Effect of chain length on aggregation of n-alkanes in CCl3F matrices at 77 K. Further ESR evidence for the occurence of hydrogen and/or proton transfer between higher alkanes and their cations

2000

J. Phys. Chem. A

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A study is made of the formation of the isomeric chlorodecanes, and a search is made for shorter-chain 1-chloroalkanes (5 ≤ n C ≤ 9) formed in CCl3F/decane by γ-irradiation at 77 K and subsequent melting at various concentrations of decane. In such systems, proton transfer from decane radical cations to decane molecules resulting in the formation of pentacoordinated decane carbonium ions (protonated decanes) is quite extensive at high decane concentration, as is evidenced by the fact that under such conditions (i) the ESR spectrum of the irradiated system at 77 K largely consists of decyl radicals and (ii) the contribution of 1-chlorodecane to chlorodecane formation after melting the sample is quite small. The contribution of decyl radicals to the paramagnetic absorption sharply increases and the contribution of 1-chlorodecane to chlorodecane formation strongly decreases with increasing concentration of decane. It is observed that at high decane concentration formation of secondary chlorodecanes is extensive, with 2-chlorodecane being by far the most prominent, but that shorter-chain 1-chloroalkanes are essentially absent. From the results obtained it is concluded that proton transfer from decane radical cations to decane molecules in γ-irradiated CCl3F/decane results in the selective formation of secondary C−H protonated decanes with a clear preference for the penultimate position and that formation of C−C and primary C−H protonated decanes by this process is negligible.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Carbon−Hydrogen vs Carbon−Carbon Protonation in the Proton Transfer from Alkane Radical Cations to Alkane Molecules. A Study in γ-Irradiated CCl₃F/Decane at 77 K

Demeyer

2000

J. Phys. Chem. A

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“… The study of proton transfer from alkane radical cations to alkane molecules (reaction 1) is possible in irradiated CCl 3 F/alkane systems, because alkanes form small aggregates in this matrix, aggregates to which positive-hole transfer still occurs efficiently . The degree of aggregation increases with increasing alkane concentration and at a specific concentration increases quite strongly with increasing chain length of the alkane solute …”

Section: Discussionmentioning

confidence: 99%

“…4 The degree of aggregation increases with increasing alkane concentration and at a specific concentration increases quite strongly with increasing chain length of the alkane solute. 12 As a result of this aggregation, a gradual transformation (with respect to the cationic species) takes place of alkane radical cations into protonated alkanes by proton transfer to neutral alkane molecules. The occurrence of such proton transfer in irradiated CCl 3 F/alkane systems at cryogenic temperatures has clearly been established by ESR spectroscopy.…”

Section: Discussionmentioning

confidence: 99%

Intrinsic Acceptor Site Selectivity in the Proton Transfer from Alkane Radical Cations to Alkane Molecules. Evidence in γ-Irradiated CCl₃F/Undecane

Demeyer

1997

J. Phys. Chem. A

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A study is made of intrinsic acceptor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The research is conducted by studying the relative degree of protonation at the various C-H bonds in undecane in irradiated CCl 3 F/undecane systems at cryogenic temperatures. It is observed that protonation occurs preferentially at the penultimate position, the difference between the other secondary C-H bonds being marginal; very little primary C-H protonation is observed under the experimental conditions.

“…CCl 3 F + e -fi CCl 2 F . + Cl - (5.6) It is now well established, however, that alkanes form small aggregates in CCl 3 F, aggregates to which positive-hole transfer still occurs efficiently [21][22][23]. The degree of aggregation increases with increasing alkane concentration and at a specific concentration increases quite strongly with increasing chain length of the alkane solute.…”

Section: Powder Epr Spectra Of Alkyl Radicalsmentioning

confidence: 99%

Proton Transfer from Alkane Radical Cations to Alkanes

Hydrogen‐Transfer Reactions

2006

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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).