Covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy is introduced, which is a new scheme for establishing nuclear spin correlations from NMR experiments. In this method correlated spin dynamics is directly displayed in terms of a covariance matrix of a series of one-dimensional (1D) spectra. In contrast to two-dimensional (2D) Fourier transform NMR, in a covariance spectrum the spectral resolution along the indirect dimension is determined by the favorable spectral resolution obtainable along the detection dimension, thereby reducing the time-consuming sampling requirement along the indirect dimension. The covariance method neither involves a second Fourier transformation nor does it require separate phase correction or apodization along the indirect dimension. The new scheme is demonstrated for cross-relaxation (NOESY) and J-coupling based magnetization transfer (TOCSY) experiments.     

Shiftless nuclear magnetic resonance spectroscopy

The acquisition and analysis of high resolution one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectra without chemical shift frequencies are described. Many variations of shiftless NMR spectroscopy are feasible. A two-dimensional experiment that correlates the dipole-dipole and dipole-dipole couplings in the model peptide , (15)N labeled N-acetyl-leucine is demonstrated. In addition to the resolution of resonances from individual sites in a single crystal sample, the bond lengths and angles are characterized by the two-dimensional powder pattern obtained from a polycrystalline sample.     

Theory of covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval's theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.     

Computer-aided spectral assignment in nuclear magnetic resonance spectroscopy

The proton-carbon correlation spectra, HMBC (heteronuclear multiple bond correlation) and HMQC (heteronuclear multiple quantum correlation), respectively, provide direct and remote connectivity information with high sensitivity. Their combination enables carbon-carbon proximity relationships to be deduced, which are formally identical to those produced by a fictitious INADEQUATE-2D experiment, where correlations would be established exclusively between atoms linked by one or two bonds. The CASA program uses these relationships, as well as DEPT spectra and elementary chemical-shift considerations to assign the ¹³C spectrum of a compound if its structure is known or assumed. If the structure conflicts with the experimental data, no assignment is produced. The CASA program serves as an aid to either spectral assignment or structural elucidation.     

Hyphenation of capillary separations with nuclear magnetic resonance spectroscopy

The hyphenation of small-volume separations to information-rich detection offers the promise of unmatched analytical information on the components of complex mixtures. Nuclear magnetic resonance (NMR) spectroscopy provides information about molecular structure, although sensitivity remains an issue for on-line NMR detection. This is especially true when hyphenating NMR to capillary separations as the observation time and analyte mass are decreased to the point where reduced information is obtained from the eluting analytes. Because of these limitations, advances in instrumental performance have a large impact on the overall performance of a separation–NMR system. Instrumental aspects and the capabilities of cLC–NMR, CEC–NMR and CE–NMR are reviewed, and applications that have used this technology highlighted. Recent trends towards small volume capillary scale separations are emphasized, as is the recent success of capillary-isotachophoresis (cITP)–NMR.     

Carbon-13 nuclear magnetic resonance spectroscopy of cephalotaxus alkaloids

Carbon-13 chemical shift assignments have been obtained for the naturally occurring Cephalotaxus alkaloids, cephalotaxine, acetylcephalotaxine, harringtonine, isoharringtonine, drupacine and cephalotaxinone.     

Carbon-13 nuclear magnetic resonance spectroscopy of conjugated polyynes

The ¹³C NMR spectra of several conjugated polyyne-aldehydes and ketones are compared with those of the corresponding alcohols. All data show considerable effects similar to those observed in conjugated polyenes.     

Fast inspection of fruits using nuclear magnetic resonance spectroscopy

High-resolution nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for obtaining chemical structure information. Its quantitative and noninvasive properties have led to its growing popularity as an analytical tool in many fields, including biology, chemistry, medicine, and food science. During transportation and storage, chemical reactions among the many nutrients lead to a loss of food quality. In these circumstances, portable NMR spectrometers can readily be used for food inspection and quality control. Because of the heterogeneous tissue distribution in food, a high-resolution NMR method is required for detailed food inspection. Therefore, in this study, we demonstrated the feasibility of using an intermolecular double-quantum coherence signal to obtain high-resolution metabolic profiles of several fruits, including grape, cantaloupe, tomato, and watermelon. The resulting high-resolution NMR spectra facilitate the identification of important metabolites, which can be used as biomarkers for food quality control. The method established here may be adapted for food inspection using portable NMR equipment.     

J-coupling nuclear magnetic resonance spectroscopy of liquids in nT fields

In ultralow magnetic fields, liquid state nuclear magnetic resonance (NMR) spectra of homonuclear spin systems exhibit line widths dominated by their natural lifetime. Chemical shifts become negligible, and heteronuclear NMR spectra show predominantly the electron-mediated field-independent J-coupling. However, weak polarization and Larmor frequencies down to a few hertz require special detectors, such as Superconducting Quantum Interference Devices (SQUID), that also enable the simultaneous detection of broad band spectra of heteronuclear spin systems. We acquired spectra of 2,2,2-trifluoroethanol and trimethyl phosphate at detection fields varying from 444 nT to 3.34 muT after prepolarizing the sample in a field of 250 muT. Down to a 1H Larmor frequency of 40 Hz, the spectra of trifluoroethanol exhibited four clearly resolvable peaks. The numerical simulation agreed well with the measured spectra. Trimethyl phosphate exhibited two major groups of nonresolved proton lines. At 1H Larmor frequencies below 150 Hz, the separation of the two groups decreased, reflecting the transition from weakly to strongly coupled spin systems. Direct determination of 3J(H,P) from the peak separation is possible only at Larmor frequencies above 150 Hz. The experimental setup allowed an easy adjustment of the detection field over several octaves. This enabled us to adapt the detection field to the best-suited measurement window providing the maximum spectral information. Low-field NMR may open new applications, such as monitoring heteronuclear reactions, low-field imaging, simultaneous NMR/magnetoencephalography measurements, or quantum computing.     

Carbon-13 nuclear magnetic resonance spectroscopy of polychlorinated diphenyl ethers

Carbon-13 n.m.r. chemical shifts are reported for diphenyl ether and several of its chlorinated analogs. Carbon shieldings were found to be dependent on the degree of steric interference to conjugative release by the oxygen atom. Substituent effects due to attached chlorine atoms seem to be more easily transmitted into the adjacent aryl moiety if the number of ortho chlorines is reduced. These results were clearly seen by a variable-by-variable plot display or by using a pattern recognition computer program. Spin-lattice relaxation times have been measured for several compounds mainly for assignment purposes, but they are also briefly discussed in view of possible interconversion mechanisms.     

Analysis of biological fluids by high-field nuclear magnetic resonance spectroscopy

The application of high-field Fourier transform nuclear magnetic resonance (NMR) spectroscopy to the analysis of biological fluids such as urine, plasma and bile is described. Applications include areas such as clinical chemistry, experimental and clinical toxicology and drug metabolism studies. In the case of proton NMR some means of attenuating or eliminating the interference due to water protons is required and suitable strategies for achieving this are discussed. The use of 2-dimensional NMR or solid-phase extraction/chromatography to enable the identification of unknowns is discussed and the potential usefulness of ¹⁹F NMR for studying the metabolism of fluorinated xenobiotics is highlighted.     

Solution nuclear magnetic resonance spectroscopy techniques for probing intermolecular interactions

Nuclear magnetic resonance (NMR) spectroscopy in solution has evolved into a powerful technique for structure determination of proteins and nucleic acids. More recently, a number of NMR-based approaches have been developed to monitor and characterize intermolecular interactions. These approaches offer unique advantages over other techniques and find their utility in both structural biology and drug discovery. We will report on basic principles and recent examples of the application of such NMR methodologies to characterize protein-protein interactions and for ligand binding studies and drug discovery.     

13C nuclear magnetic resonance spectroscopy to determine olive oil grades

¹³C nuclear magnetic resonance spectroscopy was used in a first attempt to differentiate olive oil samples by grades. High resolution ¹³C NMR Distortionless Enhancement by Polarization Transfer (DEPT) spectra of 137 olive oil samples from the four grades, extra virgin olive oils, olive oils, olive pomace oils and lampante olive oils, were measured. The data relative to the resonance intensities (variables) of the unsaturated carbons of oleate (C-9 and C-10) and linoleate (L-9, L-10 and L-12) chains attached at the 1,3- and 2-positions of triacylglycerols were analyzed by linear discriminant analysis. The 1,3- and 2- carbons of the glycerol moiety of triacylglycerols along with the C-2, C-16 and C-18 resonance intensities of saturated, oleate and linoleate chains were also analyzed by linear discriminant analysis. The three discriminanting functions, which were calculated by using a stepwise variable selection algorithm, classified in the true group by cross-validation procedure, respectively, 76.9, 70.0, 94.4 and 100% of the extra virgin, olive oil, olive pomace oil and lampante olive oil grades.     

Introduction of the Floquet-Magnus expansion in solid-state nuclear magnetic resonance spectroscopy

In this article, we present an alternative expansion scheme called Floquet-Magnus expansion (FME) used to solve a time-dependent linear differential equation which is a central problem in quantum physics in general and solid-state nuclear magnetic resonance (NMR) in particular. The commonly used methods to treat theoretical problems in solid-state NMR are the average Hamiltonian theory (AHT) and the Floquet theory (FT), which have been successful for designing sophisticated pulse sequences and understanding of different experiments. To the best of our knowledge, this is the first report of the FME scheme in the context of solid state NMR and we compare this approach with other series expansions. We present a modified FME scheme highlighting the importance of the (time-periodic) boundary conditions. This modified scheme greatly simplifies the calculation of higher order terms and shown to be equivalent to the Floquet theory (single or multimode time-dependence) but allows one to derive the effective Hamiltonian in the Hilbert space. Basic applications of the FME scheme are described and compared to previous treatments based on AHT, FT, and static perturbation theory. We discuss also the convergence aspects of the three schemes (AHT, FT, and FME) and present the relevant references.     

Tetrazoloazines. 15N nuclear magnetic resonance and infrared absorption spectroscopy

The reaction of 4-chloro-2-phenylquinazoline with K¹⁵NN₂ has been studied by ¹⁵N-NMR. spectroscopy. ¹⁵N-chemical shifts in 5-phenyl-1 (3)-[¹⁵N]-tetrazolo[1,5-c]quinazoline and -Nα(Nγ)-[¹⁵N]-4-azido-2-phenylquinazoline are reported. The characteristic IR. absorption frequencies of the tetrazole group have been determined in a series of annelated ¹⁵N-labelled compounds. From these studies and the chemistry of the labelled tetrazoles, it is concluded that all haloazines examined react with KN₃ by the direct nucleophilic substitution mechanism. An addition of nucleophile-ring opening-ring closure (ANRORC) mechanism was not observed. The synthesis of several ¹⁵N-labelled tetrazoloazines is described.     

Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses

The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins' evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing the desired line shapes both in the frequency and in the time domains are introduced and experimentally demonstrated. Sensitivity- and lineshape-wise these experiments are competitive vis-a-vis current implementations of wideline quadrupolar NMR based on hard rf pulses; additional opportunities that may derive from these ideas are also briefly discussed.     

Characterizing nitrilimines with nuclear magnetic resonance spectroscopy. A theoretical study

The 13C chemical shifts in selected nitrilimines, nitriles, acetylenes, allenes, and singlet carbenes have been calculated using density-functional theory [PBE0/6-311++G(2df,pd)] and the gauge including atomic orbital (GIAO) method. The effects of substitution on the 13C chemical shifts in nitrilimines, R1-CNN-R2, have been examined. The carbon nucleus is generally found to be deshielded by substituents in the order CH3 < NH2 < OH < F. Comparison with nitriles, acetylenes, and allenes shows that this effect is related to the presence of the cumulated functionality, C=N=N. Terminal N-substitution is found to have a larger effect than C-substitution due to a large increase in chemical shielding anisotropy. The electronic structure of nitrilimines has recently been shown to possess a carbene component whose resonance contribution varies widely with substitution, and, as previously reported, insight into the electronic structure can be gained by an analysis of the shielding tensor, especially for carbenes. Accordingly, the components of the diagonalized 13C shielding tensor for nitrilimines and stable singlet carbenes have been examined. This analysis suggests that diaminonitrilimine, H2N-CNN-NH2, may be a stable carbene, and, to the best of our knowledge, it would be the first acyclic, unsaturated stable carbene ever reported. Finally, a detailed analysis of the 13C chemical shifts shows that an increase in the dipolar character of nitrilimines induces a shielding at the carbon nucleus, while an increase in allenic or carbenic character tends to cause a deshielding.     

Assignment of juvabione diastereomers by 13C nuclear magnetic resonance spectroscopy

The determination of structures and partial assignments of stereochemistry of juvabione and some of its analogues can be made on the basis of ¹³C nuclear magnetic resonance studies. The complete ¹³C n.m.r. spectral assignments for juvabione and five analogues are reported.