INTERPRETATION OF ACTIVATION VOLUMES FOR WATER EXCHANGE-REACTIONS REVISITED - AB-INITIO CALCULATIONS FOR AL3-DATA(, GA3+, AND IN3+, AND NEWEXPERIMENTAL)

The water exchange mechanisms on the hexaaqua ions of Al3+, Ga3+, and In3+ in aqueous solution have been modeled by using ab initio calculat ions at the Hartree-Fock level. As an approximation aqua clusters in v acuo involving seven water molecules were considered. For species with five, six, or seven water molecules in the first coordination shell o f the cation, stable intermediates and transition states have been opt imized and characterized from vibrational analyses. Water exchange rea ction pathways could then be proposed via interconnected intermediates and transition states. The calculations provide theoretical evidence for a break in kinetic behavior between Al3+ Ga3+ on one side and In3 on the other. Hexaaqua complexes of Al3+ and Ga3+ show no tendency to increase their coordination number over six arid, despite the high po sitive charge on the central ion, water exchange proceeds via a D mech anism involving a pentacoordinated intermediate [M(OH2)(5).(OH2)](3+). This is in agreement with experimental volumes of activation Delta V double dagger that are the highest measured up to date for trivalent m etal cations. For [In(OH2)(6)](3+) a dissociative exchange reaction is in principle feasible, but an associative A mechanism via a 7-fold co ordinated reactive intermediate [In(OH2)(7)](3+) is energetically much more favorable. Theoretical arguments and indirect experimental evide nce in favor of an A and against an I, mechanism are discussed. The ex perimentally still lacking activation volume Delta V double dagger for water exchange on [In(OH2)(6)](3+) has been predicted to be -5 +/- 1 cm(3)mol(-1). The computed activation energies Delta E double dagger f or Al3+ and Ga3+ are in remarkable quantitative agreement with the exp erimental values for Delta H double dagger. This lends support to the applied theoretical model and suggests that for this class of aqua ion s, with a spherical and strongly bound first hydration shell, cluster calculations in vacuo are a viable approach to reproduce the structura l changes and the activation parameters for water exchange reactions i n aqueous solution. Attempts have been made to determine the water exc hange rate at In-(aq)(3+), Lu-(aq)(3+), and Zn-(aq)(2+) by using Tb-(a q)(3+) as a shift reagent. While none of these attempts proved success ful, new lower limits for water exchange on these three ions can be gi ven as 1 x 10(7) (In3+), 1 x 10(7) (Lu3+), and 5 x 10(7) s(-1) (Zn2+).