NMR characterization of cellulose acetate: Chemical shift assignments, substituent effects, and chemical shift additivity

• A cellulose acetate series was prepared by acid-hydrolysis of cellulose triacetate.
• The chemical shifts of the ring protons and carbons were precisely assigned by NMR.
• The 2-, 3-, or 6-position substituent effects on the chemical shifts were estimated.
• These effects explained the chemical shifts of di- and triacetylated cellulose.

A series of cellulose acetates (CA) with degrees of substitution (DS) ranging from 2.92–0.92 dissolved in dimethylsulfoxide (DMSO)-d6 and cellulose dissolved in tetrabutylammonium fluoride (TBAF)/DMSO-d6 were investigated by two-dimensional NMR spectroscopy. The NMR spectroscopic analysis allowed the determination of the 1H and 13C NMR chemical shifts of the eight anhydroglucose units (AGUs) that contain CA: 2,3,6-tri-, 2,3-di-, 2,6-di-, 3,6-di-, 2-mono-, 3-mono-, 6-mono-, and unacetylated AGUs. A comparative analysis of the chemical shift data revealed the substituent effect of acetyl groups at the 2-, 3-, and 6-positions on the 1H and 13C nuclei in the same AGU. In addition, chemical shift additivity could be applied to the 1H and 13C chemical shifts of CA because the chemical shifts of the diacetylated and triacetylated AGUs could be almost completely explained by the acetyl substituent effects at the 2-, 3-, and 6-positions.