Stabilisation and reactions of aliphatic radical cations produced by fast electron irradiation in solid argon matrices

Stabilisation and reactions of radical cations produced by irradiation of various aliphatic molecules (ethers and carbonyl compounds) in argon matrices in the presence of electron scavengers at 10–16 K were studied by EPR. It was found that the relative yield of trapped radical cations depended strongly on the chemical structure of ionised organic molecules. In particular, the radical cations of tetrahydrofuran and acetone are trapped with high yields at large dilution (above 1000: 1). Acetaldehyde gives both primary radical cations and fragmentation products (methyl radicals). No sign of trapping of the primary radical cations was obtained in the case of methyl tert-butyl ether; instead of this, methyl radicals are produced with high yield. Specific deuteration of the methoxy group shows that methyl radicals originate from the tert-butyl moiety. The secondary reactions of ionised molecules occurring upon irradiation of organic molecules in argon matrices were attributed to high exothermicity of the positive hole transfer from argon to solute molecules (large “IP gap”). The striking difference in the yields of primary radical cations is explained by the effect of molecular structure on intramolecular relaxation of excess energy. The implication of the obtained results for basic radiation chemistry of organic molecules in solids is discussed.