Probing the Effects of Size and Charge on the Monohydration and Dihydration of SiF₅^– and SiF₆^2– via Comparisons with BF₄^– and PF₆^–

Abstract

This study systematically examines the interactions of the trigonal bipyramidal silicon pentafluoride and octahedral silicon hexafluoride anions with either one or two water molecules, (SiF 5 − (H 2 O) n and SiF 6 2− (H 2 O) n , respectively, where n = 1, 2). Full geometry optimizations and subsequent harmonic vibrational frequency computations are performed using the CCSD(T) ab initio method with a triple-ζ correlation consistent basis set augmented with diffuse functions on all non-hydrogen atoms (cc-pVTZ for H and aug-cc-pVTZ for Si, O, and F; denoted as haTZ). Two monohydrate and six dihydrate minima have been identified for the SiF 5 − (H 2 O) n systems, whereas one monohydrate and five dihydrate minima have been identified for the SiF 6 2− (H 2 O) n systems. Both monohydrated anions have a minimum in which the water molecule adopts a symmetric double ionic hydrogen bond (DIHB) motif with C 2v symmetry. However, a second unique monohydrate minimum has been identified for SiF 5 − in which the water molecule adopts an asymmetric DIHB motif along the edge of the trigonal bipyramidal anion between one axial and one equatorial F atom. This C s structure is more than 2 kcal mol −1 lower in energy than the C 2v local minimum at the CCSD(T)/haTZ level of theory. While the interactions between the solvent and ionic solute are quite strong for the monohydrated anions (electronic dissociation energies of ≈12 and ≈24 kcal mol −1 for the SiF 5 − (H 2 O) 1 and SiF 6 2− (H 2 O) 1 global minima, respectively), these values are nearly perfectly doubled for the dihydrates, with the lowest-energy SiF 5 − (H 2 O) 2 and SiF 6 2− (H 2 O) 2 minima exhibiting dissociation energies of ≈24 and ≈47 kcal mol −1 , respectively. Structures that form hydrogen bonds between the solvating water molecules also exhibit the largest shifts in the harmonic OH stretching frequencies for the waters of hydration. These shifts can exceed −100 cm −1 for the SiF 5 − (H 2 O) 2 minimum and −300 cm −1 for the SiF 6 2− (H 2 O) 2 minimum relative to an isolated H 2 O molecule at the CCSD(T)/haTZ level of theory. This work also corrects the OH stretching frequency shifts for two dihydrate minima of PF 6− that were previously erroneously reported (J.