DFT study of 17O, 1H and 13C NMR chemical shifts in two forms of native cellulose, I α and I β

Abstract

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 I α and I β 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 I α 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.