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

A survey of the literature reporting solid-state C-13 NMR spectra of native celluloses reveals inconsistencies in the reported C-13 chemical shifts for cellulose I alpha and I beta 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 I alpha and I beta, putting them on the same chemical shift scale and establishing a definitive set of correctly referenced C-13 chemical shifts for cellulose I alpha and I beta allomorphs. In addition, 1D and 2D C-13 NMR experiments were carried out on a cellulose I alpha-rich bacterial cellulose sample (with 25% C-13 enrichment), providing additional evidence for these C-13 chemical shifts and a new peak assignment of the C-13 signals to the glucose units in cellulose I alpha. This work resolves many of inconsistencies in the cellulose solid-state NMR literature and provides a definitive set of C-13 chemical shifts that will be important for future work.

Automated summary

A survey of literature on solid-state C-13 NMR spectra of native celluloses shows inconsistencies in reported C-13 chemical shifts for cellulose I alpha and I beta allomorphs. The correct chemical shifts are unclear due to variations of up to 2 ppm. Reliable experimental data is crucial for future work, such as quantum chemical calculations or identification of cellulose phases. By digitizing previous spectra and carefully considering referencing methods, it has been possible to establish a definitive set of correctly referenced C-13 chemical shifts for cellulose I alpha and I beta allomorphs. Additional evidence and new peak assignments were obtained through C-13 NMR experiments on a cellulose I alpha-rich sample. This work resolves inconsistencies and provides essential chemical shift data for future research.