Abstract
Geometry optimization, at the B3LYP/6-311++G ** level of theory, was carried out on 4 C 1 and 1 C 4 chairs, 3,O B and B 3,O boats, and skew-boat conformations of α- and β- d-glucopyranose. Similar calculations on 1,5-anhydro- d-glucitol allowed examination of the effect of removal of the 1-hydroxy group on the energy preference of the hydroxymethyl rotamers. Stable minimum energy boat conformers of glucose were found, as were stable skew boats, all having energies ranging from ∼4–15 kcal/mol above the global energy 4 C 1 chair conformation. The 1 C 4 chair electronic energies were ∼5–10 kcal/mol higher than the 4 C 1 chair, with the 1 C 4 α-anomers being lower in energy than the β-anomers. Zero-point energy, enthalpy, entropy, and relative Gibbs free energies are reported at the harmonic level of theory. The α-anomer 4 C 1 chair conformations were found to be ∼1 kcal/mol lower in electronic energy than the β-anomers. The hydroxymethyl gt conformation was of lowest electronic energy for both the α- and β-anomers. The glucose α/β anomer ratio calculated from the relative free energies is 63/37%. From a numerical Hessian calculation, the tg conformations were found to be ∼0.4–0.7 kcal/mol higher in relative free energy than the gg or gt conformers. Transition-state barriers to rotation about the C-5–C-6 bond were calculated for each glucose anomer with resulting barriers to rotation of ∼3.7–5.8 kcal/mol. No energy barrier was found for the path between the α- gt and α- gg B 3,O boat forms and the equivalent 4 C 1 chair conformations. The α- tg conformation has an energy minimum in the 1 S 3 twist form. Other boat and skew-boat forms are described. The β -anomer boats retained their starting conformations, with the exception of the β- tg- 3,O B boat that moved to a skew form upon optimization.
Published Version
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