Calculation of solid-state C-13 NMR spectra of cellulose I alpha, I beta and II using a semi-empirical approach and molecular dynamics

Using the semi-empirical bond polarization theory (BPT) it is possible for the first time to calculate the complete solid-state C-13 NMR spectra of cr ystalline cellulose polymorphs I alpha, I beta, and II and to model the eff ect of molecular motions on the NMR spectra by molecular dynamics simulatio ns. Crystal dynamic simulations at room temperature over a time span of 20 ps were performed with a quantum-mechanical molecular-mechanical force fiel d (QM-MM) using fluctuating atomic charges to account for mutual polarizati ons. Calculations of isotropic C-13 chemical shifts as well as complete che mical shift tensors were performed. Calculated powder spectra for the indiv idual carbon atoms show significant differences between the static and dyna mic model of the cellulose polymorphs due to restricted molecular motions w ithin the crystal. The NMR line splittings of the C1 and C6 sites could cle arly be assigned to the different cellulose chains in the I beta and II pol ymorphs. The results are discussed and compared to experimental cellulose s pectra.