Computational study of a terphenyl-based Eu3+ complex: effect of small amounts of water

The shielding properties of the terphenyl-based complex L·Eu3+ in methanol solution have been studied by computer simulations with an emphasis on the influence of small amounts of water on the shielding properties. The theoretical studies were carried out in explicit solvent models by molecular mechanics and dynamics simulations (up to 2000 ps), Monte Carlo free energy perturbations, and a potential of mean force calculation. In dry methanol solutions, complex L·Eu3+ is predicted to have one methanol coordinated to the Eu3+. The presence of 1.0 ± 0.5 OH groups (from methanol) is in agreement with fluorescence measurements. Millimolar concentrations of water lead to a monohydrated complex and possibly to a dihydrated complex. At these concentrations of water in the methanol solutions of L·Eu3+, the luminescence intensities decrease, which supports the conclusion that one and possibly two waters are preferentially coordinated to the Eu3+ ion. This study also shows that molecular dynamics runs of 1 ns or more are needed to allow major transitions in the first coordination sphere of Eu3+ to occur, but even simulations up to 2 ns do not guarantee that the global energy minimum is calculated