On the formation of C2H5O2+ ions having the structure of hydroxy-protonated acetic acid

Experiments are reported which are best explained in terms of the formation of the long-sought hydroxy-protonated acetic acid, CH3C(O)OH2- This C2 H5O2+ species, generated upon dissociative ionization of 2,4-dihydroxy-2-methylpentane (consecutive losses of CH3. and C3H6), is characterized by a unique collisional activation mass spectrum and an extraordinarily small kinetic energy release for the unimolecular loss of H2O (T0.5<0.15 kJ mol−1). The latter observation is in good agreement with the results of semi-empirical molecular orbital calculations.