1930 Calculation of solid-state 13 C NMR spectra of cellulose I a,I b and II using a semi-empirical approach and molecular dynamics Frank-Thomas Koch, Wolfram Prieß, Raiker Witter, Ulrich Sternberg* Friedrich-Schiller-Universita ¨t Jena, Institut fu ¨r Optik und Quantenelektronik, Max-Wien-Platz 1, D-07743 Jena, Germany Introduction Natural as well as regenerated cellulose consists of long chains of (1-4)-b linked D-glucose residues and forms fibrous structures with crystalline and non-crystalline areas. Cellulose exhibits considerable polymorphism. The major polymorphs are native cellulose I and the regener- ated polymorph form II. Among these ordered forms the polymorph II is most intensely investigated by diffraction methods and several crystal structures have been pro- posed. According to recent studies cellulose II consists of two antiparallel chains of b-D-glucopyranose and con- tains two monomer units per unit cell. [1] In the case of native cellulose the discussion about the atomic structure of this polymorph was revived in 1984, when Atalla and VanderHart proposed two phases for the native cellulose based on results obtained by solid-state 13 C CP-MAS NMR experiments. [2–4] Consequently it is now estab- lished, that the polymorph I can be subdivided in the phase most abundant in lower plants and bacteria (the I a phase) and in the I b phase abundant in higher plants. [2–7] The structure of these two phases was resolved by Sugiyama et al. using electron diffraction. [6] Additionally some attempts have been made to model crystalline cellulose. In most cases glucose, cellobiose or longer chains have been used as a model in vacuum and in solution. [8–11] Kroon Batenburg et al. [12] and A. P. Heiner et al. [13] have simulated the I a and I b phases in a crystalline environment using molecular dynamics for the first time. Solid-state 13 C NMR studies of cellulose II as well as the I a and I b phases were performed by several authors and show different line forms and line splittings in the different polymorphs. The origin of the observed split- tings and the relatively large line width were first dis- cussed by VanderHart and Atalla [3] but most features are open for discussion. It would be of great interest to get a definite assignment of the observed peaks in 13 C CP- MAS NMR experiments to the various carbon sites in the structure model of the cellulose polymorphs. Full Paper: Using the semi-empirical bond polarization theory (BPT) it is possible for the first time to calculate the complete solid-state 13 C NMR spectra of crystalline cellulose polymorphs I a,I b, and II and to model the effect of molecular motions on the NMR spectra by mole- cular dynamics simulations. Crystal dynamic simulations at room temperature over a time span of 20 ps were per- formed with a quantum-mechanical molecular-mechanical force field (QM-MM) using fluctuating atomic charges to account for mutual polarizations. Calculations of isotropic 13 C chemical shifts as well as complete chemical shift ten- sors were performed. Calculated powder spectra for the individual carbon atoms show significant differences between the static and dynamic model of the cellulose polymorphs due to restricted molecular motions within the crystal. The NMR line splittings of the C1 and C6 sites could clearly be assigned to the different cellulose chains in the I b and II polymorphs. The results are dis- cussed and compared to experimental cellulose spectra. Macromol. Chem. Phys. 2000, 201, No. 15 i WILEY-VCH VerlagGmbH, D-69451 Weinheim 2000 1022-1352/2000/1510–1930$17.50+.50/0 Structure of cellulose II according to two chains model and the orientations of the 13 C chemical shift tensors. The 33 tensor components of carbons bond to one oxygen are marked with a (*) and the 11 tensor components of carbons bond to two oxy- gens are marked with (0). Macromol. Chem. Phys. 2000, 201, 1930–1939