Cyclic and acyclic fructose conformers in the gas phase: a large-scale second-order perturbation theory study

We performed large-scale second-order perturbation theory gas-phase calculations to study about five hundred structures of D-fructose. The two lowest energy fructose structures identified are β-pyranoses possessing (2)C5 chair, with ΔG(298 K) of 6 kJ/mol, differing in orientation of the equatorially positioned hydroxymethyl group, gt and g'g, where the gt rotamer is the global minimum, consistent with the recent microwave spectroscopy study. We have found that interconversions from the fructose global minimum to the second and third most stable β-pyranose rotamers involve the energy barriers of ca. 30 kJ/mol. Among numerous fructofuranose conformers discovered (about 250), a pair of the ((3)T2) α- and (E3) β-anomers are energetically most preferred and lie at least 12 kJ/mol above the global minimum. We also found that the fructose open-chain structures lie significantly higher in energy than the most stable cyclic species. The commonly used M06-2X density functional performs well compared to MP2 and G4 theory at identifying the low-energy fructose minima, including the global one, and at reproducing their intramolecular H-bond geometric parameters. The lowest-energy gas-phase pyranose and furanose structures of fructose benefit from stabilization due to the cooperative or quasi-linear H-bonding and both endo and exo anomeric effects.