Accurate Conformational Energy Differences of Carbohydrates: A Complete Basis Set Extrapolation.

Correlated ab initio wave function calculations have been performed, using nonrelativistic frozen core MP2 complete basis set extrapolation model chemistry. The calculations have been made for three test sets of gas-phase saccharide conformations to provide reference values for their relative energies. The remaining correlation effects are estimated from frozen core coupled-cluster singles and doubles [CCSD(T)] calculations. The test sets consist of 15 conformers of α- and β-d-allopyranose, 15 of 3,6-anhydro-4-O-methyl-d-galactitol, and four of β-d-glucopyranose. For each set, conformational energies varied by about 7 kcal/mol. These benchmark quality relative conformational energies are used to re-evaluate the performance of the best density functional methods for conformational analyses of saccharides. Our results show that the B3PW91 and PBE0 relative energies are systematically better than the B3LYP and M05-2X results. Overall, the functionals based on the exact constraints perform better for the relative energies of monosaccharide conformers than the empirically fitted functionals.