From microhydration to bulk hydration of Rb+ metal ion: DFT, MP2 and AIMD simulation study

The structures, energetics, thermodynamic parameters and vibrational spectrum of hydrated cationic rubidium clusters incorporating a single rubidium ion and up to 32 water molecules using second order Moller-Plesset and hybrid B3LYP density functional are presented. The predicted equilibrium rubidium-oxygen distance of 2.99 angstrom at the present level of theory is in excellent agreement with the diffraction result of 2.98 angstrom for a hydrated rubidium ion cluster. From geometry optimizations it has been found that Rb+ ion has seven water molecules in the first hydration sphere for n >= 24 which support the earlier experimental and simulation findings. The number of hydrogen bonds is found to be increased with an increase in the hydrated cluster size. The present calculated values of interaction enthalpies are in good agreement with the reported gas phase experimental results up to n = 5. The hydration number of rubidium metal ion is predicted to be 7 as seven water molecules are directly linked to the metal ion independently for a large cluster (n > 24) having three layer of solvation representing the bulk solvent limit. Calculation of the vibrational frequencies shows that the formation of a Rb+-water cluster induces significant shifts from the normal stretching modes of isolated water. From the ab initio molecular dynamics simulation, the average first shell coordination number was found to be 631 at both PW91 and PBE levels of theories, which is in close agreement with the QM predicted value (CN = 7) and experimental results (CN = 6.4-7.4). The average second shell coordination number is found to be similar to 21-25. (C) 2012 Elsevier B.V. All rights reserved.