Journal of physical chemistry b vol:106 issue:42 pages:10873-10883
A study is made by EPR spectroscopy of the nature of the undecyl radicals formed by gamma-irradiation of CCl3F/undecane at 77 K. It is observed that at low undecane concentration (e.g., 1 mol%) primary undecyl radicals are very prominent. With increasing undecane concentration, the (relative) contribution of primary undecyl radicals to undecyl formation decreases as a result of intermolecular radical site transfer and at 4-6 mol % undecane in CCl3F primary undecyl radicals are essentially absent. With further increases in undecane concentration primary undecyl radicals regain some of their prominence in the EPR spectra but only to a limited extent, contributing an estimated 5-10% to the total undecyl radical concentration at for instance 20 mol % undecane. In view of the documented dependence of intermolecular radical site transfer on the molecular alignment in n-alkane crystals (Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome, K. J. Phys. Chem. 1985, 89, 5278), the results allow us to conclude that (quite unlike neat undecane) undecane aggregates in CCl3F do not assume the orthorhombic crystalline structure at cryogenic temperatures, with exception of some aggregates at the highest concentrations investigated. Taking into account general crystallization characteristics of n-alkanes, it can be deduced that as a rule undecane aggregates in CCl3F are in all likelihood amorphous and this conclusion can reasonably be extended to all n-alkanes with n(c) less than or equal to 11 for concentrations below 20 mol %. Thermodynamic as well as kinetic factors are cited as causes for this general absence of crystallization.