The anomeric effect: Ab‐initio studies on molecules of the type XCH₂OCH₃

The anomeric effect of the functional groups X = CN, CCH, COOH, COO − , OCH 3 , NH 2 , and NH + 3 has been studied with ab initio techniques. Geometry effects upon rotation around the central CO bond in X CH 2 OCH 3 have been compared in the various compounds. The energy differences between the conformers with a gauche and trans ( X COC) arrangement were calculated at the 6‐31G* level in the fully optimized 4‐21G geometries. Energy differences calculated at the 4‐21G level appeared not to be reliable, especially for the groups X that contain non‐ sp 3 hybridized atoms. The 6‐31G* energy differences indicate a normal anomeric effect for X = COO − , OCH 3 , and NH 2 (g + ) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, CN, and CCH (ca. 6 kJ/mol). For X = NH 2 (t) and NH + 3 a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X COC follows roughly the same trend.