Intrinsic Conformational Preferences of Substituted Cyclohexanes and Tetrahydropyrans Evaluated at the CCSD(T) Complete Basis Set Limit: Implications for the Anomeric Effect

A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as
large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH3-, F-, OCH3-,
and OH-substituted cyclohexane and tetrahydropyran rings. The high-accuracy relative electronic energies
reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect. At
the CCSD(T) complete basis set (CBS) limit, the energy of the equatorial conformation relative to the axial
position (Δ

) is −1.75, −0.20, −0.21, and −0.56 kcal mol-1 in methyl-, fluoro-, methoxy-, and
hydroxycyclohexane, respectively, while Δ

is −2.83, +2.45, +1.27, and +0.86 kcal mol-1 for
2-methyl-, 2-fluoro-, 2-methoxy-, and 2-hydroxytetrahydropyran, respectively. Note that the equatorial and
axial conformers are nearly electronically isoenergetic in both fluoro- and methoxycyclohexane. For all eight
cyclic species, a zero-point vibrational energy correction decreases Δ

by a few tenths of a kilocalorie
per mole. Relative energies obtained with popular methods and basis sets are unreliable, including Hartree−Fock theory, the B3LYP density functional, and the 6-31G and 6-311G families of split-valence basis sets.
Even with the massive pentuple-ζ basis sets, the HF and B3LYP methods substantially overestimate the
stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol-1, respectively, for
2-methoxytetrahydropyran). Only because of a consistent cancellation of errors do these popular approaches
sometimes provide reasonable estimates of the anomeric effect.