Carbon-fluorine two-dimensional NMR correlation spectra

Direct detection heteronuclear correlation (HETCOR) ¹³C{¹⁹F} and indirect detection gHMQC and gHSQC ¹⁹F{¹³C} two-dimensional correlation spectra have been compared for 1,3-perfluoro dimethylcyclohexane. Correlations to the geminally non-equivalent fluorine atoms in the CF₂ groups are more difficult to unambiguously observe using gHMQC or HETCOR in this typical cyclic fluorochemical in which ²J_FF ∼ > ¹J_CF, but they are readily observed and assigned in the gHSQC experiment. Excellent 2D gHSQC ¹⁹F{¹³C} correlation spectra have also been obtained on H(CF₂)₄CH₂OH without proton decoupling, although half of the correlations within the several FH multiplets have zero intensity.     

Antiaromaticity proved by the anisotropic effect in 1H NMR spectra

The spatial magnetic properties (through-space NMR shieldings, or TSNMRSs) of the antiaromatic 9-oxaanthracene anion 12(-) and of the corresponding 9-dimeric dianion 11(2-) have been calculated by the gauge-invariant atomic orbitals (GIAO) perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various size and direction. The TSNMRS values, thus obtained, can be employed to indicate antiaromaticity by paratropic ring currents of the anionic compounds of 11(2-) and 12(-) studied and other neutral and ionic antiaromatic molecules from previous studies because anisotropic effects of functional groups in (1)H NMR spectra have quantitatively proven to be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS).     

Resolving overlap in two-dimensional NMR spectra: nuclear Overhauser effects in a polysaccharide

Overlapping resonances in the two-dimensional nuclear Overhauser (NOESY) spectrum of the O-antigenic polysaccharide from Escherichia coli O147 have been resolved by recording a tilted projection of a three-dimensional NOESY-HSQC spectrum, where the carbon-13 and proton evolutions are linked together. Through the introduction of small contributions from the appropriate carbon-13 shifts, a cluster of five unresolved proton peaks is separated into its components without the need to perform the full three-dimensional measurement, a time saving of at least an order of magnitude.     

Java applets for viewing one- and two-dimensional NMR spectra

Two Java applets, which allow viewing and simple reprocessing operations of one- and two-dimensional NMR spectra from within a web page, are described. For the 1D viewer, phasing, integration, peak picking and referencing are supported. Bruker, Varian and JCAMP-DX processed data files can be opened. The 2D viewer allows f2 phasing and referencing, and can read native Bruker processed data. The compiled applets are available from the author on request.     

Parallel acquisition of two-dimensional NMR spectra of several nuclear species

We show that two or more two-dimensional NMR correlation spectra can be recorded in a single shot, using a multicoil radio-frequency probe and receiver system designed for simultaneous parallel acquisition of signals from different nuclear species such as 1H, 13C, and 15N. Dubbed PANSY (parallel acquisition NMR spectroscopy), this new technique shows promise for recording several multidimensional NMR spectra of different nuclear species in a very short time.     

Non-negative matrix factorization of two-dimensional NMR spectra: application to complex mixture analysis

A central problem in the emerging field of metabolomics is how to identify the compounds comprising a chemical mixture of biological origin. NMR spectroscopy can greatly assist in this identification process, by means of multi-dimensional correlation spectroscopy, particularly total correlation spectroscopy (TOCSY). This Communication demonstrates how non-negative matrix factorization (NMF) provides an efficient means of data reduction and clustering of TOCSY spectra for the identification of unique traces representing the NMR spectra of individual compounds. The method is applied to a metabolic mixture whose compounds could be unambiguously identified by peak matching of NMF components against the BMRB metabolomics database.     

Multiple-quantum relaxation dispersion NMR spectroscopy probing millisecond time-scale dynamics in proteins: theory and application

New relaxation dispersion experiments are presented that probe millisecond time-scale dynamical processes in proteins. The experiments measure the relaxation of (1)H-(15)N multiple-quantum coherence as a function of the rate of application of either (1)H or (15)N refocusing pulses during a constant time relaxation interval. In contrast to the dispersion profiles generated from more conventional (15)N((1)H) single-quantum relaxation experiments that depend on changes in (15)N((1)H) chemical shifts between exchanging states, (1)H-(15)N multiple-quantum dispersions are sensitive to changes in the chemical environments of both (1)H and (15)N spins. The resulting multiple-quantum relaxation dispersion profiles can, therefore, be quite different from those generated by single-quantum experiments, so that an analysis of both single- and multiple-quantum profiles together provides a powerful approach for obtaining robust measures of exchange parameters. This is particularly the case in applications to protonated proteins where other methods for studying exchange involving amide proton spins are negatively influenced by contributions from neighboring protons. The methodology is demonstrated on protonated and perdeuterated samples of a G48M mutant of the Fyn SH3 domain that exchanges between folded and unfolded states in solution.     

Assignment of proton NMR spectra and conformational analysis by two-dimensional heteronuclear relayed correlation NMR spectroscopy

The method of two-dimensional heteronuclear relayed correlation spectroscopy was used to establish the assignment of the severely overcrowded part of the proton spectrum of menthol by relating it to the previously assigned carbon spectrum. Extrapolation of the signal-to-noise ratio obtained with overnight data accumulation on a 10 mM solution suggests that this experiment should be feasible on as little as 10 mg of a moderate-sized organic compound.     

1H NMR spectra and structures of some bicyclic compounds oriented in nematic phases

The ¹H n.m.r. spectra of 1,4,5-triazanaphthalene, benzothiazole and quinoline oriented in nematic liquid crystalline phases have been analysed and the parameters used to determine the interproton distance ratios.     

14N NMR spectra of molecular nitrogen in SDS/pentanol swollen lamellar phases

14N NMR spectra of air dissolved in lyotropic mesophases are reported. In order to observe the whole spectrum from a molecule in which the quadrupole coupling constant is on the order of a few megahertz, a weak alignment degree with respect to the magnetic field is mandatory. Therefore, dilute lyotropic liquid crystals, namely sodium dodecylsulphate (SDS)/pentanol swollen lamellar phases, were considered. The temperature dependence of the 14N quadrupolar splitting was followed both in the case of oil (either n-dodecane or n-heptane) and brine (a 0.2-M NaBr water solution) swelling. In the former, it paralleled the temperature dependence of the splittings of the alkane deuteria and, in both cases, it was opposite to 23Na quadrupolar splittings. Owing to the higher N2 solubility in hydrocarbons, the 14N NMR spectra provide complementary information to that obtained by means of the quadrupolar nuclei of water and hydrophilic solutes.     

Stimulated echo NMR spectra and their use for heteronuclear two-dimensional shift correlation

NMR spectra are obtained from Fourier transformation of the latter part of a Hahn stimulated echo. The sequence is 90°-τ_a-90°-τ_b-90°-τ_a-digitize versus τ₂. This is demonstrated for transfer RNA in H₂O using the semiselective J ® pulse (90°-τ_p-90°), developed by Plateau and Gueron in place of each 90° pulse. 2D correlation between ¹³N and protons is obtained by applying a 90°-τ₁-90° ¹⁵N sequence during τ_b and ¹⁵N decoupling while digitizing. The second Fourier transform is performed with respect to τ₁ after discarding the imaginary parts of the first transform, to obtain a real 2D map. Such a 2D spectrum has been obtained for 5 mM¹⁵NH₄Cl and several other small molecules. The sequence should be useful for studies of ¹⁵N labeled macromolecules.     

Factor analysis of 27Al MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers

Nanostructured materials offer enhanced physicochemical properties because of the large interfacial area. Typically, geopolymers with specifically synthesized nanosized zeolites are a promising material for the sorption of pollutants. The structural characterization of these aluminosilicates, however, continues to be a challenge. To circumvent complications resulting from the amorphous character of the aluminosilicate matrix and from the low concentrations of nanosized crystallites, we have proposed a procedure based on factor analysis of ²⁷Al MAS NMR spectra. The capability of the proposed method was tested on geopolymers that exhibited various tendencies to crystallize (i) completely amorphous systems, (ii) X‐ray amorphous systems with nanocrystalline phases, and (iii) highly crystalline systems. Although the recorded ²⁷Al MAS NMR spectra did not show visible differences between the amorphous systems (i) and the geopolymers with the nanocrystalline phase (ii), the applied factor analysis unambiguously distinguished these materials. The samples were separated into the well‐defined clusters, and the systems with the evolving crystalline phase were identified even before any crystalline fraction was detected by X‐ray powder diffraction. Reliability of the proposed procedure was verified by comparing it with ²⁹Si MAS NMR spectra. Factor analysis of ²⁷Al MAS NMR spectra thus has the ability to reveal spectroscopic features corresponding to the nanocrystalline phases. Because the measurement time of ²⁷Al MAS NMR spectra is significantly shorter than that of ²⁹Si MAS NMR data, the proposed procedure is particularly suitable for the analysis of large sets of specifically synthesized geopolymers in which the formation of the limited fractions of nanocrystalline phases is desired. Copyright © 2013 John Wiley & Sons, Ltd.     

13C NMR spectra of liquid crystalline substances in the isotropic,nematic and solid phases

A CP MAS ¹³C NMR spectrum indicates that some liquid crystalline substances containing both cyclohexane and benzene rings have a local molecular motion around the benzene plane in the solid state, and the cyclohexane ring has a rigid conformation. The reorientational diffusion of the whole molecule is also observed in the solid state.     

RUBIDIUM, a program for computer-aided assignment of two-dimensional NMR spectra of polypeptides.

Taking advantage of the rule-based expert system technology, a program named RUBIDIUM (Rule-Based Identification In 2D NMR Spectrum) was developed to accomplish the automatic 1H NMR resonance assignments of polypeptides. Besides noise elimination and peak selection capabilities, RUBIDIUM detects the cross-peak patterns of amino acid residues in the COSY spectrum, assigning these patterns to amino acid types, performing sequential assignments using combined COSY/NOESY spectra, and finally, achieving the total assignment of the 1H NMR spectrum.     

NMR spectra of organogermanium compounds: 7. NMR spectra and MNDO calculations of phenylgermacyclohexanes

¹³C nmr spectra of a variety of 1-phenylgermacyclohexanes indicates that the conformational energy of a phenyl group on germanium is essentially zero (0 – 0.1 kcal/mol), in a sharp contrast with that in phenylcyclohexanes, The results are consistent with the heats of formation of phenylgermacyclohexanes calculated by the MNDO method.     

Co-existence of magnetic phases in two-dimensional MXene

This study reports first synthesis of MXene-derived co-existing magnetic phases. New family of two-dimensional (2D) materials such as Ti₃C₂ namely MXene, having transition metal forming hexagonal structure with carbon atoms have attracted tremendous interest now a days. We have reported structural, optical and magnetic properties of un-doped and La-doped Ti₃C₂T_x MXene, synthesized using co-precipitation method. The lattice parameter (LP) calculated for La-MXene are a = 5.36 Å, c = 18.3 Å which are slightly different from the parent un-doped MXene (a = 5.35 Å, c = 19.2 Å), calculated from X-ray diffraction data. The doping of La⁺³ ions shrinks Ti₃C₂T_x layers perpendicular to the planes. The band gap for MXene is calculated to be 1.06 eV which is increased to 1.44 eV after doping of La⁺³ ion that shows its good semiconducting nature. The experimental results and density functional theory (DFT) calculations for magnetic properties of both the samples have been presented and discussed, indicating the co-existence of ferromagnetic-antiferromagnetic phases. The results presented here are novel and is first report on co-existence of magnetic properties of 2D carbides for potential applications in two-dimensional spintronics.     

Quantitative determination of magnetization exchange rate constants from a series of two-dimensional exchange NMR spectra

A method for quantitative determination of magnetization exchange rate constants (cross-relaxation and chemical exchange) from a series of two-dimensional exchange spectra is presented. The method, the least error matrix analysis (LEMA), combines a series of full matrix calculations at different mixing times in a least-squares manner. LEMA embodies the principal advantages of full-relaxation matrix analysis (FMA) and initial rate buildup (BU) analysis. Like FMA, it takes into account all the relations among the spectral matrix elements and in analogy to BU makes use of their time evolution. By means of calculations, simulations, and experiments, we have shown that LEMA provides the dynamic matrix from a given set of experimental data with errors that are smaller than in either FMA or BU calculations.     

Variable-angle spinning 1H NMR spectra of molecules in liquid-crystalline phases with positive and negative diamagnetic anisotropy

The ¹H NMR spectrum of thiophene dissolved in the liquid crystals ZLI 1167 (with negative anisotropy of diamagnetic susceptibility, Δ_χ) and ZLI 1132 (with positive Δ_χ) was studied by applying the so-called variable-angle spinning (VAS) method. In this method, the liquid-crystal sample is spun with relatively high speed around an axis forming a certain adjustable angle with the external magnetic Held. It is shown that a normal high-resolution spectrum with a varying scaling factor in the anisotropic parameters is obtained at any spin angle between 0 and 90°.