Carbon-13 NMR distinction between categories of molecular order and disorder in cellulose

Differences between values of proton rotating-frame spin relaxation time constants can be exploited to separate a solid-state¹³C NMR spectrum of cellulose into subspectra of crystalline and noncrystalline regions. Variations in chemical shifts and¹³C spin-lattice relaxation time constants can then be used to study variations in molecular order and disorder within each of the two broader categories. Mechanical damage during Wiley milling increases the content of noncrystalline cellulose and changes the nature of molecular disorder within that category. Resolution enhancement of the subspectrum assigned to crystalline cellulose reveals pairs of signals at 83.9 and 84.9 ppm (cellulose I) or 86.8 and 88.3 ppm (cellulose II) assigned to C-4 on well-ordered crystal surfaces. A broader peak in the subspectrum of crystalline cellulose I is assigned to poorly-ordered surfaces. Relative proportions in Avicel microcrystalline cellulose were estimated as: 54% in crystal interiors, 22% on well-ordered surfaces, 8% on poorly-ordered surfaces, 16% in domains of disorder extending more than a few nanometres.     

Effect of high pressure treatment on structure and properties of cellulose in eucalypt pulps

Bleached acid sulphite and kraft Eucalyptus globulus pulps were subjected to treatment at high hydrostatic pressure (400 MPa during 10 min). The associated structural changes of cellulose were evaluated by X-ray scattering, solid-state NMR and infrared spectroscopy. The high pressure treatment promoted the growth of crystalline domains predominantly via lateral aggregation (cocrystallization) and, to some extent, due to the accretion of cellulose from noncrystalline domains (recrystallization). The treated pulps exhibited increment of the amount of strongly bound water and improved accessibility to amorphous domains. The high pressure treatment of dried sulphite pulp led to restoration, at least partially, of its swelling capacity thus diminishing the hornification features. Pressure treated dried sulphite pulp showed improved fibre bonding capacity at simultaneously increased bulk of the produced handsheets. The results obtained clearly showed the potential of high pressure treatments for the modification of cellulosic fibres in different applications.     

Crystalline forms of cellulose in the silver tree fern Cyathea dealbata

Solid-state ¹³C NMR spectroscopy was used to characterize fibrous material cut from the midrib of a fern frond. Signals associated with cellulose crystallites were separated from those associated with the lignin--hemicellulosic matrix by exploiting differences in proton rotating-frame relaxation time constants. Heights of signals at 90.2 and 88.5 ppm, assigned to C-4 in cellulose I_α and I_β, indicated similar proportions of the two crystalline forms. This observation conflicts with a suggestion that plant celluloses can be grouped into the two categories of I_α-rich and I_β-rich. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular conformations at the cellulose–water interface

¹³C-NMR chemical shifts were measured for C-4 and C-6 in a collection of eight crystalline glucoses and glucosides. The influence of the hydroxymethyl conformation was greater at C-4 than at C-6, with mean chemical shifts for gauche–trans molecules displaced 3.1 ppm (C-4) and 2.5 ppm (C-6) relative to gauche–gauche molecules. This information was used to interpret ¹³C-NMR spectra of crystalline celluloses. Chemical shifts for C-4 in the crystallite cores of celluloses I and II differed by just 0.2 ppm, but the corresponding chemical shifts for well-ordered crystallite surfaces differed by 3.0 ppm. The separation between crystallite-surface signals was attributed to different hydroxymethyl conformations at the cellulose–water interface, i.e., gauche–gauche and gauche–trans on crystallites of cellulose I and cellulose II, respectively. A broad C-4 signal in the spectrum of cellulose II indicated gauche–gauche conformations in disordered cellulose. Chemical shifts for C-6 were consistent with these conformations.     

Surface Modification of Colloidal Silica by Sulfonation Route for Super-Hydrophilicity

Colloidal silica was chemically modified by a two-step method including an olefin sulfonation route as a preliminary study for super-hydrophilic applications. The hydrophobic vinyl groups were initially bonded chemically to the hydroxylated silica surface using the trichlorosilane coupling agents. The vinyl-terminated silica was then sulfonated by addition reaction with chlorosulfonic acid. The modified silica was investigated using DRIFT, TGA, element analysis, solid-state ²⁹Si- and ¹³C CP-MAS NMR. The vinyl-terminated specimen showed a characteristic IR absorption band at 1600 cm^–1 and a weight loss of approximately 3% starting at 350°C while the ²⁹Si NMR peaks at 70.9 and 79.8 ppm and ¹³C NMR at 136 ppm and 129.8 ppm were assigned to a vinyl group bonded to silica. Elemental analysis of the sulfonated silica indicated the presence of sulfur, carbon and hydrogen. Thermal decomposition in range 150–600°C was due to the presence of sulfonated organics and unreacted vinyl groups while the new signals on ¹³C NMR, which were in the range 70–15 ppm, were assigned to sulfonated carbons.     

Assignment of 23Na MAS NMR signals of ammonium- and lanthanum-exchanged sodium Y zeolites

Two distinct ²³Na MAS NMR lines in spectra of hydrated Na,NH₄-Y zeolites at about −9 and −13 ppm (referenced to crystalline NaCl) were assigned to sodium lattice cations located in the large cavities (SIII) and inside the truncated octahedra (SI′ or SII′). This assignment was supported by the appearance of these two signals in spectra of air-dried and heat-treated La,Na-Y zeolites (obtained by conventional or contact-induced solid-state ion exchange), the more so as the intensity ratios corresponded to the cation distribution known to arise after the distinctly different pretreatments. It is concluded that slowly tumbling sodium cations are located in SIII whereas Na cations in truncated octahedra show stronger quadrupole interactions. A ²³Na signal at −5.5 ppm sometimes observed in spectra of La,Na-Y zeolites was tentatively assigned to sodium cations located in the hexagonal prisms (SI).     

Carbon-13 NMR spectra of hydroxylated bile acid stereoisomers

Carbon-13 NMR signals were assigned for the complete set of the 26 theoretically possible isomers of methyl 5β-cholanates having one to three hydroxy groups at positions C-3, C-7 and/or C-12 in the nucleus. Substituent effects on the ¹³C NMR shielding data serving to characterize the position and configuration of the hydroxy groups are discussed.     

Maillard Reaction: Investigation of the Chemical Structure of Melanoioins Synthesized from D-Xylose and Glycine Using 13C and 15N Specifically Labeled Reactants

Abstract

Melanoldins were Isolated 1n 36% yield w/w from molar solution of D-xylose and glydne-2-¹³C (A); D-xylose and glycine-l-¹³C (B); D-xylose-1-¹³C and glydne (C); D-xylose and glyclne (D); D-xylose and glycine-¹⁵N (E). Each solutTon was kept at 68[ddot]C until complete disappearance of xylose as evidenced by NMR. ¹³C and ¹⁵N solid state nuclear magnetic resonance and diffuse reflectance Infrared spectrometry were used in their structural elucidation before and after basic and add hydrolysis. Both C-1 and C-2 of glycine were Incorporated Into the polymers. In the ¹³C CP-MAS NMR spectra, C-1 gave a single peak in the polymer at 171.3 ppm, while C-2 gave three at 48.1, 31.2 and 22.5 ppm. Area measurements of the respective peaks Indicated that 50% of the Incorporated glycine had undergone decarboxylatlon. C-1 of xylose was Incorporated into the polymers mainly as two types of carbons at 68.8 ppm (CHOH, C-OH) and at 133.3 ppm (unsaturated C). Hydrolysis (6N HC1) led to a 20% reduction 1n weight of the melanoldlns, a decrease of 2% in C and 10% in N. ¹³C CP-MAS NMR revealed after hydrolysis of D, the disappearance of signals at 69, 110, 152, 172 and 200 ppm. Hydrolysis of A and B reduced all signals originating from C-1 and C-2 of glydne, while hydrolysis of C reduced only the signal of 68.8 ppm. ¹⁵N CP-MAS NMR of hydrolyzed E showed a greatly reduced amide resonance at 100 ppm, with more pyrrole or imino N. DR-IR showed a reduction 1n both the 1625 and 1550 cm^-1 bands with a concurrent appearance of a 1715 cm^-1 band.     

13C NMR Spectra of tocopherols and 2,2-dimethylchromanols

Carbon-13 NMR signals have been assigned for α-γ- and δ-tocopherols,a nd their model compounds, 2,2-dimethylchromanols. The ¹³C spectra of α-tocopherol-5-CD₃ and ?8-CD₃ afforded the evidence of the assignments of Me-carbon signals. It was found that previous tentative assignments for C-6 and C-8a of α-tocopherol are not correct. Steric compression shifts have been observed at several Me-carbons and at the methylene-carbons of C-4 in their compounds.     

Definitive assignment of 13C NMR signals in tryptophan and related molecules

Selective photodeuteration of carboxyltryptamines at C-4 allowed to settle the controversy about the ¹³C nmr assignment of C-4, C-5 and C-6, which for tryptophan are definitively assigned at 118.4, 118.2 and 120.6 ppm, respectively.     

Carbon-13 NMR spectra of nucleoside 3′,5′-cyclic phosphates. Proposition for revised signal assignments

On the basis of the data obtained from ¹³C NMR spectra of 8,2′-S-cycloadenosine 3′,5′-cyclic phosphate and other nucleoside 3′,5′-cyclic phosphate analogues, it is suggested that the published assignments of the C-3′ and C-4′ signals in nucleoside 3′,5′-cyclic phosphates should be reversed. According to the revised assignments, C-4′, which is fixed very closely to the diesterified phosphate group is markedly shielded (?12.5 to ?15 ppm), and the C-3′ signal shows a downfield shift (+6 to +8 ppm) which is comparable to that for the C-5′ signal, for all compounds so far measured when compared with the chemical shifts for the corresponding nucleosides. The 3′,5′-cyclic phosphates of thymidine and 8,2′-S-cycloadenosine, which have no α-OH group on C-2′, show similar chemical shift changes for the corresponding sugar carbons which are different from those observed for ribonucleoside derivatives.     

Effect of Glucuronoxylan on the Hornification of Eucalyptus globulus Bleached Pulps

The effect of 4-O-methylglucuronoxylan (GX) on the hornification of bleached kraft and acid sulphite Eucalyptus globulus chemical pulps has been investigated. Almost straight-line dependence of kraft pulp hornification from GX content was explained through the diminishing of fibrils aggregation and better accessibility of amorphous cellulose regions to water in GX enriched pulps. The higher hornification of sulphite than kraft pulp was assigned to lower GX content in the former and to unfavourable rearrangement of cellulose molecules in crystalline and amorphous regions during acid sulphite pulping.     

Modification of crystallinity and pore size distribution in coagulated cellulose films

In this study the effects of altering the coagulation medium during regeneration of cellulose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate, were investigated using solid-state NMR spectroscopy and NMR cryoporometry. In addition, the influence of drying procedure on the structure of regenerated cellulose was studied. Complete conversion of the starting material into regenerated cellulose was seen regardless of the choice of coagulation medium. Coagulation in water predominantly formed cellulose II, whereas coagulation in alcohols mainly generated non-crystalline structures. Subsequent drying of the regenerated cellulose films, induced hornification effects in the form of irreversible aggregation. This was indicated by solid-state NMR as an increase in signal intensity originating from crystalline structures accompanied by a decrease of signal intensity originating from cellulose surfaces. This phenomenon was observed for all used coagulants in this study, but to various degrees with regard to the polarity of the coagulant. From NMR cryoporometry, it was concluded that drying induced hornification generates an increase of nano-sized pores. A bimodal pore size distribution with pore radius maxima of a few nanometers was observed, and this pattern increased as a function of drying. Additionally, cyclic drying and rewetting generated a narrow monomodal pore size pattern. This study implies that the porosity and crystallinity of regenerated cellulose can be manipulated by the choice of drying condition.     

Local fluorine environments in nanoscopic magnesium hydr(oxide) fluorides studied by F MAS NMR

¹⁹F MAS NMR signals of a variety of X-ray amorphous magnesium hydroxide (oxide) fluorides (MgF_x(OH(R))_2−x (0 < x ≤ 2)) prepared using different synthesis routes are compared and assigned. Relations between their ¹⁹F signal positions and local coordinations are elaborated on the basis of the chemical behaviour and data available for the few crystalline reference compounds known so far. Supported by the superposition model, ¹⁹F signals appearing between −150 ppm and −200 ppm were subdivided into three groups due to local [FMg₆], [FMg₅] and [FMg₄], and [FMg₃] environments, respectively.     

Investigation of the exo and endo isomers of dioxolane-type benzylidene derivatives of carbohydrates by 13C NMR spectroscopy

The absolute configurations of nine 2,3-O-benzylidene-α-L-rhamno- and α-D-mannopyranoside diasteteomeric pairs were determined and the ¹³C NMR spectra of further thirteen α-L-rhamno- and α-D-mannopyranosides, having various substituents, were completely assigned.Four ¹³C shifts were found suitable for the determination of the absolute configuration of the dioxolane skeleton. (1) The chemical shift of the acetal carbon in the endo isomers is between 103.9 and 104.7 ppm whereas for the exo isomers this region extends from 102.8 to 103.4 ppm; (2) The formation of the dioxolane ring causes a deshielding effect for the bridgehead carbons, in the exo isomers this effect is more pronounced for C-3 whereas in the endo isomers for C-2. For C-4, shielding effect was found in the exo isomers and deshielding effect in the endo ones; (3) The chemical shift of the quaternary carbon of the phenyl group is greater in the exo isomers than in the endo ones; (4) The difference between the shift of the acetal carbon and that of the quaternary carbon of the phenyl group in the exo isomers is greater than 35.4 ppm, in the endo isomers is less than 33.7 ppm.     

The substitution of germanium for silicon in AST-type zeolite

The substitution of silicon by germanium in the AST zeolite framework type, [Si_nGe_40−nO₈₀]*4(SDA⁺F⁻) expressed as unit cell content in its cubic F-centered symmetry, has been studied. Three different kinds of templates, dimethyldiethylammonium, dimethyldiisopropylammonium and isopropyltrimethylammonium cations, were used in the hydrothermal synthesis process in fluoride medium. The products were identified with XRD, MAS NMR, SEM and thermal analysis. The analysis of the X-ray powder diagrams shows that AST crystallizes in different space group symmetries depending on the nature of the SDA and the degree of Ge-substitution. The resonance signals of ¹⁹F in MAS NMR experiments for the pure Si- and Ge-end members are at −38.2 and −15 ppm, respectively, indicating that the F⁻-anion is located as co-template in the double-four-ring (D4R) of the tetrahedral framework. This is confirmed by Rietveld analysis of powder diffraction data of the pure Ge-end member. The peak splitting of the ¹⁹F NMR signal in pure GeO₂AST-type material is related to the displacement of F⁻ location inside the D4R. Two more distinct signals at −8 and −19 ppm, respectively, are observed for X-ray pure AST-samples of intermediate compositions and assigned to fluoride in D4R built of 4[GeO₄]- and 4[SiO₄]-tetrahedra (4Ge, 4Si) and to (2Ge, 6Si)-D4R, respectively. An ordered distribution of Ge in the AST-framework is proposed for cubic AST with compositions around Si/Ge=1.5–1 by correlating the intensities of ¹⁹F NMR signals and the results from chemical analysis. This model is further confirmed by the quantitative analyses of the corresponding ²⁹Si MAS NMR spectra.     

Characterization of signals in the solution 1H NMR spectrum of poly(isobutylene‐co‐p‐methylstyrene) and their utility

Poly(isobutylene‐co‐p‐methylstyrene) is an important precursor to Exxpro™ elastomers. A previous report detailed the characterization of both the proton and the carbon NMR spectra of the copolymer. 1 However, several resonances in the proton NMR spectrum of the copolymer were not assigned. Specifically, the proton methine resonance of the BSB triad sequence is now identified and used to calculate BSB triad contribution to the copolymer microstructure. This report describes the assignment of this resonance and other resonances associated with microstructural sequence distribution around p‐methylstyrene. The proton NMR signals of interest resonate at 2.8 ppm and 2.5 ppm in a typical spectrum for poly(isobutylene‐co‐p‐methylstyrene). The nature of these resonances were determined by preparation and characterization of specifically deuterated poly(isobutylene‐co‐p‐methylstyrene)s employing both one and two dimensional NMR techniques. The 2.8 ppm signal is assigned as the methine proton of a p‐methylstyrene incorporated between two isobutylene units (the BSB triad). The signal at 2.5 ppm is assigned to the meso‐BSS triad. Determination of these resonances allows for rapid evaluation of isolated p‐methylstyrene units (BSB triads) present in the copolymer using only ¹H NMR. The utility of this technique is demonstrated by comparing BSB triad values determined by ¹H and ¹³C NMR analysis. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1680–1686, 2000     

Assignment of stereochemistry to cyclohexenylidenecyanoacetates by 1H NMR spectroscopy

The stereochemistry of various pairs of isomeric 2-cyclohexen-1-ylidenecyanoacetates was assigned using ¹H NMR spectroscopy. The isomers with the γ-methylene or the γ-vinyl protons cis to the carbalkoxy group were found to have the signals of these protons at approximately 0.3 ppm and 1 ppm, respectively, downfield relative to their geometrical isomers or the corresponding 2-cyclohexen-1-ylidenemalononitriles. The observation regarding the γ-vinyl proton proved useful for the assignment of configuration to cyclohex-enylidenecyanoacetates derived form cholest-4-en-3-one. The large and constant downfield shift (c. 1 ppm) of the γ-vinyl proton when cis to the ester group results from the rigid cyclohexenylidenecyanoacetate system, in which the vinylic proton can approach more closely to the magnetically anisotropic ester carbonyl group.     

Carbon-13 NMR investigation of 2,5-diaryl-1,4-dithiins

The ¹³C NMR spectra of eight 2,5-diaryl-1,4-dithiins were recorded and signals were assigned. A linear correlation was observed between the electronegativity of the substituent groups on C-10,10′ and the chemical shifts of C-10,10′ after applying corrections for the magnetic anisotropic effect of the substituents. A Hammett correlation was found between the ¹³C chemical shifts of C-3,6 and C-7,7′ and the σ_p⁺ parameter associated with the substituents on C-10,10′. Extended electronic interaction between the π system of the aryl group and the π system of the dithiin ring was suggested by the observance of an alternating behavior in the magnitude of the substituent effects on the ¹³C shifts of C-2,5 and C-3,6. An alternating effect was also noted in the magnitude of the long-range ¹³C? F coupling constants for these same carbon signals in 2,5-(10,10′-difluoro)diphenyl-1,4-dithiin.     

13C NMR spectroscopy of [4.n.0] bicyclic compounds: Assignment of the ring junction configuration from ring junction configuration from ring junction 13C chemical shifts—application and limits

Several 6-methyl-9-carbamoyltetrahydro-4H-pyrido[1,2-α]pyrimidin-4-ones have been prepared using phosgene iminium chloride. These compounds can exist in equilibrium as the cis (3A) imine ? (3B) enamine ? trans (3C) imine. ¹H, ¹³C and ¹⁵N NMR prove that the cis- and trans-imine isomers are predominant in the equilibrium. ¹H NMR data reveal that the share of the 3B enamine form is negligible at measurable concentrations. The isomeric ratio 3A:3C is time dependent and can be monitored by measuring the CH₃? C-6 and (CH₃)₂N signals. The ¹³C NMR data show that doublets in the range 42–45 ppm for C-9 are only compatible with the imine forms 3A and 3C. The SCS values of the CH₃? C-6 and OCN(CH₃)₂ groups were calculated and used for identification of the cis and trans isomers. ¹⁵N NMR data show that the N-1 chemical shift of the imine is approximately ? 140 ppm for compound 3, whereas that of a fixed enamine is around ? 267.8. This provides additional support for the predominance of the imine tautomers in the equilibrium 3A ? 3B ? 3C. ¹⁵N data allow the stereoisomers 3A and 3C to be distinguished.