Mechanisms of water exchange between lanthanide(III) aqua ions [Ln(H2O)(n)](3+) and bulk water: A molecular dynamics simulation approach including high-pressure effects

We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (tau(res) (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12-18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)(9)](3+) aqua ion follow a dissociatively activated I-d mechanism via an eightfold-coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an I-d mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (180 degrees) and by the fact that the calculated average activation volume Delta V-not equal = + 4.5 cm(3) mol(-1) is clearly smaller than the estimated activation volume Delta V-lim(not equal) approximate to Delta V-0 = + 7.2 cm(3) mol(-1) for a process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)(8)](3+) and a [Sm(H2O)(9)](3+) aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume Delta V-0 for the coordination equilibrium [Sm(H2O)(8)](3+) +H2O reversible arrow [Sm(H2O)(9)](3+) can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)(8)](3+) exhibit the characteristics of an I-a mechanism. The water exchange takes place via a transition-state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.