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

Area of Science:

• Computational Chemistry
• Carbohydrate Chemistry
• Quantum Chemistry

Background:

• Accurate prediction of molecular conformations is essential in chemistry.
• Saccharides exhibit complex conformational landscapes.
• Computational methods are widely used to study saccharide structures.

Purpose of the Study:

• To compute benchmark quality relative conformational energies for gas-phase saccharide conformers.
• To re-evaluate the performance of density functional theory (DFT) methods for saccharide conformational analysis.
• To identify superior DFT functionals for predicting saccharide relative energies.

Main Methods:

• Correlated ab initio wave function calculations using nonrelativistic frozen core MP2 complete basis set extrapolation.
• Estimation of remaining correlation effects via frozen core coupled-cluster singles and doubles [CCSD(T)] calculations.
• Application to test sets of α- and β-d-allopyranose, 3,6-anhydro-4-O-methyl-d-galactitol, and β-d-glucopyranose conformers.

Main Results:

• Conformational energies for the tested saccharide sets varied by approximately 7 kcal/mol.
• B3PW91 and PBE0 density functional methods showed systematically better performance than B3LYP and M05-2X.
• Functionals incorporating exact constraints outperformed empirically fitted functionals for monosaccharide conformer relative energies.

Conclusions:

• The study provides high-quality reference data for saccharide conformational energies.
• DFT methods based on exact constraints are recommended for accurate conformational analysis of saccharides.
• This work aids in selecting appropriate computational tools for carbohydrate research.