Pulse radiolysis investigation of alkene radical cations

Abstract

After the electron pulse irradiation of cyclohexane or n-heptane solutions containing CCl 4 as an electron scavenger and alkenes as solutes, transient optical absorption bands were observed around 280 nm and in the wavelength range between 550 and 900 nm. In neat n-hexane , n-heptane, n-hexadecane, and cyclohexane solutions containing 10 -2 mol dm -3 CCl 4, the UV band and absorption tails extending to the red range were also measured. The 280 nm bands are assigned to π-π ∗ transitions of alkene monomer radical cations, whereas the red bands are attributed to charge resonance transitions occurring in alkene dimer radical cations. From the decay of the solvent radical cation absorption in cyclohexane after addition of alkenes, it is concluded that alkene radical cations can be formed by charge transfer from mobile holes (⊕) to alkene molecules ( S) ⊕ + S→ S +, with rate constants of about 10 11 dm 3 mol -1 s -1. The fragmentation of vibrationally excited alkane radical cations is also assumed to be a source of alkene radical cations. The alkene monomer radical cations observed add nearly diffusion-controlled to an alkene S + + S→ S + 2. Alkene dimer radical cations (S + 2)undergo further reactions, e.g. with their parent alkenes. The rate constants of such processes were determined to be 2–4 orders of magnitude less than those of the dimer radical cation formation. An effective proton transfer from the cyclohexene radical cations to alcohols was observed as well.