Post-transition-state dynamics during the deazetization of 3 resulting in two degenerate semibullvalenes (4 and 5) have been investigated with density functional theory (DFT) and quasi-classical trajectory (QCT) calculations. Removal of N-2 from 3 occurs through a synchronous and concerted pathway through an ambimodal transition state (TS1). In addition to TS2, the exclusively anticipated product from minimum energy pathway (MEP) calculations, trajectories initiated from TS1 produce 4, TS2, and 5 in 1:1:1 ratio. Isotopic substitutions (C-12(C-13/C-14)-H(D) at 1-2 positions) result in purely Newtonian kinetic isotope effects (4:5 approximate to 1.4 for C-13(1)-C-13(2)), an unequivocal evidence for dynamics controlled product formation.