The anomeric and reverse anomeric effect. A simple energy decomposition model for acetals and protonated acetals

Abstract

Geometry optimizations at the 6-31G** level were performed on various conformers of XHmCH2YHn and XHmCH2YHn+I+ (protonated), with X, Y = N, O. The resulting anomeric stabilization energies were decomposed into steric, electrostatic (lone pair – lone pair, lone pair – hydrogen), and electronic contributions. Using approximate values for steric and electrostatic terms, the electronic energy was determined to be about −2 kcal/mol for the anomeric effect to arise from O, and −2.5 kcal/mol if it arises from N. For protonated systems, an additional energy term for the reverse anomeric effect had to be added, having a value of −4 kcal/mol for O in OH-CH2-NH3+ and −5 kcal/mol for N in NH2C-H2-NH3+. The anomeric effect due to N drives NH2-CH2-OH2+ to a charge-dipole complex of the type NH2=CH2+… OH2. The energy parameters obtained have been applied to predict relative stabilities of various conformers of methanetriol, aminomethanediol, and protonated methanetriol, with good success.