Quantum Chemical Conformational Analysis of Dihydroxysiliconate Ion [H₃Si(OH)₂^-]. An Exceptionally Fluxional Pentacoordinate [10-Si-5] System

Cramer, C. J.; Squires, R. R.
J. Am. Chem. Soc. 1995, 117, 9285.

The global minimum calculated for H3Si(OH)2^- has both hydroxyl groups equatorially located in a trigonal bipyramidal (TBP) structure (large basis sets, highly correlated levels of theory). Natural bond orbital (NBO) analysis indicates that hyperconjugative interactions involving the lone pairs of the equatorial hydroxyl groups as donors slightly overcome the otherwise expected tendency for these (electronegative) groups to prefer axial positions in the TBP. A diaxially substituted local minimum structure does exist, and lies only 2.7 kcal/mol in enthalpy (298 K) above the global minimum at the highest level of theory. Another diaxial structure a mere 0.01 kcal/mol additionally higher in enthalpy is calculated to be a transition state for a double-pseudorotation that interconnects two degenerate conformations of a diequatorially substituted stereoisomer. The conformational potential energy hypersurface may thus be regarded as essentially flat, suggesting that permutation of siliconate ligand positions is facile in dialkoxysiliconates.

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