*3.6. <sup>1</sup>H-13C HSQC NMR Spectroscopy of Lignins*

The HSQC spectrum of the MWL of Paulownia wood (Figure 4a,c) shows some prominent signals that allowed to extract information about the most important features on how phenylpropanoid units are linked together. The most important correlations correspond to:

• The C–H correlations in aromatic methoxyl between δC–δH: 56.1/3.2–56.2/3.7;


Table S2 (Supplementary Material) collects the assignments of the <sup>13</sup>C–1H correlation signals in the HSQC NMR spectrum of the obtained lignin fractions.

HSQC spectrum of PFL (Figure 4b,d) reflects important structural changes after the treatment. The more apparent difference is the presence of a broad signal (around δC–δ<sup>H</sup> 63.5/4.3) which corresponds to the β-O-4′ structures with acylated γ-OH, probably as formates. The formylation of the γ-OH was referred by other researchers [9,28] treating LCM with FA solutions of different concentrations and at different temperatures. The <sup>13</sup>C NMR spectrum reflected this fact (peaks at 163.4 and 162.4 ppm already mentioned), but also the HSQC spectrum of the PFL showed a signal at δC-δ<sup>H</sup> 162.0/8.2 that confirms the formylation of the lignin.

Table 4 presents the quantification of the different types of linkages according to Sun and coworkers (Sun et al., 2014). In the MWL, β-O-4′ units were predominant and accounted for 57.6% of all identified linkages, followed by resinols (21.4%), phenylcoumarans (9.4%), and spirodienones (3.7%). In addition, minor proportions of *p*-hydroxycinnamyl with alcohol (2.2%) and aldehyde (2.3%) end-groups were quantified in the spectrum.

All the correlations corresponding to the different linkages between phenylpropane units were drastically diminished in the spectrum of PFL. The effect of delignification, through the destruction of the linkages identified in MWL, is very important. The quantification of this breakdown of bonds was 87% of the β-aryl ethers, 78% of the resinol type subunits, 65% of the phenylcoumarans and 70% of the spirodienones. The cinnamyl alcohol or aldehyde-type units completely disappeared in PFL.

Moreover, the signals derived from the aromatic nucleus (G2, G5, G<sup>6</sup> and S2,6) were less intense and extended towards smaller chemical shifts in both dimensions, which is indicative of the existence of condensation in the lignin [9,40].

**Figure 4.** HSQC spectra of MWL (**a**,**c**) and PFL (**b**,**d**).

β ′

