Characterization of structure and dynamics of the solvated indium(III) ion by an extended QM/MM molecular dynamics simulation

An extended QM/MM molecular dynamics simulation including the first and second solvation spheres into quantum mechanical treatment was performed to determine structural and dynamics of an aqueous In(III) solution. The first hydration shell composed of six water molecules around the In(III) ion was found with an average In(III)-O bond length of 2.12 Å, corresponding well with the EXAFS and LAXS results. The second hydration shell consisting of ∼14 water molecules is located at 4.25 Å, which is slightly larger than that reported from the LAXS method. The In(III)-O stretching frequency of 461 cm−1, corresponding to the force constant of 176 N/m was delivered from the enlarged QM/MM simulation. In addition, the calculated In(III)-O vibrational modes are in excellent agreement with the experimental values. Various water exchange processes were detected for the second hydration shell of the In(III) ion with an estimated mean residence time of 10.7 ps, while unsuccessful water exchange events in the first solvation shell were exhibited with several intermediate [In(H2O)6]3+⋯H2OII species, revealing a possibly associative interchange (Ia) mechanism. Finally, the enlarged QM/MM simulation shows a strong “structure-making” effect of the second solvation sphere for the In(III) ion in aqueous solution.