Two-Dimensional Nuclear Magnetic Resonance Spectra of Cellulose and Cellulose Triacetate

Abstract

An attempt was made to give a full assignment, using only NMR information, to all peaks in 1H and 13C NMR spectra for systems of cellulose in 2.5 N NaOD/D2O and of cellulose triacetate (CTA, total degree of substitution 2.92) in deuterated trichloromethane (TCM-d), and in deuterated dimethylsulfoxide (DMSO-d6). For this purpose two-dimensional (2D) homonuclear 1H shift correlation spectroscopy (COSY), heteronuclear 1H–13C shift correlation spectroscopy (C–H COSY), and long-range heteronuclear 1H–13C shift correlation spectroscopy (long-range C–H COSY) were applied to the above systems. By COSY 1H peaks of cellulose/aq. alkali solution were assigned, from lower magnetic field, to H1, H6, H6′, (H3+ H4+ H5), and H2 protons, respectively. Here, two unequivalent H6 peaks heavily overlapped in the one-dimensional spectrum were observed very separately in 2D spectrum. By C–H COSY 13C peaks of cellulose were assigned, from lower magnetic field, to C1, (C3, C4, C5), C2, and C6 carbons, respectively. C3, C4, and C5 carbon peaks were unable to be assigned separately, due to mutual and heavy overlapping of H3, H4, and H5 peaks. For cellulose triacetate, through use of COSY and C–H COSY all peaks in the region of glucopyranose backbone in 1H and 13C NMR wre able to be successfully and completely assigned. Long-range C–H COSY confirmed definitely the existence of the close correlationships between acetyl methyl proton and carbonyl carbon at the different carbon positions of CTA, which showed again the validity of the previous assignment with respect to carbonyl carbons (i.e., C6, C3, and C2 carbons from lower magnetic field).