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

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 (17)O, (1)H and (13)C 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 0.01-0.08Å 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 (17)O-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 (17)O-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 (13)C chemical shifts of the two structures and their experimental values.