Showing papers in "Magnetic Resonance in Chemistry in 2010"
TL;DR: The methodology was successfully applied to determine the fatty acid composition of several edible oils, with equivalent results to those given by the AOAC Official method by gas chromatography.
Abstract: A simple methodology for the determination of the fatty acid composition of edible oils through (1)H NMR is proposed. The method is based on the fact that all fatty acid chains are esterified to a common moiety, glycerol, and the quantification is done directly in the (1)H NMR spectra through the relationship between the areas of a characteristic signal of each fatty acid and a signal of the glycerol moiety, without the use of mathematical equations. The methodology was successfully applied to determine the fatty acid composition of several edible oils, with equivalent results to those given by the AOAC Official method by gas chromatography. Its main advantages are simplicity and the lack of need for sample pre-treatment such as derivatization or extraction.
165 citations
TL;DR: The chemical makeup of GCBE was clarified by the NMR results, and in this study, three isomers of caffeoylquinic acid were identified in the complex mixture without any separation.
Abstract: A complex mixture analysis by one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy was carried out for the first time for the identification and quantification of organic compounds in green coffee bean extract (GCBE). A combination of (1)H-(1)H DQF-COSY, (1)H-(13)C HSQC, and (1)H-(13)C CT-HMBC two-dimensional sequences was used, and 16 compounds were identified. In particular, three isomers of caffeoylquinic acid were identified in the complex mixture without any separation. In addition, GCBE components were quantified by the integration of carbon signals by use of a relaxation reagent and an inverse-gated decoupling method without a nuclear Overhauser effect. This NMR methodology provides detailed information about the kinds and amounts of GCBE components, and in our study, the chemical makeup of GCBE was clarified by the NMR results.
83 citations
TL;DR: Most of the spin–spin coupling constants under study, especially vicinal 77Se1H couplings, demonstrated a remarkable stereochemical behavior with respect to the internal rotation of the substituent in the 2‐position of the selenophene ring, which is of major importance in the stereochemical studies of the related organoselenium compounds.
Abstract: Experimental measurements and second-order polarization propagator approach (SOPPA) calculations of (77)Se-(1)H spin-spin coupling constants together with theoretical energy-based conformational analysis in the series of 2-substituted selenophenes have been carried out. A new basis set optimized for the calculation of (77)Se-(1)H spin-spin coupling constants has been introduced by extending the aug-cc-pVTZ-J basis for selenium. Most of the spin-spin coupling constants under study, especially vicinal (77)Se-(1)H couplings, demonstrated a remarkable stereochemical behavior with respect to the internal rotation of the substituent in the 2-position of the selenophene ring, which is of major importance in the stereochemical studies of the related organoselenium compounds.
77 citations
TL;DR: QUANTAS (QUANTification by Artificial Signal) is an absolute intensity external standard method for quantification by NMR that compensates for various experimental parameters, applicable to all nuclei and modern spectrometers.
Abstract: We have developed QUANTAS (QUANTification by Artificial Signal), which is a software-based protocol for concentration measurement by NMR. QUANTAS is an absolute intensity external standard method for quantification by NMR that compensates for various experimental parameters. It is applicable to all nuclei and modern spectrometers. QUANTAS is demonstrated here for (1)H and (19)F NMR, enabling heteronuclear integrals to be compared. It can be applied using fixed probe tuning, matching and pulse length, for samples with the same effective loading on the probe coil as the appropriate reference spectrum. Otherwise, an optimised tuning and matching approach is adopted for every sample together with explicit PULCON (PUlse Length-based CONcentration measurements) absolute intensity corrections.
76 citations
TL;DR: The computations have substantially aided in the assignments of both 13C and 1H resonances, as has a series of two‐dimensional spectra (HETCOR, DQ‐CRAMPS and proton–proton spin diffusion).
Abstract: This article addresses, by means of computation and advanced experiments, one of the key challenges of NMR crystallography, namely the assignment of individual resonances to specific sites in a crystal structure. Moreover, it shows how NMR can be used for crystal structure validation. The case examined is form B of terbutaline sulfate. CPMAS (13)C and fast MAS (1)H spectra have been recorded and the peaks assigned as far as possible. Comparison of (13)C chemical shifts computed using the CASTEP program (incorporating the Gauge Including Projector Augmented Wave principle) with those obtained experimentally enable the accuracy of the two distinct single-crystal evaluations of the structure to be compared and an error in one of these is located. The computations have substantiallly aided in the assignments of both (13)C and (1)H resonances, as has a series of two-dimensional (2D) spectra (HETCOR, DQ-CRAMPS and proton-proton spin diffusion). The 2D spectra have enabled many of the proton chemical shifts to be pinpointed. The relationships of the NMR shifts to the specific nuclear sites in the crystal structure have therefore been established for most (13)C peaks and for some (1)H signals. Emphasis is placed on the effects of hydrogen bonding on the proton chemical shifts. Copyright (C) 2010 John Wiley & Sons, Ltd.
73 citations
TL;DR: This study explores the concentration range over which matrix‐assisted DOSY using the surfactant SDS can achieve diffusion resolution of a simple model set of isomers, the monomethoxyphenols, supporting the idea that MAD may become a valuable tool for mixture analysis.
Abstract: Diffusion-ordered spectroscopy (DOSY) is a powerful technique for mixture analysis, but in its basic form it cannot separate the component spectra for species with very similar diffusion coefficients. It has been recently demonstrated that the component spectra of a mixture of isomers with nearly identical diffusion coefficients (the three dihydroxybenzenes) can be resolved using matrix-assisted DOSY (MAD), in which diffusion is perturbed by the addition of a co-solute such as a surfactant [R. Evans, S. Haiber, M. Nilsson, G. A. Morris, Anal. Chem. 2009, 81, 4548-4550]. However, little is known about the conditions required for such a separation, for example, the concentrations and concentration ratios of surfactant and solutes. The aim of this study was to explore the concentration range over whichmatrix-assisted DOSY using the surfactant SDS can achieve diffusion resolution of a simple model set of isomers, the monomethoxyphenols. The results show that the separation is remarkably robust with respect to both the concentrations and the concentration ratios of surfactant and solutes, supporting the idea that MAD may become a valuable tool formixture analysis.
65 citations
TL;DR: The 1H, 13C and 15N NMR coordination shifts were discussed in relation to some structural features of the title complexes, such as the type of the central atom, geometry, geometry (trans‐ or cis‐), metal‐nitrogen bond lengths and the position of both methyl groups in the pyridine ring system.
Abstract: (1)H, (13)C and (15)N NMR studies of gold(III), palladium(II) and platinum(II) chloride complexes with dimethylpyridines (lutidines: 2,3-lutidine, 2,3lut; 2,4-lutidine, 2,4lut; 3,5-lutidine, 3,5lut; 2,6-lutidine, 2,6lut) and 2,4,6-trimethylpyridine (2,4,6-collidine, 2,4,6col) having general formulae [AuLCl(3)], trans-[PdL(2)Cl(2)] and trans-/cis-[PtL(2)Cl(2)] were performed and the respective chemical shifts (delta(1H), delta(13C), delta(15N)) reported. The deshielding of protons and carbons, as well as the shielding of nitrogens was observed. The (1)H, (13)C and (15)N NMR coordination shifts (Delta(1H) (coord), Delta(13C) (coord), Delta(15N) (coord); Delta(coord) = delta(complex) - delta(ligand)) were discussed in relation to some structural features of the title complexes, such as the type of the central atom [Au(III), Pd(II), Pt(II)], geometry (trans- or cis-), metal-nitrogen bond lengths and the position of both methyl groups in the pyridine ring system.
54 citations
TL;DR: The results of calculations using recently developed quantum‐chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) are reported and compared to those calculated with Gaussian and the completion of the NMR crystallography of the zeolite ITQ‐4 is reported.
Abstract: Combining quantum-chemical calculations and ultrahigh-field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on ‘molecular fragments’ extracted from the zeolite crystal structure using Hartree–Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh-field 29Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations using recently developed quantum-chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ-4, which was previously solved from NMR data. We compare three options for the ‘refinement’ of zeolite crystal structures from ‘NMR-solved’ structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum-chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X-ray diffraction structure of ITQ-4. Copyright © 2010 John Wiley & Sons, Ltd.
53 citations
TL;DR: The bioassays showed that only compounds 14 and 16 could enhance the differentiation of 3T3‐L1 preadiocytes accompanied by secretion of adiponectin proteins among these 23 compounds.
Abstract: Five new beta-carboline alkaloids, 6,12-dimethoxy-3-(2-hydroxylethyl)-beta-carboline (1), 3,10-dihydroxy-beta-carboline (2), 6,12-dimethoxy-3-(1-hydroxylethyl)-beta-carboline (3), 6,12-dimethoxy-3-(1,2-dihydroxylethyl)-beta-carboline (4), and 6-methoxy-3-(2-hydroxyl-1-ethoxylethyl)-beta-carboline (5), and two new natural products, 6-methoxy-12-hydroxy-3-methoxycarbonyl-beta-carboline (6) and 3-hydroxy-beta-carboline (7) were isolated from the stems of Picrasma quassioides along with 16 known beta-carboline alkaloids (8-23). The structures of new compounds were determined by extensive spectroscopic analyses, and the 1D and 2D NMR data of compounds 6, 7 and 10 were reported for the first time. The bioassays showed that only compounds 14 and 16 could enhance the differentiation of 3T3-L1 preadiocytes accompanied by secretion of adiponectin proteins among these 23 compounds.
53 citations
TL;DR: A new analytical model for a quantitative description of the dependence of the silicon‐29 and boron‐11 isotropic chemical shift upon the bond angles is proposed, which incorporates possibly the effect of SiO2B2O3 intermixing.
Abstract: Boron-11 and silicon-29 NMR spectra of xSiO2 (1 − x)B2O3 glasses (x = 0.40, 0.80 and 0.83) have been calculated using a combination of molecular dynamics (MD) simulations with density functional theory (DFT) calculations of NMR parameters. Structure models of 200 atoms have been generated using classical force fields and subsequently relaxed at the PBE-GGAlevel of DFT theory. The gauge including projector augmented wave (GIPAW) method is then employed for computing the shielding and electric field gradient tensors for each silicon and boron atom. Silicon-29 MAS and boron-11 MQMAS NMR spectra of two glasses (x = 0.40 and 0.80) have been acquired and theoretical spectra are found to well agree with the experimental data. For boron-11, the NMR parameter distributions have been analysed using a Kernel density estimation (KDE) approach which is shown to highlight its main features. Accordingly, a new analytical model that incorporates the observed correlations between the NMR parameters is introduced. It significantly improves the fit of the 11B MQMAS spectra and yields, therefore, more reliable NMR parameter distributions. A new analytical model for a quantitative description of the dependence of the silicon-29 and boron-11 isotropic chemical shift upon the bond angles is proposed, which incorporates possibly the effect of SiO2B2O3 intermixing. Combining all the above procedures, we show how distributions of SiOT and BOT (TSi, B) bond angles can be estimated from the distribution of isotropic chemical shift of silicon-29 and boron-11, respectively. Copyright © 2010 John Wiley & Sons, Ltd.
48 citations
TL;DR: NMR analysis of complex mixtures can be significantly simplified using polyethyleneglycol (PEG) as resolving additive in DOSY NMR technique, which allows the extraction of individual spectra of mixture components with differing polarity.
Abstract: NMR analysis of complex mixtures can be significantly simplified using polyethyleneglycol (PEG) as resolving additive in DOSY NMR technique, which allows the extraction of individual spectra of mixture components with differing polarity. Resolving power of PEG-assisted DOSY was demonstrated with natural product mixtures. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The J‐PGSE sequence should find applications in many diffusion studies where the PGSE‐type method is required and should be a viable alternative to PGSTE especially in dilute samples due to its enhanced sensitivity.
Abstract: Peak distortion caused by homonuclear (1)H J-coupling is a major problem in many spin-echo-based experiments such as pulsed gradient spin-echo (PGSE) experiments. Although peak phase distortions can be lessened by the incorporation of anti-phase purging sequences, the sensitivity is substantially decreased. Techniques for lessening the effect of homonuclear J-coupling evolution in spin-echo-based experiments have been investigated. Two potentially useful candidates include a J-compensated inversion sequence that is efficient over a wide range of J-coupling values and a pulse sequence that refocuses homonuclear J-evolution during the spin-echo. The latter was found to work superbly on samples containing two spin (AX or AB) systems and still provided significant advantage over the standard method on samples containing more complicated spin systems. Implementation of this J-refocusing technique into a PGSE-type experiment (J-PGSE) leads to dramatic improvement of spectra and easier data analysis. The J-PGSE sequence should find applications in many diffusion studies where the PGSE-type method is required and should be a viable alternative to PGSTE especially in dilute samples due to its enhanced sensitivity.
TL;DR: The chemical study of Momordica charantia fruits led to the isolation of three new cucurbitane triterpene glycosides, momordicosides U, V, and W.
Abstract: The chemical study of Momordica charantia fruits led to the isolation of three new cucurbitane triterpene glycosides, momordicosides U, V, and W (1-3). The structures of these compounds were determined to be (19R, 23R)-5beta, 19-epoxy-19-methoxycucurbita-6,24-diene-3beta, 23-diol 3-O-beta-D-allopyranoside (1), (23R)-5beta, 19-epoxycucurbita-6,24-diene-3beta, 23-diol 3-O-beta-D-allopyranoside (2), and (19R)-5beta, 19-epoxy-19,25-dihydroxycucurbita-6,23(E)-diene-3beta-ol 3-O-beta-D-glucopyranoside (3), by chemical and spectroscopic methods.
TL;DR: High‐resolution magic angle spinning NMR spectroscopic data of smoked Atlantic salmon were fully assigned by combination of one‐ and two‐dimensional‐HRMAS experiments and for the first time, spectra were resolved sufficiently to allow semiquantitative determination in intact muscle of the highly polyunsaturated FA 22:6 ω‐3.
Abstract: Financial support from the Spanish Science and Technology Commission, Project Consolider Ingenio 2010 (CSD2007-00016-03) and the Comunidad de Madrid (Project CM S-0505/AGR-0314)
TL;DR: It is shown that DFT‐predicted paramagnetic shifts can greatly aid in obtaining and understanding NMR spectra of paramagnetic Ru(III) complexes.
Abstract: Nuclear shieldings, including the Fermi-contact and pseudocontact terms, have been calculated with density functional theory (DFT) (nonrelativistic and relativistic) methods in several Ru(III) complexes, thereby predicting 1H and 13C paramagnetic shifts. A fair agreement with experimental values is observed. Structural, magnetic and dynamic parameters have also been input to the Solomon–Bloembergen equation in order to predict signal lineshapes. It is shown that DFT-predicted paramagnetic shifts can greatly aid in obtaining and understanding NMR spectra of paramagnetic Ru(III) complexes. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: Three potentially interesting perspectives in connection with PAW/GIPAW in solid‐state NMR and pure nuclear quadrupole resonance (NQR) are presented, including the calculation of J coupling tensors in inorganic solids; the calculations of the antisymmetric part of chemical shift tensors and the prediction of 14N and 35Cl pure NQR resonances including dynamics.
Abstract: In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of (14)N and (35)Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear (2)J(P-O-Si) coupling constants in the case of silicophosphates and calcium phosphates [Si(5)O(PO(4))(6), SiP(2)O(7) polymorphs and α-Ca(PO(3))(2)]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C(3)X(4) (X = H, Cl, F) and (iii) (14)N and (35)Cl NQR predictions in the case of RDX (C(3)H(6)N(6)O(6)), β-HMX (C(4)H(8)N(8)O(8)), α-NTO (C(2)H(2)N(4)O(3)) and AlOPCl(6). RDX, β-HMX and α-NTO are explosive compounds.
TL;DR: 1H, 13C and 15N NMR studies of iron(II), ruthenium(II) and osmium( II) tris‐chelated cationic complexes with 2,2′‐bipyridine and 1,10‐phenanthroline of the general formula [M(LL)3]2+ were performed, finding significant shielding of nitrogen‐adjacent protons and metal‐bonded nitrogens was observed.
Abstract: 1H, 13C and 15N NMR studies of iron(II), ruthenium(II) and osmium(II) tris-chelated cationic complexes with 2,2′-bipyridine and 1,10-phenanthroline of the general formula [M(LL)3]2+ (M = Fe, Ru, Os; LL = bpy, phen) were performed. Inconsistent literature 1H signal assignments were corrected. Significant shielding of nitrogen-adjacent protons [H(6) in bpy, H(2) in phen] and metal-bonded nitrogens was observed, being enhanced in the series Ru(II) → Os(II) → Fe(II) for 1H, Fe(II) → Ru(II) → Os(II) for 15N and bpy → phen for both nuclei. The carbons are deshielded, the effect increasing in the order Ru(II) → Os(II) → Fe(II). Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The new integrated computational approaches developed in the recent years in the research group are reviewed, showing that proper account of stereo‐electronic, environmental and dynamical effects leads to magnetic properties in remarkable agreement with experimental results.
Abstract: Magnetic spectroscopic techniques such as electron paramagnetic resonance (EPR) and paramagnetic NMR (PNMR) are valuable tools for understanding the structure and dynamics of complex systems such as, for example, biomolecules or nanomaterials labeled with suitable free radicals. Unfortunately, such spectra do not give direct access to the radical structure because of the subtle interplay between several different effects not easily separable and evaluable by experimentalists alone. In this respect, computational spectroscopy is becoming an essential and versatile tool for the assignment and interpretation of experimental spectra. In this article, the new integrated computational approaches developed in the recent years in our research group are reviewed. Such approaches have been applied to two widely used spin probes showing that proper account of stereo-electronic, environmental and dynamical effects leads to magnetic properties in remarkable agreement with experimental results. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: It is demonstrated that the use of cryo‐probe generated 1,1‐adequate data in conjunction with HMBC dramatically improves computer‐assisted structure elucidation (CASE) both in terms of speed and accuracy of structure generation.
Abstract: The availability of cryogenically cooled probes permits routine acquisition of data from low sensitivity pulse sequences such as inadequate and 1,1-adequate. We demonstrate that the use of cryo-probe generated 1,1-adequate data in conjunction with HMBC dramatically improves computer-assisted structure elucidation (CASE) both in terms of speed and accuracy of structure generation. In this study data were obtained on two dissimilar natural products and subjected to CASE analysis with and without the incorporation of two-bond specific data. Dramatic improvements in both structure calculation times and structure candidates were observed by the inclusion of the two-bond specific data. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: A simple, cheap and flexible flowcell based on a standard NMR tube, designed for the monitoring of reactions but of wide applicability, is described, allowing the system to be employed with any conventional NMR probe and magnet.
Abstract: A simple, cheap and flexible flowcell based on a standard 5 mm NMR tube, designed for the monitoring of reactions but of wide applicability, is described. No modification of the NMR instrument is needed, allowing the system to be employed with any conventional NMR probe and magnet. The system is robust and economical in use of reagents, and can be used for studying both homogeneous and heterogeneous reactions. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The 1H, 13C and 15N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyr roles, and the NBO analysis suggests that the N�H···N hydrogen bond in the E isomer is a pure electrostatic interaction.
Abstract: The (1)H, (13)C and (15)N NMR studies have shown that the E and Z isomers of pyrrole-2-carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole-2-carbaldehyde oxime is stabilized by the N-H...N and N-H...O intramolecular hydrogen bonds, respectively. The N-H...N hydrogen bond in the E isomer causes the high-frequency shift of the bridge proton signal by about 1 ppm and increase the (1)J(N, H) coupling by approximately 3 Hz. The bridge proton shows further deshielding and higher increase of the (1)J(N, H) coupling constant due to the strengthening of the N-H...O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by approximately 3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of (1)H shielding and (1)J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the N-H...N and N-H...O hydrogen bondings to be estimated. The NBO analysis suggests that the N-H...N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the N-H bond through the N-H...O hydrogen bond occurs in the Z isomer.
TL;DR: A comparison between experimental and calculated gas‐phase as well as the conductor‐like screening model DFT 195Pt chemical shifts of a series of octahedral [PtX6−nYn]2− complexes for X = Cl, Br, F, I was carried out to assess the accuracy of computed NMR shielding and to gain insight into the dominant σdia, σpara and σSO shielding contributions.
Abstract: A comparison between experimental and calculated gas-phase as well as the conductor-like screening model DFT (195)Pt chemical shifts of a series of octahedral [PtX(6-n)Y(n)](2-) complexes for X = Cl, Br, F, I was carried out to assess the accuracy of computed NMR shielding and to gain insight into the dominant σ(dia), σ(para) and σ(SO) shielding contributions. The discrepancies between the experimental and the DFT-calculated (195)Pt chemical shifts vary for these complexes as a function of the coordinated halide ions, the largest being obtained for the fluorido-chlorido and fluorido-bromido complexes, while negligible discrepancies are found for the [PtCl(6-n)Br(n)](2-) series; the discrepancies are somewhat larger where a significant deviation from the ideal octahedral symmetry such as for the geometric cis/trans or fac/mer isomers of [PtF(6-n)Cl(n)](2-) and [PtF(6-n)Br(n)](2-) may be expected. The discrepancies generally increase in the order [PtCl(6-n)Br(n)](2-) < [PtBr(6-n)I(n)](2-) < [PtCl(6-n)I(n)](2-) < [PtF(6-n)Br(n)](2-) ≈ [PtF(6-n)Cl(n)](2-), and show a striking correlation with the increase in electronegativity difference Δχ between the two halide ligands (X(-) and Y(-)) bound to Pt(IV) for these anions: 0.09 < 0.52 < 0.63 < 1.36 ≈ 1.27, respectively. The computed (195)Pt sensitivity to Pt-X bond displacement, ∂(δ(195)Pt)/∂(ΔPt-X), of these complexes is very large and depends on the halide ion, decreasing from 24 800, 18 300, 15 700 to 12 000 ppm/A for [PtF(6)](2-), [PtCl(6)](2-), [PtBr(6)](2-) and [PtI(6)](2-), respectively.
TL;DR: In high‐resolution solid‐state 1H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:1 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.
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.
TL;DR: A novel approach for the structural analysis of heteroleptic triple‐decker (porphyrinato)(phthalocyaninato) lanthanides(III) in solutions is developed using molecular mechanics (MM+) optimization of the geometry of the complex taking into account the lanthanide‐induced shift (LIS) datasets.
Abstract: A novel approach for the structural analysis of heteroleptic triple-decker (porphyrinato)(phthalocyaninato) lanthanides(III) in solutions is developed. The developed approach consists in molecular mechanics (MM+) optimization of the geometry of the complex taking into account the lanthanide-induced shift (LIS) datasets. LISs of the resonance peaks in (1)H NMR spectra of a series of symmetric complexes [An(4)P]Ln[(15C5)(4)Pc]Ln[An(4)P], where An(4)P(2-) is 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinato-dianion, [(15C5)(4)Pc](2-) is 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninato-dianion and Ln = La, Ce, Pr, Nd, Sm, Eu, are analyzed. Analysis of LISs showed two sets of protons in the molecule with opposite signs of shift. Two-nuclei analysis of LISs testifies isostructurality of the whole series of investigated complexes in solution despite contraction of the lanthanide ions. Model-free separation of contact and dipolar contributions of LISs was performed with one-nucleus technique and did not show changes in contact and dipolar terms within the investigated series. MM+ optimization of the molecular structure allowed the interpretation of features of LIS for each particular group of protons. Parameterization of MM + -optimized model of molecule with values of structure-dependent dipolar contributions of LIS allows the development of the precise structural model of the triple-decker complex in solution. This approach allows the determination of the geometry and structure of the sandwich macrocyclic tetrapyrrolic complexes together with conformational analysis of flexible peripheral substituents in solutions. The developed method can be applied with minor modifications for the determination of structural parameters of other types of lanthanides(III) complexes with tetrapyrrolic ligands and also supramolecular systems based on them.
TL;DR: By assessing the conformation of the individual states, the thermodynamic model of a self‐association promoted conformational transition is established and is extended by a comparison with other members of the bifunctional amine‐thiourea organocatalyst family.
Abstract: The conformational diversity of the (3R,4S,8R,9R)-9-[(3,5-bis(trifluoromethyl)phenyl))-thiourea](9-deoxy)-epi-cinchonine organocatalyst is discussed. Low-temperature NMR experiments confirmed a self-association process, which promotes the quinoline rotation between two intramolecularly hydrogen-bonded monomeric conformers of the catalyst. The balanced population of the coexisting monomeric and dimeric species allowed us to conduct a structural study of a rather complex conformational dynamics of the pure catalyst. The study is extended by a comparison with other members of the bifunctional amine-thiourea organocatalyst family. Changes in the molecular structure of the catalysts influence the interplay between intra- and intermolecular hydrogen bonding, and yield different extent of catalyst self-association. By assessing the conformation of the individual states, we established the thermodynamic model of a self-association promoted conformational transition. Copyright © 2009 John Wiley & Sons, Ltd.
TL;DR: The accuracy of 13C chemical shift prediction by both DFT GIAO quantum‐mechanical (QM) and empirical methods was compared using 205 structures for which experimental and QM‐calculated chemical shifts were published in the literature.
Abstract: The accuracy of 13C chemical shift prediction by both DFT GIAO quantum-mechanical (QM) and empirical methods was compared using 205 structures for which experimental and QM-calculated chemical shifts were published in the literature. For these structures, 13C chemical shifts were calculated using HOSE code and neural network (NN) algorithms developed within our laboratory. In total, 2531 chemical shifts were analyzed and statistically processed. It has been shown that, in general, QM methods are capable of providing similar but inferior accuracy to the empirical approaches, but quite frequently they give larger mean average error values. For the structural set examined in this work, the following mean absolute errors (MAEs) were found: MAE(HOSE) = 1.58 ppm, MAE(NN) = 1.91 ppm and MAE(QM) = 3.29 ppm. A strategy of combined application of both the empirical and DFT GIAO approaches is suggested. The strategy could provide a synergistic effect if the advantages intrinsic to each method are exploited. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The structure elucidations and complete 1H and 13C NMR assignments are reported for two new xanthone derivatives that were isolated from the fermentation medium of a mangrove endophytic fungus.
Abstract: The structure elucidations and complete 1H and 13C NMR assignments are reported for two new xanthone derivatives: 1,7-dihydroxy-2-methoxy-3-(3-methylbut-2-enyl)-9H-xanthen-9-one (1) and 1-hydroxy-4,7-dimethoxy-6-(3-oxobutyl)-9H-xanthen-9-one (2). Both of these secondary metabolites were isolated from the fermentation medium of a mangrove endophytic fungus (No. ZH19). High-resolution electron impact mass spectrometry (HREIMS), Fourier transform infrared (FT-IR) absorption spectrometry, and NMR experiments including gCOSY, gHMQC, and gHMBC were used for the determination of the structures and NMR spectral assignments. Preliminary pharmacological test showed that compounds (1) and (2) inhibited KB cells with IC50 values of 20 and 35 µmol/ml, and KBV200 cells with IC50 values of 30 and 41 µmol/ml, respectively. Copyright © 2009 John Wiley & Sons, Ltd.
TL;DR: A new secondary metabolite, pterocephaline, along with the known cantleyoside, 7α‐morroniside, 3β,5α‐tetrahydrodesoxycordifoline lactam, sweroside, and loganin have been isolated from the aerial parts of P. pinardii (Dipsacaceae).
Abstract: A new secondary metabolite, pterocephaline, along with the known cantleyoside, 7α-morroniside, 3β,5α-tetrahydrodesoxycordifoline lactam, 5S-5-carboxyvincoside, sweroside, and loganin have been isolated from the aerial parts of P. pinardii (Dipsacaceae). Moreover, cantleyoside-methyl-hemiacetal and cantleyoside-dimethyl-acetal were obtained as seco-iridoid artifacts. The structures were elucidated by extensive spectroscopic methods including 1D-(1H, 13C and TOCSY) and 2D-NMR (DQF-COSY, HSQC and HMBC). Monoterpenoid glucoindole alkaloids were encountered for the first time in Dipsacaceae family. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: The findings show that the direct effect of computational parameters on 1H shifts is not big, whereas the indirect dependence via the structural data is more important, illustrating once again the sensitivity of 1H NMR observables on small changes in the local chemical structure of complex hydrogen‐bonded liquids.
Abstract: We present (1)H NMR chemical shift calculations of liquid water based on first principles molecular dynamics simulations under periodic boundary conditions. We focus on the impact of computational parameters on the structural and spectroscopic data, which is an important question for understanding how sensitive the computed (1)H NMR resonances are upon variation of the simulation setup. In particular, we discuss the influence of the exchange-correlation functional and the size of the basis set, the choice for the fictitious electronic mass and the use of pseudopotentials for the nuclear magnetic resonance (NMR) calculation on one hand and the underlying Car-Parrinello-type molecular dynamics simulations on the other hand. Our findings show that the direct effect of these parameters on (1)H shifts is not big, whereas the indirect dependence via the structural data is more important. The (1)H NMR chemical shifts clearly reflect the induced structural changes, illustrating once again the sensitivity of (1)H NMR observables on small changes in the local chemical structure of complex hydrogen-bonded liquids.
TL;DR: Two new oxidized sterols 1 and 2 were obtained from the active fraction of a mangroves fungus Aspergillus awamori isolated from the soils around the mangrove plant Acrostichum speciosum and their cytotoxic activity against A549 cell line was evaluated.
Abstract: Two new oxidized sterols 1 and 2 were obtained from the active fraction of a mangrove fungus Aspergillus awamori isolated from the soils around the mangrove plant Acrostichum speciosum. Their structures were elucidated using spectroscopic methods as 22E-7α-methoxy-5α,6α-epoxyergosta-8(14),22-dien-3β-ol (1) and 22E-3β-hydroxy-5α,6α,8α,14α-diepoxyergosta-22-en-7-one (2). The NMR data and complete assignments for both DMSO-d6 and CDCl3 were given. Their cytotoxic activity against A549 cell line was evaluated. Furthermore, the detailed conformation analysis for ring B (cyclohexene oxide system) of sterol 1 was given on the basis of NOEs. The endo-boat conformation was considered as the preferred conformation for ring B rather than half-chair conformation. Copyright © 2009 John Wiley & Sons, Ltd.