NMR spectroscopic studies on the mechanism of cellulose dissolution in alkali solutions

It was considered that the dissolution of cellulose in alkali solutions is mainly due to the breakage of hydrogen bonds. As an alkali hydroxide, KOH can provide OH^? just like LiOH and NaOH; but it is well known that LiOH and NaOH can dissolve cellulose, whereas KOH can only swell cellulose. The inability of KOH to dissolve cellulose was investigated and the mechanism of cellulose dissolving in alkali solutions was proposed. The dissolution behavior of cellulose and cellobiose in LiOH, NaOH and KOH were studied by means of ¹H and ¹³C NMR as well as longitudinal relaxation times. The structure and properties of the three alkali solutions were compared. The results show that alkali share the same interaction mode with cellobiose and with the magnitude of LiOH > NaOH > KOH; the alkalis influence the structure of water also in the same order LiOH > NaOH > KOH. The different behavior of the three alkalis lies in the different structure of the cation hydration ions. Li⁺ and Na⁺ can form two hydration shells, while K⁺ can only form loose first hydration shell. The key to the alkali solution can or cannot dissolve cellulose is whether the cation hydration ions can form stable complex with cellulose or not. K⁺ cannot form stable complex with cellulose result in the KOH solution can only swell cellulose.     

Sedum alkaloids-VI : Resolution enhanced proton NMR and 13C NMR spectroscopic studies on the solution conformation of Sedum acre alkaloids

The preferred solution conformation of 9 Sedum alkaloids and derivatives (sedinine, dihydrosedinine. 8-episedinine, diacetylsedinine, sedinone, 2-episedinone, sedacrine, 2-episedacrine and sedacryptine) was established by high resolution ¹H and¹³C NMR spectroscopy. These compounds may be divided into three classes based on the existence or the absence of an intramolecular hydrogen bond between the nitrogen atom and the hydroxyl group at C8 or at C10. In all of them (except sedacryptine) the N-CH₃ group was found to be axially oriented.     

Electrochemical impedance spectroscopic studies on niobium anodic dissolution in HF

Journal of Solid State Electrochemistry - The dissolution of niobium electrode in anodic regime in hydrofluoric acid was studied using electrochemical techniques. In the active region, complex...     

FTIR spectroscopic and NMR studies of hard elastic polypropylene

The variation of amorphous orientation and crystalline regularity of hard elastic polypropylene (HEPP) films during cyclic deformation and stress relaxation processes were studied using a FTIR spectrometer. The result proves entropic elasticity and shows the orientational hysteresis in the amorphous region or within the microfibrils, and also shows that the amorphous orientation increases, but that the crystalline regularity decreases with the increase of extension rate.Three spin-spin relaxation timesT _2f,T _2m, andT _2s and associated mass fractionsF _f,F _m, andF _s of HEPP fibers were measured with a solid echo of NMR method at different elongations and after relaxation or recovery for a long time A new possible interpretation was proposed that, while the microfibrils are formed in HEPP, the medium decay component should be ascribed to inner molecules of the microfibrils, and the slow decay component to the surface molecules of the microfibrils. According to this interpretation, the results implied that subfibrillation is the main process when HEPP is stretched up to 15% strain, and that at above 15% strain thinning and lengthening of the microfibrils become the main process. Thickening of the microfibrils was found in the recovery and relaxation processes.     

Surface enhanced Raman spectroscopic studies of 1H-indazole on silver sols.

The SER spectra of 1H-indazole adsorbed on silver hydrosol were recorded in the 1800-100 cm(-1) and in the 3200-2800 cm(-1) regions. The SERS data were interpreted on the basis of previous vibrational assignments, with the help of the results of DFT calculations carried out using the 6-31G** basis. From the comparison of SER and normal Raman spectra it can be deduced that 1H-indazole is non-dissociatively adsorbed on metal surface and that it interacts with silver sol via nitrogen atoms and ring pi-system. The molecular plane assumes a tilted orientation with respect to the silver surface. The effect of varying the concentration of adsorbate was also evaluated. The observed changes of the relative intensities of some enhanced bands suggest that the molecule assumes a more tilted orientation upon lowering the concentration of the adsorbate.     

Fourier transform Raman spectroscopic studies of a novel wood pulp bleaching system

The use of near-infrared (NIR) Fourier transform (FT) Raman spectroscopy for the study of lignocellulosic materials is discussed. An application utilizing NIR FT-Raman spectroscopy to study a novel chlorine-free process for the bleaching of wood pulps is presented in detail. The new process, still under development, entails the oxidation of residual lignin in wood pulps by vanadium-substituted polyoxometalates, and reoxidation of the reduced polyoxometalates by chlorine-free oxidants such as air, dioxygen, peroxides or ozone. Results from FT-Raman measurements of polyoxometalate-treated pulps are compared with those from chemical, spectroscopic and optical techniques commonly used in the pulp and paper industry.     

Impact of residual extractives on the thermal stability of softwood Kraft pulp

Cellulose - Wood extractives are known to cause problems during the production of pulp and paper and to impact their mechanical properties. In the present work, we systematically explored whether...     

NMR spectroscopic investigations with isatin guanylhydrazones

The synthesis of some novel (substituted) guanylhydrazones of isatin, 5-methylisatin and 1-methylisatin is decribed. Moreover, detailed nmr-spectroscopic studies (¹H-nmr, ¹³C-nmr) with these compounds and previously known congeners are presented.     

1H NMR spectroscopic studies of the conformational isomers of pyrrolidinofullerenes

Mixed bis-adduct derivatives of C60 containing a pyrrolidine and a malonate methano group were synthesized. Three regioisomers, the e', the trans-2, and the trans-3, were isolated and characterized. In-depth NMR studies of these methano-pyrrolidinofullerenes showed that the nitrogen inversion on the pyrrolidine moiety is not a fast event in the 1H NMR time scale as previously regarded. Solvent effects, variable temperature experiments, and protonation of the pyrrolidine nitrogen are addressed.     

Diepoxide treatment of softwood kraft pulp: influence on absorption properties of fibre networks

Many different approaches to the introduction of intra-fibre cross-links in fibres for use in absorption products are described in the patent literature, but relatively little has been done in terms of academic research. In this study, the long fibre fraction of a Scandinavian softwood kraft pulp has been cross-linked with the diepoxide 1,4-butanediol diglycidyl ether (BDDE). The fibre properties and the performance of the pulp in low density fibre networks were analyzed. In experiments, the cross-linking agent (BDDE) was dissolved in acetone and then mixed with the pulp. It was shown that a prior wash with NaOH (0.1 M) activates the polysaccharides and increases the extent of cross-linking, detected as a decrease in the water retention value. Chemical analysis of modified fibres also gave a clear indication of that cross-linking reactions actually occurred. The pulp properties could furthermore be influenced by varying time, temperature and the amount of BDDE in the reaction. It was also shown that it is possible to use water as a solvent for the cross-linking agent, even though the reactivity of water towards diepoxide must be considered to be high. The cross-linked pulps showed wet bulk under load comparable to that of the mechanical pulps and enhanced properties than unmodified Scandinavian softwood kraft pulp. It was also observed that fibre network test pads of the cross-linked pulp fibres, after being tested in the wet state, regained a substantially large part of their low-density structure when air-dried.     

Chiral alpha-branched benzylic carbocations: diastereoselective intermolecular reactions with arene nucleophiles and NMR spectroscopic studies

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).     

Surface enhanced Raman spectroscopic studies on 1H-1,2,4-triazole adsorbed on silver colloidal nanoparticles

1H-1,2,4-triazole is a very effective corrosion inhibitor for copper. The adsorption of this compound on silver colloidal nanoparticles has been studied by means of surface enhanced Raman scattering (SERS). SERS data are interpreted with the help of DFT calculations of models of the surface complex formed by 1H-1,2,4-triazole on the silver colloidal nanoparticles surface. It was found that this compound is adsorbed on metal surface in its anionic form and that it interacts with silver through the N₁ and N₂ atoms. The molecular plane assumes a tilted orientation with respect to the silver surface.     

Observation of quinonoid groups during the light-induced yellowing of softwood mechanical pulp

Unbleached and alkaline hydrogen peroxide bleached black spruce (picea mariana) stone groundwood samples were irradiated with a weak light source over a period of six days in an effort to probe the early photochemical events occurring during the light-induced brightness reversion. Samples withdrawn at daily intervals were subjected to oxyphosphorylation with trimethyl phosphite to labelortho-quinones and other chromophoric species with phosphorus. The solid-state³¹P NMR spectra of these samples revealed the rapid formation ofortho-quinones during the early phase of irradiation, followed by a dramatic reduction of these species at latter phases. The data suggests that during the early photochemistry,ortho-quinones are produced in high yield pulps, which subsequently react creating other more complex chromophores which do not possess the quinonic character.