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.     

A Structural Study of Epoxidized Natural Rubber (ENR-50) and Its Cyclic Dithiocarbonate Derivative Using NMR Spectroscopy Techniques

A structural study of epoxidized natural rubber (ENR-50) and its cyclic dithiocarbonate derivative was carried out using NMR spectroscopy techniques. The overlapping ¹H-NMR signals of ENR-50 at δ 1.56, 1.68-1.70, 2.06, 2.15-2.17 ppm were successfully assigned. In this work, the ¹³C-NMR chemical shift assignments of ENR-50 were consistent to the previously reported work. A cyclic dithiocarbonate derivative of ENR-50 was synthesized from the reaction of purified ENR-50 with carbon disulfide (CS₂), in the presence of 4-dimethylaminopyridine (DMAP) as catalyst at reflux temperature. The cyclic dithiocarbonate formation involved the epoxide ring opening of the ENR-50. This was followed by insertion of the C-S moiety of CS₂ at the oxygen attached to the quaternary carbon and methine carbon of epoxidized isoprene unit, respectively. The bands due to the C=S and C-O were clearly observed in the FTIR spectrum while the ¹H-NMR spectrum of the derivative revealed the peak attributed to the methylene protons had split. The ¹³C-NMR spectrum of the derivative further indicates two new carbon peaks arising from the >C=S and quaternary carbon of cyclic dithiocarbonate. All other ¹H- and ¹³C-NMR chemical shifts of the derivative remain unchanged with respect to the ENR-50.     

Structure de modèles de polyesters-4—1: Structure et conformation d'oligomères polytétraméthylène téréphtalate par diffraction des rayons X Et RMN haute résolution dans les solides (RAM-DPC)

The structures of polytetramethylene terephthalate (PTMT) and selected model compounds are described for the solid state [X-ray, ¹³C-NMR (magic angle spinning, and cross polarization)] and in solution. Compounds representative of the unstressed (α) crystalline form of PTMT have isotropic chemical shifts different from those of the stressed (β) form. The chemical shifts are interpreted in terms of conformational differences in the tetramethylene segments.     

Nitrogen bridgehead compounds. Part X (1). 1H and 13C nmr study of pyrido[1,2-a]pyrimidines

By ¹H nmr, the predominance of the conformer with a quasi-axial 6-methyl group was established for the pyrido[1,2-a]pyrimidines I-III. The ¹³C nmr spectra of I-III were completely assigned. The conjugative effect of substitution at G-3 on ¹H and ¹³C chemical shifts of the C-9 methylene group parallels its effect on reactivity.     

Carbon-13 as a tool for the study of carbohydrate structures, conformations and interactions

The application of 13C-NMR spectroscopy to problems involving the structures and interactions of carbohydrates is described. Both 13C-enriched and natural abundance compounds were used and some advantages of the use of the stable isotope are described. Carbon-carbon and carbon-proton coupling constants obtained from 1-13C enriched carbohydrates were employed in the assignment of their chemical shifts and to establish solution conformation. In all cases studied thus far, C-3 couples to C-1 only in the beta-anomers while C-5 couples to C-1 only in the alpha-anomers. C-6 and C-2 always couple to C-1 in both anomeric species. The alkaline degradation of glucose [1-13C] to saccharinic acids was followed by 13C-NMR. The conversion of glucose [1-13C] to fructose-1, 6-bisphosphate [1, 6-13C] by enzymes of the glycolytic pathway was shown as an example of the use of 13C-enriched carbohydrates to elucidate biochemical pathways. In a large number of glycosyl phosphates the 31P to H-1 and 31P to C-2 coupling constants demonstrate that in the preferred conformation and phosphate group lies between the O-5 and the H-1 of the pyranose ring. The influence of paramagnetic Mn2 + ions on the proton decoupled 13C-NMR spectra of uridine diphosphate N-acetylglucosamine indicates that the Mn2 + interacts strongly with the pyrophosphate moiety and with the carbonyl groups of the uracil and N-acetyl groups.     

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.     

Asymmetric Syntheses V: Asymmetric Alkylation of D-Camphor Imine of 1-Aminomethyl Naphthalene

Alkylation of the D-camphor imine of 1-amino-methyl naphthalene with a variety of alkylating agents gave diastereoselectivities ranging from 20–64%. The de ratio in the alkylation could be determined by the chemical shift difference of ¹H-NMR signals of the C-8 methyl groups or by ¹³C-NMR signals in the C-7 of compound (6).     

13C-NMR.-Substituenteninkremente der N3-Gruppe in Azido-1,4-benzochinonen

¹³C-NMR. Substituent Increments of the Azido-group in 1,4-Benzoquinones The empirical substituent increments of the azido-group in 1,4-benzoquinones have been derived from the ¹³C-NMR. chemical shifts of 2-azido-5-chloro-1,4-benzoquinone. The validity of the obtained values was tested by comparison of the empirical and computed ¹³C-NMR. chemical shifts of other azido-chloro-1,4-benzoquinones.     

Investigation of substituent effects on the 1H and 13C NMR spectra of (Z)-N-(arylmethylene)-arylamine N-oxides (α,N-diaryl nitrones)

Substituent effects on the ¹H and ¹³C chemical shifts of 18 differently substituted (Z)-α,N-diaryl nitrones [N-(p-X-benzylidene)phenylamine N-oxides (Series I) and N-(benzylidene)-p-Y-phenylamine N-oxides (Series II)] have been obtained. A correlation has been found between the chemical shifts of the azomethine proton (H-α) and the Hammett σ parameters and the Swain and Lupton F and R parameters. Correlations of the chemical shifts of C-1 and C-4′ in Series I, and of C-α and C-1′ in Series II, with the same parameters have been investigated. In addition, the chemical shifts of the aromatic protons and carbons of the p-disubstituted (m-disubstituted in one case) benzene rings correlated with the appropriate substituent increments (Z_i). These correlations confirm the dual behaviour of the nitrone group and the presence of through-resonance in these nitrones.     

Synthesis and characteristics of the poly(carboxybetaine)s and the corresponding cationic polymers

Two types of carboxybetaines and their corresponding cationic monomers and polymers are synthesized in this study. Comparing the chemical shifts of the methylene groups in the cationic monomers and carboxybetaines in both ¹H- and ¹³C-NMR spectra reveal that the respective methylene groups are clearly distinguished from their chemical shifts in ¹H- and ¹³C-NMR spectra. The solubilities, moisture regain properties, and solution properties of the poly(carboxybetaine)s and cationic polymers are investigated in relation to their molecular structures. Because the cationic polymers were ionized in an aqueous solution, the cationic polymers were more soluble than the poly(carboxybetaine). For the various functional groups of poly(carboxybetaine)s and cationic polymers, the order of tendency for moisture regain is  COO >  CONH . Results obtained from the reduced viscosity for cationic poly(TMMPAMS) are reversed from that for zwitterionic poly(DMAEAPL). © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3527–3536, 1997     

Melamine–formaldehyde resins: Constitutional characterization by fourier transform 13C-NMR spectroscopy

The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by ¹³C-NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A ¹³C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a ¹³C-NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.     

Carbon-13 nuclear magnetic resonance spectra. VIII—18α- and 18β-glycyrrhetic acid derivatives

The ¹³C NMR data of six pairs of 18α/18β-glycyrrhetic acid derivatives are presented. It is shown that the configuration at C-18 can easily be recognized by inspecting the chemical shifts of several characteristic carbons, e.g. C-12, C-13, C-18 and C-28. The shifts of these carbons originated by the change of the D/E ring junction proved to be largely independent of the substitution at C-3 and C-20.     

13C-NMR. Spectra of Natural and Semi-synthetic Cannabinoids

The ¹³C-NMR. spectra of one natural and ten semi-synthetic cannabinoids were analyzed in detail. Assignments of the signals are based on their chemical shifts, splitting patterns in ¹H-off-resonance decoupling experiments and comparison with ¹³C-NMR. data of related cannabinoids. With some compounds final assignments were made by selective ¹H-decoupling experiments and incremental calculations.     

Conformational Analysis of the cis- and trans-Isomers of FK506 by NMR and Molecular Dynamics

The potent immunosuppressant drug FK506 ( 2 ) has been examined by ¹H- and ¹³C-NMR spectroscopy and NOE-restrained molecular dynamics to elucidate the conformation in solution. A combination of two- and three-dimensional NMR techniques was used to completely assign the ¹H- and ¹³C-NMR chemical shifts of the two configurational isomers resulting from the cis-trans isomerization about the single amide bond. Hetero- and homonuclear coupling constants were measured to assign the diastereotopic methylene protons at C(16), C(18), and C(23). Intramolecular H? H distances were defined from NOESY spectra recorded at ?30° in CDCl₃ and used as constraints in molecular-dynamics simulations. The conformational preferences of 2 in solution are discussed in light of the constitutional features recently proposed to be necessary for binding and activity.     

Solid-state carbon-13 NMR Studies of vulcanized elastomers. V. Observation of new structures in sulfur-vulcanized natural rubber

The complementary application of the NMR inversion recovery measurements and the computer fitting of the overlapping spectral region is found to be a useful method for structural analysis of vulcanized natural rubber in the solid state. Since the linewidths in ¹³C-NMR spectra of solids are relatively broad compared with the differences between chemical shifts, some weak signals are completely obscured in the resulting spectra. If the resonances have sufficiently different relaxation times, such as methyl and methylene carbons, it is possible to detect neighboring heavily overlapped signals by using the inversion recovery delay τ value at which the interfering strong resonance has null intensity. The 20 resonances observed in the spectra of crosslinked rubbers are tentatively assigned to the structural units formed during the vulcanization process. It is found that vulcanizates containing smaller amounts of sulfur (1 and 3%) show insignificant changes in the NMR spectra for curing times of 30 and 90 min. Structural modifications in rubbers cured with 10% sulfur continuously increase with the increasing curing time up to 120 min, indicating a significant loss in double bonds in the later stages of the reaction.     

13C magnetic resonance studies of reduced proaporphine alkaloids

The natural abundance ¹³C magnetic resonance spectra of reduced proaporphines are reported, with complete assignments based on peak multiplicity and empirical calculations of chemical shifts. The chemical shift values of C-8, C-12 and C-7 are found to be strongly diagnostic of the spiro carbon configuration.     

A simple method to determine the anomeric configuration of sialic acid and its derivatives by 13C-NMR

The anomeric configuration of sialic acid and its derivatives could be determined on the basis of the coupling pattern of C-1 in the gated proton-decoupled or selective proton decoupled ¹³C-NMR spectra; the α anomer gave a doublet C-1 signal while the β gave a singlet.The α-anomers gave a doublet C-1 signal while the β gave a singlet in their selective proton-decoupled spectra.     

Phosphol-3-ene 1-oxides and -sulfides. A systematic investigation via carbon-13 nuclear magnetic resonance spectroscopy

¹³C NMR chemical shifts and ¹³C? ³¹P couplings are reported for 18 phosphol-3-ene 1-oxides and 18 corresponding sulfides. The effects of methyl substitution at positions 3 and 4 on the carbon shifts have been systematically explored and substituent parameters derived. One bond couplings from phosphorus to C-2 and C-5 have been related to the sum of the exocyclic substituent group electronegativities (covalent boundary potential values).     

Substituent effects on carbon-13 chemical shifts of para-substituted benzylbenzenes

Carbon-13 chemical shifts of fourteen para-substituted benzylbenzenes have been determined. The relative substituent chemical shifts (SCS) of the methylene carbons and the aromatic ring carbons (C-4, C-1′ and C-4′) correlated well with the Hammett substituent effects using the dual substituent parameter method. The transmission of substituent effects through the benzylbenzene system is briefly discussed.     

Über Polygermane. XIX [1]. Empirische Regelin Zur Abschätzung von 13C-NMR chemischen Verschiebungen in phenylieryten Polygermanen

On Polygermanes. XIX. Empirical rules for estimating ¹³C-NMR Chemical shifts in Phenylated polygermanes ¹³C-NMR chemical shifts are given for 85 phenyl groups independently bonded to Ge in 52 acyclic and 23 cyclic Ge compounds. ¹³C-NMR phenyl signals can be estimated from basic values for perorgano substituted homonuclear chains of Ge nd from increments for substitution at the Ge atoms.