Theoretical reaction pathways for the formation of [Si(OH) 5 ] 1- and the deprotonation of orthosilicic acid in basic solution

Details Theoretical reaction pathways for the formation of [Si(OH) 5 ] 1- and the deprotonation of orthosilicic acid in basic solution Theoretical reaction pathways for the formation of [Si(OH) 5 ] 1- and the deprotonation of orthosilicic acid in basic solution

Aug 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.3847k&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 16, pp.3847-3853

Computer Science

2

Scientific paper

Ab initio, molecular orbital calculations at the 6-31G * level including second-order Møller-Plesset electron correlation predict that the species [Si(OH) 5 ] 1- is dynamically stable in a distorted trigonal bipyramid configuration. Reaction pathways for Si ( OH ) 4 + ( OH ) - [ Si ( OH ) 5 ] 1- [( OH ) 3 SiO ] 1- -- H 2 O are also calculated. The first reaction represents the formation of pentacoordinate Si from orthosilicic acid and hydroxide. The activation energy for adding a fifth Si-(OH) - bond to the Si(OH) 4 molecule is 0.1 eV /molec ( 10kJ/mol). The second reaction is the deprotonation of the Si(OH) 4 which forms as a hydroxyl group leaves the [Si(OH) 5 ] 1- molecule. Removal of a bond from this complex requires 0.9 eV/molecule ( 85 kJ/mol). Lengthening the Si--OH 2 distance results in the isolated molecules [( OH ) 3 SiO ] 1- + H 2 O . This represents dehydration of the deprotonated orthosilicic acid. [Si(OH) 5 ] 1- and [(OH) 3 SiO] 1- - H 2 O have the same energetic stability within the accuracy of these calculations. The potential energies of the isolated molecular systems [( OH ) 3 SiO ] 1- + H 2 O and Si ( OH ) 4 + ( OH ) - are considerably higher. These results suggest that [Si(OH) 5 ] 1- may be a stable species or reaction intermediate in dissolution of silicate minerals in basic aqueous solutions.

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