Phenol-formaldehyde resol resins studied by 13C-NMR spectroscopy,gel permeation chromatography,and differential scanning calorimetry

A series of phenol-formaldehyde resins was produced in the presence of NaOH catalyst. Detailed structural and quantitative information was provided by ¹³C-NMR spectroscopy. The main interests were the relative quantities of bridge structures, methylol groups, and free phenol. Functionality and linearity of resins were also studied. The effects of the condensation F/P molar ratio on the structure and properties of the resins was studied by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) in addition to ¹³C-NMR. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1183–1193, 1997     

Optimizing the conditions of quantitative 13C-NMR spectroscopy analysis for phenol–formaldehyde resol resins

The experimental time of ¹³C-NMR quantitative analysis of phenol–formaldehyde resins was reduced so that quantitativeness was maintained. The quantitative spectra of 14 model resins were obtained using a gated decoupling technique suppressing the NOE. The paramagnetic additive, Cr(acac)₃, was used to shorten relaxation times of carbon atoms. The use of Cr(acac)₃ was optimized in two deuterated solvents, DMSO and acetone. To reach short relaxation times and further the measurement times, the concentration of relaxation reagent, the delay time, and the number of NMR scans were optimized. Quantitativeness was proved by analyzing the spectra of accurate mixture of model compounds, and the spectra of the condensed model resins. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1805–1812, 1998     

13C NMR spectroscopic characterization of trimethylol propane ester lubricants

The ability to identify the different acyl groups present in lubricants composed of mixed acid esters of trimethylol propane by¹³C nuclear magnetic resonance spectroscopy is demonstrated. The technique discriminates between esters with acyl groups containing between 5 and 10 carbon atoms and can be used similarly to identify acyl groups in pentaerythritol or neopentyl glycol ester lubricants.     

Usefulness of the carbon-13 tracer technique for characterizing terrestrial carbon pools

Application of the carbon-13 (¹³C) tracer technique to soil organic carbon emission is relatively new to many researchers and only a few results have been reported to date. This mini-review paper cites some well-documented research in organic carbon pool studies using the ¹³C tracer technique. The ¹³C abundance in soil usually remains at a stable level under a set of natural conditions. Variations in ¹³C reflect different sources and types of organic components from natural environments. An important feature of ¹³C discrimination in the soil C pool has permitted researchers to assess the dynamic nature of the pool. This discrimination may reflect a selective preference at early stage of residue decomposition by soil microbes. Crop rotation and residue input to humic substances can change the ¹³C abundance, which is a possible way to estimate soil C emission. However, the dynamic relationship between ¹³C abundance in the soil C pool and C emission is still in an early stage of development. Restrictions due to requirement of long-term experiments and duration of vegetation changes may affect its wide-spread adoptions in C emission studies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Online monitoring of synthesis and curing of phenol-formaldehyde resol resins by Raman spectroscopy

The synthesis and curing of phenol-formaldehyde resol resins were monitored online by Raman spectroscopy. The synthesis of the resins (F/P 2.0, alkalinity 4.5 wt%) was studied at rising temperature (40-90 °C) for 90 min and at constant temperatures (80 °C, 90 °C, 100 °C, and 110 °C) for 120 min. The progress of the curing was investigated isothermally (80 °C, 90 °C, 100 °C, and 110 °C) for 120 min for three resins with different degrees of condensation. The synthesis and curing of the resins were started in the reactor and the advancement of the methylolation and condensation reactions was followed through the window of the reactor in the wave number region of 2000-400 cm⁻¹ with use of a fiber optic probe for the data collection. The Raman spectra of six model compounds (formaldehyde, phenol, 2-hydroxybenzyl alcohol, 4-hydroxybenzyl alcohol, 2-benzylphenol, and 4-benzylphenol) were analyzed to facilitate the interpretation of the spectra of the resins. The consumptions of free phenol and free formaldehyde, as well as the progress of the methylolation and condensation reactions were easily monitored by following the changes in intensity of the characteristic Raman bands. The results for the cured resins obtained by Raman spectroscopy were in good agreement with the structures and residual reactivities studied by CP/MAS ¹³C NMR spectroscopy and differential scanning calorimetry (DSC), respectively. The results of the study show Raman spectroscopy to be a promising tool for the online monitoring and control of phenol-formaldehyde resol resin synthesis and curing; in addition, Raman spectroscopy offers an effective and fast method for structural study of the solid state resins.     

Improved quantitative solution state C NMR analysis of ethylene-1-octene copolymers

In order to acquire fully quantitative, high signal-to-noise ¹³C NMR spectra of ethylene-1-octene copolymers in a relatively short period of time, a detailed protocol has been developed, based on the addition of an optimised amount of the relaxation agent chromium(III)triacetylacetonate and without using nuclear overhauser enhancement (NOE). Compared to classical measurements with NOE, the proposed measuring protocol additionally offers a gain in signal-to-noise of 16%, which corresponds to a gain in experimental time of 34%. It allows precise and accurate quantification of all resonances, including small one's related to, e.g. comonomer inversion, and offers more refined information towards the determination of chemical shift and peak area measurements and the development of statistical chain microstructure models.     

C NMR study on structure, composition and curing behavior of phenol-urea-formaldehyde resole resins

Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state ¹³C NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network.     

Structural characterization of maleic anhydride grafted polyethylene by C NMR spectroscopy

Maleic anhydride grafted polyethylene, [2,3-¹³C₂] MA-g-PE, which was synthesized with ¹³C labeled maleic anhydride [2,3-¹³C₂] MA in solution, was characterized by ¹³C NMR spectroscopy in order to make clear the structure of graft groups. The results reveal that [2,3-¹³C₂] MA-g-PE has succinic anhydride oligomeric grafts with a terminal unsaturated MA ring in addition to well-known saturated succinic anhydride oligomeric grafts and that the former grafts are longer but fewer than the latter.     

On the Quantification of Lignin Hydroxyl Groups With 31P and 13C NMR Spectroscopy

Factors affecting the accuracy of the analysis of lignin hydroxyl and carboxyl groups with ³¹P NMR have been further elucidated. Two modifications of ³¹P NMR analysis of lignin, namely the protocols using 1,3,2-dioxaphospholane (PR-I) and 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (PR-II) as phosphorylation reagents with different internal standards, were studied. The previous ³¹P NMR standard protocol with PR-II underestimated OH groups by about 30%, whereas the ³¹P NMR standard protocol with PR-I tended to produce overestimated data. It has been shown that cholesterol is not an appropriate internal standard, resulting in underestimated values for OH groups due to incomplete baseline resolution. The best internal standard has been found to be endo-N-hydroxy-5-norbornene-2,3-dicarboximide. Strong care should be taken related to the stability of the internal standards to avoid inflated results due to IS degradation. Under modified optimized conditions, both methods show a good correlation with the ¹³C NMR protocol in the quantification of hydroxyl groups as average, with the variability between the methods in the range of 5–15%. However, the ³¹P NMR protocols report COOH content that is twice as low as that of ¹³C NMR data. Finally, the best approach for the use of the ³¹P and ¹³C NMR methods in lignin analysis is discussed.     

Surface complexes in zeolite-catalysed acylation reactions detected by13C MAS NMR spectroscopy

The state of the acylating agent acetyl chloride, adsorbed on a series of proton and metal ion exchanged zeolites X, Y (faujasite) and ZSM-5, was investigated by¹³C MAS NMR spectroscopy. The observed carbonyl signals were assigned to two species: chemisorbed acetyl chloride bound to lattice oxygen (signals near 182 ppm), and acetyl chloride complexed with counter cations in the lattice (signals near 172 ppm). In a few cases signals were observed which have been assigned to free acylium cation stabilized on the surface of the solid (signal 160–165 ppm). Experiments in which toluene was adsorbed on to ZnY pretreated with acetyl chloride showed the participation of various adsorbed species in the acylation reaction of toluene.     

Molecular model of Estonian kukersite kerogen evaluated by C MAS NMR spectra

The chemical structure of Estonian kukersite kerogen is evaluated using a simulation of ¹³C MAS NMR spectrum. A reasonable fit to the experimental NMR spectrum is obtained by assuming a model of the geomacromolecule with empirical formula C₄₂₁H₆₃₈O₄₄S₄NCl and a set of structural elements comprising mainly alkylated phenolic structures particularly alkyl-1,3-benzenediols and condensed alicyclic rings. From the presented model new views are coming up on the carbon skeleton of kerogen and constraints on the phenol formation pathways in the retorting process, i.e. up to 80% of methylene groups in kerogen are located in aliphatic chains and the complicated mixture of phenols in the retort oil seems to result mainly from the thermal conversion of alkyl-1,3-benzenediol units originally present in kerogen.     

Semiquantitative analysis of thiophenic compounds in light cycle oil (LCO) using C NMR spectroscopy

S-methyltetrafluoroborate salts of the thiophenic compounds (CH₃-S⁺:BF₄⁻) present in LCO petroleum fractions were obtained and analyzed by ¹H and ¹³C NMR spectroscopy. The methylation of the samples was carried out using 99.5% ¹³C enriched methyl iodine, to improve the sensitivity of the technique. The amount of the methylated derivatives was determined by the internal standard method; using dioxane as a reference, 37 sulphur compounds were detected. Among them, benzo[b]thiophene, dibenzo[b,d]thiophene, and several isomers of methyl, dimethyl and trimethyl[b]benzothiophenes were the most abundant. With this research, it was demonstrated that NMR spectroscopy can be used to analyze thiophenic compounds from petroleum medium fractions.     

Rapid and direct quantitative analysis of positional fatty acids in triacylglycerols using 13C NMR

There is increasing evidence that the positional fatty acid composition (FAC) of TAG is more important than total FAC with regard to nutrition values of edible oils and fats. A rapid and direct regiospecific analysis of positional fatty acids in TAG using ¹³C NMR was developed to overcome the tedious conventional methods which involve enzymatic hydrolysis, Grignard chemical degradation, and chromatography analysis. A set of NMR data acquisition parameters and processing methods had proven their excellent versatility and applicability on various types of oils and fats, with systematic error of 1.0 mol%. It was found that there are discrepancies between the regiospecific analysis results obtained by the current method and by conventional methods. Probable acyl migration occurring in the hydrolysis process in conventional methods is a noted problem. As the current ¹³C NMR method is a direct measurement and no hydrolysis of the sample is needed, acyl migration during the analysis is eliminated. As a result, the saturation level is always higher at sn‐1, 3 positions and lower at sn‐2 position in TAG structure of oils in the regiospecific data obtained from the current ¹³C NMR than that from conventional methods. In the absence of laborious chemical derivatization, this method is simple, accurate, and user‐friendly for researchers, especially for nutritionists to support their nutritional studies from the perspective of positional FAC of edible oils and fats.     

Synthesis and spectroscopic analysis of cellulose sulfates with regulable total degrees of substitution and sulfation patterns via C NMR and FT Raman spectroscopy

Synthesis and spectroscopic characterisation of cellulose sulfate (CS) were reported. Various CS exhibiting diverse degrees of sulfation (DS_S) were prepared through acetosulfation or direct sulfation of cellulose. During the acetosulfation, intermediate product - cellulose acetate sulfate (CAS) - was formed after the comparative esterification and subsequent deacetylation of CAS led to CS. The direct sulfation proceeded quasi-homogeneously and heterogeneously in N,N-dimethylformamide (DMF) or homogeneously in N,N-dimethylacetamide (DMAc)/LiCl mixture. The total DS_S between 0.21-2.59 and partial DS_S6 as well as DS_S2 of up to 1 were determined via elemental analysis and ¹³C NMR spectroscopy. Besides, solid-state CP/MAS ¹³C NMR could characterise CS regarding the sulfation. Subsequently, FT Raman investigation of obtained CS was conducted with the aim to establish analysis methods quantifying the total DS_S. The intensities of Raman bands ascribed to the vibrations of OSO and C-O-S groups were used as analysis parameters, yielding calibration curves with high correlation coefficients of more than 0.96.     

THE STRENGTH OF MUTAGENICITY, 13C NMR CHEMICAL SHIFTS,AND ELECTRONIC STATES OF NITROBENZANTHRONES

Mutagenic activities of nitrated benzanthrones (NBAs) vary largely with the position and the number of the nitro group. To investigate the structure-activity correlations for NBAs, we have performed nuclear magnetic resonance (NMR) measurements and molecular orbital calculations for the three nitrobenzanthrones, 2-NBA, 3-NBA, 11-NBA; the three dinitrobenzanthrones, 1,9-DNBA, 3,11-DNBA, 3,9-DNBA; and the trinitrobenzanthrone, 3,9,11-TNBA. It was confirmed that the ¹³ C chemical shifts (δ) of the ortho carbon atoms with respect to the nitro group of the compounds tend to be more upfield with decreasing mutagenic activities. The molecular orbital calculations revealed that the LUMO energies of the compounds decrease with mutagenic activities, and that the HOMO and LUMO densities tend to decrease and increase, respectively, with decreasing mutagenic activities. These results indicate that reduction is very important in the metabolism of nitrobenzanthrones.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

High resolution solution and solid state NMR characterization of ethylene/1-butene and ethylene/1-hexene copolymers fractionated by preparative temperature rising elution fractionation

Preparative TREF was used to fractionate 2 commercial LLDPE polymers. These polymers had similar MFI values, density and comonomer content, but differed in comonomer type, 1-butene vs 1-hexene. High resolution solution NMR and solid state NMR was used to characterize the copolymer fractions. Distinct differences in chemical composition distribution could be observed from solution NMR results, and these correlated well with solid state analyses. Conclusions regarding the molecular make-up and crystallization phenomena are made.     

Solid-State 13C CP/MAS NMR for Alkyl-O-Aryl Bond Determination in Lignin Preparations

Several lignin preparations (Freudenberg lignin, Björkman lignin, and Pepper lignin), technical lignins (soda, soda-AQ, Kraft, Kraft-AQ, and hydrolysis), dimeric lignin model compounds, and different polysaccharides (galactoglucomannan, arabinogalactan, xylan, and arabinan) were analyzed by means of solid-state ¹³C CP/MAS NMR. Signals assignment in solid-state NMR lignin spectra was performed on the basis of the conducted studies and earlier published data. It was established that there exists strong linear correlation (r = 0.985) between Alkyl-O-Aryl inter-unit bond content in lignin and integral signals intensity in NMR spectra in the range of chemical shifts of 96–68 ppm. The integral signals intensity was measured in correlation with the reference integral signals in the range of chemical shifts of 162–102 ppm, typical for aromatic carbon atoms. To eliminate the effect, caused by carbohydrates contained in lignin, the correction factor of 0.67% of the area of integration per 1% of carbohydrates was determined. It was shown that the solid-state ¹³C CP/MAS NMR method allowed to determine Alkyl-O-Aryl bond content in both soluble and insoluble lignin preparations, and also to determine methoxyl groups content in soluble preparations.     

Improved quantitative13C nuclear magnetic resonance criteria for determination of grades of virgin olive oils. The normal ranges for diglycerides in olive oil

A quantitative method was established to determine the presence and composition of mono-, di-, and triglycerides of olive oils of superior gradevia ¹³C NMR. The total diglyceride content and the ratio ofsn-1,2- andsn-1,3-diglycerides in extra virgin oils extracted from different olive cultivars were correlated with maturity. The correlation can be applied to identify the oils by variety. No monoglycerides were detected in the oils examined.     

Antifeedant activity againstSpodoptera litera larvae and [13C]NMR spectral assignments of the meliatoxins

The tetranortriterpene, meliatoxin A₂, induced a significant level of antifeedant activity inSpodoptera litura F. larvae, while meliatoxin B₁, which lacks the ring D epoxide, was much less active. Complete [¹³C]NMR assignments for both compounds derived from homo- and heteronuclear short- and long-range couplings are given.