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

Geometry optimizations at the 6-31G** level were performed on various conformers of XH m CH 2 YH n and XH m CH 2 YH n+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-CH 2 -NH 3 + and −5 kcal/mol for N in NH 2 C-H 2 -NH 3 + . The anomeric effect due to N drives NH 2 -CH 2 -OH 2 + to a charge-dipole complex of the type NH 2 =CH 2 + ... OH 2 . The energy parameters obtained have been applied to predict relative stabilities of various conformers of methanetriol, aminomethanediol, and protonated methanetriol, with good success.