###### Abstract
One of the non-degradative techniques that allow a more detailed description of the chemical alteration of organic matter during pyrolysis represents solid-state \({}^{13}\)C nuclear magnetic resonance (NMR) spectroscopy. With this technique, secondary reactions during sample preparation or extraction can be avoided. However, this powerful approach still has a major drawback, which is its low resolution, in particular if analyzing biochars. Most solid-state \({}^{13}\)C NMR spectra of biochars are defined by an intense but broad signal assigned to aromatic C and-in case-a smaller signal attributed to alkyl C. When analyzing the spectra in more detail, it may be possible to distinguish a small shoulder assignable to \(O\)-substituted aromatic C (\(O\)-aryl C) on the low field side of the peak of aromatic C resonance. This shoulder is commonly assigned to phenol C. Although the contribution of this phenol to the total aromaticity represents an important parameter not only for a better understanding of the chemical structure but also for elucidating the reactivity and adsorption capacity of a biochar, it is rarely considered, most probably because a highly condensed aromatic network is generally assumed [17]. On the other hand, modern solid-state NMR spectroscopy offers special pulse sequences which allow an improved assignment of signal intensity to C groups and thus a more detailed description of the nature of the aromatic network in biochars. One of those is the two dimensional (2D) \({}^{1}\)H-\({}^{13}\)C heteronuclear correlation (HETCOR) NMR experiment in which \({}^{13}\)C and \({}^{1}\)H are correlated through their spin-spin interactions [18]. This allows the identification of the connection between specific proton and carbon groups. Originally, this technique was developed for characterizing organic compounds in solution, but eventually was also successfully applied to study solid polymers and coals [19], humic substances [20; 21], kerogen [22], and biochars [23]. Although this technique has already proven its high potential for the analysis of complex and heterogeneous mixtures, the very long measurement times for obtaining meaningful spectra still limits its application to the study of the composite process, in biochar research or soil sciences and related fields. On the other hand, the information obtained from such experiments can provide the base for a better interpretation of data obtained with other routine techniques. However, many investigators in research areas not directly involved in analytical chemistry and NMR spectroscopy, may find the analysis of the complex 2D NMR spectra mysterious and at times daunting. Therefore, aside from reporting of research data, the intention of the present work is to give a short introduction how to interpret solid-state 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR spectra by applying this technique to a model char (thereafter called pyrochar) produced from commercially available cellulose pyrolyzed for 1 h at 350 \({}^{\circ}\)C. Cellulose was chosen since it represents a major player in the composition of the vegetal biomass used for biochar production. It accounts for 40% of the total biomass of wood [24] and 19-33% of grass and wheat straw residues [25].