Microsolvation of sodium ion in acetonitrile clusters: Structure and energetic trend by first principle study

Abstract

We report a systematic theoretical study on the growth pattern and energetics of Na +(CH 3CN) n clusters ( n = 1–8,12) using density functional approach at the B3LYP/6-31++G(d,p) level. Geometry optimizations for all these clusters were carried out with various possible initial guess structures without any symmetry restriction and finally the stability of the lowest energy isomer was verified from Hessian calculations. It is found that the incorporation of a sodium ion completely rearrange the equilibrium structure of the neat acetonitrile cluster. The solvated clusters favor multiple shell structure with higher symmetry. The first solvation shell is formed by six CH 3CN molecules, where the nitrogen atoms of each molecule points towards the central sodium ion. Here, the nature of binding between the solute and solvent has been attributed as strong ion–dipole interactions. The onset of the second solvation shell occurs at n = 7 and thereafter additional acetonitrile molecules interact with the molecules of the first solvation shell through N…H interaction. Such interactions are weak compared to the ion–dipole interactions leading to minimal perturbation to the inner shell structure. In consistent with this conjecture, the ion-solvation energy is found to increase very sharply upto n = 6, and becomes less steep from n = 7 onwards. Moreover, the calculated stepwise binding energies are found to be in good agreement with available experimental data, which provide confidence about the equilibrium geometries of the Na +(CH 3CN) n clusters predicted in this study.