Paramagnetic Properties of Alkane Radical Cations

The strong electronic absorption of alkane radical cations is readily understood in molecular orbital terms. Extending down from the highest occupied molecular orbital (HOMO) is a rather closely packed set of valence molecular orbitals, that are clearly displayed in the photoelectron spectra (PES) of neutral alkanes. The electronic absorption of alkane radical cations is due to transitions (induced by photon absorption) of electrons from such lower-lying molecular orbitals to the semi-occupied molecular orbital (SOMO), which is the highest-occupied molecular orbital in the ground-state ion. By illumination within the (broad and largely unstructured) absorption band of alkane radical cations, electronically excited states of alkane radical cations can thus be created in a quite convenient way. 

The paramagnetic absorption of alkane radical cations is critically dependent on their conformation. In neat n-alkane crystals, alkane molecules are in the extended all-trans conformation (see Fig. 5.1) and FDMR spectroscopy unequivocally shows that alkane radical cations retain that conformation in such systems. The extended all-trans conformation is also the preferred conformation of many n-alkane radical cations in chlorofluorocarbon and perfluorocarbon matrices. In this conformation, the unpaired electron occupies the planar cr molecular orbital and delocalizes over the entire extended chain. Only two C-H bonds (both chain-end, one on each side) are in the planar ct molecular frame in the extended structure and high unpaired-electron and positive-hole density appears only on these in-plane protons. Alkane radical cations in the extended conformation (as well as in other conformations) are thus fj-delocalized paramagnetic species. The associated hyperfme interaction with the two (equivalent) in-plane chain-end protons results in a 1 2 1 three-line (triplet) EPR spectrum. The fact that the hyperfme in- 

A second conformation that is frequently encountered in chlorofluorocarbon matrices is the gauche-at-C2 conformation, which is obtained from the extended conformer by 120° rotation around C2-C3 (see Fig. 5.1). In this conformation, the unpaired electron and positive hole delocalize over the planar part of the C-C skeleton as well as over one in-plane chain-end and penultimate C-H bond (on opposite sides of the radical cation). Other more twisted conformations (such as obtained from the extended conformer by 120° rotation around the penultimate C-C bond as well as around C2-C3, either both clockwise or one clockwise and the other counterclockwise) have also been observed, but they are less common in chlorofluorocarbon matrices. In n-alkanes in the liquid state, many conformers are present and the paramagnetic absorption reflects this by the fact that an unresolved spectrum is observed for the radical cations, resulting from hyperfine coupling with many different protons [10]. As a result of increased delocalization, the line width of such composite spectra decreases with increasing chain length. 

Alkane radical cations are very strong Bronsted acids and form an acid-base pair with neutral alkyl radicals as conjugate base according to the acid-base half-reaction 

For n-alkane radical cations, both primary and secondary alkyl radicals may act as conjugate base, depending on the conformation of the radical cation. This is a direct consequence of the fact that the site of proton donation from alkane radical cations is strictly dependent on their electronic structure, that is, a high unpaired-electron and positive-hole density in a particular C-H bond results in proton transfer from that bond (evidence for this is given in Sections 5.7 and 5.8), whereas the electronic structure of the radical cations in turn depends on their conformation. For n-alkane radical cations in the extended conformation, in which the unpaired-electron and positive-hole delocalize over the planar C-C skeleton and two in-plane chain-end C-H bonds (one on each side), proton transfer takes place exclusively from chain-end positions and the corresponding primary alkyl radicals act as conjugate base. For n-alkane radical cations in the gauche-at- 

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