ESR and optical studies of solute n-alkane cations formed in .gamma.-irradiated n-pentane and n-hexane matrixes

Ichikawa, Takahisa; Shiotani, Masaru; Ohta, Nobuaki; Katsumata, Shunji

doi:10.1021/j100346a089

Cited by 17 publications

(10 citation statements)

References 2 publications

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“…The (weak) lateral features can be attributed with certainty to 1-octyl radicals; no secondary octyl radicals are observable in the system. In association with the results of Ichikawa et al, that show that octane radical cations in pentane matrices are in the extended all-trans conformation [32], these data thus confirm the relation between the electronic structure of alkane radical cations and the site of proton donation. With respect to the site of proton acceptance in the proton transfer from matrix radical cations to solute molecules, two systems have been investigated exhaustively, viz.…”

Section: 82supporting

confidence: 88%

Proton Transfer from Alkane Radical Cations to Alkanes

Ceulemans

2006

Hydrogen‐Transfer Reactions

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Alkane radical cations are very strong Brønsted acids capable of protonating alkanes. Radiation-chemical studies have proven highly successful for the investigation of such proton transfer from alkane radical cations to alkane molecules, using n-alkane nanoparticles embedded in cryogenic CCl 3 F matrices for the study of symmetric proton transfer (RH .+ + RH fi R . + RH 2 + ) and mixed n-alkane crystals for the study of asymmetric proton transfer (R I H .+ + R II H fi R I . + R II H 2 + ). The mechanism of the radiolytic processes involved is discussed in considerable detail. Selectivity with respect to the site of both proton donation and proton acceptance has been studied, using EPR spectroscopy at 77 K for the study of proton donation and gas chromatographic analysis after melting for the study of proton acceptance. The experiments described allow one to conclude that the protondonor site is related very strictly to the structure of the semi-occupied molecular orbital of the parent radical cation, with proton transfer taking place from those C-H bonds that carry appreciable unpaired-electron and positive-hole density. With respect to the site of proton acceptance, it is observed that chemical transformation due to protonation of n-alkanes by alkane radical cations in the systems studied is restricted to C-H bonds at secondary carbon atoms (no chemical transformation resulting from proton transfer to C-C bonds or to C-H bonds at primary carbon atoms). It is argued that the absence of chemical transformation due to C-C protonation has a complex origin in which thermochemical and structural (cage) effects are involved as well as the intrinsic stability of the carbonium ions, but that the absence with respect to protonation of C-H bonds at primary carbon atoms has (in all likelihood) a purely thermochemical origin. As to protonation of n-alkanes at secondary C-H bonds, extensive evidence is presented supporting a marked preference for the penultimate position, with considerably lower (and mutually equal) transfer to the interior sites (intrinsic acceptor-site selectivity). In mixed n-alkane crystals, additional selectivity with respect to the site of proton acceptance results from structural factors in combination with the donor-site selectivity (structurally determined acceptor-site selectivity).

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Section: 82supporting

confidence: 88%

Proton Transfer from Alkane Radical Cations to Alkanes

Ceulemans

2006

Hydrogen‐Transfer Reactions

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“…Relation between the Electronic Structure of Octane Radical Cations and the Radical Site in the Octyl Radicals Formed by Proton Transfer to Pentane Molecules. The ESR spectrum of octane radical cations in pentane, obtained by difference spectroscopy after illumination of irradiated pentane-(3%) octane, 23 indicates that such cations are in the extended conformation, which appears quite reasonable. With such a structure, the imperfection in the crystal is largely limited to a small void at the end of the octane molecule (or radical cation).…”

Section: Discussionmentioning

confidence: 94%

Site selectivity in the proton-transfer reaction from alkane radical cations to alkane molecules: selective formation of 1-octyl radicals by proton transfer from octane radical cations to pentane molecules in .gamma.-irradiated n-C5D12/n-C8H18 crystals at 77 K

Stienlet¹,

Ceulemans²

1993

J. Phys. Chem.

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“…Henderson and Willard found that γ-irradiation at 77 K of singly branched alkanes with branching at the antepenultimate position of the carbon skeleton caused selective rupture of a secondary C−H bond of the −CH 2 CH 3 group but not of a tertiary C−H bond . Ichikawa and Ohta then showed that the location of the selective rupture could be changed to the tertiary C−H bond by replacing the −CH 2 CH 3 group with −CD 2 CH 3 . , Using ESR and electron spin echo spectroscopy, we found that anomaly of the C−H bond rupture came from the conversion of primary alkyl radicals and free hydrogen atoms to penultimate secondary alkyl radicals by selective hydrogen atom tunneling from the penultimate position of the carbon skeleton . We also found the selective tunneling of hydrogen atoms from the penultimate carbon of linear alkanes to free deuterium atoms …”

Section: Introductionmentioning

confidence: 83%

Matrix Effect on Hydrogen Atom Tunneling from Alkane to Free Deuterium Atoms in Cryogenic Solid

et al. 1999

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ESR study on alkyl radicals generated in deuterated organic matrixes at 77 K by hydrogen atom tunneling from alkane molecules to free deuterium atoms has been carried out for elucidating control factors for the tunneling. For small alkane molecules, the tunneling rate is determined by the height of the potential energy barrier for the tunneling. For larger molecules in glassy matrixes, the rate decreases with increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen abstracted. The tunneling rate from antepenultimate tertiary carbon is much slower than that from penultimate secondary carbon, even the potential energy barrier is lower. This abnormal effect is explained as being due to the steric hindrance by matrix molecules to the deformation of alkane molecules during the reaction. The alkyl chains surrounded by matrix molecules prevent the deformation of the chemical bonds of the carbon atom from the initial sp3 to the final sp2 configuration, which may cause the increase of the thickness of the potential energy barrier and thereby decrease of the tunneling rate. Free deuterium atoms in a crystalline adamantane matrix selectively abstract hydrogen atoms from the antepenultimate tertiary carbon, probably due to less steric hindrance to the deformation.

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ESR and optical studies of solute n-alkane cations formed in .gamma.-irradiated n-pentane and n-hexane matrixes

Cited by 17 publications

References 2 publications

Proton Transfer from Alkane Radical Cations to Alkanes

Proton Transfer from Alkane Radical Cations to Alkanes

Site selectivity in the proton-transfer reaction from alkane radical cations to alkane molecules: selective formation of 1-octyl radicals by proton transfer from octane radical cations to pentane molecules in .gamma.-irradiated n-C5D12/n-C8H18 crystals at 77 K

Matrix Effect on Hydrogen Atom Tunneling from Alkane to Free Deuterium Atoms in Cryogenic Solid

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