Showing papers in "Magnetic Resonance in Chemistry in 1988"
TL;DR: In this paper, basic stereochemical factors have been revealed which affect the 13C chemical shifts in glycosides with a chiral cyclohexane-type aglycone and in disaccharides with an Aglycone.
Abstract: Basic stereochemical factors have been revealed which affect the 13C chemical shifts in glycosides with a chiral cyclohexane-type aglycone and in disaccharides with a pyranosidic aglycone. The effects of glycosylation depend on the configuration of the anomeric centre of the glycosylating sugar residue (glycone) and on the relative absolute configuration of the glycone and aglycone. When at least one of the carbons of the aglycone, adjacent to the glycosylated carbon, bears an equatorial proton, this dependence is well accounted for by a strengthening or weakening of the spatial interaction between the protons of the glycone and aglycone, caused by a change in the conformation around the glycosidic linkage. Regularities in the glycosylation effects for disaccharides are generalized for various modes of substitution and different general configurations of the pyranosidic aglycones. Simple rules have been formulated for the prediction of the magnitudes of the glycosylation effects which can be used for the determination of some unknown structural elements in glycosides and disaccharides, in particular the absolute configuration of one of the constituent sugars.
322 citations
TL;DR: In this article, the 13C chemical shifts and a large collection of coupling constants have been measured for 169 N-substituted azoles with no other substituents on the ring.
Abstract: The 13C chemical shifts and a large collection of coupling constants have been measured for 169 N-substituted azoles with no other substituents on the ring. Simple additive models have been used to discuss both the chemical shifts and the coupling constants. The chemical shift of the substituent depends on the nature of the azole which can, accordingly, be classified into three to six families. Some structural problems (ring-chain isomerism of 1-trifluoromethylsulphonyl-1,2,3-triazole, silylotropy and stannotropy) are also discussed.
138 citations
TL;DR: In this article, the authors compared the problems inherent in the reoptimization of the Freeman-Morris sequence with the development of a number of other of experiments, most recently, including reverse (proton) detected heteronuclear multiple quantum techniques.
Abstract: Long-range heteronuclear chemical shift correlation has undergone a period of burgeoning growth during the past 4 years. Initially, such correlations were made by simply reoptimizing the conventional heteronuclear chemical shift correlation sequence originally devised by Freeman and Morris. Problems inherent in the reoptimization of the Freeman–Morris sequence have led to the development of a number of other of experiments, most recently, including reverse (proton) detected heteronuclear multiple quantum techniques. The techniques now in use are compared and the inherent problems of each are identified and weighed against advantages. Applications of each technique are also presented.
102 citations
TL;DR: In this paper, the influence of inclusion complex formation between cyclodextrins and a variety of chiral antihistaminic and analgesic agents on the 1H NMR features of both guest and host partners is reported.
Abstract: The influence of inclusion complex formation between cyclodextrins (chiefly the β-polymer) and a variety of chiral antihistaminic and analgesic agents on the 1H NMR features of both guest and host partners is reported. In many cases equivalent protons of antipodes give resonances which differ in chemical shift after inclusion, and the potential value of the data to chiral analysis and to elucidation of the structures of the inclusion complexes is demonstrated.
89 citations
TL;DR: In this article, the authors described the current status of this important field with particular reference to advancements in applications to DNA structures, and provided a comprehensive overview of the current state of the art in this field.
Abstract: Two-dimensional NMR has developed into an important tool for the study of solution structures of nucleic acids. The conformational information is generally derived from the three bond coupling constants (J) and interproton distances (r). Strategies have been developed to obtain J values using COSY and J-resolved spectroscopy. Estimation of interproton distances involves measurement of NOESY build-up curves as a function of mixing time. This information can be coupled with structural refinement techniques to obtain the three-dimensional structure of nucleic acids. Here we have described the current status of this important field with particular reference to advancements in applications to DNA structures.
78 citations
TL;DR: In this paper, the first cis-trans isomerism in monoalkylhydroxamic acids was detected by 1H, 13C and 15N NMR spectroscopy.
Abstract: The first example of cis–trans isomerism in monoalkylhydroxamic acids, detected by 1H, 13C and 15N NMR spectroscopy, is reported The 15N NMR spectrum of CH3CO15NHOH gave a clear assignment of the OH and NH protons of both Z and E isomers The assignment was confirmed by 1H and 13C NMR spectroscopy A rotational barrier of isomerization (ΔGc≠) of 178 kcal mol−1 was calculated from the variable-temperature 1H NMR spectra using the method of Shanan-Atidi and Bar-Eli
62 citations
TL;DR: In this article, the authors used pure absorption 6Li, 1H HOESY for the detection of short lithium-proton distances and for proton spectral assignments of 1,8-dilithio-1,2-diphenylhex-1-ene, which exists as a monomer-dimer equilibrium mixture in tetrahydrofuran.
Abstract: 6Li, 1H HOESY was used both for the detection of short lithium–proton distances and for proton spectral assignments of 1,8-dilithio-1,2-diphenylhex-1-ene, which exists as a monomer–dimer equilibrium mixture in tetrahydrofuran (THF-d8). Experiments using magnitude mode, pure absorption and pseudo-steady-state mode representations are described. Pure absorption 6Li, 1H HOESY proved to be the superior method.
53 citations
TL;DR: In this paper, an indirect two-dimensional (X, M) (X = 1H, 31P) triple resonance spectroscopy approach is presented for the detection of the NMR parameters of insensitive metal nuclei M = 57Fe, 103Rh, 183W and 61Ni which exhibit a scalar coupling to the more sensitive X nucleus.
Abstract: Indirect two-dimensional (X, M) (X = 1H, 31P) triple resonance spectroscopy is presented for the detection of the NMR parameters of insensitive metal nuclei M = 57Fe, 103Rh, 183W and 61Ni which exhibit a scalar coupling to the more sensitive X nucleus. The versatlity of this approach is demonstrated and compared with the previously used detection schemes by recording δ(M) and J(M, X) for a series of organometallic phosphorus tungsten(IV), rhodium(I), iron(0), iron(II) and nickel(0) complexes. The NMR data of the complexes are discussed.
48 citations
TL;DR: In this paper, long-range J(CH) values were measured for six glycoside derivatives by a selective 2D-J heteronuclear sequence and the values obtained are discussed in terms of conformational effects and constitute a valuable contribution to 2J and 3J comprehension.
Abstract: Long-range J(CH) values were measured for six glycoside derivatives by a selective 2D-J heteronuclear sequence. The values obtained are discussed in terms of conformational effects and constitute a valuable contribution to 2J and 3J comprehension.
44 citations
TL;DR: In this paper, the authors performed triple resonance studies of 1,4-dihydro-1,3-diphenyl- (1a) and 3-tert-butyl-1.4-dimethyl substituted derivatives (2a and 2b) and showed that the spin density at N-4 is the position of highest spin density which determines the 1H and 14N hyperfine coupling (HFC) pattern.
Abstract: ESR, ENDOR and triple resonance studies of 1,4-dihydro-1,3-diphenyl- (1a) and 3-tert-butyl-1,4-dihydro-1-phenyl-1,2,4-benzotriazinyl (2a) and of the corresponding radical cation 3a (2a protonated at N-4) yielded the magnitude and the sign of almost all of the 1H and 14N hyperfine coupling (HFC) constants. Their assignment was performed using various 2H labelled and/or methyl substituted derivatives. Both radical groups show major hyperfine coupling to the three 14N nuclei. In the uncharged species 1a and 2a, the spin density at N-4 apparently directs the 1H HFC pattern in the 1,2,4-benzotriazinyl component, a(H-5) and a(H-7) being larger than a(H-6) and a(H-8). In the radical cation, on the other hand, N-1, with an unusually large splitting of 9.9 G, is the position of highest spin density which determines the 1H HFC pattern, a(H-6) and a(H-8) being larger than a(H-7).
40 citations
TL;DR: In this article, a simple approach is described for simulating 13C line shapes for arbitrary values of quadrupole parameters, CX (X = 14N, 2H) distances, external field and orientation of the internuclear vector in the axis system where the electric field gradient tensor is diagonal.
Abstract: Solid-state 13C NMR spectra of 14N-containing compounds obtained under CPMAS conditions often show asymmetric doublets arising from the unaveraged 13C,14N residual dipolar coupling. A similar result has recently been noticed in deuteriated samples, whose 13C resonances showed the combined effect of 13C,2H dipolar and scalar couplings. A simple approach, easily adaptable to a desk microcomputer, is described for simulating 13C line shapes for arbitrary values of quadrupole parameters, CX (X = 14N, 2H) distances, external field and orientation of the internuclear vector in the axis system where the electric field gradient tensor is diagonal.
TL;DR: In this article, the 1H and 13C chemical shifts of secondary references for D2O and (CD3)2SO solutions were measured as a function of temperature and determined accurately and fitted to a parabola or straight line, as appropriate.
Abstract: Chemical shifts of secondary references for D2O and (CD3)2SO solutions were measured as a function of temperature. The 1H and 13C chemical shifts of dioxane, sodium 4,4-dimethyl-4-silapentanesulphonate, HOD, tetramethylammonium chloride and sodium 3-(trimethylsilyl)propionate-d4 in D2O solution were measured relative to external TMS and the chemical shifts of (CD3)2SO and H2O were measured in (CD3)2SO solution relative to internal TMS. For accurate comparison of chemical shifts it is necessary to take into account the sample temperature and, therefore, the temperature dependences of 1H and 13C chemical shifts of a number of secondary references have been determined accurately and fitted to a parabola or straight line, as appropriate.
TL;DR: In this article, the successful operation of CRAMPS techniques to obtain high-resolution proton NMR spectra of solids is discussed, including probe construction, tunning procedures, optimization of pulses, spectral calibration and referencing.
Abstract: Technical details are given for the successful operation of CRAMPS techniques to obtain high-resolution proton NMR spectra of solids. Matters addressed include probe construction, tunning procedures, optimization of pulses, spectral calibration and referencing. In practice, the MREV-8 sequences is preferred to BR-24. It is suggested that CRAMPS operation deserves more widespread application.
TL;DR: The MARCO POLO (Multiplet Anal. of Cross peaks and Ordering of Patterns in an Overdetd. by Redn. Library Organization) as discussed by the authors is a cross-peak multiplets based approach for the analysis of 2D correlation spectra of the binary alkaloid vinblastine and cyclic undecapeptide cyclosporin A.
Abstract: A recently developed approach for the anal. of cross-peak multiplets was applied to 2-dimensional correlation spectra of the binary alkaloid vinblastine and the cyclic undecapeptide cyclosporin A. The exptl. multiplet patterns are encoded in a manner that depends on the topol. of the coupling network but not on the values of the coupling consts. Multiplets that are expected to occur for various possible coupling networks are similarly encoded in a library to allow comparison with exptl. multiplets. The program is called MARCO POLO (Multiplet Anal. by Redn. of Cross peaks and Ordering of Patterns in an Overdetd. Library Organization). The outcome of this type of automated anal. is discussed in detail, with particular emphasis on remaining ambiguities. [on SciFinder (R)]
TL;DR: In this paper, the proton-coupled 13C NMR and 1H NMR spectra of azidobenzene, tetrazolo[1,5-a]pyridine, 2-azidopyrimidine, and 2-diazidopyridine were analyzed in different solvents.
Abstract: The proton-coupled 13C NMR and 1H NMR spectra of azidobenzene, tetrazolo[1,5-a]pyridine, 2-azidopyrimidine and tetrazolo[1,5-a]pyrimidine in different solvents have been analysed, and criteria for using the NMR spectral parameters (especially the 13C-1H coupling constants) for tautomeric structure determination have been elaborated. The efficiency of these criteria has been demonstrated in studies of azide--tetrazole equilibria in 2,4- and 4,6-diazidopyrimidines. The ditetrazolo tautomeric form of 2,4-diazidopyrimidine has been found in polar solvents.
TL;DR: In this paper, a pulse sequence for obtaining 13C1H shift-correlated spectra with full 1H1H decoupling is described and evaluated, and it is concluded that the sequence provides excellent resolution even for strongly coupled methylene groups.
Abstract: A pulse sequence for obtaining 13C1H shift-correlated spectra with full 1H1H decoupling is described and evaluated. It is concluded that the sequence provides excellent resolution, even for strongly coupled methylene groups. The sensitivity is superior to those of existing sequences based on 13C detection for organic compounds in the molecular weight range up to ca 500.
TL;DR: In this article, indirect, negative heteronuclear NOE enhancements were found in suitable three-spin systems of the 13C1H{1H} type, and their magnitude was found to be dependent on ring size.
Abstract: The 80 MHz 1H NMR and 20 MHz 13C NMR spectra of five 4-methylcoumarins, six 4-methyl-2(1H)-quinolones and nine 3-methylbenzo[b]furans, including six new compounds, were fully assigned. Homonuclear 1H{1H} NOEs and selective heteronuclear 13C{1H} NOEs were measured after low-power pre-saturation of the methyl protons. Indirect, negative heteronuclear NOE enhancements were found in suitable three-spin systems of the 13C1H{1H} type, and their magnitude was found to be dependent on ring size. The first examples of indirect, heteronuclear NOE enhancements on non-protonated carbons are described.
TL;DR: In this article, a series of compounds of the type XMe2SnCHRCHR'CO2Me (X = Me, Cl; R, R' = ME, Ph) were presented for multinuclear (1H, 13C, 119Sn) NMR data.
Abstract: Multinuclear (1H, 13C, 119Sn) NMR data are presented for a series of compounds of the type XMe2SnCHRCHR'CO2Me (X = Me, Cl; R, R' = Me, Ph). Values of 3J(H, H), 3J(Sn, H) and 3J(Sn, C) were used to assign the preferred conformations of these compounds and at the same time to test the empirical relationships proposed previously for 3J(Sn, C) and (indirectly) 3J(Sn, H); the validity of these relationships was confirmed.
TL;DR: In this article, solid-state 13C NMR spectra at 25 MHz of ribonucleosides are analyzed on the basis of the isotropic chemical shifts measured at 50 MHz, together with the asymmetric splittings caused by the presence of 14N calculated using a first-order equation.
Abstract: 13C CP/MAS NMR spectra of nitrogen-containing compounds often show complex signals when recorded at low field strengths. This effect is due to the 13C, 14N residual dipolar coupling which is not averaged to zero by magic-angle spinning. Solid-state 13C NMR spectra at 25 MHz of ribonucleosides are analysed on the basis of the isotropic chemical shifts measured at 50 MHz, together with the asymmetric splittings caused by the presence of 14N calculated using a first-order equation. X-ray diffraction and NQR data, together with several assumptions regarding the quadrupole tensor at each 14N site, are used to obtain a successful spectral simulation.
TL;DR: In this paper, the tautomeric equilibria in phenol-triethylamine complexes have been quantitatively investigated and a method of calculation of thermodynamic parameters based on reproducing the temperature dependences of 13C chemical shifts is described.
Abstract: The tautomeric equilibria in phenol–triethylamine complexes have been quantitatively investigated. A method of calculation of thermodynamic parameters based on reproducing the temperature dependences of 13C chemical shifts is described. The results are in agreement with UV and IR predictions for similar complexes. The chemical shifts characterizing molecular forms of the complexes depend on the type of phenol derivative.
TL;DR: In this paper, the practical aspects of proton-detected heteronuclear shift correlation spectroscopy are described in detail, with emphasis on the precautions necessary to obtain efficient suppression of signals from protons that are not coupled to 13C.
Abstract: The practical aspects of proton-detected heteronuclear shift correlation spectroscopy are described in detail, with emphasis on the precautions necessary to obtain efficient suppression of signals from protons that are not coupled to 13C. The experiment has been used to assign the quaternary carbon signals of a porphyrin and to resolve the proton spectrum of the K3 serotype polysaccharide.
TL;DR: In this paper, the second-order magnetic properties of some five-and six-membered heterocycles containing nitrogen were calculated by means of the IGLO method, and good agreement was found between calculation and experiment.
Abstract: Second-order magnetic properties, i.e. magnetic susceptibility and chemical shift tensors, of some five- and six- membered heterocycles containing nitrogen were calculated by means of the IGLO method. The molecules studied were the azoles pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, and 1H- and 2H-tetrazole, together with the corresponding N-methyl-substituted compounds, the azines pyridazine, pyrimidine, pyrazine, s-triazine, 1,2,3-triazine, s-tetrazine, pyridine and borazine.
Confirming previous investigations, good agreement was found between calculation and experiment. Only the calculated NMR shifts of nitrogen atoms that are involved in NN multiple bonds are too paramagnetic (deshielded). This effect (probably due to the neglect of electron correlation) is much less pronounced for the partial NN double bonds in aromatic rings than for genuine NN multiple bonds studied previously.
TL;DR: In this article, the 27Al chemical shift, line widths and intensities have been measured for a series of sodium aluminate solutions with a range of sodium and aluminium concentrations, and it was shown that the unusually wide lines seen in the most concentrated solutions cannot be said to arise from dimeric species.
Abstract: The 27Al chemical shift, line widths and intensities have been measured for a series of sodium aluminate solutions with a range of sodium and aluminium concentrations. The aluminium is present entirely as the anion [Al(OH)4]− in all but the most concentrated solutions, where intensity is lost owing to the formation of species with very broad lines. The viscosities of the solutions were also measured and it was shown that these accounted to a great extent for the observed changes in line width, so that the unusually wide lines seen in the most concentrated solutions cannot be said to arise from dimeric species.
TL;DR: In this paper, the change in ring current of octaethylporphyrin, meso-tetraphenylporphrin and phthalocyanine cobalt(III) complexes with axial amine ligands was studied using a double dipole network model.
Abstract: The change in ring current of octaethylporphyrin, meso-tetraphenylporphyrin and phthalocyanine cobalt(III) complexes with axial amine ligands was studied using a double dipole network model. The porphyrin ring current was found to be highest in octaethylporphyrin but decreased by 5% in meso-tetraphenylporphyrin and 29% in phthalocyanine. The ring current in the fused benzene rings of phthalocyanine was similar to that found in free benzene. An attempt was made to rationalize changes in the porphyrin ring current in terms of the porphyrin substituents.
An existing ring current program, DIPCALC, was modified to allow the calculation of ring current shifts for phthalocyanines. The parameters required to calculate ring current shifts for all three porphyrins are presented.
TL;DR: In this paper, the 13C NMR chemical shifts of some dammarane-type triterpenes, produced by oxidation of dammaran-20(S)-ol and 20 (S)-hydroxydammaran 3β-yl acetate with m-chloroperbenzoic acid, were assigned by complete analysis of these compounds using 2D long-range 13C1H correlation spectra and 2D INADEQUATE; it was found necessary to revise previous assignments.
Abstract: The 13C NMR chemical shifts of some dammarane-type triterpenes, produced by oxidation of dammaran-20(S)-ol and 20(S)-hydroxydammaran-3β-yl acetate with m-chloroperbenzoic acid, were assigned by complete analysis of these compounds using 2D long-range 13C1H correlation spectra and 2D INADEQUATE; it was found necessary to revise previous assignments. The 13C NMR signals of dendropanoxide were also analysed by 2D techniques including HHC relayed 13C1H correlation spectra.
TL;DR: It is demonstrated that a further modification of this sequence to allow the pair of 1H and 13C 180° pulses to traverse the entire fixed evolution period allows excellent f1 resolution plus a further increase in sensitivity of almost √2.
Abstract: In previous work a modified COLOC sequence combined with a selective BIRD pulse which allows one to obtain one-bond 13C1H shift-correlated spectra with full 1H1H decoupling and good (ca 15 Hz)f1 resolution, in addition to improved sensitivity over earlier sequences measured under the same conditions, was developed. In this paper it is demonstrated that a further modification of this sequence to allow the pair of 1H and 13C 180° pulses to traverse the entire fixed evolution period allows excellent (ca 7–8 Hz) f1 resolution plus a further increase in sensitivity (in the same time) of almost √2. Factors contributing to the sensitivity advantages relative to other sequences were examined. Whereas some artifacts are noted for tightly coupled CH2 groups, these may actually aid in the accurate determination of very small chemical shift differences. The precision of measurement of 1H chemical shifts is ca 1 Hz.
TL;DR: In this article, 17O NMR spectroscopic data at natural abundance in acetonitrile at 75°C were obtained for sixteen N-arylacetamides, three N-methyl-N-aryl acetamides and seven aryl acetates.
Abstract: 17O NMR spectroscopic data at natural abundance in acetonitrile at 75°C were obtained for sixteen N-arylacetamides, three N-methyl-N-arylacetamides and seven aryl acetates. The 17O chemical shifts of the N-arylacetamides are significantly influenced by both electronic and steric factors. A Hammett plot of the data for 4-substituted N-arylacetamides gives a slope of 22 ppm. Ortho substituents result in significant shielding effects, in contrast to previously reported results for conjugated ketnoes. A quantitative relationship between 17O chemical shift data and torsion angle was found for the sterically hindered arylacetamides. Electronic effects on the aryl acetate 17O chemical shifts are, in comparison, greatly reduced. Steric factors for aryl acetates show even smaller effects on the 17O chemical shifts.
TL;DR: In this article, the dependence of 1H and 13C shieldings on molecular geometry is discussed briefly and the NMR spectra of a series of derivatives of carbazole and azacarbazole (norharmane or 9H-Pyrido [3,4-b]indole) are reported.
Abstract: 13C NMR spectra of a series of derivatives of carbazole and azacarbazole (norharmane or 9H-Pyrido [3,4-b]indole) are reported. The dependence of 1H and 13C shieldings on molecular geometry is discussed briefly.
TL;DR: In this paper, it was shown that electron density non-equivalence between the two ortho and the two meta carbons is mainly responsible for the peak splittings, as manifested by the presence of an approximate linear relationship between the displcements of the 13C NMR peaks and total electron density.
Abstract: 13C NMR spectra of para-substituted methoxybenzenes and phenols were recorded in the solid state to gain an insight into the manner and origin of substantial peak splittings in the ortho (up to 9.2 ppm) and meta (up to 2.5 ppm) carbon signals. It was difficult to account for these peak splittings only by the steric interaction with the methyl group of the substituent, because their magnitude varied widely from 4.6 to 9.2 ppm with a variety of substituents at the para position, and the meta carbon peaks are also split into doublets. Instead, it was found that the electron density non-equivalence between the two ortho and the two meta carbons is mainly responsible for the splittings, as manifested by the presence of an approximate linear relationship between the displcements of the 13C NMR peaks and total electron density. The observed additional splitting in the 13C NMR spectra of meso-hexestrol and its methyl or ethyl ether(s) in the solid state was similarly explained. Stereochemical features of these molecules in the solid state are discussed on the basis of the 13C NMR data.
TL;DR: In this article, high-resolution, solid-state 13C NMR spectra of wheat gluten obtained under a variety of acquisition conditions are reported, and the relationship between the laboratory frame spin-lattice relaxation time constants and Larmor frequency is consistent with a model of macromolecular motion in which the polymer chains are held between cross-link points.
Abstract: High-resolution, solid-state 13C NMR spectra of wheat gluten obtained under a variety of acquisition conditions are reported. These spectra are compared with the results from a 1H relaxation study of dry gluten in which the transverse relaxation times, the spin–lattice relaxation times in the rotating frame and the spin–lattice relaxation times in the laboratory frame at three different Larmor frequencies are characterized. It is concluded that 13C experiments, the proton transverse relaxation measurements and the rotating frame results distinguish between protein and lipid motional environments, whilst the laboratory frame proton spin–lattice relaxation results detect two separate protein environments. The relationship between the laboratory frame spin–lattice relaxation time constants and Larmor frequency is consistent with a model of macromolecular motion in which the polymer chains are held between cross-link points. Observed changes in the various proton relaxation times on removal of lipid or disruption of disulphide cross-links are interpreted in terms of the frequency and anisotropy of molecular motions in the gluten system.