## 3 Results
### Characterization of Cellulose by 1D and 2D NMR Spectroscopy  
#### 3.2.1 Some Theoretical Background of the Cross-Polarization Experiment  
Useful information about the chemical structure of a compound can already be obtained by one dimensional (1D) solid-state \({}^{13}\)C NMR spectroscopy. However, due to the \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Weight-Loss**} & \multicolumn{2}{c}{**C**} & \multicolumn{2}{c}{**H**} & \multicolumn{2}{c}{**O \({}^{1}\)**} & \multicolumn{2}{c}{**H/C\({}_{\rm atm}\)**} & \multicolumn{2}{c}{**O/C\({}_{\rm atm}\)**} & \multicolumn{2}{c}{**pH**} \\  & **(\%)** & \multicolumn{2}{c}{**(g/kg)**} & \multicolumn{2}{c}{**(g/kg)**} & \multicolumn{2}{c}{**(g/kg)**} & \multicolumn{2}{c}{**H/C\({}_{\rm atm}\)**} & \multicolumn{2}{c}{**O/C\({}_{\rm atm}\)**} & \multicolumn{2}{c}{**pH**} \\ \hline Ce25 & 0 & 419.1 & \(\pm\)1.3 & 59.5 & \(\pm\)0.5 & 521.4 & 1.7 & 0.9 & 7.2 \\ Ce350 & 64 & 701.1 & \(\pm\)3.4 & 42.8 & \(\pm\)1.0 & 256.1 & 0.7 & 0.3 & 5.0 \\ \hline \hline \end{tabular} \({}^{\dagger}\)calculated by difference.  
\end{table} 
Table 1: Elemental composition, atomic H/C (H/C\({}_{\rm atm}\)) and O/C (O/C\({}_{\rm atm}\)), pH of cellulose (Ce25) and its pyrochar (Ce350) produced by pyrolysis at 350 \({}^{\circ}\)C for 1 h.  
low sensitivity of this technique, several single measurements have to be performed (scans) and accumulated. By doing this, the intensity of the signals adds up while the noise is reduced. In order to avoid saturation effects and thus tampering quantification, the waiting time between two scans has to be 3 to 5 times of the spin-lattice relaxation time, T\({}_{1}\). The latter is the time which is needed by the excited spin system for a complete return to the thermal equilibrium after the excitation pulse has terminated. Here, it has to be noted that due to the higher mobility of molecules in solutions than in the solid state, T\({}_{1}\) for solids can be considerably long. This is in particular true for excited \({}^{13}\)C since its low abundance (1.1 atm%) prevents homonuclear interactions which could support efficient T\({}_{1}\). For example, for \({}^{13}\)C in crystalline cellulose a T\({}_{1}\) of 266 s was reported [32]. Such components can occur as part of undegraded plant residues in immature composts and fresh soil organic matter inputs. Therefore, to ensure quantifiable solid-state NMR spectra of such samples via direct excitation, pulse delays of 13 to 22 min must be applied. Considering further that for many soil organic matter or compost samples, approximately 10,000 single scans are needed to achieve a high quality spectrum, a measurement time of at least 92 to 153 days is required. Such unfeasible long measurements times can be avoided if one takes advantage of a \"trick\" which is called cross polarization (CP) [33]. Here, one takes advantage of the highly abundant \({}^{1}\)H spins exhibiting strong homonuclear interactions and thus have comparably short T\({}_{1}\) (T\({}_{1\text{H}}\)) of approximately 2 to 3 s which shortens the total measurement time of cellulose-containing compost or soil samples to 13 h to 24 h.  
For CP, the \({}^{1}\)H spin system has first to be excited, thus magnetized (Figure 1a). Then, during the so-called contact time, t\({}_{\text{c}}\), this magnetization is transferred to the \({}^{13}\)C spin system. After termination of t\({}_{\text{c}}\), the polarized \({}^{13}\)Cs are detected during the acquisition time and their signals can be analyzed in a spectrum. Subsequently, the spin systems can relax, but being the \"engine\" of the polarization transfer a full relaxation before the next scan is only necessary for the \({}^{1}\)H spin system. Since the latter is considerably faster than for \({}^{13}\)C, the total measurement time can be largely reduced. Applying this technique, we must consider some aspects of the CP dynamics if we intend to obtain quantitative spectra.