Vibrational mode and collision energy effects on proton transfer in phenol cation–methylamine collisions

Abstract

Mass-analyzed threshold ionization has been used to prepare vibrationally state-selected phenol cations, that were then reacted with methylamine at collision energies ranging from 0.1 to 2 eV. Integral cross sections and product recoil velocity distributions are reported. Ab initio calculations of stationary points on the surface and RRKM (Rice–Ramsperger–Kassel–Marcus) analysis of complex lifetimes are also presented for comparison. The only reaction observed over the entire energy range is exoergic proton transfer (PT). For ground-state reactants, the PT cross section is reduced by increasing collision energy, such that the reaction efficiency declines from ∼71% at low Ecollision to ∼50% at 2 eV. Excitation of either v6a or v12 vibrations inhibits reaction over the entire collision energy range, with the effect being somewhat mode-specific and increasing with increasing Ecollision. At low Ecollision, both vibrational and collision energy inhibit reaction with similar efficiency. Collision energy effects diminish at high Ecollision, while vibration continues to have a strong effect. Product ion velocity distributions are approximately forward–backward symmetric at Ecollision⩽1 eV, but are backward peaked at high energies. Mechanistic implications of these results are discussed.