29Si NMR Shifts and Relative Stabilities Calculated for Hypercoordinated Silicon−Polyalcohol Complexes: Role in Sol−Gel and Biogenic Silica Synthesis

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Abstract
Penta- and hexa-coordinated silicon is rare, occurring as a transient species in some glasses, nonaqueous organosilicon solutions and organosilicon gels such as silicone, and is stable at high pressures within the earth in dense phases such as stishovite. The stable form expected in aqueous solution is quadra-coordinated silicon. A recent study proposed the existence of hypercoordinated silicon−polyalcohol complexes in aqueous solution, based on 29Si NMR shifts at −102 to −103 ppm and −145 to −147 ppm. Here, we report ab initio molecular orbital calculations of 29Si NMR chemical shifts and relative stabilities of silicon−polyalcohol monocyclic and spirocyclic complexes, from ethylene glycol (C2H6O2) to arabitol (C5H12O5) with Si in quadra-, penta- and hexa-coordination (QSi, PSi, HSi), calculated at the HF/6-311+G(2d,p)//HF/6-31G* level. Calculated shifts are accurate with a 1−8% error for QSi and 2−9% for PSi. Shifts calculated for the hypercoordinated silicon complexes having structures proposed in the literature are much more negative (−128 and −180 ppm for PSi and HSi) than observed. We propose that cyclic trimers complexed by polyalcohols can explain the −102 ppm shift, where the Si atoms are all QSi, or where two silicons are QSi and one is PSi with rapid exchange between the Si sites. The −145 ppm resonance results from structures similar to those proposed in the experimental NMR study for the −102 ppm peak. Our relative stability calculations indicate that structures proposed in the literature for hypercoordinated silicon complexes are thermodynamically unstable in aqueous solution at acidic to neutral conditions but may exist in degrading silicone-gel breast-implants. Thus, aqueous hypercoordinated silicon−polyalcohol complexes are unlikely to play an important role in biological silicon uptake and hold little promise for novel silica synthesis routes from aqueous solutions under nonextreme conditions.