Tl(I)-the strongest structure-breaking metal ion in water? A quantum mechanical/molecular mechanical simulation study

Structural and dynamical properties of the Tl(I) ion in dilute aqueous solution have been investigated by ab initio quantum mechanics in combination with molecular mechanics. The first shell plus a part of the second shell were treated by quantum mechanics at Hartree-Fock level, the rest of the system was described by an ab initio constructed potential. The radial distribution functions indicate two different bond lengths (2.79 and 3.16 A) in the first hydration shell, in good agreement with large-angle X-ray scattering and extended X-ray absorption fine structure spectroscopy results. The average first shell coordination number was found as 5.9, and several other structural parameters such as coordination number distributions, angular distribution functions, and tilt- and theta-angle distributions were evaluated. The ion-ligand vibration spectrum and reorientational times were obtained via velocity auto correlation functions. The Tl-O stretching force constant is very weak with 5.0 N m(-1). During the simulation, numerous water exchange processes took place between first and second hydration shell and between second shell and bulk. The mean ligand residence times for the first and second shell were determined as 1.3 and 1.5 ps, respectively, indicating Tl(I) to be a typical "structure-breaker". The calculated hydration energy of -84 +/- 16 kcal mol(-1) agrees well with the experimental value of -81 kcal mol(-1). All data obtained for structure and dynamics of hydrated Tl(I) characterize this ion as a very special case among all monovalent metal ions, being the most potent "structure-breaker", but at the same time forming a distinct second hydration shell and thus having a far-reaching influence on the solvent structure.