Structural factors affecting 13C NMR chemical shifts of cellulose: a computational study

Abstract

The doublet C4 peaks at * 85 and * 89 ppm in solid-state 13 C NMR spectra of native cellulose have been attributed to signals of C4 atoms on the surface (solvent-exposed) and in the interior of microfibrils, designated as sC4 and iC4, respectively. The relative intensity ratios of sC4 and iC4 observed in NMR spectra of cellulose have been used to estimate the degree of crystallinity of cellulose and the number of glucan chains in cellulose microfibrils. However, the molecular structures of cellulose responsible for the specific surface and interior C4 peaks have not been positively confirmed. Using density functional theory (DFT) methods and structures produced from classical molecular dynamics simulations, we investigated how the following four factors affect 13 C NMR chemical shifts in cellulose: conformations of exocyclic groups at C6 (tg, gt and gg), H 2 O molecules H-bonded on the surface of the microfibril, glycosidic bond angles (U, W) and the distances between H4 and HO3 atoms. We focus on changes in the d 13 C4 value because it is the most significant observable for the same C atom within the cellulose structure. DFT results indicate that different conformations of the exocyclic groups at C6 have the greatest influence on d 13 C4 peak separation, while the other three factors have secondary effects that increase the spread of the calculated C4 interior and surface peaks.

Solid state nuclear magnetic resonance studies of hydroxypropylmethylcellulose acetyl succinate polymer, a useful carrier in pharmaceutical solid dispersions

“…Figure 2a shows the 13 C CP MAS spectrum of the pulp with the assignment proposed based on the previous literature, supported by both experimental data [46] and computational study. [47] The F I G U R E 5 13 C cross polarisation spectra of the DuPont AFFINISOL™ hydroxypropylmethylcellulose acetyl succinate series at (a) L grade, (b) M grade, and (c) H grade of acetyl/succinoyl ratio. The simulated spectrum (dashed black) and the spectral deconvolution (grey line) are given for each spectrum.…”

“…The proposed assignments are based on the previous literature. [46,47] The simulated spectrum (dashed black) and the spectral deconvolution (grey line) are also given for the CP spectrum. The red dotted line in (b) marks the zero intensity signal T A B L E 1 Degree of substitution (DS) of acetate (A) and succinate (S) moieties in the various Hydroxypropylmethylcellulose-AS polymers studied here as a function of manufacturers and grade.…”

Cross-polarization dynamics and conformational study of variously sized cellulose crystallites using solid-state 13C NMR

“…Based on a computational study, it has been reported that the conformation of the exocyclic groups at C6 significantly influences the C4 peak separation, and the signal at approximately 89 ppm, corresponding to the crystalline C4 carbon, is dominated by the tg conformation [31]. Figure 4 shows the relationship between the C4-based CI value and the C6-based tg ratio of samples with different crystal sizes.…”

“…This result demonstrates that the change in the C4 crystalline signal is strongly affected by the tg conformation. The slight difference from 1.0 in the slope leads to the interpretation that the glycosidic linkage is partly restricted even when the C6 hydroxy groups sit in the noncrystalline gt conformation [31].…”

Rapid Depolymerization of Decrystallized Cellulose to Soluble Products via Ethanolysis under Mild Conditions

“…The regions between 80 and 92 ppm and 60 and 67 ppm are assigned to C4 and C6 carbons in the glucose unit . The chemical shifts of C6 and C4 represent different torsion angles of the C5−C6 bond, which are good indicators of structural changes . The regions between 87 and 92 ppm and 64 and 67 ppm, attributed to crystalline interior cellulose chains, nearly completely disappear after ball milling, indicative of conversion of interior chains to external ones with ball milling.…”