Resolving the discrepancies in reported 13C solid state NMR chemical shifts for native celluloses.

A survey of the literature reporting solid-state ¹³C NMR spectra of native celluloses reveals inconsistencies in the reported ¹³C chemical shifts for cellulose Iα and Iβ allomorphs. With reported chemical shifts varying by up to 2 ppm, it is not clear what the correct chemical shifts actually are. Since reliable experimental data are important to future work, such as quantum chemical calculations of NMR parameters or identification of cellulose phases in complex cellulosic materials, it is important that the correct experimental chemical shifts be established with confidence. Through a process of digitization of previously reported spectra and careful consideration of how chemical shifts were referenced in the past, it has been possible to correct previously reported spectra of cellulose Iα and Iβ, putting them on the same chemical shift scale and establishing a definitive set of correctly referenced ¹³C chemical shifts for cellulose Iα and Iβ allomorphs. In addition, 1D and 2D ¹³C NMR experiments were carried out on a cellulose Iα-rich bacterial cellulose sample (with 25% ¹³C enrichment), providing additional evidence for these ¹³C chemical shifts and a new peak assignment of the ¹³C signals to the glucose units in cellulose Iα. This work resolves many of inconsistencies in the cellulose solid-state NMR literature and provides a definitive set of ¹³C chemical shifts that will be important for future work. [ABSTRACT FROM AUTHOR]