Heavy-Atom Tunneling Processes during Denitrogenation of 2,3-Diazabicyclo[2.2.1]hept-2-ene and Ring Closure of Cyclopentane-1,3-diyl Diradical. Stereoselectivity in Tunneling and Matrix Effect.

Triplet cyclopentane-1,3-diyl diradical (T-) was generated via photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene () under low-temperature matrix conditions. Temperature independency of T- decay and the kinetic isotope effect of T-- provided experimental evidence in favor of heavy-atom (carbon) tunneling process during the decay of T- to bicyclo[2.1.0]pentane () via singlet S-. For the first time, the formation of was confirmed using low-temperature infrared spectra. Computations of the heavy-atom tunneling process using the small-curvature tunneling method demonstrated a fast reaction from S- to . Moreover, we observed heavy-atom tunneling during denitrogenation of . Stereoselectivity in the tunneling process of T-- was observed at 7 K to form retention-- in higher amounts compared to inversion--. Photolysis of - yielded inv-- and ret-- in environment- and temperature-dependent ratios. Moreover, because of the prominent matrix effect, T- decayed more rapidly in Ar than in glassy organic matrices.