Spectral Properties and Kinetics of Cationic Transients Generated in Electron Pulse Irradiated C7‐ to C16‐Alkanes

Abstract

AbstractAfter electron pulse irradiation of n‐heptane, 3‐methylheptane, n‐decane, n‐dodecane, n‐tetradecane, and n‐hexadecane containing CCl4 as electron scavenger, transient optical absorption bands were observed with peak wavelengths between 550 and 880 nm. The bands are assigned to the corresponding alkane radical cations. For n‐heptane radical cations a maximum extinction coefficient of 8.8 · 103 dm3 mol−1 cm−1 is estimated. Alkene radical cations were also identified in neat alkane solutions. They seem to be generated by fragmentation of vibronically excited alkane radical cations and exhibit strong optical absorption bands (π‐π*‐transitions) around 280 nm. — The kinetics of the electron transfer from n‐hexadecane as solute to cationic transients present in n‐heptane after irradiation was experimentally studied and compared with calculations using the phenomenological model of ion pair recombination presented by Infelta and Rzad [1]. From two alternative reaction mechanisms applied, differing mainly in that one assumes short‐lived mobile holes, it is concluded that positive mobile holes should also exist in n‐heptane at times <10−9 s. Then they should be transformed into massive n‐heptane radical cations. — Electron transfer efficiencies were determined in n‐heptane for 11 solutes such as alkanes, alkenes and aromatics. These efficiencies are related to rate constants of the electron transfer from the solutes to mobile positive holes. The observed dependence of the rate constants on the free energy ΔG of the electron transfer reaction is explained in terms of changes in the Franck‐Condon vibrational overlap factors.