NMR Parameters (nmr + parameter)

Distribution by Scientific Domains


Selected Abstracts


Multinuclear High-Resolution NMR Study of Compounds from the Ternary System NaF,CaF2,AlF3: From Determination to Modeling of NMR Parameters.

CHEMINFORM, Issue 9 2007
C. Martineau
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]


Ab initio Calculations of NMR Parameters of Highly Coordinated Oxygen Sites in Aluminosilicates.

CHEMINFORM, Issue 42 2004
Christel Gervais
No abstract is available for this article. [source]


Tunable Alignment for All Polymer Gel/Solvent Combinations for the Measurement of Anisotropic NMR Parameters

CHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2010
Grit Kummerlwe M.
A stretching apparatus (see figure) with arbitrary scalability of alignment strength is introduced for the measurement of anisotropic NMR parameters, which works for practically all polymer gel/solvent combinations. Its use is demonstrated in applications involving the steady incrimination of residual dipolar couplings, the distinction of enantiomers, and the conformational analysis of organic compounds. [source]


NMR Parameters and Geometries of OHN and ODN Hydrogen Bonds of Pyridine,Acid Complexes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2004
Hans-Heinrich Limbach Prof.
Abstract In this paper, equations are proposed which relate various NMR parameters of OHN hydrogen-bonded pyridine,acid complexes to their bond valences which are in turn correlated with their hydrogen-bond geometries. As the valence bond model is strictly valid only for weak hydrogen bonds appropriate empirical correction factors are proposed which take into account anharmonic zero-point energy vibrations. The correction factors are different for OHN and ODN hydrogen bonds and depend on whether a double or a single well potential is realized in the strong hydrogen-bond regime. One correction factor was determined from the known experimental structure of a very strong OHN hydrogen bond between pentachlorophenol and 4-methylpyridine, determined by the neutron diffraction method. The remaining correction factors which allow one also to describe H/D isotope effects on the NMR parameters and geometries of OHN hydrogen bond were determined by analysing the NMR parameters of the series of protonated and deuterated pyridine- and collidine,acid complexes. The method may be used in the future to establish hydrogen-bond geometries in biologically relevant functional OHN hydrogen bonds. [source]


Nuclear magnetic moments from NMR spectra,Experimental gas phase studies and nuclear shielding calculations

CONCEPTS IN MAGNETIC RESONANCE, Issue 5 2007
Karol Jackowski
Abstract NMR spectra of gaseous compounds and quantum chemical calculations are combined to determine new accurate values of magnetic dipole moments for a series of nuclei. We have analyzed shielding constants, resonance frequencies, and nuclear magnetic moments for a group of simple molecules. The chemical shifts and resonance frequencies are measured at 300 K and extrapolated to the zero-density limit in order to remove all the intermolecular effects from the experimental parameters. The absolute shielding constants in the studied molecules are obtained from ab initio calculations. Assuming the proton magnetic moment as the reference, we determine the nuclear magnetic moments of 13C, 14N, 15N, 17O, 19F, 29Si, 31P, 33S, and 73Ge. The new nuclear magnetic moments are consistent with the experimental NMR parameters, and using these new values one can for the first time predict successfully the shielding constant of a nucleus in a molecule when the corresponding resonance frequency is known. 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 246,260, 2007. [source]


The Relationship Between 27Al Quadrupolar Parameters and AlF63, Octahedron Connectivity in Crystalline and Glassy Fluoroaluminates,

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2007
Monique Body
Abstract 27Al SATRAS and MQ-MAS spectra were recorded for eight crystalline compounds, from the CaF2,AlF3 and BaF2,AlF3 binary and BaF2,CaF2,AlF3 ternary systems, and four glass compositions with different CaF2/BaF2/AlF3 contents. For the crystalline phases, the reconstruction of the spectra leads to the precise determination of the NMR parameters. For the glassy phases, the 27Al SATRAS spectra have been reconstructed using quadrupolar parameter distributions. The main finding of this study is the dependence of the quadrupolar frequency on the type of AlF63, octahedron connectivity. For crystalline phases the experimental ,Q values range between 75 kHz and 510 kHz for structures built up from isolated octahedra, and are between 560 kHz and 1250 kHz for structures built up from isolated chains of cis -connected octahedra, and are between 1530 kHz and 1580 kHz for structures built up from isolated chains of trans -connected octahedra. In the glassy phases the maximum of the quadrupolar frequency distribution shifts toward larger values and its width increases with increasing AlF3 content, and subsequently with the number of connected octahedra. The range of the ,Q values seems to indicate that when the octahedra are connected, cis connections occur most frequently. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]


AIM and NBO analysis

MAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2010

In the series of diaminoenones, large high-frequency shifts of the 1H NMR of the NH group in the cis -position relative to the carbonyl group suggests strong NHO intramolecular hydrogen bonding comprising a six-membered chelate ring. The NHO hydrogen bond causes an increase of the 1J(N,H) coupling constant by 2,4 Hz and high-frequency shift of the 15N signal by 9,10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge-independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3-bis(isopropylamino)-1-(aryl)prop-2-en-1-one (12) for conformations with the Z - and E -orientations of the carbonyl group relative to the NH group. The effects of the NHO hydrogen-bond on the NMR parameters are analyzed with the atoms-in-molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NHO hydrogen bond as compared with that of 1,1-di(pyrrol-2-yl)-2-formylethene (13) where NHO hydrogen bridge establishes a seven-membered chelate ring, and the corresponding 1J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) ,,*NH hyperconjugative interaction is weakened on going from the six-membered chelate ring to the seven-membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the 1J(N,H) value as a consequence of the NHO hydrogen bonding in the studied diaminoenones. Copyright 2010 John Wiley & Sons, Ltd. [source]


Investigation of structure and dynamics in the sodium metallocenes CpNa and CpNaTHF via solid-state NMR, X-ray diffraction and computational modelling

MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2007
Cory M. Widdifield
Abstract Solid-state 23Na NMR spectra of two organometallic complexes, cyclopentadienylsodium (CpNa) and the tetrahydrofuran (THF) solvate of CpNa (CpNaTHF), are presented. Analytical simulations of experimental spectra and calculated 23Na electric-field gradient (EFG) tensors confirm that both complexes are present in microcrystalline samples of CpNa recrystallized from THF. For the solvate, 23Na NMR experiments at 9.4 T and 11.7 T elucidate sodium chemical shielding (CS) tensor parameters, and establish that the EFG and CS tensor frames are non-coincident. Single-crystal X-ray diffraction (XRD) experiments are used to determine the crystal structure of CpNaTHF: Cmca (a = 9.3242(15) , b = 20.611(3) , c = 9.8236(14) , , = , = , = 90 , V = 1887.9(5)3, Z = 8). For CpNa, 23Na NMR data acquired at multiple field strengths establish sodium CS tensor parameters more precisely than in previous reports. Variable-temperature (VT) powder XRD (pXRD) experiments determine the temperature dependence of the CpNa unit cell parameters. The combination of 23Na quadrupolar NMR parameters, pXRD data and calculations of 23Na EFG tensors is used to examine various models of dynamic motion in the solid state. It is proposed that the sodium atom in CpNa undergoes an anisotropic, temperature-dependent, low frequency motion within the ab crystallographic plane, in contrast with previous models. Copyright 2007 John Wiley & Sons, Ltd. [source]


Multinuclear NMR study of some organoplatinum complexes containing multifunctional azines as chelating ligands,

MAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2004
Dietrich Gudat
Abstract 1H-detected indirect NMR techniques were used to determine 15N and 195Pt NMR parameters for a series of organoplatinum(IV) complexes and one platinum(II) complex containing nitrogen-based azobispyridine, bispyridyltetrazine, and bipyrimidine ligands. The inverse technique permitted the detection of small 4J(Pt,H) and 5J(Pt,H) long-range couplings and the acquisition of 15N NMR data in natural isotopic abundance via nJ(N,H) intra- and inter-ligand couplings, but failed in cases where coherence transfer is quenched by rapid relaxation of the metal atom. In one case, analysis of satellite patterns in a set of 1H,15N, 1H,195Pt and 1H,13C correlation spectra allowed a positive sign to be determined for 1J(Pt,15N). Qualitative arguments are presented to explain the observed 15N coordination shifts in complexes with different azine ligands in terms of azine,M dative bond formation and LnM,azine back-donation. Copyright 2004 John Wiley & Sons, Ltd. [source]


Influence of the C,H N intramolecular interaction on the spatial structures and 1H and 13C NMR parameters of heteroaryl vinyl ethers and sulfides

MAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2003
Andrei V. Afonin
Abstract A complete analysis of the 1H and 13C spectra of the representative series of heteroaryl vinyl ethers and sulfides and heteroaryl styryl sulfides was carried out. The electronic and spatial structures of these compounds are discussed. It was shown that the C,H N intramolecular interactions in the investigated molecules influence significantly the spectral parameters and the conformational equilibrium. Copyright 2003 John Wiley & Sons, Ltd. [source]


Electrostatic screening and backbone preferences of amino acid residues in urea-denatured ubiquitin

PROTEIN SCIENCE, Issue 2 2007
Franc Avbelj
Abstract Local structures in denatured proteins may be important in guiding a polypeptide chain during the folding and misfolding processes. Existence of local structures in chemically denatured proteins is a highly controversial issue. NMR parameters [coupling constants 3J(H,,HN) and chemical shifts] of chemically denatured proteins in general deviate little from their values in small peptides. These peptides were presumed to be completely unstructured; therefore, it was considered that chemically denatured proteins are random coils. But recent experimental studies show that small peptides adopt relatively stable structures in aqueous solutions. Small deviations of the NMR parameters from their values in small peptides may thus actually indicate the existence of local structures in chemically denatured proteins. Using NMR data and theoretical predictions we show here that fluctuating ,-strands exist in urea-denatured ubiquitin (8 M urea at pH 2). Residues in such ,-strands populate more frequently the left side of the broad , region of ,,, space. Urea-denatured ubiquitin contains no detectable ,-sheet secondary structures; nevertheless, the fluctuating ,-strands in urea-denatured ubiquitin coincide to the ,-strands in the native state. Formation of ,-strands is in accord with the electrostatic screening model of unfolded proteins. The free energy of a residue in an unfolded protein is in this model determined by the local backbone electrostatics and its screening by backbone solvation. These energy terms introduce strong electrostatic coupling between neighboring residues, which causes cooperative formation of ,-strands in denatured proteins. We propose that fluctuating ,-strands in denatured proteins may serve as initiation sites to form fibrils. [source]


Elongation of the BH8 ,-hairpin peptide: Electrostatic interactions in ,-hairpin formation and stability

PROTEIN SCIENCE, Issue 7 2001
Marina Ramrez-Alvarado
Abstract An elongated version of the de novo designed ,-hairpin peptide, BH8, has allowed us to gain insight into the role of electrostatic interactions in ,-hairpin stability. A Lys,Glu electrostatic pair has been introduced by adding a residue at the beginning and at the end of the N-terminal and C-terminal strands, respectively, of the ,-hairpin structure, in both orientations. The two resulting peptides and controls having Ala residues at these positions and different combinations of Ala with Lys, or Glu residues, have been analyzed by nuclear magnetic resonance (NMR), under different pH and ionic strength conditions. All of the NMR parameters, in particular the conformational shift analysis of C, protons and the coupling constants, 3JHN,, correlate well and the population estimates are in reasonable agreement among the different methods used. In the most structured peptides, we find an extension of the ,-hairpin structure comprising the two extra residues. Analysis of the pH and salt dependence shows that ionic pairs contribute to ,-hairpin stability. The interaction is electrostatic in nature and can be screened by salt. There is also an important salt-independent contribution of negatively charged groups to the stability of this family of ,-hairpin peptides. [source]


Recombinant decorsin: Dynamics of the RGD recognition site

PROTEIN SCIENCE, Issue 8 2000
Andrzej M. Krezel
Abstract Decorsin is an antagonist of integrin ,IIb,3 and a potent platelet aggregation inhibitor. A synthetic gene encoding decorsin, originally isolated from the leech Macrobdella decora, was designed, constructed, and expressed in Escherichia coli. The synthetic gene was fused to the stII signal sequence and expressed under the transcriptional control of the E. coli alkaline phosphatase promoter. The protein was purified by size-exclusion filtration of the periplasmic contents followed by reversed-phase high-performance liquid chromatography. Purified recombinant decorsin was found to be indistinguishable from leech-derived decorsin based on amino acid composition, mass spectral analysis, and biological activity assays. Complete sequential assignments of 1H and proton bound 13C resonances were established. Stereospecific assignments of 21 of 25 nondegenerate ,-methylene groups were determined. The RGD adhesion site recognized by integrin receptors was found at the apex of a most exposed hairpin loop. The dynamic behavior of decorsin was analyzed using several independent NMR parameters. Although the loop containing the RGD sequence is the most flexible one in decorsin, the conformation of the RGD site itself is more restricted than in other proteins with similar activities. [source]


Synthesis and molecular structures of 1-chloro-1-silacyclopent-2-enes.

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 3 2009
1-organoboration, 2-hydroboration, Combination of , protodeborylation
Abstract The reaction of alkyn-1-yl(chloro)(methyl)vinyl- and alkyn-1-yl(chloro)(phenyl)-vinylsilane with 9-borabicyclo[3.3.1]nonane (9-BBN) afforded selectively 1-silacyclopent-2-ene derivatives containing a SiCl function, as a result of consecutive 1,2-hydroboration and 1,1-organoboration. Protodeborylation with acetic acid left the SiCl functions in various 1-silacyclopent-2-enes untouched, whereas acetic acid in the presence of dipropylamine led to conversion of the SiCl into the SiOAc function. New starting materials and all products were characterized in solution by multinuclear NMR spectroscopy (1H, 11B, 13C and 29Si NMR), and the molecular structures of two 1-silacyclopent-2-ene derivatives were determined by X-ray analysis. The gas phase geometries of 1-silacyclopent-2-enes were optimized by DFT calculations [B3LYP/6-311 + G(d,p) level of theory], found to be in reasonable agreement with the results of the crystal structure determination, and NMR parameters were calculated at the same level of theory. Copyright 2009 John Wiley & Sons, Ltd. [source]


Boryl-substituted 1-silacyclobutenes.

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 1 2007
Formation, molecular structure
Abstract The 1,2-hydroboration of the chloro(hexyn-1-yl)- (1a) and chloro(phenylethyn-1-yl)diphenylsilanes (1b) with 9-borabicyclo[3.3.1]nonane afforded selectively the alkenylsilanes 2a, b, in which the boryl and the silyl groups are linked to the same olefinic carbon atom. In case of 2a, treatment with phenylethynyl lithium gave a mixture of the alkyn-1-ylborate 3a and the alkenyl(phenylethynyl)diphenylsilanes 4a. In the case of 2b, only the alkyn-1-ylsilane 4b was identified as an intermediate. Both 4a, b slowly rearranged by intramolecular 1,1-vinylboration into the silacyclobutenes 5a, b. The intermediates were characterized by 1H, 11B, 13C and 29Si NMR spectroscopy in solution, and the molecular structure of the 1-silacyclobutene 5a was determined by X-ray analysis. The gas phase geometries of model molecules corresponding to 5a were optimized by MO calculations using DFT methods [B3LYP/6-311 + G(d,p) level of theory], found to be in reasonable agreement with the results of the crystal structure determination, and NMR parameters were calculated at the same level of theory. Copyright 2006 John Wiley & Sons, Ltd. [source]


The interaction of water molecules with purple membrane suspension using 2H double-quantum filter, 1H and 2H diffusion nuclear magnetic resonance

BIOPOLYMERS, Issue 1 2004
Limor Frish
Abstract Bacteriorhodopsin is a membrane protein of the purple membrane (PM) of Halobacterium salinarum, which is isolated as sheets of highly organized two-dimensional hexagonal microcrystals and for which water molecules play a crucial role that affects its function as a proton pump. In this paper we used single- and double-quantum 2H NMR as well as 1H and 2H diffusion NMR to characterize the interaction of water molecules with the PM in D2O suspensions. We found that, under the influence of a strong magnetic field on a concentrated PM sample (0.61 mM), the PM sheets affect the entire water population and a residual quadrupolar splitting (,q ,5.5 Hz, 298 K, at 11.7 T) is observed for the D2O molecules. We found that the residual quadrupolar coupling, the creation time in which a maximal DQF signal was obtained (,max), and the relative intensity of the 2H DQF spectrum of the water molecules in the PM samples (referred to herein as NMR order parameters) are very sensitive to temperature, dilution, and chemical modifications of the PM. In concentrated PM samples in D2O, these NMR parameters seem to reflect the relative organization of the PM. Interestingly, we have observed that some of these parameters are sensitive to the efficiency of the trimer packing, as concluded from the apo-membrane behavior. The data for dionized blue membrane, partially delipidated sample, and detergent-treated PM show that these D2O NMR order parameters, which are magnetic field dependent, are sensitive to the structural integrity of the PM. In addition, we revealed that heating the PM sample inside or outside the NMR magnet has, after cooling, a different effect on the NMR characteristics of the water molecules in the concentrated PM suspensions. The difference in the D2O NMR order parameters for the PM samples, which were heated and cooled in the presence and in the absence of a strong magnetic field, corroborates the conclusions that the above D2O order parameters are indirect reflections of both microscopic and macroscopic order of the PM samples. In addition, 1H NMR diffusion measurements showed that at least three distinct water populations could be identified, based on their diffusion coefficients. These water populations seem to correlate with different water populations previously reported for the PM system. 2004 Wiley Periodicals, Inc. Biopolymers, 2004 [source]


Tunable Alignment for All Polymer Gel/Solvent Combinations for the Measurement of Anisotropic NMR Parameters

CHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2010
Grit Kummerlwe M.
A stretching apparatus (see figure) with arbitrary scalability of alignment strength is introduced for the measurement of anisotropic NMR parameters, which works for practically all polymer gel/solvent combinations. Its use is demonstrated in applications involving the steady incrimination of residual dipolar couplings, the distinction of enantiomers, and the conformational analysis of organic compounds. [source]


Toward the Complete Prediction of the 1H and 13C NMR Spectra of Complex Organic Molecules by DFT Methods: Application to Natural Substances

CHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006
Alessandro Bagno Prof.
Abstract The NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, in the case of boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the Integral Equation-Formalism Polarizable Continuum Model (IEF-PCM) continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but also by visual comparison of line spectra. The origin of the remaining discrepancies is attributed to the incomplete modeling of conformational and specific solvent effects. [source]


NMR Parameters and Geometries of OHN and ODN Hydrogen Bonds of Pyridine,Acid Complexes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2004
Hans-Heinrich Limbach Prof.
Abstract In this paper, equations are proposed which relate various NMR parameters of OHN hydrogen-bonded pyridine,acid complexes to their bond valences which are in turn correlated with their hydrogen-bond geometries. As the valence bond model is strictly valid only for weak hydrogen bonds appropriate empirical correction factors are proposed which take into account anharmonic zero-point energy vibrations. The correction factors are different for OHN and ODN hydrogen bonds and depend on whether a double or a single well potential is realized in the strong hydrogen-bond regime. One correction factor was determined from the known experimental structure of a very strong OHN hydrogen bond between pentachlorophenol and 4-methylpyridine, determined by the neutron diffraction method. The remaining correction factors which allow one also to describe H/D isotope effects on the NMR parameters and geometries of OHN hydrogen bond were determined by analysing the NMR parameters of the series of protonated and deuterated pyridine- and collidine,acid complexes. The method may be used in the future to establish hydrogen-bond geometries in biologically relevant functional OHN hydrogen bonds. [source]