Two-dimensional relayed coherence transfer experiments as a means of subgrouping protonated carbon resonances of complex polynuclear heteroaromatics on the basis of vicinal proton-proton connectivities

Application of autocorrelated homonuclear (COSY) and proton-carbon chemical shift correlation two-dimensional nmr experiments to the problem of assigning the ¹H- and ¹³C-nmr spectra of phenanthro[4,3-a]dibenzothiophene fails despite their considerable utility in the assignment of the spectra of the smaller phenanthro[b]thiophenes. The failure of these techniques in the case of phenanthro[4,3-a]dibenzothiophene is predominantly due to the inability of the COSY spectrum to provide the means of subgrouping the proton resonances into their component spin subsystems. Two-dimensional relayed coherence transfer nmr experiments which first established coherence between vicinally coupled protons which is then transferred to the carbon spins which are ultimately observed circumvent these problems. The application of two-dimensional relayed coherence transfer to phenanthro[4,3-a]dibenzothiophene is described, the technique leading to the sub-grouping of all five of the proton spin subsystems and thus providing a means of beginning the total assignment.     

Double quantum filtered zero quantum coherence with broadband homonuclear f1 Decoupling

A pulse sequence is described which combines double quantum filtration and broadband homonuclear F₁ decoupling in a two-dimensional zero quantum (ZQCOSY) nmr experiment.     

Spin state selective detection of single quantum transitions using multiple quantum coherence: simplifying the analyses of complex NMR spectra

NMR spectra of molecules oriented in liquid-crystalline matrix provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used as an orienting media result in the spectra of spins that are generally strongly coupled. The number of allowed transitions increases rapidly with the increase in the number of interacting spins. Furthermore, the number of single quantum transitions required for analysis is highly redundant. In the present study, we have demonstrated that it is possible to separate the subspectra of a homonuclear dipolar coupled spin system on the basis of the spin states of the coupled heteronuclei by multiple quantum (MQ)-single quantum (SQ) correlation experiments. This significantly reduces the number of redundant transitions, thereby simplifying the analysis of the complex spectrum. The methodology has been demonstrated on the doubly 13C labeled acetonitrile aligned in the liquid-crystal matrix and has been applied to analyze the complex spectrum of an oriented six spin system.     

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.     

Probing proton-proton proximities in the solid state: high-resolution two-dimensional 1H-1H double-quantum CRAMPS NMR spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.     

Ultrafast two-dimensional NMR spectroscopy using constant acquisition gradients

Multidimensional NMR spectroscopy plays an important role in the characterization of molecular structure and dynamics. A new methodology for acquiring this kind of spectra has been recently demonstrated, endowed with the potential to compress arbitrary multidimensional NMR acquisitions into a single scan. This "ultrafast" nD acquisition protocol is based on a spatiotemporal encoding of the indirect-domain spin evolution, followed by a repetitive decoding and re-encoding of the information thus stored employing a train of alternating-sign gradients. Such train of switching gradients extending throughout the course of the data acquisition may pose extreme demands on a magnetic resonance system, particularly when dealing with nonshielded gradients, strong eddy currents, or rapidly relaxing spin systems. Limits to the in vivo applicability of such fast-switching scheme may also arise due to gradient-induced perineural stimulation. The present study describes a new approach to ultrafast nD NMR that reduces the number of gradient switchings during the acquisition period to zero, leading in essence to a constant-gradient acquisition scheme. This approach operates on the basis of a novel spatiotemporal encoding including discrete, temporally overlapping, frequency-shifted pulses. Principles and examples of this new approach are given; sensitivity limitations and signal-enhancing prospects of such constant-gradient acquisitions are also discussed and exemplified.     

Fingerprints of damped quantum rotation observed in solid-state proton NMR spectra.

(1)H NMR spectra of the methyl group in an oriented crystal sample of methylmalonic acid with all three non-methyl protons replaced by deuterons are interpreted in terms of the damped quantum rotation (DQR) theory of NMR line shapes. The DQR approach offers a perfect theoretical reproduction of the observed spectra while the conventional Alexander-Binsch line-shape model shows evident defects in the present case. The temperature trends of the quantities characterizing the coherent and incoherent dynamics of the methyl group in the DQR approach (the effective tunnelling frequency and two coherence-damping rates) derived from the spectra are fairly reproduced using a model reported previously. The present findings provide further evidence of limitations to the validity of the common belief that molecular rate processes in condensed phases are necessarily classical.     

Epoxy resin curing reaction studied by proton multiple‐quantum NMR

This study demonstrates that the curing reaction of thermosetting polymers, in particular, epoxy resins cured with aliphatic amines, can be characterized by different multiple‐quantum (MQ) NMR experiments performed in low‐field spectrometers. Measurements of the curing reaction at 40 °C, 70 °C, and 100 °C are carried out to obtain the kinetic parameters like induction time, vitrification time, polymerization rate, and activation energy being consistent with data obtained by other traditional and complementary techniques. Besides, it is demonstrated that ¹H MQ‐NMR spectroscopy is a successful approach to overcome the experimental challenge arising from the characterization of high dipolar‐coupled polymers to study the network structure and segmental dynamics of thermosetting polymer networks. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1324–1332     

Assignment of proton NMR spectrum of triglycidylether of para-aminophenol (TGPAP), using two-dimensional techniques: SUPERCOSY and J-resolved

One- and two-dimensional 400-MHz proton NMR experiments are reported for the epoxy resin monomer, triglycidylether of para-aminophenol (TGPAP). Assignment of the epoxy protons becomes difficult due to many overlapping signals from the glycidylamine and glycidylether regions. To solve this problem we employed the homonuclear proton shift-correlated (SUPERCOSY) and J-resolved two-dimensional techniques. The combination of these methods permits the complete elucidation of the complex spectrum.     

Selective detection of multiplets via double quantum coherence: A 19F NMR study of perfluorodecalin

Commercially obtained perfluorodecalin (octadecafluorodecahydronaphthalene) contains a mixture of cis- and trans-isomers, the ¹⁹F NMR signals of which overlap. The spectrum can be simplified and assignments can be made by using techniques based on double quantum coherence. The INADEQUATE method eliminates peaks due to the cis-isomer, and the DOUBTFUL method selectively detects two quartets of the trans-isomer.     

Conformational studies on 2-fluoro-1,2-disubstituted ethanes by NMR spectroscopy. Influence of electronegativity on vicinal proton–proton and fluorine–proton coupling constants

The analysis of the ABKX spectra of thirteen compounds of the series RC(H-K)(F-X)C(H-A)(H-B)X gave the four vicinal proton-proton and fluorine-proton coupling constants. These coupling constants of conformationally mobile structures were used (i) to calculate the populations of the rotational states of the ? CHF? CH₂? bond, (ii) to calculate the vicinal trans proton-proton J(HH)^t and gauche and trans fluorine-proton coupling constants J(FH)^g and J(FH)^t and (iii) to give the unambiguous assignment of protons H-A and H-B. The dependence of the gauche and trans coupling constants with substituent electronegativity is explored. The results extend known correlations towards smaller electronegativity values. More quantitatively, the results and those in the literature, excluding those where deformations of torsional or bond angles occur, give a good fit of the data: a linear fit for J(HH)^t = 15.0-0.77 Σ(ΔE), an exponential fit for J(FH)^g = 15.35 exp [-0.266 Σ (ΔE)] and a linear fit for J(FH)^t = 65.75 - 7.52 Σ (ΔE), where Σ (ΔE) is the sum of the electronegativity difference between hydrogen and the six atoms or groups on the CH? CF fragment.     

Methyl proton contacts obtained using heteronuclear through-bond transfers in solid-state NMR spectroscopy

A two-dimensional proton-mediated carbon-carbon correlation experiment that relies on through-bond heteronuclear magnetization transfers is demonstrated in the context of solid-state NMR of proteins. This new experiment, dubbed J-CHHC by analogy to the previously developed dipolar CHHC techniques, is shown to provide selective and sensitive correlations in the methyl region of 2D spectra of crystalline organic compounds. The method is then demonstrated on a microcrystalline sample of the dimeric protein Crh (2 x 10.4 kDa). A total of 34 new proton-proton contacts involving side-chain methyl groups were observed in the J-CHHC spectrum, which had not been observed with the conventional experiment. The contacts were then used as additional distance restraints for the 3D structure determination of this microcrystalline protein. Upon addition of these new distance restraints, which are in large part located in the hydrophobic core of the protein, the root-mean-square deviation with respect to the X-ray structure of the backbone atom coordinates of the 10 best conformers of the new ensemble of structures is reduced from 1.8 to 1.1 A.     

Total assignment of the proton and carbon NMR spectra of the alkaloid quindoline — utilization of HMQC-TOCSY to indirectly establish protonated carbon-protonated carbon connectivities

The reisolation of the indoloquinoline alkaloid quindoline (also known as norcryptolepine) from Cryptolepis sanguinolenta is reported. The structure was unequivocally confirmed by two-dimensional nmr methods; the proton and carbon spectra were assigned for the first time. Because of congestion in the proton spectrum HMQC-TOCSY was used as an alternative to the more familiar COSY experiment. In addition to establishing proton-proton connectivities, HMQC-TOCSY affords the added benefit of providing, in an indirect sense, connectivity information between protonated carbons.     

Kinetics from indirectly detected hyperpolarized NMR spectroscopy by using spatially selective coherence transfers

An important recent development in NMR spectroscopy is the advent of ex situ dynamic nuclear polarization (DNP) approaches, which are capable of yielding liquid‐state sensitivities that exceed considerably those afforded by the highest‐field spectrometers. This increase in sensitivity has triggered new research avenues, particularly concerning the in vivo monitoring of metabolism and disease by NMR spectroscopy. So far such gains have mainly materialized for experiments that focus on nonprotonated, low‐γ nuclei; targets favored by relatively long relaxation times T₁, which enable them to withstand the transfer from the cryogenic hyperpolarizer to the reacting centers of interest. Recent studies have also shown that transferring this hyperpolarization to protons by indirectly detected methods could successfully give rise to ¹H NMR spectra of hyperpolarized compounds with a high sensitivity. The present study demonstrates that, when merged with spatially encoded methods, indirectly detected ¹H NMR spectroscopy can also be exploited as time‐resolved hyperpolarized spectroscopy. A methodology is thus introduced that can successfully deliver a series of hyperpolarized ¹H NMR spectra over a minutes‐long timescale. The principles and opportunities presented by this approach are exemplified by following the in vitro phosphorylation of choline by choline kinase, a potential metabolic marker of cancer; and by tracking acetylcholine’s hydrolysis by acetylcholine esterase, an important enzyme partaking in synaptic transmission and neuronal degradation.     

Application of double quantum filtered phase sensitive two-dimensional NMR spectroscopy: Chemistry of pentamethylcyclopentadienyl- and indenyl-platinum complexes

Two-dimensional double quantum filtered phase sensitive (DQPH)COSY NMR spectroscopy has been used to aid characterization of new η-pentamethylcyclopentadienyl- and η-indenyl-platinum complexes. The syntheses of the first mononuclear η-pentamethylcyclopentadienylplatinum complexes are reported.     

Assignment of the 13C-NMR spectrum of phenanthro[4,3-A]dibenzothiophene through the utilization of two-dimensional relayed coherence transfer and double quantum coherence spectroscopy

Total assignment of the ¹³C-nmr spectrum of the helical molecule phenanthro[4,3-a]dibenzothiophene at 125.762 MHz is reported. Assignments were made by the combined application of ¹³C-¹³C autocorrelated double quantum coherence and heteronuclear relayed coherence transfer (RELAY) experiments.     

Automated analysis of proton NMR spectra from combinatorial rapid parallel synthesis using self-organizing maps

It is now quite routine to acquire proton NMR spectra of compounds in 96-well plates prepared in a rapid parallel synthesis fashion using a flow-NMR automation setup. However, the analysis of 96 NMR spectra obtained in this manner is often laborious and painstakingly slow. We have developed a new, automated method for rapidly analyzing 96 NMR spectra of compounds synthesized in an 8 x 12 matrix using self-organizing maps (SOM). This unsupervised neural network is capable of clustering together NMR spectra containing a common pattern of -R groups and identifying outliers from within such clusters. Analysis of these outlier spectra can quickly help indicate the presence of undesired products, impurities, starting materials, and other unexpected errors in a 96-well plate synthesis by focusing the chemists' attention on the aberrant NMR spectra. Thus, SOM can be a valuable tool in performing efficient quality control on combinatorial libraries.     

Application to classification of mulberry leaves using multivariate analysis of proton NMR metabolomic data

Recently, NMR-based metabolomic analysis has been used to acquire information based on differentiation among biological samples. In the present study, we examined whether multivariate analysis was able to be applied to natural products and/or material field. Each extraction of 24 leaf samples, divided into six locations from the tip of the stem in each of four strains, was analyzed by pattern recognition methods, known as Principal Component Analysis (PCA) and Soft Independent Modeling of Class Analogy (SIMCA). Twenty-four extracts from mulberry leaf showed independent spectra by 1H NMR. The separation of leaf extraction data due to the difference at six locations was achieved in the PCA score plot as correlation PC1 (86.1%) and PC3 (4.6%) and showed two loading plots, suggesting classification by leaf position as an independent variable in the loading plot. Moreover, the difference among six locations clarified the seven highest discrimination powers by the SIMCA method. Meanwhile, the PCA score plot obtained classification by the variety of mulberry strains with three loading plots, but the SIMCA method did not give a peak by classification.