The anomeric effect: the dominance of exchange effects in closed-shell systems

• Published in: Organic & biomolecular chemistry Vol. 11; no. 2; pp. 299 - 308
• Main Authors: Bauerfeldt, Glauco F, Cardozo, Thiago M, Pereira, Márcio S, da Silva, Clarissa O
• Format: Journal Article
• Language: English
• Published: England 14.01.2013
• Subjects: Cyclohexanes - chemistry; Electrons; Isomerism; Molecular Conformation; Pyrans - chemistry; Solutions; Solvents - chemistry; Static Electricity; Thermodynamics
• ISSN: 1477-0520; 1477-0539
• DOI: 10.1039/c2ob26818c

The origin of the anomeric effect has remained an open question. After Mo demonstrated that hyperconjugation is not responsible for the anomeric effect [Y. Mo, Nature Chem., 2010, 2, 666.], electrostatic interactions and Pauli repulsions have been at the center of this debate. In this work, the total energies of the most stable rotamers of the equatorial and axial anomers of fluoro, hydroxyl, cyano and amino groups in cyclohexane and 2-substituted tetrahydropyran rings are decomposed into their fundamental kinetic, electrostatic and exchange components. In this partitioning scheme, the differences in the total energies among the most stable rotamers of each anomer correlate very well with the differences in the exchange components, revealing that the anomeric effect has no electrostatic origin. Indeed, the anomeric effect is dominated by the exchange energy. This proposal for the origin of the anomeric effect brings new insights that, once incorporated, may improve qualitative chemical models. Implications of this new proposal for the origin of the anomeric effect on geometric parameters and solvation are also discussed.