Theoretical investigations of 13C chemical shifts in glucose, cellobiose, and native cellulose by quantum chemistry calculations

Abstract

The effects of the conformation and hydrogen bonding on 13C isotropic chemical shifts have theoretically been investigated for β- d-glucose, d-cellobiose, and the cellobiose units of native cellulose by quantum chemistry calculations based on the DFT method. The linear relationship between the chemical shift of the C6 carbon and the torsion angle around the C6 O6 bond in the CH 2OH side group, which was previously obtained in experiments, is successfully reproduced for β- d-glucose by the theoretical calculations. A similar linear relationship is also found to hold for the C4 carbon, supporting the previous finding in experiments. Moreover, the C5 chemical shift also depends on the conformation of the side group, but the conformation of the O6H hydrogen atom at the γ position may mainly contribute to the dependence for the C5 carbon through the possible formation of intramolecular hydrogen bonding. The γ H- gauche effect produced by the OH hydrogen atom ( γ-H) at the γ position is found, for the first time, to induce 3–5 ppm downfield shift for the carbon in question, and this effect reduces by 2–3 ppm when the intramolecular hydrogen bonding associated with γ-H is formed. Similar calculations for d-cellobiose and the cellobiose units in native cellulose reveal appreciable dependences of the C1 and C4 chemical shifts on the torsion angles ϕ and ψ around the (1 → 4)- β-glycosidic linkage. In contrast, no significant effects of different intramolecular and intermolecular hydrogen bondings forming between neighboring glucose residues are recognized on the chemical shifts of the respective carbons associated with these hydrogen bondings.