Revised solid-state 13C NMR peak assignments for cellulose II and cellulose Iβ from two-dimensional double-quantum dipolar recoupling experiments

Abstract

The crystalline forms of native cellulose (cellulose Iα and cellulose Iβ) and regenerated or mercerized cellulose (cellulose II) have characteristic solid-state ¹³C NMR spectra. An important task is to assign each of the ¹³C signals to the two sets of six signals arising from the two crystallographically inequivalent glucose units present in each of their structures. Such peak assignments have previously been proposed for cellulose Iα, cellulose Iβ, and cellulose II from through-bond ¹³C 2D NMR correlation spectra. We have recently demonstrated that the reported ¹³C chemical shifts for cellulose Iα and Iβ require adjustment in order to be on a chemical shift scale with liquid tetramethylsilane at 0 ppm and have presented a revised peak assignment for cellulose Iα. We had reason to believe that similar revisions were required for cellulose II and cellulose Iβ. For cellulose II, it is shown here that a key correlation involving C2 and C3 carbons has been systematically absent from previously reported correlation spectra and is only observed when a through-space dipolar recoupled experiment is performed. For cellulose Iβ, the ambiguity in assigning the C1 signals from only the C1–C2 correlations in the correlation spectrum is overcome by detecting longer range C1–C3, C1–C4, and C1–C5 correlations with a through-space dipolar recoupling experiment. In the end, a revised set of ¹³C peak assignments for cellulose II and cellulose Iβ is presented whose chemical shifts are correctly referenced to tetramethylsilane at 0 ppm and fully supported by 2D NMR correlation spectra.