Dissociation dynamics of energy-selected acetic acid ions: The gas phase heat of formation of the acetyl ion

The dissociation of energy-selected acetic acid ions (CH3COOH+) has been investigated by Threshold Photoelectron-photoion Coincidence (TPEPICO) spectroscopy. The lowest energy dissociation pathway for CH3COOH+ is OH center dot loss, and the 0K onset (E-0) of this process is measured to be 11.641 +/- 0.008 eV. The reaction rate for this step is instantaneous on the time-scale of the experiment so that no rate theory is required to extract its onset. Because the heats of formation of acetic acid and the OH center dot radical are known to an accuracy of +/- 0.5 kJ/mol, we can use the OH center dot loss onset to obtain an accurate heat of formation of the acetyl ion, Delta H-f degrees(298) (K)[CH3CO+] = 658.5 +/- 1.0 kJ/mol, which agrees to within 0.9 kJ/mol with an earlier determination based on CH3 center dot loss from the acetone ion. At higher energies, the acetic acid ion loses CH3 center dot to form the HOCO+ ion in competition with the lower energy OH-loss. Two versions of the statistical reaction rate theory failed to reproduce the branching ratios for these higher energy parallel reactions. Because of the magnitude of this disagreement, we conclude that the acetic ion may dissociate non-statistically at these higher energies. (C) 2010 Elsevier B.V. All rights reserved.