Pulsed nitrogen laser excitation has been used to determine rate and activation parameters for the reaction of singlet oxygen with α-tocopherol (α-T; vitamin E), 6-hydroxy-2,2,5,7,8-pentamethylchroman (HPMC), and 2,4,6-tri-tert-butylphenol (TTBP) in five solvents. The experimental activation enthalpies for α-T and HPMC, predominantly physical quenchers, are significantly more negative than values previously determined for chemical quenchers of 1Δg, and it is concluded that these reactions proceed via the intermediacy of an exciplex which is formed reversibly. It is concluded that the balance between physical and chemical quenching is a sensitive function of spin-orbit coupling properties and entropy factors which play a dominant role as a consequence of a flat enthalpy profile along the critical portions of the reaction coordinate.
ASJC Scopus subject areas
- Colloid and Surface Chemistry