Delocalized .pi. radical cations of acetals

Snow, Larry D.; Wang, Jih Tzong; Williams, Ffrançon

doi:10.1021/ja00371a063

Cited by 33 publications

(12 citation statements)

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“…The structure of the frozen rigid conformation of the s-trioxane radical cation is comparable to the structure of the radical cation of 1,3-dioxane ( cf. Figure 3a,b), for which coupling constants a (H)/mT of 14.6, 13.7, 2.45 (2H) and 1.0 (2H) were determined [1,2,8]. With respect to energetically (and magnetically) equivalent conformers of both, the radical cation of s-trioxane—owing to its high symmetry—has six equivalent conformers of its low-energy half-boat structure ( cf.…”

Section: Resultsmentioning

confidence: 99%

“…The technique of low-temperature EPR spectroscopy in freon matrices is well established and has been used successfully to investigate a large variety of radical cations, generated radiolytically through positive charge transfer from matrix to solute. Radical cations of saturated six-membered rings with oxygen atoms have already attracted much attention [1,2,3,4], but the results obtained are sometimes contradictory, regarding both species assignment and structure. It is well known that a “rigid” form of these species can exist as a chair, boat or envelope conformation and the puckering motion has as a rule a relatively small activation energy for interconversion between different puckered structures.…”

Section: Introductionmentioning

confidence: 99%

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The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study

Naumov

Knolle²,

Naumov³

et al. 2014

Molecules

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Abstract:The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10-140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely "rigid" half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the "rigid" half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, OPEN ACCESSMolecules 2014, 19 17306 characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study

Naumov

Knolle²,

Naumov³

et al. 2014

Molecules

View full text Add to dashboard Cite

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“…Most remarkable and surprising is the observation of only a single large splitting constant ($4.4 mT), as usually oxygen centred radical cations of cyclic ethers are expected to have two or more large b-proton couplings. 3,10,13 Bonazzola et al 6 assigned the spectrum to the neutral THP radical, with an asymmetric three-electron s* bond between the oxygen and the unpaired electron on the carbon atom at position 4 of a boat conformer (the hyperfine splitting constants are given in Table 1). The matter was further analysed quantum chemically by Humbel and Hiberty, 24 who concluded, however, that such a through-space interaction is not likely to occur and the unpaired electron remains fully localised on carbon 4 of the most stable chair conformer of the THP radical.…”

Section: A Tetrahydropyranmentioning

confidence: 99%

“…Radical cations of various rings containing oxygen atoms radiolytically generated in low-temperature freon matrices have been investigated before, [3][4][5][6][7] but the results obtained are sometimes contradictory, regarding both species assignment and structure. In some cases with cyclic molecules, besides interconversion through ring puckering, also inversion of the ring through the molecular plane was observed.…”

Section: Introductionmentioning

confidence: 99%

Radical cations of tetrahydropyran and 1,4-dioxane revisited: quantum chemical calculations and low-temperature EPR results

Naumov

Janovský

Knolle

et al. 2003

Phys. Chem. Chem. Phys.

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“…Methylal (dimethoxymethane) appears to be a very suitable model for the investigation of this kind. Indeed, methylal radical cation exhibits a p − σ − p SOMO delocalized over the −OCH 2 O− fragment, which manifests itself in the appearance of characteristic, extremely large hyperfine coupling with the protons of the CH 2 − group . In the frame of semiquantitative quantum chemical consideration, such a structure may imply certain weakening of both CO and CH bonds upon ionization of the parent methylal molecule .…”

Section: Introductionmentioning

confidence: 99%

Effect of Matrix Electronic Characteristics on Trapping and Degradation of Organic Radical Cations in Solid Rare Gases: A Case Study of Methylal Radical Cation

et al. 2000

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The matrix effects on trapping and degradation of methylal radical cation generated in solid argon, krypton, and xenon doped with an electron scavenger at 16 K were investigated by EPR spectroscopy. A relatively weak characteristic signal from trapped methylal radical cations was recorded in an argon matrix. However, the most intense signals in this matrix result from the products of degradation of the primary cations, mainly methyl and methoxy radicals. A comparatively low g max value for methoxy radical (g = 2.032) was explained by formation of a π-complex [CH3O•...CH2O+CH3] upon fragmentation of the parent cation in solid argon. In the case of a xenon matrix, the main observed species is the CH3O•CHOCH3 radical, which appears to result from proton loss in the primary cation. Both deprotonation and fragmentation products were found in krypton. The matrix effects were attributed to the variations in ionization potentials and polarizability (or basicity) of matrix atoms. Fragmentation predominating in argon was explained by excess energy resulting from highly exothermic positive hole transfer. Deprotonation in xenon is favored by basicity of the matrix atoms. Two possible mechanisms were discussed for the latter case, i.e., thermodynamic effect (deprotonation to matrix) and kinetic effect (catalysis of intramolecular H shift by matrix atoms).

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Delocalized .pi. radical cations of acetals

Cited by 33 publications

References 0 publications

The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study

The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study

Radical cations of tetrahydropyran and 1,4-dioxane revisited: quantum chemical calculations and low-temperature EPR results

Effect of Matrix Electronic Characteristics on Trapping and Degradation of Organic Radical Cations in Solid Rare Gases: A Case Study of Methylal Radical Cation

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