Femtosecond Dynamics of Norrish Type-II Reactions: Nonconcerted Hydrogen-Transfer and Diradical Intermediacy

Norrish type‐II and McLafferty rearrangements, which both involve an intramolecular transfer of a γ H atom, can be differentiated on the femtosecond time scale. The McLafferty rearrangement results in ion fragmentation of the parent ketone, whereas the Norrish type‐II reaction leads to a diradical species, which then either cyclizes or fragments (see scheme). For Norrish type‐II reactions, the reaction time for the transfer of the hydrogen atom is within 70 – 90 fs, and the lifetime of the diradical intermediate is in the range of 400 – 700 ps at the total energy studied.