## 3 Results
### Characterization of Cellulose by 1D and 2D NMR Spectroscopy
#### 3.2.2 Solid-State 1D CPMAS \({}^{13}\)C NMR Spectrum of \(\alpha\)-Cellulose  
The spectrum in Figure 2a was obtained from commercially available \(\alpha\)-cellulose using the CP technique. It shows seven clearly distinguishable resonance lines and a small shoulder at 96 ppm. The latter can be assigned to terminal C1. The most pronounced signals at 74.7 ppm and 72.4 ppm are caused by C2, C3, and C5 in cellulose. Intensity derived from C6 contributes to the signal at 65 ppm and 62 ppm. The resonance line at 104 ppm corresponds to the anomeric C1 carbon in a glycosidic bond [36]. The equivalent resonance in hemicellulose would appear at 103 ppm. The signal at 89 ppm is assigned to C4 in crystalline units, whereas C4 in amorphous structures results in a signal at 83 ppm. The crystallinity of cellulose is due to its abundant hydroxyl groups which are involved in intra and intermolecular H-bonds. The ratio between those two signals can be used as a measure for the relative crystallinity of cellulose [37; 38]. A higher ratio indicates a higher crystallinity. For the cellulose analyzed here, the ratio was obtained after integration of the signals in the chemical shift regions 95 to 85 ppm and 85 to 80 ppm. A value of 1 was calculated indicating that approximately half of the cellulose units occur in crystalline structures. Figure 1: Scheme of the cross polarization process (**a**) and the \({}^{13}\)C intensity of O-alkyl C as a function of contact time measured for a soil sample (Cambisol, Ah horizon from the Sierra de Aznalcöllar, Southern Spain) with and without demineralization with hydrofluoric acid (HF) (10% \(v/v\)) (**b**) modified from [34]. 2.3 D Solid-State \({}^{1}\)H-\({}^{13}\)C HETCOR NMR Spectra of \(\alpha\)-Cellulose and Its T\({}_{1\rm H}\)  
Although 2D NMR spectroscopy has many different applications, all pulse sequences are based on the introduction of a preparation time during which the magnetization of a spin system is adjusted into a state appropriate to whatever properties needs to be detected. Then, the spins are allowed to evolve with the given conditions and after their additional manipulation during a mixing period the modulated magnetization is detected. Figure 2: Solid-state 1D \({}^{13}\)C NMR spectrum of \(\alpha\)–cellulose, its intensity distribution, the T\({}_{1\rm H}\) determined for the respective signal and the chemical structure of cellulose (**a**), the respective 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR spectra obtained with a contact time, \(\rm t_{c}=50\)\(\rm\mu s\) (**b**), \(\rm t_{c}=200\)\(\rm\mu s\) (**c**) and \(\rm t_{c}=1000\)\(\rm\mu s\) (**d**). Assembling several 1D spectra with incrementing evolution time (t1) creates a data set which is two-dimensional in time. Fourier transformation of both dimensions leads to a 2D contour plot correlating the interactions detected in the indirect dimension F1 with the signals detected in the direct dimension F2. The HETCOR NMR spectrum detects the \({}^{13}\)C signals which are modulated by \({}^{1}\)H chemical shift information. For its application in solid-state, the preparation and mixing times are followed by the \({}^{1}\)H-\({}^{13}\)C polarization-transfer step during which the encoded \({}^{1}\)H chemical-shift modulation is transferred coherently to \({}^{13}\)C. Then, the respective \({}^{13}\)C intensities are recorded. For the spectrum in Figure 2b, 128 single spectra were acquired with t\({}_{\text{c}}\) = 50 \(\upmu\)s an increment of t1 by 22.9 \(\upmu\)s. They were Fourier-transformed both the indirect \({}^{1}\)H (F1) (relative to t1) and the direct \({}^{13}\)C (F2) dimension.