DFT study of 17O, 1H and 13C NMR chemical shifts in two forms of native cellulose, Iα and Iβ

This computational study is intended to shed light on the crystalline and molecular structure, together with the hydrogen bonding (H-bonding) differences between two forms of native cellulose. DFT calculations were carried out to characterize the 17O, 1H and 13C nuclear magnetic resonance (NMR) parameters in cellulose Iα and Iβ with the B3LYP functional employing the 6–311++G∗∗ and 6–31+G∗ basis sets. Geometry optimization revealed that the average HB length is shortened by 0.01–0.08 Å when the chains are aligned, whereas the average bond angle increases by about 4–8° exhibiting the enhancement of HB strength. For the isolated cellotetramer chains, the isotropic 17O–H chemical shifts were plotted as a function of HB length. Our results indicated that as the HB length in cellotetramer Iα increases, the 17O–H chemical shift isotropy increases, but this parameter changes in the opposite direction for the other structure. Moreover, B3LYP/6–311++G∗∗ calculations reveal that there is an acceptable correlation between the calculated 13C chemical shifts of the two structures and their experimental values.

Graphical abstractH-bonded cluster of cellulose I in the crystalline phase.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Influence of HBs interactions on NMR parameters of 17O, 1H, and 13C nuclei. ► Correlation of 17O and 1H chemical shifts with HBs distances. ► 13C NMR parameters can differentiate between Iα and Iβ structures in solid phase.