The 13C NMR signals from the aromatic ring carbons in a series of lignin model compounds of the arylglycerol beta-aryl ether type in DMSO solution have been assigned. The model compounds investigated are representative of the erythro and threo forms of differently substituted arylglycerol beta-aryl ethers.

Residual and dissolved lignins from maritime pine pulps obtained by laboratory batch reactor (conventional and modified with surfactant) and flow-through reactor kraft cookings, were studied and compared. The same experimental conditions were used in order to achieve the same degree of delignification. The lignin samples were characterised by elemental analysis, residual carbohydrate content, permanganate oxidation and quantitative 13C NMR analysis. These studies confirmed that the pulping procedure influences the lignin structure, namely the lateral chain degradation, the content of hydroxyl groups and condensed lignin units. It was also shown that the amount of condensed lignin structures was lower in the case of flow-through reactor and batch modified cookings when compared with batch conventional cooking.

Making the hypothesis that the superiority of chlorine dioxide (D) over oxygen (0) delignification was due to the in situ formation of ClOH, some ClOH treatments (h) were performed in combination with oxygen treatments (0) in a multi-step sequence. Extensive delignification was observed after 0(hO)(hO) type sequence where h stands for a hypochlorous acid treatment with low charges of hypochlorous acid. After final bleaching (DED), the viscosity of the 0(hO)(hO) treated pulps was at a good level. Moreover, the O(hO)(hO)DED sequence did not produce more chloroform and AOX than the conventional ODEDED process.

Intramolecular H‐bonding interactions were investigated in solution for the threo and erythro diastereomeric forms of a guaiacyl β‐O‐4 lignin model compound by using the NMR data obtained from hydroxyl protons. Temperature coefficients of the chemical shifts (dδ/dT) and coupling constants (3JHCOH) were measured in aprotic and protic solutions: DMSO‐d6, acetone‐d6 and acetone‐d6–water. The NMR parameters do not support the existence of strong and persistent intramolecular H‐bonds that could participate in the stabilization of the guaiacyl β‐O‐4 structure in solution, but instead indicate that intermolecular H‐bonds to solvent predominate. 1D NOE experiments nevertheless revealed the presence of a direct chemical exchange between the hydroxyl protons, suggesting the possible existence of weak and transient intramolecular H‐bonding interactions. The conformational flexibility of the threo structure was also investigated in acetone solution from the measurement of long‐range 1H, 1H and 1H, 13C coupling constants and from NOESY experiments. The NMR data are not consistent with any single conformation, indicating that different conformers co‐exist in solution. The experimental results support the conformational flexibility predicted by molecular dynamics simulations performed in a previous study. Finally, both experimental and theoretical approaches indicate that weak intramolecular H‐bonds can exist transiently in solution, breaking and reforming as the β‐O‐4 molecule undergoes conformational interconversion, but cannot be invoked as possible means of conferring rigidity to the β‐O‐4 structure. Copyright © 2004 John Wiley & Sons, Ltd.

A softwood kraft pulp was submitted to successive oxygen stages (O, OO, OOO). Residual lignin after treatment was extracted by acid hydrolysis and characterised by C-13 NMR and GPC. The progressive decrease in the efficiency of the O treatment in a multiple O sequence was explained by the poor reactivity of some of the free phenolic lignin structures with oxygen. Oxygen (O) and chlorine dioxide (D) delignification were compared. Contrary to OOO.... the DEDEDE... process led to total delignification. Both chemicals attacked the free phenolic groups in lignin as evidenced by the important decrease in their number measured after treatment. However, their disappearance was more complete in the case of ClO2. Moreover a higher number of COOH groups were formed in the latter case. According to mechanisms given for chlorine dioxide delignification, hypochlorous acid (ClOH) is produced as by-product during a D stage and would react with residual lignin. This would explain the better efficiency of D compared to O stages.

SummaryThe conformational preferences of thethreoanderythrodiastereomeric forms of a guaiacyl β-O-4 dimer have been investigated by molecular modeling using the CHARMM force field. Many low energy conformations have been identified for each diastereomer, showing that β-O-4 dimers can adopt a large variety of shapes. A consistent structural model has emerged that indicates different conformational behavior for thethreoanderythroforms, corresponding to a preferential extended overall shape for thethreoform. All the low energy conformers are stabilized by intramolecular H-bonds. In particular, the highly directional H-bond between the α or γ hydroxyl hydrogen and the aromatic methoxy oxygen governs the B aromatic ring orientation. However, it has appeared that the conformational preferences are predominantly governed by local steric interactions rather than by differences in the H-bonding pattern. From the satisfactory agreement between computed data (geometries and the Boltzmann distribution of the low energy conformers) and the experimental data reported in the literature (X-ray crystal structures and3JHαHβNMR coupling constant), the CHARMM force field has been validated for the study of β-O-4 structures. Clearly, the molecular modeling calculations have led to an improved rationalization of the conformational data collected by experimental techniques.

Results of a comprehensive study on the chemical structure of lignin from plantation Eucalyptus globulus Labill are presented. Lignin has been isolated by a modified mild acidolysis method and thoroughly characterized by functional group analysis, by a series of degradation techniques (nitrobenzene oxidation, permanganate oxidation, thioacidolysis, and Py-GC-MS), and (1)H and (13)C NMR spectroscopy. Plantation Eucalyptus globulus lignin was found to be of the S/G type with an extremely high proportion of syringyl (S) units (82-86%) and a minor proportion of p-hydrophenyl propane (H) units (roughly 2-3 mol %). Unknown C-6 substituted and 4-O-5' type syringyl substructures represent about 65% of lignin "condensed" structures. Eucalypt lignin showed high abundance of beta-O-4 (0.56/C(6)) structures and units linked by alpha-O-4 bonds (0.23/C(6)). The proportion of phenylcoumaran structures was relatively low (0.03/C(6)). Different kinds of beta-beta substructures (pino-/syringaresinol and isotaxiresinol types) in a total amount of 0.13/C(6) were detected. ESI-MS analysis revealed a wide molecular weight distribution of lignin with the center of gravity of mass distribution around 2500 u.

A series of isolated lignin samples from an OQP sequence has been analyzed by quantitive C-13 NMR (nuclear magnetic resonance) spectroscopy. It was found that the residual lignin from unbleached and oxygen-bleached pulps had undergone several structural changes. A corresponding lignin isolated after the subsequent hydrogen peroxide stage, on the other hand, had many of the features of a native lignin, except for a substantial increase in the amount of carboxyl groups. For the corresponding dissolved lignins, the spectra were clearly different from each other and the changes are more dramatic. This implies that, in the various stages, the most degraded lignin is dissolved whereas the lignin remaining in the fibres is much less affected. Oxygen bleaching seems to further oxidize partly degraded lignin structures resulting in dissolution of these fragments. Hydrogen peroxide bleaching, in the other hand, results in an extensive formation of hydrophilic groups, predominantly aliphatic carboxyl groups, together with a limited degradation of the lignin polymer.

Carbon–carbon connectivity spectra of 13C-enriched aspen lignin recorded using the 2D INADEQUATE experiment revealed cross peaks which can be assigned to four types of arylglycerol β-aryl ethers (β-O-4 structures): erythro forms of arylglycerol β-syringyl ethers, threo forms of arylglycerol β-syringyl ethers, erythro forms arylglycerol β-guaiacyl ethers and threo forms of arylglycerol β-guaiacyl ethers. The intensities of the cross peaks suggest larger amounts of β-syringyl ethers than β-guaiacyl ethers. The erythro isomers dominate among the β-syringyl ethers. Erythro and threo forms of β-guaiacyl ethers are present in similar amounts. © 1998 John Wiley & Sons Ltd.

The structure of an acetylated 13C-enriched poplar wood lignin preparation was studied using three-dimensional HMQC−HOHAHA NMR spectroscopy. This method takes advantage of the large dispersion of 13C chemical shifts to resolve individual 1H chemical shifts. The whole spin system of a 1H−13C correlation observed in an HMQC spectrum, even for minor components and unknown structural units, can be traced out. It is shown here that both trans- and cis-isomers of 6,7-dihydrodibenzo(e,g)(1,4)-dioxocin, a recently discovered prominent linkage in softwood lignins, occur also in hardwood lignin. Moreover, the poplar lignin preparation contains small amounts of noncyclic α-aryl ether linkages. Signals from a side chain of unknown structure are tentatively assigned to a spiro-cyclohexadienone. Keywords: NMR; 3D HMQC−HOHAHA; lignin

The reactions between chlorine dioxide and the residual lignin in oxygen-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxidation at different pH values and subsequently analysed by oxidative degradation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins isolated from kraft pulps after each of the bleaching stages in the OD(EOP)DD sequence.

The three-dimensional HMQC-HOHAHA experiment was successfully applied to an acetylated C-13-enriched poplar wood lignin preparation. The resolution of this technique proved to be sufficient to give unambiguous assignments for most of the side-chain structures, which overlap heavily in one- and two-dimensional NMR spectra. By this technique it was possible to show, for the first time, that alpha,beta-diaryl ether structures exist in lignin, although in low abundance. These structures have been considered to be of importance in lignin structure, since they may act as reactive crosslinking groups providing the lignin polymer with a branched network structure.

C-13-NMR spectroscopy has been used to characterize the lignins extracted by an organosolv flow-through process from spruce TMP, bleached TMP (BTMP) and yellowed BTMP (YBTMP). In general, the C-13-NMR spectra of these lignins are similar to that of spruce MWL, indicating that the lignins underwent limited structural changes. Thus they may be an acceptable substrate for lignin structural study. However, these lignins appear to have a significant amount of condensed aromatic carbons assigned mainly to beta-5, 5-5 and diarylmethane structures. These were likely formed during the organosolv cook, or initially present in TMP and extracted with this organosolv process. On the other hand, the hydrogen peroxide bleaching causes little change in the lignin overall structure. As a favorable effect, elimination of the coniferaldehyde structures can clearly be seen. The photoyellowing appears to result in the creation of aromatic carbonyl and carboxyl groups. This suggests that oxidative cleavage of Calpha, Cbeta bonds is one of the important photodegradation reactions of lignin.