When anomeric effects collide

Rotational coordinates about the C(3)-O(4) bonds of 2,4-dioxaheptane (DOH) and 2,4,6-trioxaheptane (TOH) are compared at correlated levels of electronic structure theory for gauche and trans orientations of the O(2)-C(3) bonds. TOH has overlapping anomeric effects, while DOH does not. The overlapping stereoelectronic effect shows its largest impact on the length of the O(2)-C(3) bond, which is typically 0.02 Å longer in DOH than in TOH. However, the energetic consequences of the overlapping anomeric effect in TOH are very small, as judged by total conformational energies and analysis of delocalization energies within a natural bond orbital framework.