Mechanisms of Water Exchange between Lanthanide(III) Aqua Ions [Ln(H 2 O) 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 (τ res (1st shell) = 1577, 170 and 410 ps for Nd 3+ , Sm 3+ and Yb 3+ , 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(H 2 O) 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 0 ) and by the fact that the calculated average activation volume Δ V ≠ = + 4.5 cm 3 mol −1 is clearly smaller than the estimated activation volume Δ V ≠ lim ≈Δ V 0 = + 7.2 cm 3 mol −1 for a limiting D process. In the case of Sm 3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H 2 O) 8 ] 3+ and a [Sm(H 2 O) 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 Δ V 0 for the coordination equilibrium [Sm(H 2 O) 8 ] 3+ + H 2 O → [Sm(H 2 O) 9 ] 3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H 2 O) 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.