Phenolic Dimers Research Articles

Effects of side-chain hydroxyl groups on pyrolytic β-ether cleavage of phenolic lignin model dimer

Effects of side chain hydroxyl groups on pyrolytic β-ether cleavage of phenolic model dimers were studied with various deoxygenated dimers under pyrolysis conditions of N2/400°C/1 min. Although phenolic dimer with hydroxyl groups at the C α − and C γ −positions was much more reactive than the corresponding nonphenolic type, deoxygenation at the C γ -position substantially reduced the reactivity up to the level of the nonphenolic type. These results are discussed with the cleavage mechanism via quinone methide intermediate formation, which is activated through intramolecular hydrogen bonds between C α − and C γ − hydroxyl groups.

Synthesis of dimeric phenol derivatives and determination of in vitro antioxidant and radical scavenging activities

In this study, di(2,6-dimethylphenol) (Di-DMP), di(2,6-diisopropylphenol) (Di-DIP, dipropofol) and di(2,6-di-t-butylphenol) (Di-DTP) were synthesized by the reaction of monomeric phenol derivatives with catalytic CuCl(OH). TMEDA and Na2S2O4. Their antioxidant capacity and radical scavenging activity were examined using different in vitro methodologies such as 1,1-diphenyl-2-picryl-hydrazyl (DPPH·) free radical scavenging, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activity, total antioxidant activity by ferric thiocyanate, total reducing power by potassium ferricyanide reduction method, superoxide anion radical scavenging, hydrogen peroxide scavenging and ferrous ions chelating activities.

Relative distributions of phenol dimers and hydroxy acids in a cultivated soil and above ground maize tissue

The distributions of lignin-derived phenol dimers and cutin/suberin-derived hydroxy carboxylic acids were studied in a single analysis using CuO oxidation of maize leaves and stems as well as the plough layer of a silty soil cultivated with maize. Their relative contributions in soils and plants were compared to that of lignin monomers reported in a previous study. The x,16-dihydroxyhexadecanoic acid isomers and 9,10,18-trihydroxyoctadecanoic acid were the most abundant hydroxy acids in the soil. An x-hydroxy hexadecane-1,16-dioic acid was relatively enriched in soil compared to maize leaves. Hydroxy acids were relatively enriched in soils compared to lignin-derived phenols. Lignin dimers were relatively enriched in soil with regard to lignin monomers. Lignin dimers with 5,5 aryl–aryl and α,2-methyl linkages were the most concentrated dimers in soil compared to the plant tissue.

Application of genetic algorithms in automated assignments of high-resolution spectra

This paper describes an automated assignment and fitting procedure for high-resolution rotationally resolved spectra. The method is based on the application of genetic algorithms (GA) and both frequency and intensity information of these spectra is used. The basic ideas behind the GA technique is introduced and the particular fitness function critical for the success of the GA evaluation is discussed. The meta-optimization of the internal GA parameters for an optimal exploration of the error landscape of the spectrum fitting is investigated. A number of typical examples are given in which the use of automated spectrum assignments with the GA method is of crucial importance in the analysis. Examples are given for fits of very dense spectra due to overlap of a number of rovibronic spectra in conformers and isotopomers and strongly congested spectra in dimer systems. It is also shown that since the GA performs an overall fit of the complete spectrum very good information on the orientation of the transition dipole moments is gathered. Contents PAGE 1. Introduction 354 2. The genetic algorithm 357 2.1. Introduction 357 2.2. The fitness function 358 2.2.1. Definition 358 2.2.2. Numerical evaluation of the fitness function 359 3. Optimization of the GA fits 360 3.1. Effect of the width (Δ w) of the weight function w(r) on the convergence of the GA 360 3.2. Meta-optimization of the internal GA parameters 361 3.3. Uncertainties in and correlations between the parameters from a GA fit 362 4. Methods 363 4.1. Experimental 363 4.2. The calculated spectra 364 4.2.1. The asymmetric rotor Hamiltonian 364 4.2.2. Internal rotation Hamiltonian 365 4.2.3. Intensities 366 4.3. The structure determination 367 4.4. The software used 368 5. Examples of the success of GA automatic assignments 368 5.1. GA fits of very dense rovibronic spectra 368 5.1.1. [7-D]phenol-N2 368 5.1.2. Benzonitrile-Ar 369 5.1.3. 4-methylphenol and its water complex 371 5.2. Simultaneous GA fits of a number of overlapping rovibronic spectra 373 5.2.1. 7-azaindole (Pyrrolo[2,3-b]pyridine) 374 5.2.2. Resorcinol (1,3-dihydroxybenzene) 375 5.2.3. Conformers and isotopomers of tryptamine 377 5.3. Strongly congested spectra in dimer systems 379 5.3.1. Benzoic acid dimer revisited 380 5.3.2. Benzonitrile dimer 385 5.3.3. Phenol dimer 387 5.4. Orientation of the transition dipole moment 389 5.5. GA fit of rotational contours 391 5.5.1. The FTIR spectrum of benzotriazole 391 5.5.2. The low-resolution LIF spectrum of azaindole-water 393 6. Summary 394 Acknowledgements 395 Appendix A 395 Appendix B 396 References 400 Supporting online material 403

Evaluation of CuO oxidation parameters for determining the source and stage of lignin degradation in soil

The composition of phenols and other aromatic compounds in organic and mineral soil horizons and their respective source vegetation from different climatic zones of the Canadian Prairies were analyzed using CuO oxidation and gas chromatography-mass spectrometry (GC-MS) to investigate the stage of lignin degradation. Parameters based on the CuO oxidation products were calculated for the soils and corresponding vegetation to determine the lignin sources and to monitor the lignin degradation. In addition to the widely used lignin monomer parameters, parameters resulting from lignin-derived phenolic dimers are used for the first time to assess lignin degradation in soils. The composition of lignin-derived phenols (S/V, C/V) in soil closely matches the composition observed in their respective source plants (grass, Aspen, Pine) reflecting the preservation of characteristic lignin patterns in soils. Degradation parameters based on lignin phenols and benzenes derived from tannins or other phenolic biomolecules indicate a progressive degradation from the vegetation to the soil horizons. In addition to commonly used lignin monomer indicators, parameters based on the lignin dimers are applied. Lignin degradation is found to be lowest in the Pine forest, intermediate in the grassland soils and highest in the Aspen-grassland transition soil. Degradation parameters based on non-lignin aromatic derivatives (3,5-dihydroxybenzoic acid, benzenepolycarboxylic acids) demonstrate a similar trend. The lignin from samples in the cooler climate (Black Chernozems) is observed to be more oxidized than in the soils from the warmer climate (Brown Chernozems) suggesting that abiotic processes may be in involved in the alteration of lignin and other phenolic biomolecules in soils. The results indicate that the comparative analysis of CuO oxidation products of soils and source vegetation is a valuable tool to assess the sources and degradation of lignin in soils.

Determining the Intermolecular Structure in the S0 and S1 States of the Phenol Dimer by Rotationally Resolved Electronic Spectroscopy

The rotationally resolved UV spectra of the electronic origins of five isotopomers of the phenol dimer have been measured. The complex spectra are analyzed using a fitting strategy based on a genetic algorithm. The intermolecular geometry parameters have been determined from the inertial parameters for both electronic states and compared to the results of ab initio calculations. In the electronic ground state, a larger hydrogen-bond length than in the ab initio calculations is found together with a smaller tilt angle of the aromatic rings, which shows a more pronounced dispersion interaction. In the electronically excited state, the hydrogen-bond length decreases, as has been found for other hydrogen-bonded clusters of phenol, and the two aromatic rings are tilted less toward each other.

Syntheses of Novel Hexahomotrithiacalix[3]arenes

The first synthesis of the C 3 -symmetrical p-tert-butylhexahomotrithiacalix[3]arene 7d via one-pot procedure by the reaction of 2,6-bis(chloromethyl)-p-tert-butylphenol 8d with sodium sulfide nanohydrate under high-dilution conditions is reported. Also, hexahomotrithiacalix[3]arenes 7d-f (where d-f designate the type of the substituents R at the p-positions of the phenolic rings: d, R=t-Bu; e, R=CH3; f, R=Cl) were synthesized via a convergent approach in good yield by the [2+1] cyclization reaction of 2,6-bis(chloromethyl)phenol monomers 8d-f and bis(chloromethyl)phenol dimers 15d-f in presence of sodium sulfide nanohydrate under high-dilution conditions. The structures of 7d-f were determined by 1H, 13C NMR, MS and elemental analysis. A preliminary study of the binding properties of 7d with alkali- and heavy metal cations using biphasic picrate extraction method showed only weak abilities to bind the cations examined.

Resonance enhanced multiphoton ionization and time-of-flight mass spectra of jet-cooled 3-chlorophenol dimer

The resonance enhanced multiphoton ionization (REMPI) excitation spectrum of jet-cooled 3-chlorophenol (3-ClP) was measured in combination with time-of-flight (TOF) mass spectrometry. In the TOF mass spectrum, 3-ClP, 3-ClP dimer and 3-ClP-water cluster cations were observed. The REMPI excitation spectrum of the dimer was observed for the first time. The low-frequency intermolecular vibrations show the hydrogen bonding of the dimer of 3-ClP to be similar to the phenol dimer. The hydrogen bonding was found to be weaker than that of the phenol dimer. The structures of the dimer and the 3-ClP-water cluster were obtained from ab initio calculations, which suggest that dimer has a sandwich-type structure with weak hydrogen bond.

Competition between cation-π interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. I. Phenol dimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)2 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. The spectra showed the presence of hydrogen bonding in the colder M+(phenol)2Ar cluster ions but the absence of hydrogen bonding in the warmer M+(phenol)2 species. For the cold species, the IR spectra were compared with minimum-energy ab initio calculations to elucidate the hydrogen-bonded structures. In the dominant hydrogen-bonded configurations observed experimentally, the phenol molecules form hydrogen-bonded dimers and the alkali-metal ions bind to the phenol via a cation-pi interaction with the aromatic ring. Increasing the strength of the cation-pi interaction by decreasing the ion size forces the distance between the phenol O-H groups to increase, thus weakening the intermolecular hydrogen bond. Free-energy differences of different configurations relative to the ground state demonstrate that hydrogen-bonded structures are enthalpically favored, while non-hydrogen-bonded structures are entropically favored and are thus observed in the warm cluster ions.

Structural elucidation of new ferulic acid-containing phenolic dimers and trimers isolated from maize bran

Four new phenolic dimers and trimers that contain ferulic acid moieties were isolated from the alkaline hydrolyzate of insoluble maize bran fiber and their structures were established by 1D/2D NMR and mass spectrometry. The biological role of one dimer remains unclear whereas the dimeric vanillin-ferulic acid-cross-product probably represents an oxidative degradation product from the corresponding diferulate. Both ferulic acid dehydrotrimers are able to cross-link polysaccharide chains. However, the 5-5/8-O-4(H 2O)-triferulic acid may be a cross-link in its identified structure whereas we assume that the identified 8-O-4/8-5(non-cyclic)-triferulic acid arose from a natural 8-O-4/8-5(cyclic)-triferulate analog during the saponification process.

Dissolved Lignin and Other Aromatic Substances in Thermomechanical Pulp Waters

Dissolved aromatic substances in water suspensions of unbleached and peroxide‐bleached spruce thermomechanical pulp (TMP) were isolated by sorption on XAD‐8 resin and fractionated into lignin and oligomeric aromatic substances (OAS). The fractions were characterized by chromatographic (GC and HP‐SEC) and spectrometric (FT‐IR, 13C‐NMR, GC‐MS) methods, as well as by GC after direct on‐line pyrolysis and thermally assisted hydrolysis and methylation. The dissolved lignin in unbleached TMP water was structurally similar to spruce milled‐wood lignin (MWL), but its average molar‐mass was lower and the mass distribution more narrow. The oligomeric aromatic substances included phenolic dimers, trimers, and tetramers that were structurally different from those of MWL. After peroxide bleaching the amount of dissolved semipolar (MTBE) extractives and oligomeric aromatic substances was much lower. The amount of dissolved lignin from wood fibers was, on the contrary, much higher. The lignin released from TMP after bleaching was extensively oxidized and had a slightly higher average molar‐mass than lignin released from unbleached TMP.

Novel Phenolic Glycoside Dimer and Trimer from the Whole Herb of Pyrola rotundifolia

From the water-soluble constituents of the whole herb of Pyrola rotundifolia (Pyrolaceae), one novel phenolic glycoside dimer, pyrolaside A (1), and one novel phenolic glycoside trimer, pyrolaside B (2), together with two known phenolic glycosides homoarbutin (3) and isohomoarbutin (4), were isolated. The structures were elucidated by spectroscopic analysis and confirmed with chemical degradation. In vitro tests for antimicrobial activity showed pyrolaside B (2) to possess significant activity against two Gram-positive organisms, Staphylococcus aureus and Micrococcus luteus.

Antioxidant Activity of Propofol and Related Monomeric and Dimeric Compounds

This study was carried out to investigate the antioxidant activity of propofol (2,6-diisopropylphenol) and its related compounds, butylated hydroxyanisole (BHA), 2,6-dimethylphenol, 2,6-di-t-butylphenol, and their dimeric compounds. The degree of antioxidant activity was evaluated based on the degree of peroxidation induced with Fe-ascorbic acid in egg phosphatidylcholine through the determination of thiobarbituric acid-reactive substances (TBARS) formed during peroxidation. Their antioxidant activities were in the order of dipropofol>di(2,6-di-t-butylphenol)>diBHA>di(2,6-dimethylphenol). Dipropofol, a dimeric compound of propofol, showed the highest antioxidant activities. Dimeric compounds had higher activities than monomeric compounds, and the 1,1-diphenyl-p-picryhydrazyl-trapping ability of dimeric compounds was also greater than those of monomeric compounds (4-10-fold). These results suggest that dimeric phenols may increase their antioxidant activities along with increments in the conjugation system and play a inhibitory role in the propagation of free radical chain reactions.

Antibacterial Activity of Dipropofol and Related Compounds

Phenolic compounds, in general, exhibit antioxidant and antibacterial activities. We studied antimicrobial activity of the phenolic antioxidants, propofol (2,6-diisopropylphenol), tocopherol, eugenol, butylated hydroxyanisole (BHA), and several of their dimer compounds. Dipropofol (dimer of 2,6-diisopropylphenol) showed strong antibacterial activity against gram-positive strains including methicillin resistant Staphylococcus aureus (MRSA) and vancomycin resistant Enterococci (VRE), while propofol and other monomeric and dimeric phenols having methyl or tert-butyl groups showed no remarkable activity. The results indicated that the dimeric structure of 2,6-diisopropylphenol moiety may play an important role in the antibacterial activity.

Structure and Morphology of Polyamide 66 and Oligomeric Phenolic Resin Blends: Molecular Modeling and Experimental Investigations

Interaction between polyamide 66 (PA66) and phenol formaldehyde resin (PFR) was studied by molecular modeling for oligomeric model compounds and by experimental methods. Complexation between N-methylacetamide (NMA) and phenol was analyzed by calculating the equilibrium structure, hydrogen bond length, and interaction energy. Additionally, electrostatic potential (ESP) derived atomic charges were computed. Quantum mechanical calculations combined with an experimental Fourier transform infrared spectroscopy (FTIR) study revealed that there exists a strong hydrogen bonding between the carbonyl group of NMA and the hydroxyl group of phenol. Furthermore, the interaction energy of the NMA−phenol complex was found to exceed that of the NMA and phenol dimers and also their complexes with water molecules, which provides an explanation for the decreased water absorption of polyamides upon blending with PFR. FTIR and differential scanning calorimetry (DSC) studies of PA66-PFR blends demonstrated, in addition, miscib...

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters.

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong "sigma-type H-bond" such as phenol-(dimethyl ether)(n=1) and phenol dimer, and the second is phenol-(ethylene)(n=1) having a weak "pi-type H-bond." It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.

Phenolic Resins (II) – Influence of the Chemical Structure of High Molecular Weight Molecules on the Mechanisms of Cross-linking and on the Final Structure of the Resins

The physicochemical characterization of the structures of the oligomers (n < 4) in resols has been carried out by fragmenting monomers in LC/UV/MS and LC/UV/MS/MS. Fragmentation mechanisms are related to the numbers and positions of substituents on the aromatic ring and to the types of oligomer junctions. It was more difficult to determine the structures of phenol hemiacetals and dimer hemiacetals because of the large number of position isomers. The resols were prepared with differing molar ratios R = Formaldehyde/Phenol and catalysts. They were cross-linked using two industrially recommended heat cycles. The progression of resin cross-linking was determined by solid state 13C NMR (CP/MAS). The residual percentage of monomers and oligomers at n < 4 was determined in leachates (water and methanol) and characterized by LC/UV/MS. The results for cross-linking advancement were correlated with the various synthesis parameters (ratio R, type of catalyst and heat cycle).

Investigation of Potential Taste Reduction of Catechin and Grape Seed Dimeric Phenols in Water by Wine Proteins

Soluble glycoproteins were isolated from Semillon wines made over a 5-y period, from 1990 to 1994, by precipitation with ammonium sulfate and fractionated by chromatography on Sephadex G-100. The total phenols were extracted with 50% aqueous ethanol from Semillon seeds, and the four seed phenol fractions (catechins, dimeric phenols, and oligomeric and polymeric tannins) were fractionated by diatomaceous earth partition chromatography of the total phenols. The proteins, catechin, and dimeric fraction alone appeared unchanged during storage in a wine-like model solution (aqueous 10% ethanol solution of pH 3.0) at 20 °C from 3 to 48 ch. However, the oligomeric and polymeric tannins, as well as the total phenols, gave rise to appreciable turbidity. The possible sensory effects of added proteins were investigated using paired comparisons between pure compounds (catechins and dimers) and the same compounds mixed with proteins. The hypothesis was that the proteins would decrease the taste of the catechins and dimers by decreasing their availability to interact with the taste receptors. Results corroborated the hypothesis when lower concentration of phenols and proteins were used. However, at higher concentrations, the reverse trend was observed. It is hypothesized that higher protein concentrations increased the total taste intensity by adding a taste of their own.

High-resolution pump–probe rotational coherence spectroscopy – rotational constants and structure of ground and electronically excited states of large molecular systems

The application of high-resolution pump–probe rotational coherence spectroscopy (RCS) provides precise rotational constants of large molecules and clusters for both the ground and the electronically excited states. A variety of experimental implementations are now available and described in the text. We give examples for these implementations and emphasize recent developments in this field. In particular, state-selective, mass-selective and four-wave mixing schemes are discussed for the molecules p-cyclohexylaniline ( pCHA), cyclohexylbenzene (CHB), benzene, p-difluorobenzene ( pDFB) and the clusters phenol dimer and pDFB–Ar. New results for CHB, benzene and pDFB by pump–probe photoionization and degenerate four-wave mixing are reported. The combination of different experimental methods and the comparison with theory based on ab initio calculations leads to a detailed description of the corresponding molecular structure. Finally, an outlook for future directions of RCS is presented.

Structure and binding energy of the phenol dimer: correlated ab initio calculations compared with results from rotational coherence spectroscopy

Correlated ab initio calculations are compared with experimental data from time-resolved rotational spectroscopy for the rotational constants of the phenol dimer. This system poses a challenge to theory since both hydrogen bonding and dispersion interaction are determining its unique geometry. Excellent agreement is obtained at the RI-MP2/TZVPP level of theory, demonstrating that higher order polarization functions are necessary for an accurate structural description. The computed intermolecular frequencies, stabilization energy and enthalpy are also reported.