text: If the lone pairs of electrons on the oxygens at the anomeric center of 2-methoxypyran are shown, then a brief examination of the conformations of the anomers reveal that the β-anomer always has at least one pair of eclipsing (coplanar 1,3-interacting) lone pairs, this n-n repulsion is a high energy situation.  On the other hand, the α-anomer has conformations in which there are no n-n repulsions, and that is true in the exo-anomeric conformation. The energetically unfavourable n-n repulsion present in the β-anomer, coupled with the energetically favourable hydrogen bond between the axial H-5 and a lone pair of electrons on the axial α-anomeric substituent (C-H/n hydrogen bond), have been suggested [references 7 and 8] to account for most of the energetic difference between the anomers, the anomeric effect.  The molecular mechanics program StruMM3D, which is not specially parameterized for the anomeric effect, estimates that the dipolar contributions to the anomeric effect (primarily the n-n repulsion, and C-H hydrogen bonding discussed above) are about 1.5 kcal/mol.