A 13C-NMR spectral study of cellulose and glucopyranose dissolved in the NH3/NH4SCN solvent system

The ¹³C-NMR chemical shifts of a cellulose with a DP_w of 23 dissolved in the NH₃/NH₄SCN solvent system were found to be very similar to those of cellulose dissolved in DMSO (cellulose oligomers), in the LiCl/DMAC system and in the N-methylmorpholine N-oxide/DMSO system. It was concluded from this that cellulose does not react with the NH₃/NH₄SCN solvent. It was found, however, that glucose reacts with the solvent at C-1 to form β-D -glucopyranosy-lamine. Separation of this compound from the solvent resulted in another compound which was determined to be β,β-di-D -glucopyranosylamine. The compounds β-D -glucopyranosylamine, N-acetyl-2,3,4,6-tetra-O-acetyl-β-D -glucopyranosylamine, β,β-di-D -glucopyranosylamine, α,β-di-D -glucopyranosylamine, 2,3,4,6,2′,3′,4′,6′-octa-O-acetyl-α,β-di-D -glucopyranosylamine were all synthesized and the ¹³C-NMR chemical shifts of these compounds are reported. It was also found that for the low-DP cellulose sample which was used the reducing end group existed and had reacted with the solvent to form an amine at C-1.     

Interaction of cellulose with amine oxide solvents

Cellulose I, mainly as ramie or as Avicel microcrystalline cellulose, has been monitored by optical microscopy and by ¹³C CPMAS NMR, over the course of its dissolution in hot N-methylmorpholine N-oxide solvent. Its interaction with the near-solvent N-ethylmorpholine N-oxide and related non-solvents has also been investigated. NMR shows that N-methylmorpholine N-oxide partly converts crystalline cellulose I into amorphous solid cellulose. The changes in chemical shift imply increased flexibility at the glycosidic bonds. In contrast, N-ethylmorpholine N-oxide converts cellulose I to cellulose III_I, without dissolution. Microscopy shows that the ramie fibres swell laterally, and at least some also shorten longitudinally, during dissolution. Model studies using methyl--d-glucopyranose show no evidence from ¹³C chemical shifts for different modes of binding with different solvents. However, N-methylmorpholine N-oxide binds more strongly to methyl--d-glucopyranose in DMSO than does N-ethylmorpholine N-oxide, whereas N-ethylmorpholine N-oxide binds better to H₂O. Also, ¹³C T ₁ values for aqueous cellobioside show increasing rotational freedom of the –CH₂OH sidechains as N-methylmorpholine N-oxide is added. Together, these observations imply the initial penetration of solvents and near-solvents between the molecular cellulose sheets. Subsequently, N-methylmorpholine N-oxide breaks H-bonds, particularly to O-6, just sufficiently to loosen individual chains and then dissolve the sheets.     

Acylation of Cellulose with N,N′‐Carbonyldiimidazole‐Activated Acids in the Novel Solvent Dimethyl Sulfoxide/Tetrabutylammonium Fluoride

Summary: Carboxylic acids were efficiently activated with N,N′‐carbonyldiimidazole (CDI) and applied for the acylation of cellulose under homogeneous conditions using dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride trihydrate (TBAF) as solvent. The simple and elegant method is a very mild and easily applicable tool for the synthesis of pure aliphatic, alicyclic, bulky, and unsaturated cellulose esters with degrees of substitution of up to 1.9. Products are soluble in organic solvents, e.g., DMSO or N,N‐dimethylformamide (DMF). The cellulose esters were characterized by elemental analysis, FT‐IR, ¹H and ¹³C NMR spectroscopy and show no impurities or substructures resulting from side reactions.

The esterification of cellulose using carboxylic acids activated in situ with N,N′‐carbonyldiimidazole.     

Solid-state NMR studies of methyl celluloses. Part 1: regioselectively substituted celluloses as standards for establishing an NMR data basis

Three methyl celluloses with completely uniform substitution pattern, 2-O-methyl cellulose (1), 3-O-methyl cellulose (2) and 6-O-methyl cellulose (3), were prepared according to the cationic ring opening polymerization approaches starting from substituted 1,2,4-orthopivalate derivatives of d-glucose. These samples allowed for the first time to sort out the methyl substitution effects on solid-state NMR chemical shifts and relaxation. Dipolar dephasing experiments allowed the detection and assignment (¹H, ¹³C) of the methyl groups. In 1 and 2, these resonances overlapped with those of C-6, whereas in 3, the methyl signal experienced a low-field shift into the region of C-2,3,5. ¹³C T₁ experiments were used to verify different relaxation behavior of the carbon sites, particularly the short relaxation time of at the carbon substitution site next to the methyl groups. This effect was used to unambiguously identify the ¹³C chemical shifts of the carbons carrying the methoxyl substituent, although they overlap with all resonances in the C-2,3,5 region. The data obtained for the standard samples with uniform substitution will now be used as the basis for determining methylation patterns and substitution degree in commercial methyl celluloses.     

NMR spectroscopic studies on dissolution of softwood pulp with enhanced reactivity

N-methylmorpholine N-oxide (NMMO) is a known cellulose solvent used in industrial scale (LyoCel process). We have studied interactions between pretreated softwood pulp fibers and aqueous NMMO using nuclear magnetic resonance (NMR) spectroscopic methods, including solid state cross polarisation magic angle spinning (CP-MAS) ¹³C and ¹⁵N spectroscopies, and ¹H high resolution MAS NMR spectroscopy. Changes in both cellulose morphology and in accessibility of solvents were observed after the pulp samples that were exposed to solvent species were treated at elevated temperature. Evidence about interactions between cellulose and solvent components was observed already after a heat treatment of 15 min. The crystalline structure of cellulose was seen to remain intact for the first 30 min of heat treatment, at the same time there was a re-distribution of solvent species taking place. After a 90 min heat treatment the crystalline structure of cellulose had experienced major changes, and potential signs of regeneration into cellulose II were observed.     

Carbon-13 NMR chemical shifts in some substituted 1,2,4-triazol-3-ones

Carbon-13 NMR data for 15 substituted 1,2,4-triazol-3-ones are presented and discussed with regard to enolization in the neutral molecules. The coupling constant and chemical shift data show that the proton at N-2 is not exchanging rapidly in the DMSO-d₆ solvent. Using D₂O? OD^? as a solvent, it is found that the C-3 and C-5 resonances are shifted downfield by nearly the same amount, suggesting that the proton at N-4 is being removed. Enolization in the neutral molecules does not occur to any significant extent.     

1H, 13C and 7Li nuclear magnetic resonance study of the lithium chloride–N,N-dimethylacetamide system

The complex solvent obtained by dissolving 5-10% of lithium chloride in N,N-dimethylacetamide (DMA) presents a good method for dissolving highly insoluble polymers, such as cellulose. ¹H, ¹³C and ⁷Li NMR spectroscopy have been used, together with viscosity and conductivity measurements, for the study of this complex solvent. The ¹H and ¹³C chemical shift variations of DMA, on increasing the lithium chloride concentration, are found to be in opposite directions. The T₁ relaxation times show a large decrease in the mobility of DMA in the presence of lithium chloride. Methyl-β-D -glucopyranoside has been used as a model for cellulose in order to investigate the mechanism of solution of this polymer. It was found that each hydroxy group of the solute interacts with one lithium chloride molecule in solution.     

Phase transformations in microcrystalline cellulose due to partial dissolution

All-cellulose composites were prepared by partly dissolving microcrystalline cellulose (MCC) in an 8.0 wt% LiCl/DMAc solution, then regenerating the dissolved portion. Wide-angle X-ray scattering (WAXS) and solid-state ¹³C NMR spectra were used to characterize molecular packing. The MCC was transformed to relatively slender crystallites of cellulose I in a matrix of paracrystalline and amorphous cellulose. Paracrystalline cellulose was distinguished from amorphous cellulose by a displaced and relatively narrow WAXS peak, by a 4 ppm displacement of the C-4 ¹³C NMR peak, and by values of T₂(H) closer to those for crystalline cellulose than disordered polysaccharides. Cellulose II was not formed in any of the composites studied. The ratio of cellulose to solvent was varied, with greatest consequent transformation observed for c < 15%, where c is the weight of cellulose expressed as % of the total weight of cellulose, LiCl and DMAc. The dissolution time was varied between 1 h and 48 h, with only small additional changes achieved by extension beyond 4 h.     

Peptide conformations. Part 30. Assignment of the 1H-, 13C-, and 15N-NMR spectra of cyclosporin A in CDCl3 and C6D6 by a combination of homo- and heteronuclear two-dimensional techniques

The ¹H-, ¹³C-, and ¹⁵N-NMR spectra of the immunosuppressive cyclic undecapeptide cyclosporin A ( 1 ) have been analyzed at 300 MHz in CDCl₃, C₆D₆, and mixtures of these solvents. A combination of different homo- and heteronuclear two-dimensional NMR techniques enable complete assignment of all H-, C- and 4 N-signals. Recognition of the proton spin systems has been achieved via ¹H,¹H–COSY and double-quantum-¹H-NMR spectroscopy. NOESY spectra yield some sequence assignments, but two techniques using coupling across amide bonds have been applied to get independent assignments of all amino acids in the sequence: (i) An ¹H,¹H-COSY spectrum optimized for small coupling constants enables the detection of long-range couplings from N-methyl groups to both α-protons attached to that amide bond. (ii) An ¹H, ¹³C-COSY spectrum optimized for C,H-long-range couplings (J = 5 to 10 Hz) to the eleven CO groups again yields coupling to both α-protons attached to that amide bond. Additionally these two experiments yield the assignment of N-methyl protons and carbonyl C-atoms. Normal and relayed ¹H,¹³C-COSY in both solvents have been applied to assign all C-atoms via their directly attached and remote protons. An ¹H,¹³C-COLOC spectrum at 500 MHz in CDCl₃, which uses H,C-long-range couplings confirms the assignment of all proton spin systems as well as the C-signals of each individual amino acid. Ambiguities in the assignment of the C(δ)'s of MeLeu have thus been removed. An ¹H,¹⁵N-COSY spectrum enables the assignment of the 4 NH N-atoms.     

Three nuclei n.m.r. spectroscopy of dimethoate compounds. A large solvent effect on the 31P?S?C?1H vicinal coupling

Proton, phosphorus and carbon magnetic resonance spectra of dimethoate, dimethoxon, des-N-methyldimethoate, ω-hydroxydimethoate, trimethyldithiophosphate and O,O-dimethyldithiophosphate in different solvents have been measured. Most of the n.m.r. parameters were characteristic of the structural environment of the corresponding nucleus and solvent-independent. However, in the amide structures dimethoate, dimethoxon, des-N-methyldimethoate and ω-hydroxydimethoate the ³¹P? S? C? ¹H vicinal coupling showed an unusually large solvent effect of a 2.3 to 6.5 Hz decrease when CDCI₃ was replaced by acetone-d₆ or DMSO-d₆.     

Proton-transfer effect in hydrogen-bonded complexes of phospho-diester groups with amines in non-aqueous solvents

The proton transfer in 1 : 1 mixtures of phosphoric acid-di-n-butylester with various aromatic amines has been studied in deutero-chloroform and DMSO-d₆ by ¹H NMR spectroscopy. The 50% equilibrium was found for both solvents at ΔpK_a(50%)-values from 4.4 to 5.2, i.e. the strength of the base is 4.4 to 5.2 higher than that of the acid. The titration curves are steeper in DMSO than in chloroform, indicating a higher potential barrier in POH … N ? PO^? … H⁺N hydrogen bonds in the more polar solvent.     

The preparation of new oxoniobium(V) complexes from hydrated Niobium(V) Oxide: the crystal and molecular structure of Oxotris(2-pyridinolato-<Emphasis Type="Italic">N</Emphasis>-oxide)niobium(V)

Three oxoniobium(V) complexes ONbL₃ with HL = tropolone (1), 2-pyridinol-N-oxide (2) or, 3-hydroxy-1,2-dimethyl-4(1H)-pyridone (3) are obtained in good yield by one step reactions from commercially available hydrated niobium(V) oxide in aqueous media. The products are characterized by elemental analysis, FT-IR spectroscopy, NMR spectroscopy (¹H- and ¹³C-) and thermogravimetry. The crystal structure of oxotris(2-pyridinolato-N-oxide)niobium(V) is determined by single crystal X-ray diffraction, showing discrete molecules with pentagonal bipyramidal coordinated niobium. The ¹H-NMR spectra of CDCl₃ solutions indicate a fluxional behaviour for the three complexes.     

Linearity relationship between solvatochromic shifts of stilbazolium-like dyes and modified reaction field function

We analyzed statistically the linear correlation of the solva-tochromic shifts of the stilbazolium-like dyes in the nonselected solvents with the reaction field function, L(εr) - bL( n2), and the solvent polarity parameter, ETN, respectively, and observed that there were not perfectly linearity relationships between them, so we introduced ETN into L(εr) - bL(n2) to form a new reaction field function, L(εr) - bL(n2) g ETN, called as the modified reaction field function, which can be perfectly linearly correlated with the solvatochromic shifts of the stilbazolium-like dyes in the nonselected solvents.     

Solvent Effects on the NMR Chemical Shifts of Imidazolium-Based Ionic Liquids and Cellulose Therein

The interactions of ionic liquids (IL) with solvents usually used in liquid-state nuclear magnetic resonance (NMR) spectroscopy are studied. The ¹H- and ¹³C-NMR chemical shift values of 1-n-butyl-3-methyl (BM)- and 1-ethyl-3-methyl (EM)-substituted imidazolium (IM) -chlorides (Cl) and -acetates (Ac) are determined before and after diluting with deuterated solvents (DMSO-d₆, D₂O, CD₃OD, and CDCl₃). The dilution offers structural modifications of the IL due to the solvents capacity to ionization. For further investigation of highly viscous cellulose dopes made of imidazolium-based IL, solid-state NMR spectroscopy enables the reproducibility of liquid-state NMR data of pure IL. The correlation of liquid- and solid-state NMR is shown on EMIM-Ac and cellulose/EMIM-Ac dope (10 wt %).     

Regioselective Dendritic Functionalization of Cellulose

Summary: A series of regioselectively dendronized cellulose derivatives has been prepared by the treatment of cellulose in a N,N‐dimethylacetamide/LiCl solvent system with dendrons possessing isocyanate focal substituents. These new materials were characterized using FT‐IR and ¹³C NMR spectroscopies, and thermogravimetric analysis; the products were highly soluble in a wide range of organic solvents.

Substitution pattern of the dendronized cellulose.     

The effect of solvents on the 13C NMR chemical shifts of the carbonyl carbon and the rotational barriers of N,N-dimethylbenzamide

The ¹³C solvent induced chemical shifts (SICS) of the carbonyl carbon and the thermodynamic barriers to rotation about the C? N bond of N,N-dimethylbenzamide are linearly related to the solvent parameter, E_T(30). A multi-parametric solvent parameter approach indicates that the SICS are influenced equally by polar effects and hydrogen-bond donor effects. Rotational barriers for N,N-dimethylbenzamide may, in principle, be determined by measurement of the ¹³C chemical shift of the carbonyl carbon in a particular solvent.     

Kinetics and Mechanism of Izomerization of N-Alkoxycarbonyl-5-aroxytetrazoles

The kinetics and mechanism of the N²-N¹-isomerization of 2-methoxycarbonyl-5-(p-X-phenoxy)- tetrazoles (X = H, CH₃, NHCOCH₃, Cl, Br, NO₂) were studied by ¹H NMR spectroscopy in a DMSO-d ₆-CDCl₃ mixture (25:75). The rate of isomerization of the N²-isomer into N¹-isomer fit the first-order equation (after three half-conversion periods). The isomerization is accompanied by hydrolysis and decarboxylation. The Hammett plot of ln(k _X k _H) for the isomerization showed a good correlation with ^- values (^- = 1.33, r = 0.965). A poor correlation with values was obtained. The kinetic data, the effect of solvent polarity, the substituent effects, and the results of AM1 quantum-chemical calculations suggest an ionic mechanism of the isomerization in polar solvents and a concerted mechanism in nonpolar solvents.     

Metal Complexes with Macrocyclic Ligands. XV. The Complexation Kinetics of Open Chain and Cyclic Tetraazaligands with Ni2+ in DMSO and DMF

The complexation kinetics of 2,6,9, 13-tetraazatetradecane (1) , 1,4,8, 11-tetraazacyclotetradecane (2) and N,N′,N″,N'-tetramethyl-1,4,8, 11-tetraazacyclotetradecane (3) with Ni²⁺ were studied by the stopped-flow technique in DMSO and DMF. The biomecular rate constants k_L^Ni (Table 2) follow in both solvents the order 1 ? 2 > 3. The similar complexation rates of 1 and 2 in their unprotonated form indicate that for both the open chain and the cyclic ligand the same mechanism holds. By comparison with the solvent exchange the rate determining step of the complexation is the dissociation of the first solvent molecule in the outer-sphere complex. The lower reactivity of 3 is probably due to steric effects. In the case of 2 a second step in the complexation process was observed and explained by a rearrangement of the ligand already coordinated to the metal ion.     

Interaction and Scale of Mixing in Cellulose Acetate/Poly(<Emphasis Type="Italic">N</Emphasis>-vinyl Pyrrolidone-co-vinyl Acetate) Blends

Binary blends and pseudo complexes of cellulose acetate (CA) with vinyl polymers containing N-vinyl pyrrolidone (VP) units, poly(N-vinyl pyrrolidone) (PVP) and poly(N-vinyl pyrrolidone-co-vinyl acetate) [P(VP-co-VAc)], were prepared, respectively, by casting from mixed polymer solutions in N,N-dimethylformamide as good solvent and by spontaneous co-precipitation from solutions in tetrahydrofuran as comparatively poor solvent. The scale of miscibility and intermolecular interaction were examined for the blends and complexes by solid-state ¹³C-NMR spectroscopy. It was revealed that the formation of complexes was due to a higher frequency of hydrogen-bonding interactions between the residual hydroxyl groups of CA and the carbonyl groups of VP residues in the vinyl polymer component. From measurements of CP/MAS spectra and proton spin-lattice relaxation times (T_1ρ^H) in the NMR study, the existence of the hydrogen-bonding interaction was also confirmed for the miscible blends and the homogeneity of the mixing was estimated to be substantially on a scale within a few nanometers.     

Phase diagram of the ternary system NMMO/water/cellulose

The phase diagram of the system N-methylmorpholine-N-oxide(NMMO)/H₂O/cellulose has been measured at 80 °C by establishing a solubility map (observation of the mixtures under the microscope), by the analysis of coexisting phases and determining the critical point. These experiments manifest a continuous reduction of the two phase area existing for the subsystem H₂O/cellulose upon the addition of NMMO, where a weight fraction of NMMO in the mixed solvent exceeding 75 wt% is required for Solucell 400 to reach the critical composition. The critical cellulose concentration is only 0.34 wt%, i.e., more than an order of magnitude lower than for the solutions of typical vinyl polymers in mixed solvents. All experimental observations can be well modeled on the basis of composition dependent binary interaction parameters by means of recently established mixing rules. For the subsystems H₂O/cellulose and NMMO/water the corresponding data are known from independent earlier measurements. The adjustment of two parameters to the ternary phase diagram was required to obtain this information for NMMO/cellulose, the third binary subsystem.