Probing the Effects of Size and Charge on the Monohydration and Dihydration of SiF 5 - and SiF 6 2- via Comparisons with BF 4 - and PF 6

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 (H O) and SiF (H O) , respectively, where = 1, 2). Full geometry optimizations and subsequent harmonic vibrational frequency computations are performed using the CCSD(T) 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 (H O) systems, whereas one monohydrate and five dihydrate minima have been identified for the SiF (H O) systems. Both monohydrated anions have a minimum in which the water molecule adopts a symmetric double ionic hydrogen bond (DIHB) motif with symmetry. However, a second unique monohydrate minimum has been identified for SiF 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 structure is more than 2 kcal mol lower in energy than the 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 for the SiF (H O) and SiF (H O) global minima, respectively), these values are nearly perfectly doubled for the dihydrates, with the lowest-energy SiF (H O) and SiF (H O) minima exhibiting dissociation energies of ≈24 and ≈47 kcal mol , 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 for the SiF (H O) minimum and -300 cm for the SiF (H O) minimum relative to an isolated H 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 that were previously erroneously reported ( 2020, 124, 8744-8752, DOI: 10.1021/acs.jpca.0c06466).