The ring-closure reactions of a variety of triplet 1,3-cyclobutanediyls (1) have been observed by EPR spectroscopy under matrix-isolation conditions. It is shown that matrix-site effects can be very well modeled by a Gaussian distribution of activation energies with a standard deviation of ca. 0.5 kcal/mol. Several new techniques have been developed to model such dispersive kinetics. Structures 1 with delocalizing substituents (vinyl, phenyl) show conventional Arrhenius behavior, with log A = 6–8 and Ea = 1–2 kcal/mol. Fully localized cyclobutanediyls ring close via quantum mechanical tunneling. A model emphasizing the differing well depths of the singlet biradicals is developed to rationalize these contrasting behaviors.
ASJC Scopus subject areas
- Colloid and Surface Chemistry