posted on 2023-11-02, 19:34authored byJacob Stamm, Sung Kwon, Shawn Sandhu, Moaid Shaik, Rituparna Das, Jesse Sandhu, Bradley Curenton, Clayton Wicka, Benjamin G. Levine, Liangliang Sun, Marcos Dantus
We report femtosecond time-resolved measurements of the
McLafferty
rearrangement following the strong-field tunnel ionization of 2-pentanone,
4-methyl-2-pentanone, and 4,4-dimethyl-2-pentanone. The pump–probe-dependent
yields of the McLafferty product ion are fit to a biexponential function
with fast (∼100 fs) and slow (∼10 ps) time constants,
the latter of which is faster for the latter two compounds. Following
nearly instantaneous ionization, the fast time scale is associated
with rotation of the molecule to a six-membered cyclic intermediate
that facilitates transfer of the γ-hydrogen, while the ∼50–100
times longer time scale is associated with a π-bond rearrangement
and bond cleavage between the α- and β-carbons to produce
the enol cation. These experimental measurements are supported by ab initio molecular dynamics trajectories, which further
confirm the time scale of this important stepwise reaction in mass
spectrometry.