NMR Resonances (nmr + resonance)

Distribution by Scientific Domains

Selected Abstracts

How to determine the number of inner-sphere water molecules in Lanthanide(III) complexes by 17O NMR spectroscopy.

A technical note
Abstract Lanthanide(III) complexes of polyaminocarboxylates are widely used in MRI as contrast agents. The paramagnetic properties of the metal ion contribute to the increase of 1H relaxation rates, while the chelate offers a stable binding with the metal. The number of water molecules, coordinated directly to the Ln(III) ion, is very important for the relaxivity and, thus, the efficacy of these contrast agents. Here, we describe convenient methods to determine this parameter by measurement of Ln(III)-induced shifts of the water 17O NMR resonance. Copyright 2007 John Wiley & Sons, Ltd. [source]

Pyrazolate-Based Dinucleating Ligands in L2M2 Scaffolds: Effects of Bulky Substituents and Coligands on Structures and MH,C Interactions

Jens C. Rder
Abstract A series of nickel(II) and palladium(II) complexes [L2M2]2+ have been prepared and structurally characterized, where L is a pyrazolate ligand with bulky 2,6-dimethyl- or 2,6-di(isopropyl)anilinomethyl side arms. Coordinating counter anions such as chloride can bind to axial sites of the dinickel species in a solvent-dependent process, giving rise to five-coordinate high-spin metal ions. In the case of weakly coordinating anions, the metal ions are found in roughly square-planar environments, and the structures are governed by the tendency of the bulky aryl groups to avoid each other, which forces the methyl or isopropyl substituents in the aryl 2- and 6-positions to approach the metal ions from the axial directions. This leads to drastic low-field shifts of the respective 1H NMR signals, e.g. , = 7.86 ppm for the isopropyl ,CH which comes in close proximity to the low-spin nickel(II) center. The relevance of such low-field NMR resonances of protons close to the axial sites of d8 metal ions for possible three-center four-electron MH,C hydrogen bonds involving the filled d orbital of the metal ion is discussed. In the present case, attractive MH interactions are assumed to be of no major significance. This was corroborated by the structure of a further [L2Ni2]2+ type complex where the anilinomethyl side arms bear only a single 2-isopropyl group, which was found rotated away from the metal. Additional spectroscopic and electrochemical properties of the various complexes are reported. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

X-ray Crystal Structure of a Sodium Salt of [Gd(DOTP)]5,: Implications for Its Second-Sphere Relaxivity and the 23Na NMR Hyperfine Shift Effects of [Tm(DOTP)]5,

Fernando Avecilla
Abstract The X-ray structure of the sodium salt of [Gd(DOTP)]5, shows two different chelates, [Gd(1)(DOTP)]5, and [Gd(2)(DOTP)]5,, bound at either surface of a sheet formed by a cluster of hydrated Na+ ions. Each [Gd(1)(DOTP)]5, anion binds directly to four Na+ ions of this cluster through the free oxygen atoms of the phosphonate groups of the adjacent ligand, while each [Gd(2)(DOTP)]5, unit is connected to the cluster via hydrogen bonds only. The Gd3+ ions in the two moieties do not have any inner-sphere water molecules, and are eight-coordinate. Their coordination polyhedra are twisted square antiprisms, with slightly different twist angles. These m, isomers are found in the crystal structure as racemic mixtures of enantiomers. Only one set of NMR resonances is observed in aqueous solution, corresponding to an averaged m, isomer. In this crystal structure, the Na+ ions bind the phosphonate oxygen atoms of the [Gd(1)(DOTP)]5, anion at positions far removed from the main symmetry axis. This is significantly different from the binding mode(s) previously proposed to be occurring in solution between Na+ and [Tm(DOTP)]5,, based on the interpretation of solution paramagnetic 23Na NMR shifts. This could arise as a result of the effects of the cluster of hydrated Na+ ions that are present, which may hinder axial binding modes and distort lateral binding modes. Further, in the crystal structure, both types of Gd3+ centers have four second-sphere water molecules that are located at distances (4.2,4.5 ) significantly longer than those previously proposed from the analysis of the NMRD data of [Gd(1)(DOTP)]5,. This is a result of the coordination of Na+ by these water molecules, thus preventing their direct interaction with the phosphonate oxygen atoms. However, in solution such second-sphere water molecules can interact strongly with the phosphonate ligand oxygen atoms, resulting in efficient relaxation if their binding has relatively long lifetimes (> 50 ps). Rotational immobilization will amplify this contribution, thus making it similar to outer-sphere relaxation. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Oxidation of ,4 - and ,5 -Steroids with Hydrogen Peroxide Catalyzed by Porphyrin Complexes of MnIII and FeIII

Susana L. H. Rebelo
Abstract In this paper we describe a new environmentally friendly method to promote the stereoselective epoxidation of ,4 - and ,5 -steroids. Metalloporphyrins efficiently catalyze the epoxidation reactions of 17,-acetoxy-4-androstene (1), 4-cholestene (2) and 3,-acetoxy-5-cholestene (3) in the presence of H2O2 as oxygen donor. Modeling the molecular structure of the porphyrin as well as the central metal allows the control of the preferential formation of ,- or ,-epoxides. Porphyrins with bulky, electron-withdrawing groups in the ortho positions of the meso phenyls and with MnIII as the central metal ion, such as [Mn(TDCPP)Cl], gave preferentially the ,-epoxide of ,4 - and ,5 -steroids. [Fe(TPFPP)Cl] catalyzes preferentially the ,-epoxidation of ,4 -steroids and also increases the stereoselectivity for the ,-epoxide in ,5 -steroids, similar to the results obtained with m -CPBA (m -chloroperbenzoic acid) as oxidant. The substrate structure strongly influences the chemoselectivity of the reactions. The X-ray structures of two main products were determined, and two-dimensional NMR techniques allowed the full assignment of 1H and 13C NMR resonances as well as the stereochemistry of these products. A mechanistic proposal involving oxo species for the ,-approach and peroxy species for the ,-approach is proposed. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Structural model for an AxxxG-mediated dimer of surfactant-associated protein C

FEBS JOURNAL, Issue 11 2004
Visvaldas Kairys
The pulmonary surfactant prevents alveolar collapse and is required for normal pulmonary function. One of the important components of the surfactant besides phospholipids is surfactant-associated protein C (SP-C). SP-C shows complex oligomerization behavior and a transition to ,-amyloid-like fibril structures, which are not yet fully understood. Besides this nonspecific oligomerization, MS and chemical cross-linking data combined with CD spectra provide evidence of a specific, mainly ,-helical, dimer at low to neutral pH. Furthermore, resistance to CNBr cleavage and dual NMR resonances of porcine and human recombinant SP-C with Met32 replaced by isoleucine point to a dimerization site located at the C-terminus of the hydrophobic ,-helix of SP-C, where a strictly conserved heptapeptide sequence is found. Computational docking of two SP-C helices, described here, reveals a dimer with a helix,helix interface that strikingly resembles that of glycophorin A and is mediated by an AxxxG motif similar to the experimentally determined GxxxG pattern of glycophorin A. It is highly likely that mature SP-C adopts such a dimeric structure in the lamellar bilayer systems found in the surfactant. Dimerization has been shown in previous studies to have a role in sorting and trafficking of SP-C and may also be important to the surfactant function of this protein. [source]

Hydrogen bonds and local symmetry in the crystal structure of gibbsite

Anastasia Vyalikh
Abstract First-principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the 27Al MAS NMR resonances in gibbsite. The 27Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished 27Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al-I site characterized by a CQ = 4.6 MHz is surrounded by OHgroups participating in four intralayer and two interlayer hydrogen bonds, while the Al-II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH-bonds are aligned toward the interlayer gallery. In high-resolution solid-state 1H CRAMPS (combination of rotation and multiple-pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite-like layer,intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright 2010 John Wiley & Sons, Ltd. [source]

Quantum chemistry investigation of electronic structure and NMR spectral characteristics for fluorides of dialkylamidosulfoxylic acids and related compounds

A. B. Rozhenko
Abstract The parent (H2NSF) and N,N -dialkyl-substituted fluorides of amidosulfoxylic acid (R2NSF, RMe or R2NMorph) as well as the related compounds XSF (XCH3, OH, F, SiH3, PH2, SH, Cl) have been investigated with quantum chemical calculations at the ab initio (MP2) level of approximation. The geometries, electronic structures, molecular orbital (MO) energies and NMR chemical shift values have been calculated to evaluate the role and extent of the polarization and delocalization effects in forming of the high-field fluorine NMR resonances within the series of interest. The ,F magnitudes for all investigated fluorides of amidosulfoxylic acid as well as the ,N value calculated for Me2NSF are in the good agreement with the 19F and 14N NMR chemical shift values measured experimentally. For the parent compounds, H2NSF and H2NSO2F, the orientation of principal axes of the magnetic shielding tensors and the corresponding principal ,ii values along these axes have been qualitatively interpreted basing on the analysis of the MO interactions in the presence of the rotating magnetic field. Copyright 2009 John Wiley & Sons, Ltd. [source]

Investigation of the correlations between 19F and 1H NMR signals for various mono- and di-substituted octafluoro[2.2]paracyclophanes

Alex J. Roche
Abstract A selection of mono- and pseudo ortho di-substituted octafluoro[2.2]paracyclophane derivatives were analyzed using 19F- 1H HOESY, 1H COSY and 19F COSY techniques. This resulted in the unambiguous assignment of the 19F and 1H NMR resonances, and also revealed interesting solvent effects and noteworthy coupling patterns for various JHH, JHF, and JFF interactions, including observable through bond 7JFF and 8JFF couplings. For the four mono-substituted derivatives, the assignments were achieved through the combination of 19F- 1H HOESY, 1H COSY and 19F COSY techniques. The C2 symmetry of the six pseudo ortho di-substituted derivatives that were examined produced simplified spectra, and careful inspection of the characteristic 1H coupling patterns led to the assignment of 1H signals. Therefore only 19F- 1H HOESY experiments were required to complete the assignments for those molecules. Refinements and alternative strategies for previous protocols are presented for the molecules that were less responsive to nuclear Overhauser effect (nOe) experiments. Copyright 2009 John Wiley & Sons, Ltd. [source]

Complete assignments of 1H and 13C NMR resonances of oleanolic acid, 18,-oleanolic acid, ursolic acid and their 11-oxo derivatives

Werner Seebacher
Abstract Complete assignments of 1H and 13C NMR chemical shifts for oleanolic acid, 18,-oleanolic acid, ursolic acid and their 11-oxo derivatives based on 1H, 13C, 2D DQF-COSY, NOESY, HSQC, HMBC and HSQC-TOCSY experiments were achieved. Copyright 2003 John Wiley & Sons, Ltd. [source]

15N NMR spectroscopy as a method for comparing the rates of imidization of several diamines,

James C. Johnston
The relative rates of the conversion of amide-acid to imide were measured for a series or aromatic diamines that have been identified as potential replacements for 4,4,-methylene dianiline (MDA) in high-temperature polyimides and polymer composites. These rates were compared with the 15N NMR resonances of the unreacted amines. The initial rates of imidization track with the difference in chemical shift between the amine nitrogens in MDA and those in the subject diamines. This comparison demonstrated that 15N NMR spectroscopy is appropriate for the rapid screening of candidate diamines to determine their reactivity relative to MDA, and can serve to provide guidance to the process of creating the time,temperature profiles used in processing these materials into polymer matrix composites. POLYM. COMPOS. 27:723,729, 2006. 2006 Society of Plastics Engineers [source]

Synthesis and molecular structure of 2,4,6-tri[bis(diisopropylamino)boryl(methylamino)]borazine, [(NiPr2)2B(Me)N]3B3N3H3

Brangre Toury
Abstract Reaction of bis(diisopropylamino)(methylamino)borane, (NHiPr)2B(NHMe), with 2,4,6-trichloroborazine (ClBNH)3 affords 2,4,6-tri[bis(diisopropylamino)boryl(methylamino)]borazine, 2,4,6-[(NiPr2)2B(Me)N]3B3N3H3, which is the first boryl-borazine structurally characterized. According to the X-ray single crystal structure and the chemical shifts of 11B NMR resonances of boron atoms, compared with the aminoborane and borazine analogs, the borazine and boryl ,-systems are not coplanar either in the solid state or in organic solution. Copyright 2002 John Wiley & Sons, Ltd. [source]

Head-to-Head Cross-Linked Adduct between the Antitumor Unit Bis(,- N,N,-di- p -tolylformamidinato)dirhodium(II,II) and the DNA Fragment d(GpG)

Helen T. Chifotides Dr.
Abstract Reactions of the compound cis -[Rh2(DTolF)2(CH3CN)6](BF4)2, a formamidinate derivative of the class of antitumor compounds [Rh2(O2CR)4] (R=Me, Et, Pr), with 9-ethylguanine (9-EtGuaH) or the dinucleotide d(GpG) proceed by substitution of the acetonitrile groups, with the guanine bases spanning the RhRh bond, in a bridging fashion, through sites N7/O6. In the case of 9-EtGuaH, both head-to-head (HH) and head-to-tail (HT) isomers are formed, whereas with the tethered bases in d(GpG), only one right-handed conformer HH1R [Rh2(DTolF)2{d(GpG)}] is present in solution. For both cis -[Rh2(DTolF)2(9-EtGuaH)2](BF4)2 and [Rh2(DTolF)2{d(GpG)}], the absence of N7 protonation at low pH and the substantial decrease of the pKa values for N1-H deprotonation, support N7/O6 binding of the bases to the dirhodium core. The N7/O6 binding of the bases is further corroborated by the downfield shift by ,,,4.0 ppm of the 13C NMR resonances for the C6 nuclei as compared to the corresponding resonances of the free ligands. The HH arrangement of the guanine bases in [Rh2(DTolF)2{d(GpG)}] is indicated by the intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. Complete characterization of the [Rh2(DTolF)2{d(GpG)}] conformer by 2D NMR spectroscopy supports anti- orientation and N (C3,- endo) conformation for both deoxyribose residues. The N-pucker for the 5,-G base is universal in such cross-links, but it is very unusual for platinum and unprecedented for dirhodium HH cross-linked adducts to have both deoxyribose residues in the N-type conformation. The bulk, the nonlabile character, and the electron-donating ability of the formamidinate bridging groups spanning the dirhodium core affect the nature of the preferred dirhodium DNA adducts. Molecular modeling studies performed on [Rh2(DTolF)2{d(GpG)}] corroborate the structural features obtained by NMR spectroscopy. [source]

X-ray Diffraction and Solid-State NMR Studies of a Germanium Binuclear Complex

Lus Mafra
Abstract A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2],4,H2O (where pmida4,=N -(phosphonomethyl)iminodiacetate and C4H12N22+=piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2, complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H,X(13C,31P) heteronuclear correlation (HETCOR) and 1H,1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee,Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic,organic hybrids. We assigned the NMR resonances with the help of C,,,H and P,,,H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry). [source]