Showing papers in "Magnetic Resonance in Chemistry in 1992"
TL;DR: In this paper, the magic angle solid-state NMR spectra of SnO 2 and SnO were recorded and analyzed, and it was shown that the chemical shift tensor anisotropy is much smaller for SnO2 (136 ppm) than SnO (975 ppm).
Abstract: The magic angle solid-state NMR spectra of SnO 2 and SnO were recorded and analysed. The chemical shift tensor anisotropy is much smaller for SnO 2 (136 ppm) than for SnO (975 ppm). Simple calculations suggest that the latter is mainly due to the asymmetry in the valence electron cloud of the tin atom in SnO. An Sn-Sn indirect coupling of 8300 Hz was found in SnO, indicating the possibility of Sn 5s orbitals at the top of the valence band of SnO
88 citations
TL;DR: In this article, the 1H and 13C NMR spectra of the anthraquinones physcion, emodin, chrysophanol, aloe-emodin and rhein were assigned.
Abstract: The 1H and 13C NMR spectra of the anthraquinones physcion, emodin, chrysophanol, aloe-emodin and rhein were assigned. The previously preferred assignments for C-2 and C-4 in physcion were reversed. The NOE technique was used for the assignment of the 1H signals and SEFT and/or heteronuclear decoupling for 13C resonances.
85 citations
TL;DR: In this paper, a simple Pake doublet due only to the directly bonded CH pair was obtained by measuring the local field at the position adjacent to a 13C nucleus under proton homonuclear decoupling.
Abstract: A method is proposed which allows a simple Pake doublet due only to the directly bonded CH pair to be obtained by measuring the local field at the position adjacent to a 13C nucleus under proton homonuclear decoupling. This approach allows precise determination of the CH distance even in compounds with high proton density.
41 citations
TL;DR: In this paper, an NMR spectroscopic study of the internal dynamics of a mucin glycoprotein-derived di-antennary octasaccharide was performed.
Abstract: Most complex carbohydrates can take a large number of shapes in solution. Determining the favored solution conformations and characterizing the internal motions of complex carbohydrates is essential to understanding their biological function, particularly their interactions with proteins. This paper describes an NMR spectroscopic study of the internal dynamics of a mucin glycoprotein-derived di-antennary octasaccharide,
1H NMR spectra of this oligosaccharide in D2O and H2O were completely assigned by the combination of TOCSY, ROESY and triple-quantum spectroscopy experiments. Specific {1H,1H} cross-relaxation rates and 1H longitudinal relaxation times were measured at various temperatures and magnetic field strengths. The data show that the NeuAcα(2 3)Galβ arm of the octasaccharide can adopt two conformations around the (2 3)-bond. Most important, the NeuAcα(2 3)Galβ and Galα(1 4)Galβ linkages both have pronounced conformational flexibility; their rearrangements are fast on the time scale of the overall molecular tumbling.
40 citations
TL;DR: Deuterium isotope effects on 13C nuclear shielding were investigated for a series of o-amino and acetamido azo compounds and for amino- and leucoanthraquinones as mentioned in this paper.
Abstract: Deuterium isotope effects on 13C nuclear shielding were investigated for a series of o-amino and acetamido azo compounds and for amino- and acetamido-9,10-anthra- and leucoanthraquinones. A variety of these compounds form intramolecular hydrogen bonds and are possibly tautomeric in nature. Deuterium isotope effects on 13C chemical shifts were used to probe both the tautomerism and the nature of the hydrogen bond. Deuterium isotope effects over two bonds reflect the strength of the hydrogen bond. In general, isotope effects do not depend on the solvent and, in this study, are independent of temperature. In the acetamido compounds isotope effects on the internal aromatic carbons, 2ΔCar(ND), vary much more than the 2ΔCO(ND) values of the external carbonyl carbon. Plots of 2Δ vs. δ(H) accordingly show different slopes. Investigations of these types of compounds allow a correlation between the hydrogen-bond enthalpy and 2ΔCO(ND) of amides. 2ΔCO(ND) values therefore form a basis for extending the use of deuterium isotope effects to, for example, biomolecules. Diamino-9,10-leucoanthraquinone represents, to our knowledge, the first case of a compound in which isotope effects from both a hydrogen-bonded and a free ND deuterium can be observed. Bis(phenylamino)leucoanthraquinone exhibits interesting equilibrium isotope effects over many bonds. Many of these isotope effects are non-additive. Hydrogen bonding is discussed in general, based on these observations.
38 citations
TL;DR: In this article, the connectivity between the equivalent carbons from the viewpoint of isotopomeric asymmetry was investigated in the C2 symmetrical tetrastilbene hopeaphenol from Carex pumila.
Abstract: In the structure determination of the C2 symmetrical tetrastilbene hopeaphenol, isolated from Carex pumila, the elucidation of the C--C connectivity between equivalent carbons was needed. From an isotopomeric point of view, three isotopomers, 12C12C, 12C13C and 13C13C, are present with respect to the equivalent carbons connecting each other. Among them, the the 12C12C species is undetectable and the 13C13C species is excluded owing to its low natural abundance. Therefore, only the 12C13C isotopomer is detected in 13C NMR and this species is not symmetrical. In the H12C13H system, the separate detection of 1J(CH) and 2J(CH) was achieved by the INEPT-LSPD, COLOC and HMBC methods to demonstrate the connectivity between the equivalent carbons from the viewpoint of isotopomeric asymmetry.
35 citations
TL;DR: In this article, the 13C and 31P NMR chemical shifts and coupling constants are reported and assigned for the two cyclohexylphenylphosphines, tricyclohex-phosphine and triphenylphophosphine, which are completely assigned on the basis of one-bond and long-range 13C13C 2D INADEQUATE experiments.
Abstract: 13C and 31P NMR chemical shifts and coupling constants are reported and assigned for the two cyclohexylphenylphosphines, tricyclohexylphosphine and triphenylphosphine. The 13C NMR spectra of all compounds except dicyclohexylphenylphosphine could be assigned on the basis of APT experiments, substituent effects on the chemical shifts and the values of the 13C31P spin–spin coupling constants. The 13C NMR spectrum of dicyclohexylphenylphosphine, which shows six non-equivalent (both in chemical shift and in spin–spin coupling) cyclohexyl carbons, was completely assigned on the basis of one-bond and long-range 13C13C 2D INADEQUATE experiments. The variable-temperature spectra indicate a hindered motion in this compound. The one-bond 13C13C spin–spin coupling constants of the cyclohexyl carbons vary only slightly with the distance from the substituent. An increase in the 31P chemical shift with an increasing number of cyclohexyl groups parallels the increasing catalytic activity of rhodium complexes containing the studied molecules as ligands.
34 citations
TL;DR: In this article, the NMR assignments for a series of 12 thermo- and photochromic 1,3,3-trimethylspiro(indoline-2,2′-benzopyrans) dyes are reported.
Abstract: The proton NMR assignments for a series of 12 thermo- and photochromic 1,3,3-trimethylspiro(indoline-2,2′-benzopyrans) dyes is reported. All of the protons in the dye molecule were assigned through a combination of homonuclear decoupling experiments and correlation spectroscopy. The relative stereochemistry of the indolino gem-dimethyl groups was assigned so that, for the S-epimer, the pro-R methyl was found to resonate at 1.24 ppm while the pro-S methyl appeared at 1.37 ppm for compound 1.
34 citations
TL;DR: In this article, scalar coupling and cross-relaxation were considered for magnetization transfer in 2D NMR spectroscopy, and two distinct mechanisms were proposed to obtain information about the spatial proximity and molecular motion.
Abstract: Two-dimensional (2D) NMR spectroscopy has become a routine part of molecular structure determination in liquids. In most modern experiments there is some correlation of transition frequencies between the two dimensions, and the mechanism responsible for the correlation dictates the information obtainable from the 2D spectrum. Two distinct mechanisms for magnetization transfer are considered: scalar coupling and cross-relaxation. The former gives rise to information about the proximity of spins through the electronic framework of the molecule and the latter provides information on spatial proximity and molecular motion
34 citations
TL;DR: In this article, the 13 C and 1 H NMR spectra of the aglycone tomatidine were used for the assignment of the sugar moiety in tomatine and the procedure used is of general applicability for the structural elucidation of sugars and glycosidic biomolecules.
Abstract: Heteronuclear correlation techniques and homonuclear E.COSY spectra were used to obtain a complete assignment of the 13 C and 1 H NMR spectra of the aglycone tomatidine. In addition, 1D/2D TOCSY, NOESY and ROESY spectra allowed the assignment of the sugar moiety in tomatine. The procedure used is of general applicability for the structural elucidation of sugars and glycosidic biomolecules
32 citations
TL;DR: In this paper, the coexistence of monomer and dimers of quinine was established by investigating the temperature and concentration dependence of the NMR spectral parameters, and their conformations were determined by a combination of accurate 2D NOESY and proton-selective relaxation rate measurements.
Abstract: The coexistence of monomer and dimers of quinine was established in quinine solutions by investigating the temperature and concentration dependence of the NMR spectral parameters. Their conformations were determined by a combination of accurate 2D NOESY and proton-selective relaxation rate measurements.
TL;DR: In this article, NMR studies of polyene macro-lides have long been hampered by their low solubility and high tendency for association, for which reason DMSO-d is the most commonly used solvent.
Abstract: and roxaticin3 have been determined by x-ray analysis, because most of the polyene macrolides exhibit poor crystallinity. On the other hand, NMR studies of polyene macro- lides have long been hampered by their low solubility and high tendency for association, for which reason DMSO-d, is the most commonly used solvent. The res- onance line width in DMSO-d, was 7-8 Hz? and this almost prevented coupling constant analysis.
TL;DR: The 1H and 13C NMR spectra of 22 pinane derivatives have been recorded and completely assigned in this article, and spectral data for this set of compounds provide information about the conformation of these molecules and allow the deduction of the various structural effects observed in the bicyclo[311]heptane series
Abstract: The 1H and 13C NMR spectra of 22 pinane derivatives have been recorded and completely assigned The spectral data for this set of compounds provide information about the conformation of these molecules and allow the deduction of the various structural effects observed in the bicyclo[311]heptane series
TL;DR: In this paper, the performance and sensitivity characteristics of a standard 5 mm inverse detection probe and a new micro inverse detector were compared on identical amounts of material at 500 MHz. And the latter consistently afforded approximately twice the signal-to-noise performance of the former in two-dimensional experiments, corresponding to a factor of four improvement in performance times.
Abstract: Performance and sensitivity characteristics of a standard 5 mm inverse detection probe and a new micro inverse detection probe design were compared on identical amounts of material at 500 MHz. The latter consistently afforded approximately twice the signal-to-noise performance of the former in two-dimensional experiments, corresponding to a factor of four improvement in performance times to obtain comparable signal-to-noise ratio projections of HMQC data. The latter also facilitated the acquisition of HMQC spectra on much smaller amounts of material than was possible in the conventional 5 mm inverse detection probe.
TL;DR: Substituted phenol-trimethylamine N-oxide (TMAO) systems were studied in this paper, where the 1H chemical shift of the hydrogen-bonded proton first increases with decreasing pKa of the phenols and, after a sharp maximum, it then decreases.
Abstract: Substituted phenol–trimethylamine N-oxide (TMAO) systems were studied. The 1H chemical shift of the hydrogen-bonded proton first increases with decreasing pKa of the phenols and, after a sharp maximum, it then decreases. Thus, the shielding of the hydrogen-bonded proton by the O atom of the phenol first decreases, and this proton is most deshielded in the 3,4-dinitronphenol–TMAO system. With a further decrease in the pKa of the phenol this proton becomes shielded again by the O atom of the TMAO molecule. This behaviour of the 1H NMR signal is in very good agreement with the changes in the shape of the proton potential obtained from IR spectra. In the case of the strongest hydrogen bond, in the 3,4-dinitrophenol-TMAO system, the 13C signals are considerably broadened, probably owing to the long lifetime of this bond. Δ14 Values were calculated from the 13C NMR signals, which should also reflect the changes in the nature of the phenol—TMAO hydrogen bonds. A small difference is obtained between the results obtained from the 13C and 1H NMR spectra. This difference is caused by the effects of the substituents on the 13C NMR signals, which are not taken into account in the calculation of the Δ14 values.
TL;DR: In this paper, double selective and single selective proton relaxation rates have been shown to provide directly the dipolar cross-relaxation rate of any pair of protons in a ligand molecule.
Abstract: Measuring double-selective and single-selective proton relaxation rates has been shown to provide directly the dipolar cross-relaxation rate of any pair of protons in a ligand molecule. The same measurements for a ligand exchanging between a free and a macromolecule-bound state yield the cross-relaxation rate of the bound ligand provided that the rate of exchange is relatively fast, thus allowing one to obtain dynamic and conformational features of the bound ligand. As an example, rigid binding of sulphisomidine to bovine serum albumin has been demonstrated.
TL;DR: In this paper, the authors used perturbation theory at second, third and fourth order to extract the general features of higher order terms of the Hamiltonian of the molecule in the external magnetic field and the two nuclear spin operators.
Abstract: As the magnetic fields used in NMR spectrometers become greater, it is expected that a field dependence of nuclear spin–spin coupling will be observed. A part of this, which would be independent of temperature, is due to the contributions of higher terms in the Hamiltonian of the molecule in the external magnetic field and the two nuclear spin operators. Perturbation theory at second, third and fourth order is used to extract the general features of these higher order terms. Contributions are due (a) to diamagnetic and paramagnetic terms of each of the four familiar parts of the spin coupling— Fermi contact, spin–dipolar, and orbital paramagnetic and orbital diamagnetic, and (b) to cross-terms determined by operators for the diamagnetic and paramagnetic shielding at the two coupled nuclei. Explicit expressions are presented for the more important contributions. It is estimated for strongly magnetic nuclei in large molecules that the coupling varies as 0.5 × 10−5B2 Hz, so that a field of about 14 T should produce a change in the coupling of 0.001 Hz. These are, respectively, the field used in high-field NMR spectrometers and the upper limit of precision in the measurement of spin–spin coupling constants at the present time.
TL;DR: The 1H and 13 C NMR spectra of brassinolide and 24-epi-brassinolide were assigned using 1D and 2D NMR techniques as discussed by the authors.
Abstract: The 1 H and 13 C NMR spectra of the two phytohormones, brassinolide and 24-epi-brassinolide, and the two synthetic analogues 22,23,24-trisepi-brassinolide and (22S,23S)-28-homo-brassinolide, were assigned using 1D and 2D NMR techniques. Independent assignment strategies are presented for the 1 H and 13 C chemical shifts of the steroidal side-chain
TL;DR: In this paper, the absolute coupling signs of 1J(119Sn, 13C) ( O) in (n5-C5Me5)2Sn (2) were determined using advanced direct and indirect two-dimensional methods.
Abstract: Absolute coupling signs of 1J(119Sn, 13C) ( O) in (n5-C5Me5)2Sn (2) were determined using advanced direct and indirect two-dimensional methods These are based on Ψ-BIRD techniques and z-filtering, leading to pseudotriple-resonance experiments of the type 1H{119Sn(13C)} and 13C{1H(119Sn)}, respectively A mixed n5-cyclopentadienyltin(II) amide was prepared and characterized by 1H, 13C, 29Si and 119Sn NMR in solution in the presence of its symmetrical redistribution products 1 and [(Me3Si)2N]2Sn Solid-state 119Sn CP/MAS NMR spectra of 1 shows the presence of two tin sites (in accord with the x-ray analysis) with a 622 ppm difference in 119Sn chemical shifts, the mean value being in close agreement with the solution-state δ119Sn value The symmetries of the shielding tensor for the two tin sites are different and there appears to be a correlation between this property and the ring centroid—Sn—ring centroid angle α 13C CP/MAS NMR proved insensitive to the structural differences between the two types of molecules of 1 in the unit cell
TL;DR: In this article, a series of (η 5 -cyclopentadienyl)trialkylplatinum(IV) complexes are reported for NMR data, and chemical shift differences are rationalized primarily on the basis of substituent electronic properties.
Abstract: 195 Pt NMR data are reported for a series of (η 5 -cyclopentadienyl)trialkylplatinum(IV) complexes. Chemical shift differences are rationalized primarily on the basis of substituent electronic properties. Changes in solvent produce relatively modest changes in chemical shift, and the dependence of the chemical shift on temperature and concentration is sufficiently strong to warrant control of these variables for comparative measurements. An improved synthesis of [IPtMe 3 ] 4 is also described
TL;DR: The NMR spectra of the N-borane adducts of these imines can be used to determine their configuration as discussed by the authors, which can also be used for configuration determination.
Abstract: phenylethylidene)ethylenediamine and N-borane-his(a-phengIben~ylidene)ethylenediamine, and four imines, Nborane-a-methylethylidene-2-phenylethylamine, two N-borane-a-phenylethylidenon-hutylamine and N-borane-aphenglbenzylidene-n-hutylamine, are reported. The NMR spectra of the N-borane adducts of these imines can be used to determine their configuration.
TL;DR: In this article, the Pt-HN bonding modes in two cycloplatinated complexes, [Pt(C6H4CH2NMe2-2-C,N), and [PT(1-C10H6NMe 2-8C, N), were characterized by 15N-filtered and 2D NOESY 1H measurements.
Abstract: New Pt … HN bonding modes in two cycloplatinated complexes, [Pt(C6H4CH2NMe2-2-C,N)(C6H4CH2NMe2H-2-C,H)Br] (1) and [Pt(1-C10H6NMe2-8-C,N)(1-C10H6NMe2H-8-C,H)Br] (2), were characterized by 15N-filtered and 2D NOESY 1H measurements. Both 1 and 2 show markedly different interactions of an RNMe2H+ group with a platinum centre; the 1J(15N,1H) values of 69 and 59 Hz for 1 and 2, respectively, are considerably less than those of the related free and protonated organic amine systems. The corresponding 1J(195Pt,1H) values are 72 and 180 Hz, respectively. An approach both to understanding and recognizing these Pt … HN interactions is presented.
TL;DR: In this paper, the relative configurations and conformations of the spiropyrazolines were elucidated by different NMR methods, and the spiro-2-pyrazoline rearrangement was performed on proton catalysis.
Abstract: 1,3-Dipolar cycloaddition of (E)- and (Z)-arylidene-1-tetralone derivatives affords trans- and cis-spiro-1-pyrazolines, respectively, regio- and stereo-selectively in a one-step reaction. These rearrange into spiro-2-pyrazolines on proton catalysis. The relative configurations and conformations of the spiropyrazolines were elucidated by different NMR methods.
TL;DR: In this paper, the solution behavior and redistribution reactions of tetraorganodistannoxanedimers were investigated by 19 F and 119 Sn NMR spectroscopy, and the variable-temperature NMR data suggest that both the ladder and staircase structures exist in solution.
Abstract: The solution behaviour and redistribution reactions of tetraorganodistannoxanedimers were investigated by 19 F and 119 Sn NMR spectroscopy. The variable-temperature NMR data suggest thatboth the «ladder» and« staircase» structures exist in solution. Redistribution between two different tetrabutylstannoxanes is rapid on the NMR time scale. The 119 Sn 2D EXSY NMR data for [Bu 2 Sn(F)OSnBu 2 (Cl)] 2 indicate that there is no exchange between exo- and endocyclic tin centres
TL;DR: In this article, a natural abundance 15N NMR study was carried out on a series of 1- and 2-mono- and 1,5-and 2-disubstituted tetrazoles in different media and direct proof that N-4 is the protonation site is presented, to the best of our knowledge.
Abstract: A natural abundance 15N NMR study was carried out on a series of 1- and 2-mono- and 1,5- and 2,5-disubstituted tetrazoles in different media. An erroneous assignment given in the literature for 2,5-disubstituted tetrazoles was corrected using MNDO molecular orbital calculations and the analysis of changes in the 15N chemical shifts on protonation. Earlier assignments for 1- and 2-mono- and 1,5-disubstituted tetrazoles were confirmed by the same means and by the use of 15N,H couplings. Direct proof that N-4 is the protonation site is presented, to our knowledge for the first time.
TL;DR: In this article, a model-free analysis of carbon-13 spin-lattice relaxation times and NOE factors for tetrasaccharide, lacto-N-neotetraose, in solution as a function of temperature and magnetic field is presented.
Abstract: Carbon-13 spin–lattice relaxation times and NOE factors are reported for a tetrasaccharide, lacto-N-neotetraose, in solution as a function of temperature and magnetic field. The data that are outside the extreme narrowing regime are analysed using the ‘model-free’ approach of Lipari and Szabo. The global reorientation is assumed to be isotropic. The experimental data for the inner rings can be fitted very well using a simplified version of the equations, neglecting the local motion term in the spectral densities. The complete expression for the spectral densities is required for the outer rings, and three different fitting procedures are compared.
TL;DR: It is shown that the expression accounts for the main physical effects of the protein peptide group on the proton chemical shifts, and explains the main characteristics of the C‐α, H and amide NH chemical shifts in proteins.
Abstract: It has been found that protein NH protons on top of a peptide group plane experience large upfield conformation shifts. The joint analysis of this effect and other known effects of the peptide group on proton chemical shifts has led to a two-term empirical expression for peptide group proton chemical shift computation as a function of protein backbone conformation. Both terms are expressed by McConnell's point-dipole shielding expressions, one referred to an axis perpendicular to the peptide plane and origin in the coordinate centre of the OCN atoms and the other referred to an axis along the carbonyl bond and origin close to the oxygen atom. Values for the two constants have been determined by least-squares fitting of the C-α, H and amide NH chemical shifts of the protein ubiquitin. As a cross-check on the validity of the expression, the C-α, H and NH shifts of ribonuclease and BPTI (basic pancreatic trypsin inhibitor) have been computed. The general agreement between the observed and computed shifts and the correlation coefficients found (0.72 on average) indicate that the expression accounts for the main physical effects of the protein peptide group on the proton chemical shifts. It is shown that, together with ring current shifts, the expression explains the main characteristics of the C-α, H and amide NH chemical shifts in proteins.
TL;DR: In this paper, the full structure of astellatol, a new complex sesterterpene isolated from Aspergillus variecolor, was determined solely by the use of one-and two-dimensional NMR techniques.
Abstract: The full structure of astellatol, a new complex sesterterpene isolated from Aspergillus variecolor, was determined solely by the use of one- and two-dimensional NMR techniques. The carbon skeleton was deduced from a two-dimensional INADEQUATE spectrum, and the proton shifts were deduced from a two-dimensional CH correlation (HMQC) experiment. Analysis of a two-dimensional proton DQFCOSY spectrum and a series of homonuclear NOE difference spectra allowed the determination of the full stereochemistry.
TL;DR: In this article, the 13C NMR signals for some 2-ethylthio-, 2-methylsulphinyl- and 2 -ethylsulphonyl-4′-substituted acetophenones were assigned.
Abstract: The 13C NMR signals for some 2-ethylthio-, 2-ethylsulphinyl- and 2-ethylsulphonyl-4′-substituted acetophenones were assigned. The carbonyl carbons exhibit a progressive upfield shift on going from the ketosulphides to the ketosulphoxides and to the ketosulphones. The α-methylene carbons fro the three classes of compounds are shielded by almost the same amount in relation to the corresponding calculated values. The chemical shifts of the aromatic ring carbons are in close agreement with those calculated using substituent chemical shifts.
TL;DR: In this paper, the crystal structure of the [Pt(N,N-Me2en)(μ-OH)]2(NO3)2 dimer was determined and the results showed that this is the trans isomer with an inversion centre between the two Pt atoms.
Abstract: The hydrolysis reactions of complexes of the type Pt(L)X2 where L is ethylenediamine (en), N,N- dimethylenediamine (N,N-Me2en), N,N-dimethylenediamine (N,N′-Me2en) and N,N,N′, N′-tetramethylethylenediamine (Me4en) were studied The halo ligands were removed by precipitation with AgNO3 and the pH of the resulting filtrate, which was about 2, was adjusted to 58 (pD = 62) The study of the products formed was carried out by multinuclear NMR spectroscopy The 195Pt NMR spectra showed the presence of several hydrolysed products, especially with en The hydroxo-bridged dimer can be synthesized in a pure state with all the ligands except en The results on the latter ligand contradict the published results on the synthesis of the en hydroxo-bridged dimer whose biological properties were studied When Pt(L)I2 is used as starting material a small quantity of chloro species was always observed, indicating that the reactant contained a small quantity of Pt(L)ICI The crystal structure of the [Pt(N,N-Me2en)(μ-OH)]2(NO3)2 dimer was determined The compound belongs to the P21/c space group, with a = 8262(5), b = 10960(8), c = 11405(8), β = 12107(5)°, Z = 2 and ρcalc = 2720 mg m−3 The results showed that this is the trans isomer with an inversion centre between the two Pt atoms There is a considerable strain inside the four-membered ring, as shown by the reduced internal angles OPtO′ = 791(6)0 and PtOPt′ = 1010(6)°