Electron impact ionization mass spectrometry enables detection of cluster ions from a water-alcohol binary mixture, by which it was previously reported that a pure alcohol solution gave only small cluster ions, while the addition of water in a small amount (ca. 20wt%) resulted in the appearance of much larger cluster ions. We have theoretically examined the effect of existing water on the formation of alcohol clusters, by taking a typical cyclic cluster composed of three ethanol and one water molecules. The calculations were carried out with the Hartree-Fock method with 6-31G** basis set. As a result, we showed that the ionization of alcohols is followed by proton (H+) donation by the water close to the ionized alcohol via energy transfer. The excess ionization energy is taken by the dissociation of the water molecule so that the alcohol-rich cluster ions will be stabilized. In the absence of water, the excess energy is distributed throughout the cluster, so that large cluster ions, not possible to observe, will be fragmented into small pieces in a vacuum.
