Heavy-Atom Tunneling in Ring-Closure Reactions of Beryllium Ozonide Complexes

Quantum mechanical tunneling (QMT) has long been recognized as crucial for understanding chemical reaction mechanisms, particularly in reactions involving light atoms like hydrogen. However, recent findings have expanded this understanding to include heavy-atom tunneling reactions. In this report, we present the observation of two heavy-atom tunneling reactions involving the spontaneous conversions from end-on bonded beryllium ozonide complexes, OBeOOO (A) and BeOBeOOO (C), to their corresponding side-on bonded ozonide isomers, OBe(η²-O₃) (B) and BeOBe(η²-O₃) (D), respectively, in a cryogenic neon matrix. This discovery is supported by the weak temperature dependence of the rate constants and unusually large ¹⁶O/¹⁸O kinetic isotope effects. Quantum chemistry calculations reveal extremely low barriers (<1 kcal/mol) for both ring-closure reactions. Additionally, instanton theory calculations on both reactions unveil that the tunneling processes involve the concerted motion of all four oxygen atoms.