Water-assisted interconversions and dissociations of the acetaldehyde ion and its isomers. An experimental and theoretical study

The gas-phase reactions of the ion CH(3)CHO/H(2)O](+*) have been investigated by mass spectrometry. The metastable ion (MI) mass spectrum reveals that this ion-molecule complex decomposes spontaneously by the losses of H(2)O, CO, and (*)CH(3). The structures of stable complexes and transition states involved in the potential energy surface (PES) have been studied by the G3//B3-LYP/6-31+G(d) computational method. Hydrogen-bridged water complexes have been found to be the major products of the losses of CO and (*)CH(3). The CO loss produces the (*)CH(3)...H(3)O(+)] ion and involves a "backside displacement" mechanism. The products corresponding to (*)CH(3) loss have been assigned by theory to be OC...H(3)O(+)] and CO...H(3)O(+)], and their 298 K enthalpy values, calculated at the G3 level of theory, are Delta(f)HOC...H(3)O(+)] = 420 kJ/mol and Delta(f)HCO...H(3)O(+)] = 448 kJ/mol. The PES describing the interconversions among water-solvated CH(3)CHO(+*), CH(3)COH(+*), and CH(2)CHOH(+*) have been shown to involve proton-transport catalysis (PTC), catalyzed 1,2 H-transfer, and an uncatalyzed H-atom transfer mechanism, respectively.