Symmetric and asymmetric proton transfer from heptane and octane radical cations to heptane molecules in γ-irradiated n-C7H16–n-C8H18–2-C6H13Cl crystals: structural disorder in mixed alkane crystals

Abstract

A study has been made of the isomeric composition of secondary chloroheptanes formed upon γ-irradiation at 77 K and subsequent melting of heptane containing 1 mol% 2-chlorohexane and various concentrations of octane. It is observed that the relative importance of 2-chloroheptane increases as a result of the presence of octane in the crystallites. This increase is attributed to selective proton transfer from ground state heptane radical cations to penultimate C–H bonds in heptane molecules. The proton transfer is induced by (partial) dislocation of heptane molecules adjacent to octane solute molecules, which brings the penultimate heptane C–H bonds into close contact with planar chain-end C–H bonds in heptane radical cations. Proton transfer from extended all-trans octane radical cations (on which the positive hole temporarily resides) to heptane molecules may also contribute to the observed effect. The results have important implications with respect to the molecular packing in binary n-alkane crystals. In binary n-alkane crystals, in which the shorter component is predominant, molecules of the longer component cannot be fully accommodated in one molecular layer of the crystal, even with a chain length mismatch of only one methylene unit; instead (at least partial) dislocation of adjacent molecules of the shorter component takes place. Such dislocations do not extend indefinitely over the crystal, however, and crystal order is restored by squeezing, deformation and changes in conformation of the appropriate molecules, including molecules of the shorter component.