The radicals produced by γ radiolysis of adamantane and adamantane containing 2-methyltetrahydrofuran (MTHF), 2-methyltetrahydrothiophene (MTHT), and tetrahydrothiophene (THT) have been investigated for the purpose of obtaining a better understanding of radical reactions in the adamantane matrix and of clarifying the identity of the radical formed by γ irradiation of pure glassy MTHF. Thermal and photochemical changes in the identity of the radicals produced by γ irradiation of pure adamantane can be induced at 77 K. If solutes are present the adamantyl radicals formed at 77 K are quantitatively replaced by solute radicals on warming, apparently as a result of diffusion of solute molecules to the adamantyl radical centers followed by abstraction of an H atom. For the solutes MTHT, MTHT, and THT, the radicals form only by H abstraction from carbon atoms adjacent to the heteroatom. MTHT forms only the secondary α radical; MTHF forms both the secondary and tertiary α radicals. MTHT radicals in adamantane decay by second-order kinetics with a rate constant of ∼0.17 M-1 s-1 at 295 K. The tertiary and secondary MTHF radicals decay by first-order kinetics with half-lives of ∼1.5 and 3.4 min, respectively, at ∼245 K, accompanied by growth of new radicals. The evidence suggests a unimolecular ring opening followed by H atom abstraction from the matrix. Reversible changes in the ESR spectrum of the MTHT radical in adamantane indicate that the axial methyl conformation is favored relative to the equatorial at 77 K, and that the enthalpy and entropy differences for the two conformations are 1.7 kcal mol-1 and -4.2 cal deg-1, respectively. Analyses of the ESR spectra of the MTHF radicals in adamantane as a function of temperature indicate that the ESR spectrum of the matrix radical in γ-irradiated glassy MTHF is attributable to the tertiary α radical.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry