## 3 Results
### Characterization of Cellulose by 1D and 2D NMR Spectroscopy
#### 3.3.1 General Information Obtained by 1D Solid-State CPMAS \({}^{13}\)C NMR Spectroscopy
For a further evaluation of the importance of furan units within the aromatic network in the cellulose char, the ratio O-aryl C-to-total aryl C (I([160-140])/I([160-95])) may be used. Bearing in mind that phenol C plays a minor role, it was estimated that approximately 30% of the aromatic C are attributable to C\(\alpha\) in furan units. **The role of alkyl C for the chemical structure of the cellulose char:** According to the 1D NMR spectrum in Figure 3, the \(O\)-alkyl and alkyl C (100 to 0 ppm) corresponds to 44% of the total organic C. This divides into 31% for methylene C (45 to 0 ppm) and 13% for O-alkyl C (Table S2). However, for a better evaluation of the role of alkyl C for the properties of the pyrochar, it is important to reveal if this alkyl C represents an integral part of the organic network or occurs as physically and chemically separated residues of only partially degraded cellulose units. A first indication for the former is revealed from the fairly small variation of T\({}_{\rm IH8}\) determined for the different chemical shift ranges. They vary between 270 and 330 ms (Table S2) evidencing efficient \({}^{1}\)H spin diffusion among the \({}^{1}\)H bound to carbonyl, carboxyl, aryl and alkyl C which is only the case if alkyl C is chemically connected the aromatic units. The high uniformity of T\({}_{\rm IH}\) suggests that the thermally altered cellulose residues maintained their polymeric or oligomeric feature, rather than being the product of depolymerized units that randomly re-condensed to structures with random molecular size distribution. Note that our pyrolysis reactor allowed the escape of volatile compounds released during pyrolysis, which prevented their condensation within the reactor during its cooling after the pyrolysis. However, compared to the unheated cellulose, the T\({}_{\rm IH8}\) of the cellulose char are considerably shorter which is in agreement with a loss of molecular rigidity during heating most likely due to loss of stabilizing H-bonds and shortening of the polymer length. A closer look at the respective HETCOR NMR cross plots unveils further interesting information. In line with their efficient polarization by their adjunct protons, alkyl C was already identified in the HETCOR spectrum obtained with t\({}_{\rm c}\) = 50 \(\upmu\)s (Figure 5a). Increasing t\({}_{\rm c}\) to 200 \(\upmu\)s correlates the alkyl \({}^{1}\)H chemical shift region between 2.0 ppm and 5.0 ppm with the aromatic \({}^{13}\)C chemical shift region between 150 and 100 ppm demonstrating that a considerable proportion of alkyl H has strong interaction with aromatic units. Thisconfirms the conclusion obtained from the relaxation data that rather than being physically separated, a considerable portion of the alkyl C forms an integrated part of the aromatized char structure. In line with it, the HETCOR spectrum obtained with t\({}_{\rm c}\) = 1000 \(\upmu\)s (Figure 5c) correlates the aromatic \(\delta\)H region (6.5 to 7.5 ppm) to the alkyl C region (60 to 14 ppm). Note that there are no indications for interactions between outer aryl H in PAHs (>7.5 ppm) and alkyl C, allowing the assumption that only a low amount of higher molecular weight PAHs are connected via alkyl bridging. The cross peaks at \(\delta\)C/\(\delta\)H 137/2.6 ppm or \(\delta\)C/\(\delta\)H 133 ppm/2.1 ppm in Figure 5b, demonstrate that unprotonated aryl C is connected to methyl C (\(\delta\)C: 32 and 15 ppm). Bearing in mind that with the information of Figure 5c, this aryl C cannot be associated to high molecular weight PAHs, since no clear cross signals are seen at \(\delta\)C/\(\delta\)H 45 to 0 ppm/ \(>\) 7.5 ppm. However, since the aryl H signal at 2.1 ppm shows a further correlation with the \({}^{13}\)C signal at 69 ppm, the bridging methyl C must be in close vicinity to \(O\)-alkyl C, possibly because they are connected via ether bond (Figure 4, structure 17). As mentioned above, we have no evidence that the signals in the \(O\)-alkyl C region derive from remaining cellulose derivatives as they are described by [PERSON] et al.