Role of defects in radiation chemistry of crystalline organic materials. 2. Effective and selective formation of solute radicals and its dependence on the carbon-chain difference in the radiolysis products of CmH2m+2/C10D22 normal hydrocarbon mixed crystals as studied by ESR spectroscopy

Radiolysis of the mixed crystals of binary n-alkanes has been studied by ESR spectroscopy to understand the role of defects in the radical formation in crystalline organic compounds. Highly efficient (10-25%) and selective formation of solute radicals depending on the carbon-chain difference between solute and matrix molecules was found in the CmH2m+2 (1 mol %)/C10D22 mixed crystals (m = 7-12) irradiated at 77 K. The terminal type of radical C·H2CH2CH2- (I) is formed only in the mixed crystals with shorter solute molecules than the matrix (m < 10) and is the only solute radical in the case of m ≤ 8. Penultimate type, CH3C·HCH2- (II), and interior type, -CH2C·HCH2- (III), of radicals are formed selectively in the case of m ≥ 10. These efficient and selective formations were observed neither in the mixed crystals irradiated at 4 K nor in the protiated-alkane matrix (C10H22) irradiated at 77 or 4 K. These results suggest that the D-atom reaction in the molecular layer boundary regions plays an important role in the solute radical formation. The details of this process are as follows. Deuterium atoms radiolytically produced from the matrix molecules can diffuse at 77 K, and some of them escape to the boundary regions. The escaped D atoms can migrate for some distance in the boundary regions (with looser molecular packings than those in the bulk regions) and efficiently abstract a H atom from a protiated solute molecule via a tunneling reaction with a large isotope effect. In the case of m ≤ 8, the escaped D atoms can react with only the terminal CH3 groups of the solute molecule in the boundary regions, forming selectively radical I