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George P , Bock CW , Glusker JP
Protonation of Monosubstituted Oxiranes - a Computational Molecular-Orbital Study of Oxonium Ion Versus Carbonium-Ion Formation
Journal of Physical Chemistry. 1992 Apr 30;96(9) :3702-3708
PMID: ISI:A1992HR49000027   
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The effect of substituents on the nature of the more stable species formed in the initial protonation of oxiranes-whether oxonium or carbonium ions-has been investigated by computational molecular orbital techniques. The geometries, electronic energies, and the distributions of total atomic charge for oxirane, for monosubstituted fluoro-, hydroxy-, amino-, methyl-, cyano-, formyl-, formaldimino-, and vinyloxiranes, and for the corresponding protonated species have been calculated using the 6-31G* basis set with full geometry optimization. Single-point energy determinations have also been made using Moller-Plesset perturbation theory at the MP2, MP3, MP4 (SDQ), and MP4 (SDTQ)/6-31G* parallel-to RHF/6- 31G* levels, and, in the case of oxirane and fluorooxirane, up to the MP4 (SDTQ)/6-31G** parallel-to RHF/6-31G* level. In the case of oxirane itself and the fluoro, methyl, cyano, formyl, and formaldimino derivatives, oxonium ion structures-both the anti and the syn invertomers-are more stable than carbonium ion structures. On the other hand, in the cases of hydroxy, amino, and vinyl derivatives, carbonium ion structures are more stable than oxonium ion structures and may be further stabilized by the formation of hydrogen bonds. The protonation energies for those derivatives that form carbonium rather than oxonium ions are the most negative, as expected in view of the stabilization of such carbonium ions by charge delocalization. The withdrawal of charge from the rest of the substituted oxirane, necessary to bond the added proton, is greater in the formation of the carbonium ions than the oxonium ions. This withdrawal of charge is greatest from -CH = CH2, compared to that from other unsaturated groups with a carbon atom bonded to the oxirane ring (-C = N, -CH = O, and -CH = NH). This is presumably a result of the larger number of covalently bonded H atoms (from which charge is most readily withdrawn) and to the less electronegative terminal heavy atom. It may therefore be inferred that carbonium ion formation would be favored over oxonium ion formation in the case of hydrocarbon epoxides where the oxirane ring is joined to an extensive conjugated system, for example, in the epoxides of polycyclic aromatic hydrocarbons.
English Article HR490 J PHYS CHEM