Hydrogen cluster ion equilibria

The equilibria H3++H2=H5+ and H5++H2=H7+ have been studied using both hydrogen and deuterium in a cooled corona discharge ion source fitted with a very small ion exit aperture. Enthalpies of reaction are in relatively good agreement with results obtained in earlier studies by Hiraoka and Kebarle using a pulse electron impact ion source. Absolute values of equilibrium constants and entropies of reaction are in somewhat poorer agreement. The enthalpy determined for the reaction yielding H5+ was found to be in slightly better agreement with the recent theoretical study of Yamaguchi, Gaw, and Schaeffer than results obtained in earlier examinations. A discussion of potential errors in the experimental determination of these equilibria is presented.