 3 radical is expected to be the one and only dissociation product mobile enough to leave a matrix cage.…”

Section: Discussionmentioning

confidence: 99%

“…1-MeV electron bombardment of trifluoroacetamid crystal, Rogers and Kispert;19 photolysis of [(CF 3 ) 2 CF] 2 C 2 F 5 radicals in a hexafluoropropylene trimer matrix in Allayarov et al;29 photolysis of hexafluoroacetone adsorbed on 13X zeolite in Svejda;32 and γ-irradiation of CF 4 in solid Xe, Florin et al33 In g-Factors for Several Solid Gas Matrices According to Relevant Literature Data a,b Compare to the computed g-factor = 2.002739 for methyl in vacuum in the present work. b In case of the axially symmetric g-tensor,g iso = [(2g ⊥ + g ∥ )/3].…”

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confidence: 99%

Rotation Dynamics Do Not Determine the Unexpected Isotropy of Methyl Radical EPR Spectra

et al. 2015

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A simple first-principles electronic structure computation, further qc (quantum chemistry) computation, of the methyl radical gives three equal hf (hyperfine) couplings for the three protons with the unpaired electron. The corresponding dipolar tensors were notably rhombic and had different orientations and regular magnitude components, as they should, but what the overall A-tensor was seen by the electron spin is a different story! The final g = (2.002993, 2.002993, 2.002231) tensor and the hf coupling results obtained in vacuum, at the B3LYP/EPRIII level of theory clearly indicate that in particular the above A = (-65.19, -65.19, 62.54) MHz tensor was axial to a first approximation without considering any rotational dynamics for the CH3. This approximation was not applicable, however, for the trifluoromethyl CF3 radical, a heavier and nonplanar rotor with very anisotropic hf coupling, used here for comparison. Finally, a derivation is presented explaining why there is actually no need for the CH3 radicals to consider additional rotational dynamics in order for the electron to obtain an axially symmetric hf (hyperfine) tensor by considering the simultaneous dipolar couplings of the three protons. An additional consequence is an almost isotropic A-tensor for the electron spin of the CH3 radical. To the best of our knowledge, this point has not been discussed in the literature before. The unexpected isotropy of the EPR parameters of CH3 was solely attributed to the rotational dynamics and was not clearly separated from the overall symmetry of the species. The present theoretical results allowed a first explanation of the "forbidden" satellite lines in the CH3 EPR spectrum. The satellites are a fingerprint of the radical rotation, helping thus in distinguishing the CH3 reorientation from quantum rotation at very low temperatures.

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Reactions of Atoms with Magnesium Oxide Surfaces Studied in a Flow System by ESR‐Spectroscopy

Svejda

Haul

Mihelcic

et al. 1975

Ber Bunsenges Phys Chem

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Energy transfer from excited metastable He‐ and Ar‐atoms to MgO surfaces as well as reactions of atomic hydrogen, oxygen and nitrogen respectively with these surfaces in a flow system produce paramagnetic centers which are detected in situ by ESR. The signals observed are attributed to S‐ and SH‐centers as well as to O 3−‐radicals and to Vb‐type centers of clustered O‐‐radicals. The reaction of S‐centers with N2O‐gas yielding O 3− was observed. Mechanisms for the interaction of excited rare gas atoms and of H‐ O‐ and N‐atoms with the defect oxide surface are discussed. At high microwave power the ESR‐signal of the S‐center appears in emission. Fast valence change processes of Mn‐ions in MgO were observed on interaction with H‐ or O‐atoms.

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Electron spin resonance study of adsorption stabilization of photolytic trifluoromethyl radicals on zeolites

Cited by 10 publications

References 4 publications

Electron spin resonance studies of fluoroalkyl radicals in solution. III. Photolysis of perfluoroketones and adduct formation

Electron spin resonance studies of fluoroalkyl radicals in solution. III. Photolysis of perfluoroketones and adduct formation

Rotation Dynamics Do Not Determine the Unexpected Isotropy of Methyl Radical EPR Spectra

Reactions of Atoms with Magnesium Oxide Surfaces Studied in a Flow System by ESR‐Spectroscopy

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