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1.
Using several N-methyl pyrazole analogs the utility of 13C nmr in determining isomeric structures was examined. The chemical shift assignments of the pyrazole carbons and thus the isomeric structures were determined using a combination of proton coupled and proton decoupled 13C nmr spectra.  相似文献   

2.
Four solvents of cellulose have been studied by using 13C-NMR spectroscopy. All these solvents, N-methyl morpholine-N-oxide, methylamine, hydrazine, and paraformaldehyde (PF), contained dimethyl sulfoxide (DMSO) as a cosolvent. Oligomers of cellulose of DP = 10 soluble in hot DMSO have been used as model compounds. 13C chemical shifts and line shapes show that three of the mentioned solvents are “true solvents” of cellulose. On the other hand, dissolution of cellulose in DMSO-PF system occurs by the formation of a statistical derivative of cellulose. Enriched 13C bacterial cellulose on C-1 and C-6 positions have been used to identify the 13C positions mainly in DMSO-N-methyl morpholine-N-oxide system. This solvent has been found to be degradative for the macromolecule when the solution is kept at 100°C over a long period. Viscosity measurements show a reduction of the molecular weight in these conditions. Polarimetry indicates that no glucose is present in solution and hence there is a statistical break of the chain. Enriched cellulose solution in DMSO–N-methyl morpholine-N-oxide has been also used for relaxation time (T1) determination both of the solvent and of the enriched carbons of the polymer. Nuclear Overhauser enhancement (NOE) was found to be 1.8 for C-1 and 2.1 for C-6 showing that relaxation phenomenon is not purely dipolar. T1 values of 97 and 65 msec are found for C-1 and C-6 of cellulose, in good agreement with the values known for polysaccharides. Determination of T1 for the different carbon atoms of the solvent DMSO-N-methyl morpholine-N-oxide with and without cellulose shows a large reduction of T1 for N-methyl morpholine-N-oxide molecule. This denotes a slower molecular motion of this molecule and a preferential interaction with the cellulose macromolecule.  相似文献   

3.
The 13C NMR spectra of a series of 4-t-butylcyclohexane-O-methyl oximes and N,N-dimethylhydrazones containing α-methyl substituents have been measured. The effects of α-methyl substitution (syn-axial, anti-axial and anti-equatorial) on the carbons α, β and γ to the substitution site are similar to those previously found in cyclohexanones. Use has been made of these substituent parameters to determine the conformational equilibria of the mobile 2-methylcyclohexanone derivatives.  相似文献   

4.
The natural abundance 13C NMR spectra of 2,2′-dimethyl-, 2,2′-dimethoxy- and 2,2′-dihydroxybiphenyls, and a series of 2,2′-dimethoxy-5,5′-disubstituted biphenyls were recorded. Unambiguous signal assignments of the carbons ortho and para to an aryl ring in biphenyls were made by selective deuteration and/or the graphical method for 1H single frequency off-resonance decoupled spectra. Contrary to the reported assignments, it was shown that the signal for C-6 in 2,2′-dimethylbiphenyl clearly appears at lower field than that for C-4. The signals for the ortho carbons (C-6) of 2,2′-dimethoxy-5,5′-disubstituted biphenyls generally appeared at lower fields than those for the para carbons (C-4). The validity of applying deuterium isotope shifts to the assignments of 13C chemical shifts of di- and tetra-substituted biphenyls is also discussed.  相似文献   

5.
The 1H and 13C spectra of fluorene, fluorenone, phenanthrene and their 4-methyl and 4,5-dimethyl derivatives have been examined. To complete the analyses for fluorenone and 4-methylfluorenone, 1H spectra were recorded at 270 MHz. The results from the 1H spectra permitted unequivocal assignments for the protonated aryl carbons by selective proton decoupling. A consistent set of assignments for the quaternary carbons was obtained through consideration of the dominant relaxation processes operative at these centres. This series of compounds was examined to investigate the shielding effects produced by the close approach of methyl groups separated by five bonds for comparison with the contrasting trends found for methyl carbons separated by three and four bonds. The results indicate that the relative orientation of the methyl groups is an extremely critical factor governing their shieldings and those of neighboring centres.  相似文献   

6.
The NMR spectra of ten disubstituted amides have been recorded at 0°C in carbon tetrachloride and in benzene solutions. The benzene dilution shifts (ASIS) and proton spin decoupling were used to make the chemical shift assignments. A time-averaged solvent cluster model for the association between the amide and benzene is consistent with the observed ASIS values. The assignments for the N-methine and N-methyl resonance peaks in RCON[CH(CH3)2]2, where R is methyl, ethyl or propyl are inverted from the assignments for N,N-diisopropylformamide (R ? H).  相似文献   

7.
The 13C n.m.r. spectra of 11 derivatives of 2,3-dimethylenenorbornane, 1–11, of 5 derivatives of 2,3-dimethylene-7-oxanorbornane, 12–16, and of 2,3,5,6-tetramethylene-7-oxanorbornane (17) have been measured and the chemical shifts have been assigned. The effects of 1-methyl, 5-hydroxy, 5-acetoxy, 5-para-bromobenzenesulphonyloxy and 5-keto substituents on the olefinic carbons of the s-cis-butadiene group are compared with the same substituent effects reported for model compounds. Apparent linear correlations between the reciprocals of the VN transition energies of the butadiene chromophores and the differences of the chemical shifts ΔδC between the quaternary and methylene olefinic carbons are found for the dienes 1–3, 12–14, butadiene and 2,3-dimethylbutadiene. The ΔδC of the olefinic carbons of the tetraene 17 also falls on the correlation line if the average of the two absorption hands at 250 and 228 nm is taken for the VN transition energy of this compound. The chemical shift of the carbon of the methano bridge H2C-7 is almost insensitive to the presence of one or two methylene groups at C-2,3, in contrast with the downfield shift of 10–14 ppm observed when an endocyclic double bond is introduced into the norbornane skeleton.  相似文献   

8.
Efficient procedures were developed for N-deethylation of lappaconitine to give N(20)-deethyllappaconitine. Alkyl derivatives of N(20)-deethyllappaconitine, including labeled lappaconitine, and N(20)-acetoxy-N(20)-deethyllappaconitine were prepared for the first time. The assignments of the signals for the carbon atoms in the 13C NMR spectra of lappaconitine and related lappaconine were refined using 13C—13C 2D INADEQUATE and 2D 13C—1H correlation experiments.  相似文献   

9.
N-Mono and N,N-dialkyl/diarylbarbituric acids exist in solution as a single tautomer. The 13C nmr spectroscopy shows that they are present in the triketo form in a number of polar and non-polar solvents. 2-Thiobarbituric acid derivatives, however, show extensive tautomerization. Their 13C chemical shift assignments were achieved by utilizing models 11a , 11c , 12b and 12d and from which relative tautomer distribution ratios were determined. These ratios were correlated with the dielectric constant of the various solvents (?). Thio-barbituric acids also formed adducts with solvents having carbonyl groups, characteristic observed only with barbiturates possessing the thione or thiophenolic group. 6-Amino and 6-methyluracils and thiouracils exist in DMSO solution as stable “ene” forms as do orotic acid, 24 , and its thio analogue 25. Compound 25 undergoes disproportionantion and tautomerization when heated or on prolonged standing in solution. Literature contradictions regarding the structure of “4,6-dihydroxypyrimidine,” 26 , were resolved and its tautomers in solution correctly assigned by 13C nmr. Anions of barbiturics and related systems exist in one of the two possible types A and B, depending on whether ring nitrogens are substituted (type A), or not (type B). Rapid H/D exchange at C5 was evident from C-deuterium coupling. The redistribution of charge through C4(C6) carbonyl groups shown by 13C shifts of carbonyl carbon atoms of up to 10 ppm as compared to the CO carbons of the neutral species was evident.  相似文献   

10.
Carbon-13 chemical shift assignments are reported for benzo[b]thiophene and 1-(X-benzo[b]thienyl)ethyl acetate derivatives, where X=? CH(OAc)CH3 substituted at positions 2-7. Substituent chemical shift (SCS) effects for the ethyl acetate group are additive at all positions. A substantial upfield shift was observed at C-3, arising from the peri interaction of H-3 and the 4-ethyl acetate substituent. Carbon-13 relaxation times (T1) and nuclear Overhauser enhancements (η) have been measured for benzo[b]thiophene and its derivatives, and the contributions of dipolar, TDD1, and spin rotation, TSR1, relaxation have been determined. Intramolecular dipole–dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism whenever protons are bound directly to the carbons under investigation. Nonprotonated ring carbons are relaxed by both DD and SR mechanisms. Anisotropic motion has an easily observable effect on the DD contribution to T1, and can form the basis for spectral assignments, as in 1-phenylethyl acetate. Long-range 13C? 1H coupling constants were observed both between ring carbons and between ring carbons with ring side-chain hydrogens. These results have been used for the structure determination of the title compounds.  相似文献   

11.
An efficient approach to NMR assignments in intrinsically disordered proteins is presented, making use of the good dispersion of cross peaks observed in [15N,13C′]‐ and [13C′,1HN]‐correlation spectra. The method involves the simultaneous collection of {3D (H)NCO(CAN)H and 3D (HACA)CON(CA)HA} spectra for backbone assignments via sequential HN and Hα correlations and {3D (H)NCO(CACS)HS and 3D (HS)CS(CA)CO(N)H} spectra for side‐chain 1H and 13C assignments, employing sequential 1H data acquisitions with direct detection of both the amide and aliphatic protons. The efficacy of the approach for obtaining resonance assignments with complete backbone and side‐chain chemical shifts is demonstrated experimentally for the 61‐residue [13C,15N]‐labelled peptide of a voltage‐gated potassium channel protein of the Kv1.4 channel subunit. The general applicability of the approach for the characterisation of moderately sized globular proteins is also demonstrated.  相似文献   

12.
1H NMR chemical shifts for some α-hetero-substituted N,N-diethylacetamides were recorded. The resonance assignments for the syn- and anti-methylene and -methyl protons have been made unambiguously through their aromatic solvent induced shifts and are opposed to the literture assignments for the N-methylene protons. An empirical relationship between the Charton polar (σL) and steric (V) parameters and the α-methylene proton resonances was found. The N-methylene proton chemical shifts also showed a qualitative dependence on the α-substituent electronegativity, while the N-ethyl methyl proton chemical shifts were related to the α-substituent steric effects. The Paulsen and Todt anisotropic model and the more populated rotamers proposed seem to explain the results very well.  相似文献   

13.
The 1H and 13C NMR chemical shifts of 1,2,3,4,5,6,7,8‐octahydroacridine, 12 of its 9‐substituted derivatives, and of the corresponding N‐oxides were determined, assigned, and discussed in terms of 9‐substituent effects and effects of N‐oxidation. A good linear correlation was found between the 13C chemical shifts of the aromatic carbons in octahydroacridines and those of respective carbons in the corresponding N‐oxides. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

14.
The calculation of the 13C and 15N NMR chemical shifts by a combined molecular mechanics (Pcmodel 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31 + G(d)) procedure is used to investigate the conformations of a variety of alkyl substituted anilines. The 13C shifts are obtained from the GIAO isotropic shielding (Ciso) with separate references for sp3 and sp2 carbons (δc = δref − Ciso). The 15N shifts are obtained similarly from the GIAO isotropic shielding (Niso) with reference to the 15N chemical shift of aniline. Comparison of the observed and calculated shifts provides information on the molecular conformations. Aniline and the 2,6-dialkylanilines exist with a rapidly inverting symmetric pyramidal nitrogen atom. The 2-alkylanilines have similar conformations with the NH2 group tilted away from the 2-alkyl substituent. The N,N-dialkylanilines show more varied conformations. N,N-dimethylaniline has a similar structure to aniline, but N-ethyl, N-methylaniline, N,N-diethylaniline, and N,N-diisopropylaniline are conformationally mobile with two rapidly interconverting conformers. In contrast, the anilines substituted at C2 and the nitrogen atom exist as one conformer where the steric interaction between the C2 substituent and the N substituent determines the conformation. In 2-methyl-N-methylaniline, the nitrogen atom is pyramidal as usual with the N-methyl opposite to the 2-methyl, but in 2-methyl-N,N-dimethyl aniline, the NMe2 group is now almost orthogonal to the phenyl plane. This is also the case with 2-methyl-N,N-diethylaniline and 2,6-diisopropyl-N,N-dimethylaniline. The comparison of the observed and calculated 15N chemical shifts confirms the above findings, in particular the pyramidal conformation of aniline and the above observations with respect to the conformations of the N,N-dialkylanilines.  相似文献   

15.
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.  相似文献   

16.
The 13C NMR spectrum (at natural abundance) of monomeric chlorophyll α in acetone-d6 has been recorded to re-examine the assignments of the low field (aromatic-olefinic) region of the spectrum. The assignments, made by the examination of the fully coupled spectrum and by the use of long-range selective 1H decoupling (LSPD) with low-power irradiation, were compared with those of the previous reports. The results of the present work clarify the ambiguities previously encountered in the assignment of the 10a-ester, 7c-propionyl, P-2-phytyl, 2b-vinyl, γ- and β-methine carbon atoms, as well as the β-pyrrolic carbon-6 and α-pyrrolic carbons ?16 and ?17 of chlorophyll α. Reassignment of the three last carbons was found necessary. Knowledge of the chemical shifts of these carbon atoms was considered to be particularly valuable, as it yields relevant information on the delocalized π electron system which is crucial for the function of chlorophyll in photosynthesis.  相似文献   

17.
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.  相似文献   

18.
The 13C NMR spectrum of 6H-pyrido[4,3-b]-5,11-dimethylcarbazole has been re-examined to confirm the assignments using normal decoupled and coupled INEPT techniques. The results indicate that there were several errors in previous assignments. J(13C, 1H) for the seven methine carbons have also been obtained.  相似文献   

19.
The 13C NMR spectra of some derivatives of bryonolic acid (1) (D:C-friedoolean-8-en-3β-ol-29-oic acid) were assigned by means of 13C-enrichment, lanthanide-induced shifts (LIS) and comparison of chemical shift data between derivatives. The 13C-enriched species of 1, i.e., 1a, 1b and 1c were biosynthesized by Luffa cylindrica (Cucurbitaceae) callus fed with [1-13C]-, [2-13C]- or [1,2-13C2]-acetate, respectively. Methyl acetylbryonolates 2, 2a, 2b and 2c, methyl bryonolates 3, 3a, 3b and 3c, methyl bryononates 4 and 4a, diacetyl-3β,29-diols (3,29-diacetyl-D:C-friedoolean-8-en-β,29-diol) 5, 5a, 5b and 5c, and 3-acetyl-3β,29-diols 6, 6a and 6b were prepared from 1, 1a, 1b and 1c, and their 13C NMR spectra were recorded. The 13C concentration of the 13C-enriched species was high enough to exhibit the satellite peaks clearly, and the analysed data were very useful for this study. Thus, total assignments for 2, 3, 4, 5 and 6 were established. It was found that conversion of the methoxycarbonyl group at C-29 into an acetoxymethyl group caused complex changes in the chemical shifts of the C, D- and E-ring carbons and those of the methyl carbons linked to these rings.  相似文献   

20.
A link between a substructure searching system and a 13C NMR data base has been established and permits the retrieval and examination of the chemical shifts associated with specific substructures. The means by which these searches are accomplished is described and the results from the searches are presented and discussed. The system is interactive, and can be used to locate in the data base the chemical shifts of carbon atoms in precisely defined environments. Alternatively, it may be used to learn the range of the chemical shifts possessed by particular types of carbon atoms, such as N-methyl or O-methyl carbons.  相似文献   

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