A determination of the stabilities of H+5, H+7, H+9, and H+11 from measurement of the gas phase ion equilibria H+n + H2 = H+n+2 (n = 3,5,7,9)

Abstract

The ion equilibria H+n + H2 = H+n+2 for n = 3,5,7,9 were determined in a pulsed electron beam high ion source pressure mass spectrometer. Van’t Hoff plots of the temperature dependence of the equilibrium constants Kn,n+2 lead to the corresponding ΔH°, ΔS°, and ΔG° changes. The ΔH values obtained were ΔH3,5 = −9.6, ΔH5,7 = −4.1, ΔH7,5 = −3.8 ΔH9,11 = −2.4 kcal/mole; ΔS°3,5 = −24.6, ΔS°5,7 = −19.8, ΔS°7,9 = −20.2, ΔS°9,11 = −19.3 e.u. (standard state 1 atm). These results are compatible with predictions from theoretical calculations in which the protonated hydrogen clusters are represented by a three center bonded H+3 to which H2 molecules are weakly attached. The first three H2 molecules go to the three corners of the H+3 triangle and the fourth goes to an outer position.