Variable Contact Time Experiments Research Articles

Influence of grape marc extract on tuning the intermolecular interactions in the high‐density polyethylene

AbstractIntermolecular interactions and molecular dynamics in thin films of high‐density polyethylene (HDPE) containing a high loading of Grape Marc Extract (GME) were analyzed by solid‐state NMR. Optical microscopy (OM), Scanning Electron Microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) studies were used to corroborate the findings. The OM and SEM images and 1H‐13C RAMP CP/MAS (ramped amplitude cross‐polarization/magic‐angle spinning) spectra of HDPE, HDPE‐GME, and GME confirmed the incorporation of the GME into the HDPE structure. The results were compared to those for an unfilled HDPE film. The influence of GME on enhancing the amorphous nature of HDPE, altering the crystalline melting behavior and possible intermolecular interactions were evident from the DSC and FTIR. The NMR spin‐diffusion and T1(1H), T2(1H), T1ρ*(13C), T1ρ(1H) relaxation and variable contact time experiments revealed the interactions and dynamics at the sub‐micron level.

Solid state NMR investigation of the roman Acqualadroni rostrum: tenth year assessment of the consolidation treatment of the wooden part

This work follows a previous one dealing with the state of conservation study of the wooden part of the roman Acqualadroni rostrum soon after its finding in the seabed of Acqualadroni (Messina, Italy). The archaeological survey and recovery were particularly relevant since this artefact is one of the two rostrums, nowadays known, found together with its wooden part. Following the recovery, it was consolidated by immersion in a melamine-formaldehyde resin (Kauramin) aqueous solution for eight months at the “Centro di Restauro del Legno Bagnato” (Pisa, Italy). The present investigation is aimed to determine at microscopic scale the wood state of conservation and to highlight interactions of the consolidant with wood components, by using solid-state NMR spectroscopy ten years after the consolidation treatment. Sampling by coring was exceptionally authorized, and the wood core was divided into seven aliquots as a function of depth. 13C cross-polarization magic angle spinning (13C{1H} CP-MAS NMR) spectra and variable contact time (VCT) experiments have been carried out to determine the cellulose crystallinity index, the lignin condensation degree and the holocellulose-lignin ratio. The interactions between the resin and the wood components were highlighted by T1ρH relaxation times determination and two-dimensional 13C–1H correlation (FSLG CP HETCOR) NMR experiments. Findings revealed specific interactions between the aromatic part of the resin and the lignin, as well as a network of hydrogen bonds involving all components of the system.

Quantitative characterization of coal structure by high-resolution CP/MAS 13C solid-state NMR spectroscopy

A method for quantitatively characterizing the carbon skeletal structure of coal by variable contact time experiment using high-resolution CP/MAS 13C solid-state NMR spectroscopy is proposed in this paper. The initial polarization transfer intensity from protons directly bonded with carbons, instead of dipolar-dephasing techniques which had to run on a lower frequency NMR spectrometer (100.02 MHz for proton), was used to divide the bridgehead and protonated aromatic carbons, making all the NMR data in this paper obtained on a high frequency NMR spectrometer (500.12 MHz for proton). On this basis, the fractions of different carbons in coal were further divided by the initial polarization transfer intensity from spin diffusion of protons unbonded with carbons. The structure of Naomaohu coal, a subbituminous coal from China, was measured. The change of polarization transfer with contact time was analyzed quantitatively. The fractions of aromatic, aliphatic, carboxyl and carbonyl carbons, and corrective aromaticity are 0.61, 0.39, 0.1 and 0.51, respectively. The fractions of protonated and bridgehead aromatic carbons are 0.22 and 0.09, respectively. These results agreed with literatures, and bond concentration calculated by the carbon skeletal structure distribution of coal was reasonable.

A structural study of Acacia nilotica and Acacia modesta gums

Superficially similar carbohydrate polymers from similar sources can have dramatically different characteristics. This work seeks to examine the molecular properties responsible for these differences. Protons responsible for cross-polarization in the anomeric region of Acacia nilotica (AN) were replaced easily by deuterium, but not for Acacia modesta (AM). Time constants describing the mobility and cross-polarization transfer were both found to be lower for AM. Variable contact time experiments showed poorer fits and more heterogeneity for AN. Solution state HSQC experiments showed a lower number of environments in the anomeric region for AM. The relaxation time T2 of AM solutions had a lower value consistent with a higher viscosity. The Tg′ of solutions were −14.5°C AN and −18.5°C AM. These results form a largely self-consistent picture of molecular differences between AN and AM, suggesting a more compact but heterogeneous structure for AN and more branching in the case of AM.

Conformational analysis of capsaicin using 13C, 15N MAS NMR, GIAO DFT and GA calculations

Capsaicin produced by plants from genus Capsicum exerts multiple pharmacological effects and has found applications in food and pharmaceutical industry. The alkaloid was studied by a combined approach: solid-state NMR, GA conformational search and GIAO DFT methods. The 13C CPMAS NMR spectra were recorded using variable contact time and dipolar dephasing experiments. The results of cross-polarization (CP) kinetics, such as TCP values and long T1ρH (100–200 ms), indicated that the capsaicin molecule is fairly mobile, especially at the end of the aliphatic chain. The15N MAS NMR spectrum showed one narrow signal at −255 ppm. Genetic algorithm (GA) search with multi modal optimization was used to find low-energy conformations of capsaicin. Theoretical GIAO DFT calculations were performed using different basis sets to characterize five selected conformations. 13C CPMAS NMR was used as a validation method and the experimental chemical shifts were compared with those calculated for selected stable conformers. Conformational analysis suggests that the side chain can be bent or extended. A comparison of the experimental and the calculated chemical shifts indicates that solid capsaicin does not have the same structure as those established by PWXRD.

Genotype evaluation of cowpea seeds (Vigna unguiculata) using 1H qNMR combined with exploratory tools and solid-state NMR

The ultimate aim of this study was to apply a non-targeted chemometric analysis (principal component analysis and hierarchical clustering analysis using the heat map approach) of NMR data to investigate the variability of organic compounds in nine genotype cowpea seeds, without any complex pre-treatment. In general, both exploratory tools show that Tvu 233, CE-584, and Setentão genotypes presented higher amount mainly of raffinose and Tvu 382 presented the highest content of choline and least content of raffinose. The evaluation of the aromatic region showed the Setentão genotype with highest content of niacin/vitamin B3 whereas Tvu 382 with lowest amount. To investigate rigid and mobile components in the seeds cotyledon, 13C CP and SP/MAS solid-state NMR experiments were performed. The cotyledon of the cowpea comprised a rigid part consisting of starch as well as a soft portion made of starch, fatty acids, and protein. The variable contact time experiment suggests the presence of lipid-amylose complexes.

Comprehensive multiphase NMR spectroscopy: A new analytical method to study the effect of biodiesel blends on the structure of commercial rubbers

Biodiesel has been incorporated into the energy matrix in several countries, but its compatibility with some materials used in automotive engines is of growing concern. In the present study, comprehensive multiphase (CMP) NMR is applied to understand the different phases in fuel–rubber systems. CMP NMR is a novel technology that integrates all the hardware from solution-, gel- and solid-state into a single NMR probe, permitting all phases to be studied in samples in their natural unaltered state. Transverse relaxation experiments in combination with inverse diffusion editing permit the increasing mobility of the rubbers chains with biodiesel exposure to be monitored. Conversely diffusion editing and RADE experiments highlight the more rigid domains. In summary NR and NBR showed the absence of highly rigid domains after exposure to biodiesel whereas SBR and EPDM especially better retained their structural integrity. 13C editing protocols and 13C–1H HSQC confirmed the increasing gel-like properties of NR, NBR and SBR with exposure to biodiesel. However, variable contact time experiments showed that biodiesel penetrates even the most resilient EPDM pores causing relaxation of the polymer chains and demonstrates that NMR is sensitive enough to highlight even the very slight swelling of the EPDM. Through these results it is possible to observe that the elastomer that exhibits the lowest susceptibility to biodiesel in molecular terms was EPDM, followed by SBR, NR, and NBR. It was also observed that biodiesel molecules were present in the EPDM structure despite the literature reporting its general resilience to biodiesel.

More insight into characterization of the waterlogged wooden part of Acqualadroni Roman Rostrum by solid-state NMR

Solid-state NMR spectroscopy was applied to characterize the wooden part of a roman Rostrum recovered in the Tyrrhenian Sea in the Acqualadroni area (Messina, Italy). The Acqualadroni Rostrum has been, in the recent past, investigated to establish its provenance and conservation. In this paper, solid-state NMR was used to obtain information on the conservation state of the wood as a preliminary step for the conservation process.A wooden sample of this artifact, collected by coring, was divided in four parts in order to correlate the conservation state to the depth. Results were compared with those obtained for a modern wood of the same species.A structural study was performed by acquiring 13C cross-polarization magic angle spinning spectra to evaluate the cellulose crystallinity degree and the lignin condensation degree. In addition, the holocellulose–lignin ratio and the cellulose–lignin residual interactions were determined through variable contact time experiments and relaxation times determination respectively.Being the solid-state NMR a non-destructive technique, all measurements were performed with no modification of the samples such as solvent extraction or other chemical treatment, making the samples available for further analytical investigation.

Solid-State 1H and 13C Nuclear Magnetic Resonance Spectroscopy of Athabasca Oil Sands Asphaltenes: Evidence for Interlocking π-Stacked Nanoaggregates with Intercalated Alkyl Side Chains

Solid-state 1H and 13C nuclear magnetic resonance (NMR) spectroscopic studies on heptane-extracted Athabasca oil sands asphaltenes show strong support for the “island” or “Yen–Mullins” model. Structural calculations on the deconvolved 13C direct-polarization magic-angle spinning (DP-MAS) NMR spectrum reveal the existence of large, peri-condensed polycyclic aromatic hydrocarbon (PAH) systems. Further support for this model was obtained from the variable contact time and dipolar dephasing experiments. Variable contact time experiments demonstrated decreased mobility of the alkyl groups with increasing proximity to the aromatic core. Furthermore, it was shown that all of the aromatic groups are highly rigid, with no evidence for mobile aromatic moieties pendant from the main aromatic core. The cross-polarization discrimination induced by variable amplitude minipulses (CP-DIVAM) nutation curves enable discrimination between signals from mobile and rigid components and show that the alicyclic groups are conden...

The Use of Solid State NMR to Evaluate the Carbohydrates in Commercial Coffee Granules

Coffee brings many health benefits due to its chemical constituents. Based on this information, it is essential to know the main chemical compounds from coffee granules; the intermolecular interaction among the coffees compounds and the molecular components homogeneity. In this study six types of roasted commercial coffee were evaluated by solid state nuclear magnetic resonance (NMR), employing carbon-13 (13C) and hydrogen (1H) nucleus. Carbon-13 was analyzed applying high field NMR techniques, such as: magic angle spinning (MAS); magic angle spinning with cross-polariza- tion (CPMAS) and magic angle spinning with cross-polarization and dipolar dephasing (CPMASDD). The hydrogen was evaluated via relaxation times. Proton spin-lattice relaxation time in the rotating frame was deter-mined through the carbon-13 decay, during the variable contact-time experiment, using high field NMR. Proton spin-lattice relaxation time was determined through the inversion-recovery pulse sequence, using low field NMR. Considering all NMR results, it was concluded that the major coffee compounds are: a) triacilglycerides, which constitute the mobile region in the granule coffee and b) Carbohydrates such as: polysaccharides and fibers that belong to the rigid domain. These constituents belong to different molecular mobility domain, although they have strong intermolecular interactions due to the granule organization.

The Evaluation of Polyethylene/Clay Composite from Solid State NMR

Polymeric nanocomposites based on polyethylene (PE) and Brazilian natural montmorillonite clay (MN) were obtained by melt processing, using a twin-screw extruder. The main objective of this work is focusing on the characterization of composites materials by solid-state nuclear magnetic resonance (NMR). The solid-state NMR measurements were used to observe both polymer matrix (through carbon-13 and hydrogen nuclei) and the clay (silicon-29 and aluminum-27). The polymer matrix analyses were carried out applying solid state techniques, such as: cross-polarization magic angle spinning (CPMAS), variable contact time (VCT) and by the proton spin-lattice relaxation time in the rotating frame parameter (T1ρH), detected from the resolved carbon-13 decay of the VCT experiment and through the determination of spin-lattice relaxation time, T1H (using low field NMR). The clay was analyzed by 29Si and 27Al, employing MAS NMR technique. From those techniques we can have principally response on clay dispersion in the polyethylene matrix, as well as the interactions between both components in the nanostructured material. The T1H response was an important result which showed, that the materials formed, presented different molecular domains (according to the domain size that varied from 25 to 50 nm, measured by relaxation), considering the clay dispersion mode in terms of intercalation and/or exfoliation in the polymer matrix.

Structural studies on lithocholyl-N-(2-aminoethyl)amide in the solid state

The synthetic procedure of lithocholyl-N-(2-aminoethyl)amide yielded a mixture of several forms detected by solid state 13C CP/MAS NMR although the solution state NMR unambiguously ascertained that the compound was pure. By recrystallization from various solvents one pure polymorph alongside with four solvates were isolated. The structures of the pure polymorph and the solvates were characterized by 13C and 15N CP/MAS NMR and powder X-ray diffraction (PXRD) methods. Variable contact time and dipolar dephasing experiments were employed to obtain optimized CP parameters and to distinguish various CHn (n = 0–3) resonances. CSA analyses of spinning side bands at different spinning rates showed small variations in the shielding tensor values of the carbonyl group between the pure polymorph (recrystallized from acetonitrile, tetrahydrofuran and 1,4-dioxane) and p-xylene solvate.

Solid-state NMR of polyaniline nanofibers

The structure of nanofibers of polyaniline (PANI) formed by oxidation of aniline with ammonium persulfate in the presence of HCl have been determined by solid-state 13C and 15N NMR experiments. The nanofibers synthesized in this way by rapid mixing and in the presence of strong acid resemble standard PANI in structure. Due to rapid mixing the reactants are consumed very quickly at the very beginning of the reaction, preventing secondary growth. Solid-state 15N and 13C CP MAS NMR results suggest that the emeraldine base form of the nanofibers exists mainly as an alternating copolymer of reduced and oxidized repeat units. A broad shoulder between 80 and 150 ppm in the 15N spectra is attributed to the presence of positively charged radical centers distributed along the polymer backbone, due to protonation of the imine nitrogens. Removal of the shoulder by dedoping with LiOH confirms that it is due to the positively charged imine nitrogens. Non-quaternary suppression 15N NMR experiment indicates the cross-linking and the presence of tertiary nitrogens in the nanofibers. The imine to amine ratio obtained from a variable contact time experiment is 0.8.

Structural studies of 4,5,6,7-tetrabromobenzimidazole derivatives by means of solid-state 13C, 15N NMR spectroscopy and DFT calculations

Tetrabromobenzimidazole derivatives containing sulfur or nitrogen atom in a five- or six-membered saturated ring were synthesized as potential ligands of casein kinase (CK2). Structural data of these compounds are crucial for understanding their inhibitory activity as inhibitors. Solution and solid-state 13C NMR spectra were recorded for six compounds, and 15N MAS spectra – for two of them. 13C CPMAS spectra were assigned by comparison with solution data and with the aid of dipolar dephasing and variable contact time experiments. The correctness of assignments of 13C and 15N chemical shifts was verified by GIAO DFT calculations of shielding constants. The differences between the solution and solid-state chemical shift values were explained in terms of intermolecular interactions. The doublet resonances occurring in the solid-state 13C and 15N NMR spectra of N 1, N 2-propylene-2-amino-4,5,6,7-tetrabromobenzimidazole indicate the presence of two molecules in crystallographic unit cell. Shielding constants calculated for a dimer with two N1′ H…N3 hydrogen bonds suggest that the association type does not influence carbon shielding. The coexistence of two tautomers with N1′ H and N3 H is less probable.

13C CPMAS NMR and DFT calculations of anthocyanidins

Anthocyanidins, red dyes from flower petals and fruits, are beneficial to human health. They attract considerable attention owing to their strong antioxidant and radical scavenging properties, however they are unstable in solution and available in small amounts only. 13C CP MAS NMR spectra were recorded to characterize solid-state conformation of nine anthocyanidins: apigenidin, pelargonidin, cyanidin, delphinidin, peonidin, malvidin robinetidin, luteolinidin and diosmetinidin chlorides. For some carbons, the solid-state chemical shifts were different from those obtained for solutions, indicating differences in conformation and intermolecular interactions. The principal elements of the 13C chemical shift tensor were measured for pelargonidin, cyanidin, delphinidin and malvidin chlorides using PASS-2D NMR technique. DFT GIAO calculations of shielding constants were performed for apigenidin and several geometric isomers of pelargonidin. Comparison of experimental 13C δ ii with the theoretical shielding parameters was helpful in predicting the most reliable geometry in the solid state. The cross-polarization parameters were obtained from variable-contact time experiments; T CH are longer and the values of T 1 ρ H are shorter in the order: pelargonidin<cyanidin<delphinidin. It is probable that solid anthocyanidins become less ordered as the number of OH groups increases.

Solid‐state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable.

Spectroscopic and conformational properties of size-fractions separated from a lignite humic acid

A lignite humic acid (HA) was fractionated by preparative high performance size-exclusion chromatography (HPSEC) in seven different size-fractions. The size-fractions were characterized by cross polarization (CP) magic angle spinning (MAS) 13C NMR spectroscopy and a further analytical HPSEC elution under UV and fluorescence detection. The alkyl hydrophobic components mainly distributed in the largest molecular-size-fraction, whereas the amount of oxidized carbons increased with decreasing size of fractions. Cross polarization time ( T CH) and proton spin-lattice relaxation time in the rotating frame ( T 1 ρ (H)) were measured from variable contact time (VCT) experiments. The bulk HA was characterized by the shortest T CH values and the longest T 1 ρ (H) values which suggested, respectively, one. an aggregation of components in a large conformation that favored a fast H–C cross polarization, and, two. consequent steric hindrances that prevented fast local molecular motions and decreased proton relaxation rates. Conversely, the separated size-fractions showed longer T CH values and shorter T 1 ρ (H) values than the bulk HA, thereby indicating that they were constituted by a larger number of mobile molecular conformations. The UV and fluorescence absorptions were both low in the large size-fractions that mainly contained alkyl carbons, whereas they increased in the olephinic- and aromatic-rich fractions with intermediate molecular-size, and decreased again in the smaller fractions which were predominantly composed by oxidized carbons. These results support the supramolecular structure of humic substances and indicate that the observed variation in conformational distribution in humic association may be used to explain environmental processes with additional precision.

NMR Characterization of 13C-Benzene Sorbed to Natural and Prepared Charcoals

We investigated how the NMR properties of uniformly 13C-labeled benzene molecules are influenced by sorption to charcoals produced in the laboratory and collected from the field following wildfires. Uniformly 13C-labeled benzene was sorbed to two charcoals produced in the laboratory at 450 and 850 degrees C. The chemical shift of benzene sorbed to the higher-temperature charcoal was 5-6 ppm lower than that of benzene sorbed to the lower-temperature charcoal. This difference was attributed to stronger diamagnetic ring currents (which cause a shift to lower ppm values) in the more condensed or "graphitic" high-temperature charcoal. The chemical shift of benzene sorbed to two charcoals collected from the field following wildfires indicated a degree of charcoal graphitization intermediate between that of the two laboratory-prepared charcoals. Variable contact time and dipolar dephasing experiments showed that the molecular mobility of sorbed benzene molecules increased with increasing charcoal graphitization, and also increased with increasing benzene concentration. We propose that the chemical shift displacement of molecules sorbed to charcoal could be used to identify molecules sorbed to black carbon in heterogeneous matrixes such as soils and sediments, and to establish how condensed or "graphitic" the black carbon is.

Intimate association between O/N-alkyl carbon and iron oxides in clay fractions of forest soils

Iron oxides with high specific surface areas promote the stabilisation of soil organic matter. We compared solid state 13C nuclear magnetic resonance (NMR) spectra of clay fractions from A horizons before and after demineralisation with 10% hydrofluoric acid (HF) to study selective associations between Fe oxides and functional groups in soil organic matter. The samples were collected from soils under temperate forest conditions, classified as Cambisols, Leptosols, Luvisols and Phaeozem. After removal of Fe oxides by demineralisation, solid state 13C NMR spectra of clay fractions from Leptosols and Luvisols showed an increase in the relative O/N-alkyl C intensity that ranged from 3% to 20%. A variable contact time experiment with a sample from a Luvisol without HF treatment showed that protons in the vicinity of O/N-alkyl C relaxed faster than in the vicinity of other functional groups. Therefore, part of the O/N-alkyl C was invisible unless very short contact times were applied. This demonstrated that Fe oxides and O/N-alkyl C are preferentially associated in soils. The O/N-alkyl C signal loss in the untreated samples was strongly correlated with the ratio of dithionite-extractable iron oxides and the organic carbon (OC) concentration. Our study showed that OC:Fe ratios >1 do not necessarily assure that reasonable solid state 13C NMR spectra will be obtained. In fact, clay fractions with OC:Fe ratios between 2.2 and 3.8 were affected by association between Fe oxides and O/N-alkyl C and had to be treated with HF to circumvent a selective signal loss of O/N-alkyl C.

1H, 13C NMR studies and GIAO/DFT calculations of substituted N-(4-aryl-1-piperazinylbutyl) derivatives, new analogues of buspirone

13C cross-polarisation (CP) magic angle spinning (MAS) NMR data are reported for seven piperazinylbutyl derivatives of 1,4-dichloro-dibenzo[ e, h]bicyclo[2,2,3]octane-2,3-dicarboimide, new analogues of buspirone (anxiolytic drug). The assignment of solid state 13C NMR spectra were made with an aid of variable contact time experiments, as well as by comparison with solution data and calculated shielding constants. 13C CPMAS NMR spectra showed a disorder of methylene carbons in solids of 1– 7, in 1 and 3 two molecules differing in conformation of n-butyl chain are probably present in the asymmetric unit cell. In CDCl 3 solution, the barrier to piperazine ring inversion is 50 kJ/mol for 2, and lower than 46 kJ/mol for 1 and 3. Satisfactory agreement between the experimental chemical shifts (both in solution and solid state) and theoretical values of shielding constants (calculated by GIAO/DFT and GIAO/HF methods) was obtained (correlation coefficients R 2>0.98).