An ab Initio Study of Fructose in the Gas Phase

Abstract

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 (α1−α9 and β1−β9), were selected for a detailed study of geometry, energy, atomic charges, and hydrogen bonding. The preferred furanose ring conformations were found to center around 3T2 and 3T4 for the respective α and β anomers, both of which support a quasi-axial 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 α and β anomers at 298.15 K, α1 and β1, 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 α1 and β1 anomers. Results indicate that basis extension diminishes, whereas electron correlation enhances, hydrogen bonding. Relative electronic and Gibbs free energies of the 11 most populated α- and β-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.