An ab initio study of fructose in the gas phase

Seventy-two electronic; structures of D-fructofuranose (D-FF) in the gas phase were determined by full geometry optimizations at the HF/6-31G* level. Twenty-nine structures, including the lowest energy species of nine distinct hydroxymethyl conformations of each anomer (alpha 1-alpha 9 and beta 1-beta 9), were selected for a detailed study of geometry, energy, atomic charges, and hydrogen bonding. The preferred furanose ring conformations were found to center around T-3(2) and T-3(4) for the respective alpha and beta anomers, both of which support a quasiaxial position of the anomeric C2-O2 bond. These findings are consistent with the results from calculations on tetrahydrofuran (THF) and 2-hydroxytetrahydrofuran (2-HO-THF) at the same level. Calculated geometries are in reasonable agreement with the solid-state data on the fructose residues of di-D-fructose anhydride III, 1-kestose, and sucrose. The most stable alpha and beta anomers at 298.15 K, alpha 1 and beta 1, have the gauche-gauche (GG) orientation of the hydroxymethyl C6-O6 bond relative to the ring C5-O5 and C4-C5 bonds and a gauche-trans (GT) orientation of the hydroxymethyl C1-O1 bond relative to the ring C2-O5 and C2-C3 bonds. Effects of basis set and electron correlation on calculated energies were deduced from HF, MP2, and MP4 calculations using the 6-31G**, 6-31+G**, and 6-311++G** basis sets on two 2-HO-THF conformers and the D-FF alpha 1 and beta 1 anomers. Results indicate that basis extension diminishes, whereas electron correlation enhances, hydrogen bonding. Relative electronic and Gibbs free energies of the 11 most populated alpha- and beta-D-FF conformers at 298.15 K were estimated at the MP2/6-311++G** composite level based on HF/6-31G** geometries. This study provides physical data for parametrizing carbohydrate force fields in molecular modeling and promotes understanding of the anomeric and conformational properties of fructose structures.