Adiabatic ionization potential of acetic acid and torsional dynamics of its cation

Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy and supersonic cooling are used to investigate the CH(3) torsional dynamics of the acetic acid cation and to determine an accurate value for the first adiabatic ionization potential of acetic acid (IP=85 912+/-5 cm(-1)), which has been the subject of debates for more than 40 yr. A doubling of the torsional barrier upon ionization is due to a significant shortening of the C-C bond and reduces the tunneling efficiency by an order of magnitude.