Separation Of NMR Signals Research Articles

Enantiomeric NMR signal separation behavior and mechanism of samarium(III) and neodymium(III) complexes with (S,S)-ethylenediamine-N,N'-disuccinate.

Enantiomeric 1 H and 13 C NMR signal separation behaviors of various α-amino acids and DL-tartarate were investigated by using the samarium(III) and neodymium(III) complexes with (S,S)-ethylenediamine-N,N'-disuccinate as chiral shift reagents. A relatively smaller concentration ratio of the lanthanide(III) complex to substrates was suitable for the neodymium(III) complex compared with the samarium(III) one, striking a balance between relatively greater signal separation and broadening. To clarify the difference in the signal separation behavior, the chemical shifts of β-protons for fully bound D- and L-alanine (δb (D) and δb (L)) and their adduct formation constants (Ks) were obtained for both metal complexes. Preference for D-alanine was similarly observed for both complexes, while it was revealed that the difference between the δb (D) and δb (L) values is the significant factor to determine the enantiomeric signal separation. The neodymium(III) and samarium(III) complexes can be used complementarily for higher and smaller concentration ranges of substrates, respectively, because the neodymium(III) complex gives the larger difference between the δb (D) and δb (L) values with greater signal broadening compared to the samarium(III) complex.

Application of random coherence order selection in gradient-enhanced multidimensional NMR

Development of multidimensional NMR is essential to many applications, for example in high resolution structural studies of biomolecules. Multidimensional techniques enable separation of NMR signals over several dimensions, improving signal resolution, whilst also allowing identification of new connectivities. However, these advantages come at a significant cost. The Fourier transform theorem requires acquisition of a grid of regularly spaced points to satisfy the Nyquist criterion, while frequency discrimination and acquisition of a pure phase spectrum require acquisition of both quadrature components for each time point in every indirect (non-acquisition) dimension, adding a factor of 2N-1 to the number of free- induction decays which must be acquired, where N is the number of dimensions. Compressed sensing (CS) ℓ1-norm minimisation in combination with non-uniform sampling (NUS) has been shown to be extremely successful in overcoming the Nyquist criterion. Previously, maximum entropy reconstruction has also been used to overcome the limitation of frequency discrimination, processing data acquired with only one quadrature component at a given time interval, known as random phase detection (RPD), allowing a factor of two reduction in the number of points for each indirect dimension (Maciejewski et al. 2011 PNAS 108 16640). However, whilst this approach can be easily applied in situations where the quadrature components are acquired as amplitude modulated data, the same principle is not easily extended to phase modulated (P-/N-type) experiments where data is acquired in the form exp (iωt) or exp (-iωt), and which make up many of the multidimensional experiments used in modern NMR. Here we demonstrate a modification of the CS ℓ1-norm approach to allow random coherence order selection (RCS) for phase modulated experiments; we generalise the nomenclature for RCS and RPD as random quadrature detection (RQD). With this method, the power of RQD can be extended to the full suite of experiments available to modern NMR spectroscopy, allowing resolution enhancements for all indirect dimensions; alone or in combination with NUS, RQD can be used to improve experimental resolution, or shorten experiment times, of considerable benefit to the challenging applications undertaken by modern NMR.

2-Substituted and 2,2-disubstituted adamantane derivatives as models for studying substituent chemical shifts and C–Hax⋯Yax cyclohexane contacts—results from experimental and theoretical NMR spectroscopic chemical shifts and DFT structures

The complete 1H and 13C NMR chemical shifts assignment for various 2-substituted and 2,2-disubstituted adamantane derivatives 1–38 in CDCl3 solution was realized on the basis of NMR experiments combined with chemical structure information and DFT-GIAO (B3LYP/6-31+G(d,p)-GIAO) calculations of chemical shifts in solution. Substituent-induced 13C NMR chemical shifts (SCS) are discussed. C–Hax⋯Yax contacts are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts will be further investigated in this work on basis of new adamantane derivatives, which are substituted at C-2 to provide models for 1,4-C–Hax⋯Yax and 1,5-C–Hax⋯Yax contacts. The B3LYP/6-31+G(d,p) calculations predicted the presence of NBO hyperconjugative attractive interactions between C–Hax and Yax groups along C–Hax⋯Yax contacts. The 1H NMR signal separation, Δδ(γ-CH2), reflects the strength of the H-bonded C–Hax⋯Yax contact.

Improper Hydrogen Bonded Cyclohexane C–Hax···Yax Contacts: Theoretical Predictions and Experimental Evidence from 1H NMR Spectroscopy of Suitable Axial Cyclohexane Models

C-H(ax)···Y(ax) contacts are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts is investigated in this work. MP2/6-31+G(d,p) calculations predicted the presence of improper hydrogen bonded C-H(ax)···Y(ax) contacts of different strength in substituted cyclohexane rings. To support the theoretical predictions with experimental evidence, several synthetic 2-substituted adamantane analogues (1-24) with suitable improper H-bonded C-H(ax)···Y(ax) contacts of different strength were used as models of a substituted cyclohexane ring. The (1)H NMR signal separation, Δδ(γ-CH(2)), within the cyclohexane ring γ-CH(2)s is raised when the MP2/6-31+G(d,p) calculated parameters, reflecting the strength of the H-bonded C-H(ax)···Y(ax) contact, are increased. In molecules with enhanced improper H-bonded contacts C-H(ax)···Y(ax), like those having sterically crowded contacts (Y(ax) = t-Bu) or contacts including considerable electrostatic attractions (Y(ax) = O-C or O═C) the calculated DFT steric energies of the γ-axial hydrogens are considerably reduced reflecting their electron cloud compression. The results suggest that the proton H(ax) electron cloud compression, caused by the C-H(ax)···Y(ax) contacts, and the resulting increase in Δδ(γ-CH(2)) value can be effected not just from van der Waals spheres compression, but more generally from electrostatic attraction forces and van der Waals repulsion, both of which are improper H-bonding components.

Measuring Screw-Sense Preference in a Helical Oligomer by Comparison of 13C NMR Signal Separation at Slow and Fast Exchange

While an unequal population of rapidly interconverting left- and right-handed conformers of a helical oligomer can be detected by circular dichroism, precise quantification of a conformer ratio has not previously been achieved. We demonstrate, using a set of labeled peptide analogues, that simple analysis of peak separation in their (13)C NMR spectra at slow and fast exchange allows an accurate value for the ratio of helical conformers to be obtained. The method reports the ratio of conformers at the site of the label and can therefore be used to investigate local variations in helical conformational control.

Improper hydrogen-bonded cyclohexane C–Hax⋯Yax contacts: experimental evidence from 1H NMR spectroscopy of suitable axial cyclohexane models

Stronger improper H-bonded cyclohexane C–H ax ⋯O contacts cause an increase in the 1 H NMR signal separation within the γ-CH 2 s of cyclohexane rings. B3LYP/6-31+G(d,p) calculations predicted the presence of improper hydrogen-bonded C–H ax ⋯Y ax contacts of different strength in cyclohexane derivatives; 1 it was predicted that the addition of an appropriate bridging fragment X ax between an axial substituent Y 1 and a cyclohexane carbon would strengthen the improper hydrogen-bonded contact C–H ax ⋯Y 1 when the X ax –Y 1 bond vector bisects the cyclohexane ring. To support the theoretical predictions with experimental evidence for this effect, several 2-substituted adamantane analogues with suitable improper H-bonded C–H ax ⋯O contacts of different strength were synthesized, as models of the corresponding cyclohexane derivatives, and their 1 H NMR spectra were recorded at 298 K. The 1 H NMR signal separation within the cyclohexane ring γ-CH 2 s is increased when the B3LYP/6-31+G(d,p)-calculated strength of the H-bonded C–H ax ⋯O=C ax contact interaction is increased.

Diffusion-Ordered NMR Spectroscopy in the Structural Characterization of Functionalized Carbon Nanotubes

The emerging applications of functionalized carbon nanotubes (CNTs) in various research domains necessitate the use of many different analytical techniques to confirm their structural modifications in a fast and reliable manner. Thus far, NMR spectroscopy has not been among the main tools for characterization of organically modified carbon nanostructures. (1)H analysis is limited because the signals in these derivatives are typically weak and broad, resulting in uncertainties of a few parts per million, and because of the strong interference of residual solvent signals. To overcome these limitations, we investigated the applicability of proton NMR spectroscopy based on gradient-edited diffusion pulse sequences (1D diffusion-ordered spectroscopy, DOSY) in the characterization of CNT derivatives. In general, diffusion NMR experiments allow the separation of NMR signals of different species present in a mixture, according to their own diffusion coefficients, merging spectroscopy information with size analysis. In the present study, a selected set of CNT derivatives was synthesized and analyzed using 1D DOSY experiments by applying strong magnetic field gradients (up to 42.6 G cm(-1)). Colorimetric tests (i.e., Kaiser test) and TGA analysis support the NMR findings, which are related to isolated and/or bundled short SWNTs, on the basis of TEM and AFM characterization. The overall results show that the diffusion-based NMR spectroscopy is a fast and promising approach for the characterization of covalently modified CNT derivatives.

NMR determination of crystallinity for poly(ɛ- l-lysine)

Crystallinity of poly(ɛ- l-lysine) (ɛ-PL) was discussed by analyzing the differences in the 1H spin–spin relaxation times ( T 2 H), the 13C spin-lattice relaxation times ( T 1 C), and the 13C NMR signal shapes between the crystalline and the non-crystalline phases. The observed 1H relaxation curve (free induction decay followed by solid-echo method) showed the sum of Gaussian and exponential decays. Similarly, the observed 13C relaxation curves obtained from the Torchia method were double-exponential. The 13C NMR spectrum of ɛ-PL was divided into the narrow and the broad lines by utilizing the intrinsic differences in the 1H spin-lattice relaxation times in the rotating-frame between them, which are attributed to the crystalline and the non-crystalline phases, respectively. Even though the crystallinity is obtained from the identical NMR measurements, the estimated values are different with each other. The crystallinity estimated from the T 2 H differences was 75.8 ± 0.1% at 333 K and 60.7 ± 0.4% at 353 K. From the T 1 C differences, the value was estimated to be 62 ± 11%. Furthermore, the value estimated from the NMR signal separation was 54 ± 5%. In this study we have explained these discrepancies by the difference in susceptibility among the experiments for the inter-phase, which exists in-between the crystalline and the amorphous phases. Furthermore, the estimated crystallinity was ascertained by the X-ray diffraction experiment.

Direct determination of tautomerism in purine derivatives by low-temperature NMR spectroscopy

An investigation of the tautomerism of the purine derivatives N, N-dimethyl- N′-(7(9)- H-purin-6-yl)-formamidine 1 , 6-chloropurine 3 and 6-methoxy purine 5 at low temperatures by NMR spectroscopy has been carried out. Knowledge of tautomeric equilibria is important for predicting N-alkylation positions, hydrogen-bonding patterns, and interactions with biological targets. In the NMR spectra of 1 and 5 at 213 K we observed two sets of signals, whereas at laboratory temperature there was only a single set of signals, reflecting the time-averaged contribution of both components. Based on characteristic values of 13C and 15N chemical shifts and of vicinal 1H– 13C scalar coupling constants, the two components of 1 were determined to be the N7–H (71%) and N9–H (29%) tautomers and those of 5 as the N7–H (18%) and N9–H (82%) tautomers. The investigation of 3 revealed a substantial predominance of the N9–H tautomer without any separation of NMR signals at 213 K.

New chirality recognizing reagents for the determination of absolute stereochemistry and enantiomeric purity by NMR

The new chiral aryl methoxyacetic acids 3–8 were prepared and used as reagents for recognition of the chirality of alcohols and amines by 1H-NMR and for analysis of enantiomeric purity. Separation of NMR signals by compounds 6–8 is two to three times greater than by the standard reagents MTPA ( 1) and MPA ( 2). The influence of conformational and magnetic factors in this effectiveness is discussed.