Heteronuclear Chemical Shift Correlation Research Articles

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy.

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.

Multiple acquisitions in a single scan: exhausting abundant 1H polarization at fast MAS

Proton-detected solid-state NMR spectroscopy is emerging as a unique tool for atomic characterization of organic solids due to the boost of resolution and sensitivity afforded by the combined use of high magnetic field and ultrafast magic angle spinning (MAS). Here, we proposed a new set of proton-detected solid-state NMR sequences that hybrid multi-dimensional 1H–1H homonuclear chemical shift correlation (HOMCOR) and two-dimensional 1H–13C heteronuclear chemical shift correlation (HETCOR) sequences into a single experiment, enabling the simultaneous acquisition of multidimensional HOMCOR and HETCOR spectra and thus significant time savings. Based on the core idea of exhausting 1H polarization in each transient scan, we firstly demonstrated that 3D 1H multiple-quantum (MQ) HOMCOR sequence can be combined with 2D HETCOR sequence into a single experiment, leading to the simultaneous acquisition of a 3D 1H MQ HOMCOR and a 2D 1H–13C HETCOR spectrum. Besides, we also showed that 2D 1H/1H double-quantum/single-quantum (DQ/SQ) and single-quantum/single-quantum (SQ/SQ) HOMCOR sequence can be simultaneously combined with HETCOR sequence either, and thus three spectra can be simultaneously obtained from one experiment, including 2D 1H DQ/SQ, 2D 1H SQ/SQ and 2D 1H–13C HETCOR spectra. Since there is only one recycle delay in each experiment, experimental time is substantially reduced compared to separate acquisition of each multi-dimensional solid-state NMR spectrum. Furthermore, those new sequences can be implemented on any standard solid-state spectrometer with only one receiver. Thus, we foresee that these approaches can be valuable for the study of a broad range of molecular systems, including polymers, pharmaceuticals, covalent-organic frameworks (COF) and so on.

A scalable solid-state nanoporous network with atomic-level interaction design for carbon dioxide capture.

Carbon capture and sequestration reduces carbon dioxide emissions and is critical in accomplishing carbon neutrality targets. Here, we demonstrate new sustainable, solid-state, polyamine-appended, cyanuric acid–stabilized melamine nanoporous networks (MNNs) via dynamic combinatorial chemistry (DCC) at the kilogram scale toward effective and high-capacity carbon dioxide capture. Polyamine-appended MNNs reaction mechanisms with carbon dioxide were elucidated with double-level DCC where two-dimensional heteronuclear chemical shift correlation nuclear magnetic resonance spectroscopy was performed to demonstrate the interatomic interactions. We distinguished ammonium carbamate pairs and a mix of ammonium carbamate and carbamic acid during carbon dioxide chemisorption. The coordination of polyamine and cyanuric acid modification endows MNNs with high adsorption capacity (1.82 millimoles per gram at 1 bar), fast adsorption time (less than 1 minute), low price, and extraordinary stability to cycling by flue gas. This work creates a general industrialization method toward carbon dioxide capture via DCC atomic-level design strategies.

Exploiting heterogeneous time scale of dynamics to enhance 2D HETCOR solid-state NMR sensitivity.

Multidimensional solid-state NMR spectroscopy plays a significant role in offering atomic-level insights into molecular systems. In particular, heteronuclear chemical shift correlation (HETCOR) experiments could provide local chemical and structural information in terms of spatial heteronuclear proximity and through-bond connectivity. In solid state, the transfer of magnetization between heteronuclei, a key step in HETCOR experiments, is usually achieved using cross-polarization (CP) or insensitive nuclei enhanced by polarization transfer (INEPT) depending on the sample characteristics and magic-angle-spinning (MAS) frequency. But, for a multiphase system constituting molecular components that differ in their time scales of mobilities, CP efficiency is pretty low for mobile components because of the averaging of heteronuclear dipolar couplings whereas INEPT is inefficient for immobile components due to the short T2 and can yield through-space connectivity due to strong proton spin diffusion for immobile components especially under moderate spinning speeds. Herein, in this study we present two 2D pulse sequences that enable the sequential acquisition of 13C/1H HETCOR NMR spectra for the rigid and mobile components by taking full advantage of the abundant proton magnetization in a single experiment with barely increasing the overall experimental time. In particular, the 13C-detected HETCOR experiment could be applied under slow MAS conditions, where a multiple-pulse sequence is typically employed to enhance 1H spectral resolution in the indirect dimension. In contrast, the 1H-detected HETCOR experiment should be applied under ultrafast MAS, where CP and heteronuclear nuclear Overhauser effect (NOE) polarization transfer are combined to enhance 13C signal intensities for mobile components. These pulse sequences are experimentally demonstrated on two model systems to obtain 2D 13C/1H chemical shift correlation spectra of rigid and mobile components independently and separately. These pulse sequences can be used for dynamics based spectral editing and resonance assignments. Therefore, we believe the proposed 2D HETCOR NMR pulse sequences will be beneficial for the structural studies of heterogeneous systems containing molecular components that differ in their time scale of motions for understanding the interplay of structures and properties.

Hybridization of TEDOR and NCX MAS solid-state NMR experiments for simultaneous acquisition of heteronuclear correlation spectra and distance measurements.

Magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy is a major technique for the characterization of the structural dynamics of biopolymers at atomic resolution. However, the intrinsic low sensitivity of this technique poses significant limitations to its routine application in structural biology. Here we achieve substantial savings in experimental time using a new subclass of Polarization Optimized Experiments (POEs) that concatenate TEDOR and SPECIFIC-CP transfers into a single pulse sequence. Specifically, we designed new 2D and 3D experiments (2D TEDOR-NCX, 3D TEDOR-NCOCX, and 3D TEDOR-NCACX) to obtain distance measurements and heteronuclear chemical shift correlations for resonance assignments using only one experiment. We successfully tested these experiments on N-Acetyl-Val-Leu dipeptide, microcrystalline U-13C,15N ubiquitin, and single- and multi-span membrane proteins reconstituted in lipid membranes. These pulse sequences can be implemented on any ssNMR spectrometer equipped with standard solid-state hardware using only one receiver. Since these new POEs speed up data acquisition considerably, we anticipate their broad application to fibrillar, microcrystalline, and membrane-bound proteins.

Synthesis, characterization, spectroscopic properties, DFT calculations and antimicrobial activities of p-toluenesulfonamide,N,N′-1,3-propanediylbis(ptspr) and [Cu(II) (phenanthroline)2]ptspr

In this paper, p-toluenesulfonamide,N,N′-1,3-propanediylbis (ptspr) and [Cu(II) (phenanthroline)2]ptspr compounds has been synthesized for the first time and they characterized by X-ray diffraction, elemental analysis, FT-IR, 1H, 13C, heteronuclear chemical-shift correlation (HETCOR) and correlation spectroscopy (COSY) NMR spectroscopies. The optimized molecular structures of the compounds were also calculated by using the DFT/B3LYP method with 6–311++G (d,p)/LanL2DZ basis sets. The vibrational frequencies, intensities of vibrational bands and, HC nuclear magnetic resonance (NMR) chemical shifts of the molecules were calculated. Additionally, nonlinear optical properties (NLO), fragments contributions in HOMO and LUMO molecular orbitals were investigated with the same level of theory. All of the calculated properties were compared with the experimental values of this study and it is seen that all physical and chemical parameters calculated are very consistent with the experimental results. In addition, the antimicrobial activities of synthesized compounds were studied against some Gram-positive and Gram-negative bacteria by using the microdilution and disk diffusion method. It was determined from the results of this study that the antimicrobial activity screening showed the complex have more activity than ligand against the tested bacteria.

Computer-aided Dereplication and Structure Elucidation of Natural Products at the University of Reims.

Natural product chemistry began in Reims, France, in a pharmacognosy research laboratory whose main emphasis was the isolation and identification of bioactive molecules, following the guidelines of chemotaxonomy. The structure elucidation of new compounds of steadily increasing complexity favored the emergence of methodological work in nuclear magnetic resonance. As a result, our group was the first to report the use of proton-detected heteronuclear chemical shift correlation spectra for the computer-assisted structure elucidation of small organic molecules driven by atom proximity relationships and without relying on databases. The early detection of known compounds appeared as a necessity in order to deal more efficiently with complex plant extracts. This goal was reached by an original combination of mixture fractionation by centrifugal partition chromatography, analysis by 13 C NMR, digital data reduction and alignment, hierarchical data clustering, and computer database search.

Sequential acquisition of multi-dimensional heteronuclear chemical shift correlation spectra with 1H detection

RF pulse schemes for the simultaneous acquisition of heteronuclear multi-dimensional chemical shift correlation spectra, such as {HA(CA)NH & HA(CACO)NH}, {HA(CA)NH & H(N)CAHA} and {H(N)CAHA & H(CC)NH}, that are commonly employed in the study of moderately-sized protein molecules, have been implemented using dual sequential 1H acquisitions in the direct dimension. Such an approach is not only beneficial in terms of the reduction of experimental time as compared to data collection via two separate experiments but also facilitates the unambiguous sequential linking of the backbone amino acid residues. The potential of sequential 1H data acquisition procedure in the study of RNA is also demonstrated here.

Hydrogen bond interaction and dynamics in PMMA/PVPh polymer blends as revealed by advanced solid-state NMR

The hydrogen bond interactions and dynamics of the poly (methyl methacrylate) (PMMA) and poly (4-vinyl phenol) (PVPh) polymer blends were studied by a variety of advanced solid-state NMR techniques. The possible hydrogen bond interactions between the carbonyl group of PMMA and hydroxyl group of PVPh were successfully elucidated by two-dimensional 1H–1H spin-exchange 13C–1H heteronuclear chemical-shift correlation (HETCOR) NMR experiment. The 13C 2D separation of undistorted powder patterns by effortless recoupling (SUPER) experiments combined with quantum chemical calculations for the theoretical predictions of chemical-shift anisotropy (CSA) parameters were utilized to investigate the intermolecular hydrogen-bonding interactions and molecular conformation of the blends. 13C–1H PISEMA (polarization inversion and spin-exchange at magic angle) experiments at different temperatures were used to reveal the heterogeneous dynamics resulting from the cooperative motion associated with the hydrogen bond interaction. PISEMA analysis shows that the motion of aromatic group in PVPh is affected by the rotating motions of methyl in PMMA.

X-ray topography, photopyroelectric and two-photon absorption studies on solution grown benzimidazole single crystal

Benzimidazole (BMZ) is an organic nonlinear optical material, and it is possible to grow it in the laboratory by a slow evaporation solution growth technique using methanol as the solvent. Its nonlinear optical harmonic generation efficiency is 4.5 times higher than that of standard potassium dihydrogen phosphate (KDP) single crystal. It crystallizes with orthorhombic structure, which is noncentrosymmetric in nature, with lattice parameters a=13.58 A, b=6.85 A, c=6.97 A. The functional groups of the grown specimen are confirmed by heteronuclear chemical shift correlation (HETCOR) analysis and it is found that other elements are not incorporated in the compound. The crystalline perfection is examined by high resolution X-ray diffraction (HRXRD), and its topographic image was recorded for the as-grown single crystal indicates the presence of some grain boundaries in the grown specimen. Thermal parameters such as thermal diffusivity (α), thermal conductivity (k), thermal effusivity (e), and heat capacity (cp) have been determined by a photopyroelectric thermal wave method. In addition, piezoelectricity, birefringence, and third-order nonlinear optical properties have been examined. The two-photon absorption coefficient of the title compound is determined by the open-aperture Z-scan technique.

Structural Characterization of a Phenoxazine Analog Formed as a By-product of the Synthesis of a 3-Amino-N,N-dimethylsalicylamide

A highly colored impurity formed at low levels during the preparation of 3-amino N,N-dimethyl salicylamide was isolated and identified as 2-amino-N,N,N′,N′-tetramethyl-3-oxo-3H-phenoxazin-4,6-dicarboxamide using mass spectrometry and two-dimensional NMR methods that included long-range 1H-15N heteronuclear chemical shift correlation data to assemble the proton-deficient “right-half” of the 2-aminophenoxazin-3-one nucleus. The structure was independently confirmed using computer-assisted structure elucidation methods and by synthesis.

Synthesis, characterization and antimicrobial activity of m-toluenesulfonamide, N,N′-1,2-ethanediylbis (mtsen) and [Cu(II)(phenanthroline)2]mtsen complex

M-toluenesulfonamide, N,N′-1,2-ethanediylbis (a disulfonamide compound, mtsen) and [Cu(II)(phenanthroline)2]mtsen compounds were newly synthesized. The molecular structure of mtsen was investigated by using elemental analyses, liquid chromatography–mass spectrometry (LC–MS), X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), dispersive Raman spectroscopy, 1H, 13C, heteronuclear chemical-shift correlation (HETCOR) and correlation spectroscopy (COSY) NMR spectroscopies. The FT-IR, dispersive Raman and far-infrared spectra of mtsen were recorded at room-temperature and discussed assisted with B3LYP/6-311G(d,p) level of theory along with scaled quantum mechanics force field (SQM-FF) method. Furthermore, 1H and 13C NMR analyses were performed at B3LYP/6-311++G(d,p) theory level using gauge including atomic orbital (GIAO) method and compared with the experimental findings. Further analyses were also made for [Cu(II)(phenanthroline)2]mtsen complex by using elemental analysis, LC–MS, magnetic susceptibility; conductivity measurement and FT-IR. The antibacterial activities of synthesized compounds were studied against some Gram-positive and Gram-negative bacteria by using the microdilution and disk diffusion method. The biological activity screening showed that complex have more activity than ligand against the tested bacteria.

Molecular Optimization of Multiply-Functionalized Mesoporous Films with Ion Conduction Properties

Sequential processing of multiply functionalized mesoporous films is shown to yield materials that are compositionally and structurally heterogeneous on mesoscopic and molecular length scales, both of which must be controlled to optimize macroscopic ion-conduction properties. Cubic mesoporous silica films prepared from strongly acidic solutions were subsequently functionalized under highly alkaline conditions to incorporate hydrophilic aluminosilica surface moieties, followed by nonaqueous conditions to introduce perfluorosulfonic-acid surface groups. Such sequential combination of individually incompatible steps yielded stable mesoporous films with high surface hydrophilicities and strong acid functionalities that exhibited high proton conductivities (ca. 9 × 10(-2) S/cm) at elevated temperatures (120 °C). Molecular, mesoscopic, and macroscopic properties of the multiply functionalized films were monitored and correlated at each stage of the syntheses by nuclear magnetic resonance (NMR) spectroscopy, small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), elemental analysis, adsorption, and ion conductivity measurements. In particular, variable-temperature solid-state two-dimensional (2D) (27)Al{(1)H}, (29)Si{(1)H}, (27)Al{(19)F}, and (29)Si{(19)F} HETeronuclear chemical-shift CORrelation (HETCOR) NMR spectra reveal separate surface adsorption and grafting sites for the different functional surface species within the mesopore channels. The hydrophilic aluminosilica and acidic fluoro-group loadings and interaction sites are demonstrated to be strongly affected by the different synthesis and functionalization treatments, which must be separately and collectively optimized to maximize the proton conductivities.

Chemical shift correlation at high MAS frequencies employing low-power symmetry-based mixing schemes

An approach for conveniently implementing low-power CN ( n ) (ν) and RN ( n ) (ν) symmetry-based band-selective mixing sequences for generating homo- and heteronuclear chemical shift correlation NMR spectra of low γ nuclei in biological solids is demonstrated. Efficient magnetisation transfer characteristics are achieved by selecting appropriate symmetries requiring the application of basic RF elements of relatively long duration and numerically tailoring the RF field modulation profile of the basic element. The efficacy of the approach is experimentally shown by the acquisition of (15)N-(13)C dipolar and (13)C-(13)C scalar and dipolar coupling mediated chemical shift correlation spectra at representative MAS frequencies.

Heteronuclear chemical shift correlation and J‐resolved MAS NMR spectroscopy of lipid membranes

Direct observation of J-couplings remains a challenge in high-resolution solid-state NMR. In some cases, it is possible to use Lee-Goldburg (LG) homonuclear decoupling during rare spin observation in MAS NMR correlation spectroscopy of lipid membranes to obtain J-resolved spectra in the direct dimension. In one simple implementation, a wide line separation-type (13)C-(1)H HETCOR can provide high-resolution (1)H/(13)C spectra, which are J-resolved in both dimensions. Coupling constants, (1)J(HC), obtained from (1)H doublets, can be compared with scaled (1)J(θ)(CH)-values obtained from the (13)C multiplets to assess the LG efficiency and scaling factor. The use of homonuclear decoupling during proton evolution, LG-HETCOR-LG, can provide J-values, at least in the rare spin dimension, and allows measurements in less mobile membrane environments. The LG-decoupled spectroscopic approach is demonstrated on pure dioleoylphosphatidylcholine (DOPC) membranes and used to investigate lipid mixtures of DOPC/cholesterol and DOPC/cholesterol/sphingomyelin.

Unsymmetrical indirect covariance processing of hyphenated and long‐range heteronuclear 2D NMR spectra ‐ Enhanced visualization of 2JCH and 4JCH correlation responses

Abstractmagnified imageRecent reports have demonstrated the unsymmetrical indirect covariance combination of discretely acquired 2D NMR experiments into spectra that provide an alternative means of accessing the information content of these spectra. The method can be thought of as being analogous to the Fourier transform conversion of time domain data into the more readily interpreted frequency domain. Hyphenated 2D‐NMR spectra such as GHSQC‐TOCSY, when available, provide an investigator with the means of sorting proton‐proton homonuclear connectivity networks as a function of the 13C chemical shift of the carbon directly bound to the proton from which propagation begins. Long‐range heteronuclear chemical shift correlation experiments establish proton‐carbon correlations via heteronuclear coupling pathways, most commonly across three bonds (3JCH), but in more general terms across two (2JCH) to four bonds (4JCH). In many instances 3JCH correlations dominate GHMBC spectra. We demonstrate in this report the improved visualization of 2JCH and 4JCH correlations through the unsymmetrical indirect covariance processing of GHSQC‐TOCSY and GHMBC 2D spectra.

Synthesis, Potentiometric, Kinetic, and NMR Studies of 1,4,7,10-Tetraazacyclododecane-1,7-bis(acetic acid)-4,10-bis(methylenephosphonic acid) (DO2A2P) and its Complexes with Ca(II), Cu(II), Zn(II) and Lanthanide(III) Ions

A cyclen-based ligand containing trans-acetate and trans-methylenephosphonate pendant groups, H 6DO2A2P, was synthesized and its protonation constants (12.6, 11.43, 5.95, 6.15, 2.88, and 2.77) were determined by pH-potentiometry and (1)H NMR spectroscopy. The first two protonations were shown to occur at the two macrocyclic ring N-CH 2-PO 3 (2-) nitrogens while the third and fourth protonations occur at the two phosphonate groups. In parallel with protonation of the two -PO 3 (2-) groups, the protons from the NH (+)-CH 2-PO 3 (2-) are transferred to the N-CH 2-COO (-) nitrogens. The stability constants of the Ca (2+), Cu (2+), and Zn (2+) (ML, MHL, MH 2L, and M 2L) complexes were determined by direct pH-potentiometry. Lanthanide(III) ions (Ln (3+)) form similar species, but the formation of complexes is slow; so, "out-of-cell" pH-potentiometry (La (3+), Eu (3+), Gd (3+), Y (3+)) and competitive spectrophotometry with Cu(II) ion (Lu (3+)) were used to determine the stability constants. By comparing the log K ML values with those of the corresponding DOTA (H 4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and DOTP (H 8DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephosphonic acid) complexes, the order DOTA < DO2A2P < DOTP was found for all the metal ion complexes examined here with the exception of the Ca (2+) complexes, for which the order is reversed. The relaxivity of Gd(DO2A2P) decreases between pH 2 and 7 but remains constant in the pH range of 7 < pH < 12 ( r 1 = 3.6 mM (-1) s (-1)). The linewiths of the (17)O NMR signals of water in the absence and presence of Gd(DO2A2P) (at pH = 3.45 and 8.5) between 274 and 350 K are practically the same, characteristic of a q = 0 complex. Detailed kinetic studies of the Ce (3+) and Gd (3+) complexes with DO2A2P showed that complex formation is slow and involves a high stability diprotonated intermediate Ln(H 2DO2A2P)*. Rearrangement of the diprotonated intermediate into the final complex is an OH (-) assisted process but, unlike formation of Ln(DOTA) complexes, rearrangement of Ln(H 2DO2A2P)* also takes place spontaneously likely as a result of transfer of one of the protons from a ring nitrogen to a phosphonate group. The order of the OH (-) assisted formation rates of complexes is DOTA > DO2A2P > DOTP while the order of the proton assisted dissociation rates of the Gd (3+) complexes is reversed, DOTP > DO2A2P > DOTA. (1)H and (13)C NMR spectra of Eu(DO2A2P) and Lu(DO2A2P) were assigned using two-dimensional correlation spectroscopy (2D COSY), heteronuclear multiple quantum coherence (HMQC), heteronuclear chemical shift correlation (HETCOR), and exchange spectroscopy (EXSY) NMR methods. Two sets of (1)H NMR signals were observed for Eu(DO2A2P) characteristic of the presence of two coordination isomers in solution, a twisted square antiprism (TSAP) and a square antiprism (SAP), in the ratio of ~93% and ~7%, respectively. Line shape analysis of the (1)H NMR spectra of Lu(DO2A2P) gave lower activation parameters compared to La(DOTP) for interconversion between coordination isomers. This indicates that the Ln(DO2A2P) complexes are less rigid probably due to the different size and spatial requirements of the carboxylate and phosphonate groups.

Application of biselective refocusing soft pulses to the simplification of heteronuclear correlation spectra

The biselective spin echo technique allows the signals of coupled proton pairs to be extracted from crowded liquid state proton NMR spectra. Its use as a preparation sequence in heteronuclear chemical shift correlation experiments requires the removal of the heteronuclear coupling interaction during the biselective echo time. The discrimination between coupled and uncoupled protons signals is achieved by double quantum filtration, which delivers antiphase magnetization states. The latter are not directly compatible with the design of an HSQC-like pulse sequence. The conversion of antiphase to in-phase magnetization states by a second biselective echo sequence solves this problem. The optimization of spin echo delays is also discussed. Lastly, the article presents modified HSQC and HMBC pulses sequences in which information is obtained solely for the biselectively selected proton pairs. A peracetylated trisaccharide was used as a test molecule.

13C‐15N Connectivity networks via unsymmetrical indirect covariance processing of 1H‐13C HSQC and 1H‐15N IMPEACH spectra

Abstractmagnified imageLong‐range 1H‐15N heteronuclear shift correlation methods at natural abundance to facilitate the elucidation of small molecule structures have assumed a role of growing importance over the past decade. Recently, there has also been a high level of interest in the exploration of indirect covariance NMR methods. From two coherence transfer experiments, A→B and A→C, it is possible to indirectly determine B⟷C. We have shown that unsymmetrical indirect covariance methods can be employed to indirectly determine several types of hyphenated 2D NMR data from higher sensitivity experiments. Examples include the calculation of hyphenated 2D NMR spectra such as 2D GHSQC‐COSY and GHSQC‐NOESY from the discrete component 2D NMR experiments. We now wish to report the further extension of unsymmetrical indirect covariance NMR methods for the combination of 1H‐13C GHSQC and 1H‐15N longrange (GHMBC, IMPEACH‐MBC, CIGAR‐HMBC, etc.) heteronuclear chemical shift correlation spectra to establish 13C‐15N correlation pathways.

Application of unsymmetrical indirect covariance NMR methods to the computation of the 13C↔15N HSQC‐IMPEACH and 13C↔15N HMBC‐IMPEACH correlation spectra

Utilization of long-range (1)H--(15)N heteronuclear chemical shift correlation has continually grown in importance since the first applications were reported in 1995. More recently, indirect covariance NMR methods have been introduced followed by the development of unsymmetrical indirect covariance processing methods. The latter technique has been shown to allow the calculation of hyphenated 2D NMR data matrices from more readily acquired nonhyphenated 2D NMR spectra. We recently reported the use of unsymmetrical indirect covariance processing to combine (1)H--(13)C GHSQC and (1)H--(15)N GHMBC long-range spectra to yield a (13)C--(15)N HSQC-HMBC chemical shift correlation spectrum that could not be acquired in a reasonable period of time without resorting to (15)N-labeled molecules. We now report the unsymmetrical indirect covariance processing of (1)H--(13)C GHMBC and (1)H--(15)N IMPEACH spectra to afford a (13)C--(15)N HMBC-IMPEACH spectrum that has the potential to span as many as six to eight bonds. Correlations for carbon resonances long-range coupled to a protonated carbon in the (1)H--(13)C HMBC spectrum are transferred via the long-range (1)H--(15)N coupling pathway in the (1)H--(15)N IMPEACH spectrum to afford a much broader range of correlation possibilities in the (13)C--(15)N HMBC-IMPEACH correlation spectrum. The indole alkaloid vincamine is used as a model compound to illustrate the application of the method.