Ab Initio Studies on the Molecular Conformation of Lignin Model Compounds I. Conformational Preferences of the Phenolic Hydroxyl and Methoxy Groups in Guaiacol (original) (raw)
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Ab Initio Studies on the Molecular
Summary. The conformational preferences of the lignin guaiacyl structural unit were studied at the MP2=6-311G(d,p) level of theory using guaiacol (2-methoxyphenol) as model compound. The potential energy surface of guaiacol was investigated by the ab initio method with full geometry optimization by varying the torsion angles of the guaiacol functional groups (hydroxyl and methoxy). An overall of nine stationary points were located, four of which were found to be minima and the other five transition structures between them. The energy minima of guaiacol can adopt one cisoid and three transoid conformations for the hydroxyl and methoxy groups. The transoid structures differ by the orientation of the methoxy group inside and outside of the aromatic plane. The most stable cisoid conformer has an intramolecular hydrogen bond between phenolic hydrogen and methoxy oxygen with a binding energy of 18.09–18.51 kJ=mol as calculated with the second-order (MP2) and fourth-order (MP4SDQ) Møller-Plesset methods and with larger polarized basis sets including diffuse functions. When comparing the geometrical parameters of the global energy structure with relevant experimental data from crystallographic structures good agreement between the data was found. The saddle points, the effect of calculation level on the energy relative stability, the rotational barrier heights, and the relative concentrations of the conformers are also discussed.
Advances in Molecular Relaxation and Interaction Processes, 1979
The semiempirical PCILO method has been used to study stereochemistry of the intramolecular hydrogen bond O-H...0 in 2-methoxyphenol (guaiacol) and 2-methoxy-6-fonmyl phenol (o-vanillin). According to my calculations, the hydrogen bond in guaiacol is weak and has energy of 2.7 kJ/mole and forms a planar five membered ring. The hydrogen bond fonmed by carbonyl oxygen of the formyl gTOUp in o-vanillin is more stable and fonms a nonplanar six membered ring with an angle of rotation of the -CHO group out-of-the-plane of the benzene ring equal to 15O. The energy of this hydrogen bond is 6.07 kJ/mole. +FOT PaTt VII see Tef. L81 l ++Present address:
Reactivity of lignin subunits: the influence of dehydrogenation and formation of dimeric structures
Journal of Molecular Modeling, 2019
Lignin is one of the most abundant natural materials around the world, accounting for about a quarter of the woody tissue. In general, it is well known that these highly branched aromatic bio-polymers are formed from the polymerization of pcoumaryl, coniferyl, and sinapyl alcohols; however, the connection between these structures are still not known in detail. In this work, we have employed electronic structure calculations to investigate local reactivities and details regarding the connectivity between the basic structures of lignin (unmodified mono and dilignols as well as dehydrogenated monolignols). Condensed-to-atoms Fukui indexes, local softness and hard and soft acids and bases principle were employed in the analyses. The results allow identifying reactive sites on the lignin subunits and access details on the synthesis and degradation of this bio-material. In particular, it is possible to identify a strong influence of the dehydrogenation and monomer dimerization on the monolignols reactivities, which activate the O-C4 and C5 positions.
A supramolecular proposal of lignin structure and its relation with the wood properties
Anais Da Academia Brasileira De Ciencias, 2009
In spite of the great importance of cellulose the lignin is considered the second most abundant substance of the wood. However, little attention has been given it, mainly to wood properties. The lignin as well as other structural compounds (cellulose and hemicelluloses), has obviously an important role on the wood properties, probably due its composition and existent bonds. In general lignins have β-O-4 (Alkyl Aril Ether) as majoritary bond. This bond in a continued structure form big molecules with spiral conformation as virtual model. Based on this idea, lignins that have high/low β-O-4 content may have differentiated spiraled structures,suggesting different behaviors on the wood properties,which shows that the lignins (Guaicyl:Syringyl (GS)) of angiosperms, for example, which have higher β-O-4 content would present higher spiral conformation than gymnosperms lignins(HG). On the other hand HG lignins have chance of being more anchored on the matrix compound than GS lignins. In this context, the β-O-4 bonds of lignins possibly affect the wood properties, therefore, it is considered relevant for wood technology science discussion.
Coupling and Reactions of Lignols and New Lignin Monomers: A Density Functional Theory Study
ACS Sustainable Chemistry & Engineering, 2020
This perspective summarizes and compares computational results for the thermodynamics of bond dissociation, coupling, and rearomatization for a number of noncanonical lignin monomer− lignol combinations that have been found to occur experimentally. The noncanonical lignin monomers discussed are tricin, caffeyl alcohol, 5hydroxyconiferyl alcohol, and piceatannol. Among dimeric combinations, the results for bond dissociation are generally similar, but in cases for which trimers have been reported (tricin-lignol adducts), this value can be quite variable, with stereochemical and structural preferences. Among the adducts examined thus far, the energies associated with quinone methide formation and rearomatization are not dissimilar and would not impede subsequent polymerization. These fundamental studies may help to elucidate how lignin monomers are incorporated into the lignin polymer, provide leads for targeted genetic modification, and be of use in deconstruction for the production of commodity chemicals.
Structural Analysis of Lignins from DifferentSources
Five lignin samples were fractionated with Acetone/Water mixtures and the obtained fractions were subjected to extensive structural characterization, including Fourier Transform Infrared (FT-IR), Gel permeation Chromatography (GPC) and Phosphorus-31 NMR spectroscopy ( 31 P-NMR). The results showed that for all studied lignins the solubility increases with the increment of the acetone concentration. Wheat straw lignin has the highest solubility in 90/10 (v/v) Acetone/Water mixture, 400 mg lignin being dissolved in 1 mL mixture. The weight average molecular weight of the obtained fractions increased with the increment of acetone concentration and thus with solubility. 31 P-NMR analysis based on lignin modification by reactive phospholane into phosphitylated compounds was used to differentiate and quantify the different types of OH groups (aromatic, aliphatic, and carboxylic) found in the fractions obtained with 70/30 (v/v) Acetone/Water mixture.
The Journal of Physical Chemistry Letters, 2015
Plant biomass recalcitrance, a major obstacle to achieving sustainable production of second generation biofuels, arises mainly from the amorphous cell-wall matrix containing lignin and hemicellulose assembled into a complex supramolecular network that coats the cellulose fibrils. We employed the statistical-mechanical, 3D reference interaction site model with the Kovalenko−Hirata closure approximation (or 3D-RISM-KH molecular theory of solvation) to reveal the supramolecular interactions in this network and provide molecular-level insight into the effective lignin−lignin and lignin−hemicellulose thermodynamic interactions. We found that such interactions are hydrophobic and entropy-driven, and arise from the expelling of water from the mutual interaction surfaces. The molecular origin of these interactions is carbohydrate−π and π−π stacking forces, whose strengths are dependent on the lignin chemical composition. Methoxy substituents in the phenyl groups of lignin promote substantial entropic stabilization of the ligno-hemicellulosic matrix. Our results provide a detailed molecular view of the fundamental interactions within the secondary plant cell walls that lead to recalcitrance.