Geometric and electronic structures of silicon fluorides (N = 4 – 6) and potential energy surfaces for dissociation reactions → SiF₄ + F^– and →  + F^–

Abstract

The geometric and electronic structures of a series of silicon fluorides SiFnfalse(n−4false)− (n = 4 − 6) were computationally studied with the aid of density functional theory (DFT) method with B3LYP and M06‐2X functionals and coupled cluster (CCSD and CCSD(T)) methods with 6‐311++G(d,p) basis set. The nature of the Si‐F bonds in these compounds was analyzed in the framework of the natural bond orbital theory and natural resonance theory. Energy characteristics (heats of reactions and energy barriers) of the dissociation reactions SiF5− → SiF4 + F– and SiF62− → SiF5− + F– were calculated using the DFT and CCSD methods. The potential energy surface of elimination of a fluoride anion from SiF62− has a specific topology with valley‐ridge inflection points corresponding to bifurcations of the minimal energy reaction path. © 2016 Wiley Periodicals, Inc.

Probing the Effects of Size and Charge on the Monohydration and Dihydration of SiF5– and SiF62– via Comparisons with BF4– and PF6–

“…The formation of a shorter H-bond to the axial position of SiF 5 – than the equatorial position (1.88 vs 2.31 Å) in the C s global minimum monohydrate structure is consistent with the charge distribution on the F centers. A natural bond orbital (NBO) analysis has shown that there is a slightly greater concentration of negative charge on the axial F atoms …”

“…For the SiF 5 – (H 2 O) 1 system, the identified C 2 v DI 1 minimum features a typical, symmetric DIHB with the water molecule bridging two equatorial F atoms of the anion. Because of the anion’s trigonal bipyramidal structure with slightly more negative charge accumulated on the axial F atoms, another unique monohydrate minimum was identified in which water binds in an asymmetric DIHB motif with one axial and one equatorial F atom. Interestingly, this second minimum, C s DI 1 , has an electronic energy that is more than 2 kcal mol –1 lower than that of the C 2 v DI 1 local minimum.…”

“…The closely related silicon hexafluoride dianion, SiF 6 2– , is also a molecule of interest as it has been shown to dissociate into SiF 5 – and F – . , SiF 6 2– also shares an octahedral geometry with the isoelectronic PF 6 – ion, and both have histories of use in coordination chemistry. − The microscale hydration of the SiF 6 2– ion provides an opportunity to probe the charge dependence of microhydration patterns between the doubly charged SiF 6 2– ion and the singly charged SiF 5 – ion as well as the BF 4 – and PF 6 – ions, both of whose hydration patterns have been studied previously at both the microscale − and in the bulk phase. ,, …”

Anhydrous Silicophosphoric Acid Glass: Thermal Properties and Proton Conductivity

“…In the case of six‐fold‐coordinated Si atoms, the 3 s, 3p, and 3d atomic orbitals hybridize to form sp 3 d 2 hybrid orbitals. The four valence electrons of a Si atom and the four valence electrons of four ligand atoms are shared, forming four covalent bonds, whereas the remaining two hybrid orbitals are occupied by two electron pairs donated by two ligand atoms and form two covalent bonds, as in the SiF 6 2− ion of hexafluorosilicic acid (H 2 SiF 6 ) [31,32] . Therefore, the SiO 6 units are dianionic in nature (expressed as (SiO 6/2 ) 2− ), and two positive charges are required to compensate for the formation of each (SiO 6/2 ) 2− unit [28] .…”

“…The four valence electrons of a Si atom and the four valence electrons of four ligand atoms are shared, forming four covalent bonds, whereas the remaining two hybrid orbitals are occupied by two electron pairs donated by two ligand atoms and form two covalent bonds, as in the SiF 6 2À ion of hexafluorosilicic acid (H 2 SiF 6 ). [31,32] Therefore, the SiO 6 units are dianionic in nature (expressed as (SiO 6/2 ) 2À ), and two positive charges are required to compensate for the formation of each (SiO 6/2 ) 2À unit. [28] In H 5 Si 2 P 9 O 29 glass, only protons that bind to the NBOs of PO 4 units surrounding Si (6) atoms (i. e., Q 3 (1Si) units) and form OÀ H bonds can be involved in charge compensation.…”