EVAPORATION OF DIMERS FROM PROTON-BOUND ACETIC-ACID CLUSTERS

Proton-bound clusters of acetic acid and mixed clusters of acetic acid and water evaporate neutral dimers of acetic acid. This attribute is shared with liquid acetic acid. Clusters are considered to bridge the gap between the gas phase and condensed phases. The evaporation of dim ers begins to be observed with the heptamer. Cluster binding energies to monomer and dimer units were deduced from kinetic energy releases. The values for the low members of the series are in quite good agreeme nt with previous data from high pressure mass spectrometry. Kinetic en ergy releases were remeasured and re-evaluated also for proton-bound f ormic acid clusters. Thermochemical cycles lead to the following bond energies of the evaporated neutral dimers: 0.58 +/- 0.07 eV for acetic acid and 0.66 +/- 0.13 eV for formic acid. These values indicate, con trary to previous conclusions, that the neutral dimers evaporate in th e ring-closed, two hydrogen-bonded, form.