Linear alkane radical cations prepared in synthetic zeolites by irradiation at 4 K: ESR evidence for ion molecule reaction to form 1-alkyl radicals

A new method is explored for preparing radical cations of alkanes in the system other than halogenated and neon matrices. The radical cations are produced from n-C/sub 6/H/sub 14/ and n-C/sub 8/H/sub 18/ adsorbed in synthetic zeolite (ZSM-5) by X-irradiation at 4 K. The ESR spectra show that the cations exhibit the planar extended structure without showing coexistence of the gauche conformer. With increasing the concentration of alkanes, the parent molecules are increasingly adsorbed as a dimer, from which the 1-alkyl radical is preferentially formed during irradiation at 4 K. This indicates that the n-alkane radical cation undergoes prompt deprotonation via the ion-molecular reaction when a neighboring alkane molecule exists as a proton acceptor in the adsorbed site. The formation of the 1-alkyl radical is consistent with the high unpaired electron density in the in-plane C-H bonds at the chain end of the radical cations. These results are closely related to their previous finding of the selective formation of the 1-alkyl radicals in the 4 K radiolysis of crystalline neat linear alkanes.