## 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
The respective \({}^{1}\)H chemical shifts are: 9.5 ppm (H6), 7.2 ppm (H2), 6.5 ppm (H3), 4.6 ppm (H5) ([[https://www.chemicalbook.com/SpectrumEN_67-47-0_11](https://www.chemicalbook.com/SpectrumEN_67-47-0_11) HNMR.htm]([https://www.chemicalbook.com/SpectrumEN_67-47-0_11](https://www.chemicalbook.com/SpectrumEN_67-47-0_11) HNMR.htm), accessed on 10 January 2021). However, in the spectrum in Figure 5a, signals assignable to C5 at 8C/\(\delta\)H 57 ppm/4.6 ppm, or to aldehyde C around 8C/\(\delta\)H 178 ppm/9.5 ppm cannot be identified, suggesting that if present, the contribution of HMF units is below the detection limits. A further important information deducible from this spectrum is based on the occurrence of the strong signal in the \({}^{13}\)C region from 130 to 120 ppm correlating with \({}^{1}\)H resonating between 7 and 8 ppm, which evidences that a considerable part of the aromatic network must be protonated. This would be the case for \({}^{1}\)H in benzofuran, dibenzofuran, diphenyl or naphthalene (Figure 4, structure 3, 11, 13, 14). Thus, the aromatic signal in the respective 1D solid-state \({}^{13}\)C NMR spectrum cannot be solely explained by polycondensed aromatic hydrocarbons (PAHs) (Figure 4, structure 15). The outer protonated C of the latter may contribute to the cross peak at 8C/\(\delta\)H 130 to 110/8.8 to 9.5 ppm [49]. As determined by [49], pyrene results in \({}^{13}\)C NMR signals at 125 ppm, 127 ppm and 131 ppm, its \({}^{1}\)H chemical shifts are between 7.6 to 8.2 ppm. \({}^{13}\)C and \({}^{1}\)H resonances of coronene (Figure 4, structure 15) were reported to occur at 123 ppm, 127 ppm, 125 ppm and 9.2 ppm, respectively [46]. **Phenols and the structure of the aromatic network:** Increasing \(\mathrm{t_{c}}\) to 200 \(\mathrm{\SIUnitSymbolMicro\secondcontrast to former reports suggesting that the aromatic units were connected to the alkyl chains via aromatic ether [50]. Aside from phenol C, the new signals at \(\delta\)C/\(\delta\)H 156, 154, 144 ppm /7.4 ppm which correlate with signals at \(\delta\)C/\(\delta\)H 125-122 ppm, 117 ppm-111 ppm/7.4 ppm can derive from benzofuran or dibenzofuran units (Figure 4, structure 3, 9, 11) as they were suggested to be formed during the pyrolysis of cellulose by [PERSON] and [PERSON] [51]. [PERSON] et al. [45] proposed that during hydrothermal carbonization, HMF units fuse which leads to a loss of the two side groups (Figure 4, structure 7). Substitution of their C2 or C3 shifts their signal to 117 ppm which can be polarized by a neighboring C (\(\delta\)C/\(\delta\)H 110-113 ppm/6.3 ppm). A cross peak can be identified at \(\delta\)C/\(\delta\)H 117 ppm/6.3 ppm, but no clear cross signal appears at \(\delta\)C/\(\delta\)H 117 ppm/6.3 ppm. Further condensation of the polyfuran chain shifts the signal further to 127 ppm [43] (Figure 4, structure 8), attributed to a nonprotonated aromatic C. The latter would receive polarization from H2 or H3 in furan (around 6.3 ppm), but our spectrum allows no unbiased support of such an interpretation. In order to estimate the contribution of intensity of furan structures to the chemical shift region commonly assigned to phenol C, the ratio of the intensities assigned to aromatic C adjunct to \(O\)-substituted aromatic C in the chemical shift regions 115 to 100 ppm (I([15-95])) and the total O-substituted aromatic C 160 to 140 ppm (I([160-140])) was calculated. For uncondensed furans and benzofurans, this value is 1 (Figure 4, structure 3, 5). Crosslinking of furan units decreases the value to 0.5 (Figure 4, structure 7, 8). For dibenzofurans a value \(<\) 0.5 is expected. For uncondensed phenols this ratio increases to 2, whereas condensation can reduce it to 1 (Figure 4, structure 15). For the present sample a value of 0.5 was obtained which corresponds to furan structures with at least one crosslinked C2 furan or benzofuran. This supports the observation, obtained from the HETCOR NMR spectra, that phenol plays a negligible role within the aromatic network of the studied pyrochar.