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. Influences While the anomeric effect is a general explanation for this type of stabilization for a molecule, the type and amount of stabilization can be affected by the substituents being examined as well as the solvent being studied. Substituent effect In a closed system, there is a difference observed in the anomeric effect for different substituents on a cyclohexane or tetrahydropyran ring (Y=Oxygen). When X=OH, the generic anomeric effect can be seen, as previously explained. When X=CN, the same results are seen, where the equatorial position is preferred on the cyclohexane ring, but the axial position is preferred on the tetrahydropyran ring. This is consistent with the anomeric effect stabilization. When X=F, the anomeric effect is in fact observed for both rings. However, when X=NH2, no anomeric effect stabilization is observed and both systems prefer the equatorial position. This is attributed to both sterics and an effect called the reverse anomeric effect (see below). Solvent effect One common criticism of the hyperconjugation theory is that it fails to explain why the anomeric effect is not observed when substituted tetrahydropyran molecules are placed in polar solvents, and the equatorial position is once again preferred. It has been shown, however, that hyperconjugation does depend on the solvent in the system. Each of the substituted systems described above were tested in the gas phase (i.e. with no solvent) and in aqueous solution (i.e. polar solvent). When X=F, the anomeric effect was observed in both media, and the axial position was always preferred. This is attributed to hyperconjugation. When X=OH or CN, the anomeric effect was seen in the gas phase, when the axial position was preferred. However, in aqueous solutions, both substituents preferred the equatorial position. This is attributed to the fact that there are more electrostatic repulsions with the axial positioned substituent and the polar