Cryogenic Rearrangements\nof Spiroheptadiyl: Light-\nor Heavy-Atom Quantum Tunneling?

Triplet\n1,4-spiro[2.4]heptadiyl (<b>SHD</b>) has been shown\nexperimentally to undergo rapid ring-opening and subsequent 1,2-hydrogen\nshift upon generation via photolysis of a diazene precursor at cryogenic\ntemperatures. Modern computational tools elucidate the potential energy\nsurface and kinetics behind this cascade reaction, disproving the\nearlier hypothesized mechanism invoking hot molecule effects in the\nfirst ring-opening step and tunneling in the second hydrogen transfer\nstep. Instead, our results assign the <b>SHD</b> instability\nto heavy-atom tunneling and a subsequent photochemical hydrogen shift.\nThis essentially is the opposite of the originally proposed mechanism.\nThis case study thus addresses common misconceptions about the fundamental\nprinciples of tunneling involving hydrogen or carbon.