REACTIONS OF PROTONATED ACETIC-ACID CLUSTERS (CH3COOH)(N)H-3)( (N=1)

Reactions of(CH3COOHH+ (n = 1-3) with a series of base molecules B wer e studied using a selected ion dow tube. All the reactions observed pr oceed at near gas kinetic collision rates for bases having higher prot on affinities than acetic acid. Different reaction channels are observ ed with increasing proton affinity of the neutral: association and pro ton transfer for the proton-bound monomer; association, ligand switchi ng and proton transfer for the dimer; association, ligand switching, s witching with solvent evaporation and proton transfer for the trimer. A thermochemical comparison of the appearance enthalpy and free energy for proton transfer from (CH3COOHH+ and (C2H5OHH+ established that bo th sets of reactions take place by forming BH+ with evaporation of thr ee monomeric units (of acetic acid or ethanol, respectively) rather th an evaporation of neutral dimers. Proton transfer from (CH3COOHH+ is f ast provided that the reaction is exothermic; proton transfer from (CH 3COOHH+ is fast even if the reaction is endothermic (but exergonic).