Interpretation Of NMR Spectra Research Articles

A Potentiometric and 113Cd NMR Study of Cadmium Complexation by Natural Organic Matter at Two Different Magnetic Field Strengths

The binding of cadmium to Suwannee River natural organic matter (NOM) has been investigated across a broad range of Cd/C ratios (0.00056-0.0056) and pH values (3.5-11) by (113)Cd NMR spectroscopy at two magnetic field strengths (B(0) = 9.4 and 11.7 T). Caused by the very peculiar and highly complex nature of the Cd-NOM exchanging system, these (113)Cd NMR spectra are characterized by a pH- and concentration-dependent superposition of slow, intermediate, and fast chemical exchange. The complex interplay of solution chemistry and chemical exchange requires a thorough mapping of this Cd-NOM chemically exchanging system through NMR acquisition at two magnetic field strengths and a systematic variation of Cd/C ratios and pH values. The interpretation of (113)Cd NMR spectra is greatly facilitated and constrained by simultaneous measurements of pH and pCd, which allows a model-independent calculation of organically bound Cd(2+) under all experimental conditions. Within the range of chemical conditions applied in this study, (113)Cd NMR spectrometric evidence is consistent with coordination of cadmium by oxygen, nitrogen, and sulfur ligands in NOM. Under all experimental conditions, cadmium is primarily coordinated to oxygen; however, several lines of evidence point to the participation of nitrogen ligands, even in acidic solutions where nitrogen ligands are primarily bound to protons. Under alkaline conditions, up to one-third of cadmium may be coordinated to nitrogen, and a small, but unquantifiable, percentage of cadmium is coordinated to sulfur ligands, as evidenced by far-low-field (113)Cd NMR resonances.

Dammarane-type glycosides from Gynostemma pentaphyllum.

Fifteen new dammarane glycosides (1-15), together with four known compounds, gypenosides IV, VIII, LXXI, and XLIX, were isolated from the MeOH extract of the aerial parts of Gynostemma pentaphyllum. Their structures were elucidated by 1D and 2D NMR spectra interpretation as well as by chemical degradation.

Is It Possible to Extend the Coleman−Fox Method for Polymer Sequence Determination by NMR to Copolycondensates?

A new theory is presented for the interpretation of NMR spectra of copolycondensates, with special attention to those obtained by melt mixing of two macromolecular chains (reactive blending). The repeat unit is split into two parts, referred to as half-monomers. The present theory gives a highly accurate description of the changes in the sequence distribution during the reactive blending reaction, since the effect of the first and second neighbors along the macromolecular backbone (half-monomer penultimate effect) is explicitly considered. The theoretical predictions are compared with experimental data taken from the literature and, more specifically, NMR data concerning five copolymer systems, namely, a copolymer obtained by reactive blending of poly(ethylene terephthalate) and poly(ethylene adipate), an almost alternating copolymer with units of ether-sulfone and ether-ketone, a copolymer derived from 6-methyl-2,5-morpholinedione, and two copolymers obtained by reactive blending of poly(butylene terephthalate) with poly(bisphenolA carbonate) and Nylon6, respectively.

Quantum calculations of the electronic structure and NMR quadrupolar interaction parameters for tugtupite

Tugtupite [Na 8 (Al 2 Be 2 Si 8 O 24 )Cl 2 ] is an extremely ordered mineral containing 27 Al, 23 Na, and 9 Be quadrupolar nuclei. The quadrupolar interaction parameters (QI), C q and η, for these nuclei were calculated from the electronic structure by full-crystal linearized augmented plane-wave (FC LAPW) or ab initio molecular orbital (MO) calculations. The orientation of the EFG tensors, calculated using FC LAPW correspond with the crystallographic axes a1, a2, and c, and η at the Al site is equal to 0.00 as required by the symmetrical electron density distribution in the a-c planes. The electron density of states (DOS) shows the EFG to originate from the valence band of the p-p, p-s electrons. The agreement between the calculated QI values and experimental results for tugtupite shows that FC LAPW is very effective to evaluate the electronic structure of an ordered mineral, and aid in the interpretation of MAS NMR spectra. The combination of computational efficiency and accuracy of ab initio cluster MO calculations were examined by using different cluster sizes, basis sets, theoretical levels, and charge backgrounds using the Gaussian 98W program. A small cluster embedded in a point charge background gave sufficiently accurate 27 Al EFG tensors, with much greater computational efficiency than with WIEN2k. The MO cluster calculation became closer to the experimental results as the number of atoms in the clusters increased, but the calculation cost increased as well. The point-charge background was found to contribute about 10% to the EFG. MO calculations, with B3LYP level and basis set 6-311G+d** for the cluster of two atom shells around the central Al nucleus, gave quadrupole interaction values within 13% of that found by the FC LAPW method using the WIEN2k program. Therefore, MO cluster calculations can provide a more efficient but less accurate alternative to FC LAPW for quantum calculations of mineral structures.

Novel Dammarane-Type Glycosides from Gynostemma pentaphyllum

Three new dammarane glycosides (1-3), together with five known compounds, gypenoside LXIX (4), gylongiposide I (5), gypenoside XLVIII (6), allantion (7) and vitexin (8) were isolated from the MeOH extract of the aerial parts of Gynostemma pentaphyllum. Compounds 5, 7, and 8 were isolated from this plant for the first time. Their structures were elucidated by 1D and 2D NMR spectra interpretation as well as by chemical degradation.

Himalomycin A and B: isolation and structure elucidation of new fridamycin type antibiotics from a marine Streptomyces isolate.

In our screening of marine Streptomycetes for bioactive compounds, in addition to the known metabolites rabelomycin (1), fridamycin D (2b), N-benzylacetamide and N-(2'-phenylethyl)acetamide, two new anthracycline antibiotics designated as himalomycin A (2c) and B (2d) were isolated from the culture broth of the marine Streptomyces sp. isolate B6921. The structure of the new antibiotics was determined by comparison of the NMR data with those of fridamycin D (2b) and by detailed interpretation of mass, 1D and 2D NMR spectra.

Chandrananimycins A approximately C: production of novel anticancer antibiotics from a marine Actinomadura sp. isolate M048 by variation of medium composition and growth conditions.

In our screening of marine actinomycetes for bioactive principles, three novel antibiotics designated as chandrananimycin A (3c), B (3d) and C (4) were isolated from the culture broth of a marine Actinomadura sp. isolate M045. The structures of the new antibiotics were determined by detailed interpretation of mass, 1D and 2D NMR spectra.

What is X? A Classroom Exercise

The last stage of three-step synthetic sequence to obtain an alkylated Birch product gave an unexpected and undesired product. From the discussion of all the reactions involved rather than from the interpretation of NMR spectra, the structure of the product X is elucidated.

Structure validation of natural products by quantum-mechanical GIAO calculations of 13C NMR chemical shifts.

Geometry optimization and GIAO (gauge including atomic orbitals) (13)C NMR chemical shift calculations at Hartree-Fock level, using the 6-31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental (13)C NMR chemical shifts for fourteen low-polar natural products, containing 10-20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of (13)C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical (13)C chemical shifts of both correct and incorrect structures matched the experimental data.

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited: a new assignment of structural changes and crystal-field effects 25 years later.

Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes [Ln(L1-2H)(3)](3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)(3)](3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2<r (2)> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Water-soluble glycosides from Ruta graveolens.

An EtOH extract of the dried aerial parts of Ruta graveolens was suspended in water and then partitioned with EtOAc. Three new glycosides, 3'-sinapoyl-6-feruloylsucrose (4), methylcnidioside A (5), and methylpicraquassioside A (6), together with four known glycosides, 3',6-disinapoylsucrose (1), cnidioside A (2), rutin, and picraquassioside A (3), were isolated from the water-soluble part. Their structures were elucidated by interpretation of IR, MS, and 1D and 2D NMR spectra and comparison with literature data.

Advances in Structural Determination of Saponins and Terpenoid Glycosides

Saponins and triterpenoid glycosides are polar constituents of plants many of which exhibit a broad spectrum of biological activities. They often occur as intricate mixtures difficult to separate by conventional procedures. Improvements in liquid chromatographic techniques and availability of new matrices have improved the possibility of purifying even the isomeric saponins. The application of newer isolation and purification processes are illustrated in the present review with examples both from the literature and from the reviewers laboratories. Advances in mass spectrometry, for example tandem mass spectrometry, offer considerable advantages in structure determination of complex saponins, and their use has been illustrated. The major structural information however comes from NMR spectra. Due to the complex structures, interpretation of NMR spectra recorded even with high field instruments is not easy. The present review summarizes the available 2D NMR techniques particularly HMBC, NOESY and HOHAHA for simplifying spectral interpretation with suitable examples. In special cases, use of molecular mechanics and molecular dynamics studies of structure leads to valuable information on conformation of molecules. Their application was found to be of considerable advantage for saponins possessing polysubstituted monosaccharides. Keywords: Saponins, Terpenoid Glycosides, Esins, Triterenoid, Aesculus hippocastanum, Saponaria officinalis, Mimusops elengi, Ms-saoponis, Silene rubicunda, Negative FABMS of anaggallisin

ZeoTsites: a code for topological and crystallographic tetrahedral sites analysis in zeolites and zeotypes

The program zeoTsites has been developed as a complementary tool to some solid state computer codes with the aim of obtaining detailed analyses of TO and TT distances, and TOT angles in zeotypes (T ≡ four connected atom), as well as T atom connectivity and vertex symbols. From the crystallographic data the program finds the zeotype geometry for individual and average TOT units. Two kinds of averages are considered by taking into account the topologic and the crystallographic nature of the T sites. The framework distortions upon isomorphic substitution can be analyzed from the results of computational calculations or experimental data. The coordination sequences of the T atoms and the vertex symbols are also calculated by the program. Connected atoms in the first and second coordination shells are also calculated for assistance in the interpretation of NMR spectra of pure AlPO 4 and SiO 2 structures as well as chemically substituted forms.

MUNIN: a new approach to multi-dimensional NMR spectra interpretation.

A new method, MUNIN (Multi-dimensional NMR spectra interpretation), is introduced for the automated interpretation of three-dimensional NMR spectra. It is based on a mathematical concept referred to as three-way decomposition. An NMR spectrum is decomposed into a sum of components, with each component corresponding to one or a group of peaks. Each component is defined as the direct product of three one-dimensional shapes. A consequence is reduction in dimensionality of the spectral data used in further analysis. The decomposition may be applied to frequency-domain or time-domain data, or to a mixture of these. Features of MUNIN include good resolution in crowded regions and the absence of assumptions about line shapes. Uniform sampling of time-domain data, a prerequisite for discrete Fourier transform, is not required. This opens an avenue for the processing of NMR data that do not follow oscillating behaviour, e.g. from relaxation measurements. The application of MUNIN is illustrated for a 1H-15N-NOESY-HSQC, where each component is defined as the set of all NOE peaks formed by a given amide group. As a result, the extraction of structural information simply consists of one-dimensional peak picking of the shape along the NOE-axis obtained for each amide group.

MUNIN: application of three-way decomposition to the analysis of heteronuclear NMR relaxation data.

MUNIN (Multidimensional NMR Spectra Interpretation), a recently introduced approach exploiting the mathematical concept of three-way decomposition, is proposed for separation and quantitative relaxation measurements of strongly overlapped resonances in sets of heteronuclear two-dimensional spectra that result from typical relaxation experiments. The approach is general and may also be applied to sets of two-dimensional spectra with arbitrary modulation along the third dimension (e.g., J-coupling, diffusion). Here, the method is applied for the analysis of 15N rotating frame relaxation data.

Haliclonyne, a new highly oxygenated polyacetylene from the marine sponge Haliclona species.

Haliclonyne (1), a new polyacetylene carboxylic acid, has been isolated from the marine sponge Haliclona sp. collected in the Gulf of Eilat. The structure of haliclonyne, a C(47) oxo-octahydroxy-dienetetrayne carboxylic acid, was elucidated by interpretation of NMR and mass spectra of 1 and two of its derivatives.

Quantum mechanical calculation of 23 Na NMR shieldings in silicates and aluminosilicates

To assist in the assignment and interpretation of 23Na NMR spectra in silicate and aluminosilicate minerals and glasses we have calculated the 23Na NMR shieldings and the electric field gradients (EFG) at the Na for a number of Na-containing species. Included are Na(OH2) n +, n = 1, 2, 4, 5, 6 and 8, and Na+ complexes with SiH3OH, SiH3ONa and O(SiH3)2. We have also evaluated shieldings and EFGs for Na-centered clusters extracted from crystalline Na2SiO3 and anhydrous sodalite, Na6[AlSiO4]6. Using 6-31G* SCF optimized geometries and the GIAO method with a 6-31G* basis set [and 6-311(2d,p) bases for the smaller clusters] we find a calculated increase in shielding with coordination number (CN) for the Na(OH2) n +, n = 4, 6, 8 series that agrees reasonably well with experimental trends. Calculated changes in the Na shielding as water is replaced by bridging or nonbridging silicate O atoms are also consistent with experimental observations. The deshielding of Na (with respect to gas-phase Na+) which is produced by an O-containing ligand is a strongly decreasing function of the R(Na–O) and a weakly decreasing function of the underbonding or free valence of the O. Deshielding contributions to the isotropic shielding from different ligands are additive to good approximation for low CN species, so that the total deshielding can be calculated accurately by summing the contributions from the individual ligands. However for the larger CN species the directly calculated deshieldings are substantially smaller than those obtained using such an additivity approximation. We further test this approximation by calculating the deshieldings for Na in 12 different sites in silicate and aluminosilicate minerals which have recently been studied experimentally, using our calculated deshielding contributions for individual O-containing ligands and experimental values for the Na–O distances. Correlation coefficients between the experimental shifts and the calculated deshieldings are around 0.9 and the slope of the correlation is almost 1.0 . Calculations on large Na-centered clusters extracted from the crystal structures of Na2SiO3 and anhydrous sodalite reproduce the experimental values for both NMR shieldings and electric field gradients but at considerable computational cost. Comparison with recent 23Na NMR studies on hydrous albite glasses indicates that coordination of either H2O or OH− to the Na could give the magnitude of deshielding observed, depending upon the detailed Na–O distances within the hydrous glass.

A Safe Simple Halogenation Experiment

This experiment is designed to be a safe and experimentally simple procedure appropriate to the early weeks of a course when halogenation is the only reaction which has been discussed in the lecture. It can also provide some early experience with simple interpretation of NMR spectra.

Ab Initio Calculations of the17O NMR Chemical Shift of Hydronium and Dihydroxonium Ions in Their Fluoroborates: Comparison with Experiment

The geometries of the hydronium and dihydroxonium cations in ion pairs with fluoroborate anions were examined by ab initio calculations at the MP2/6-31G* level. It was found that the representation of the hydronium ion in the field of an anion as an equilateral triangle, employed in the literature for the interpretation of low-temperature broad-band NMR spectra of water in solid acids, is an oversimplification, particularly for the composition H5O2+ (dihydroxonium). Chemical shift calculations (DFT−GIAO−B3LYP at the dzvp, tzp, tz2p, and qz2p levels) were conducted for 17O in H3O+·BF4- (1) and H5O2+·BF4- (2). The signal of 2 was predicted to appear at higher frequency (downfield) than the signal for 1. For experimental verification, the 17O NMR spectra were recorded for various mixtures of hydronium fluoroborate and water. A nonmonotonic variation of the 17O chemical shift with the increase in water content was observed; the signal moved first toward higher frequency and had the highest chemical shift for a water-to-hydronium ratio of 1:1 (H5O2+), after which a monotonic variation toward lower frequency (upfield) was observed. Along both branches of the δ 17O vs composition plot (H3O+·BF4- to H5O2+·BF4- and H5O2+·BF4- to H2O) the chemical shift variation was nonlinear. Thus, the experiments and the calculations were in qualitative agreement (the signal for 1 at lower frequency than the signal for 2), but the chemical shift difference predicted by the calculations was larger than the experimental result. Better agreement between the calculated and measured chemical shift differences is obtained for an orientation of ions in 2 with two fluorine atoms hydrogen bonded with the cation. Likewise, a better agreement is obtained for the pyramidal form than for the planar form of 1, in agreement with the geometry optimization results.

Empirical Investigation on the Reproducibility of13C NMR Shift Values

Automated NMR spectra interpretation packages are becoming increasingly important especially now that high-throughput synthesis often vastly increases the amount of data to be analyzed in a given time frame. It is vitally important for these interpretation packages to be robust, and this means that it essential to know how the quality of NMR data varies is real life experiments. A practical test concerning the reproducibility of chemical shift information from recorded 13C NMR spectra was carried out using eight test substances. Two hundred seventy spectra were contributed from both Academia and Industry and have been subjected to a standardized interpretation. For these spectra the confidence limits of their chemical shifts have been calculated. The standard deviations found vary from 0.2 up to about 2.5 ppm, and the range of values for the shifts reported vary from ±0.7 up to more than ±7 ppm around the respective average shift. Test substances recorded in more polar solvents generally showed higher sta...