Heteronuclear Two-dimensional Experiments

To return to linear sampling, Jeannerat presents a method using reference spectra enabling the resolution of ambiguities in heteronuclear two-dimensional experiments where the spectral window can be reduced up to the point where... 

The heteronuclear two-dimensional experiment described here provides... 

In many cases, the analytical tasks are simply to detect and quantify a specific known analyte. Examples include the detection and quantification of commonly used buffer components (e.g., Tris, acetate, citrate, MES, propylene glycol, etc.). These simple tasks can readily be accomplished by using a standard one-dimensional NMR method. In other situations, the analytical tasks may involve identifying unknown compounds. This type of task usually requires homonuclear and heteronuclear two-dimensional NMR experiments, such as COSY, TOCSY, NOESY, HSQC, HMBC, etc. The identification of unknown molecules may also require additional information from other analytical methods, such as mass spectrometry, UV-Vis spectroscopy, and IR spectroscopy.14... 

An aspect of general interest in organometallic chemistry is the equilibrium between contact and solvent-separated ion pairs, because metal cations which are sun ounded by an individual solvent cage are expected to show different reactivity towards basic centres than those closely attached to carbanions or amines. At the same time, the anionic centre is less shielded in an SSIP than in a CIP and thus expected to be more reactive. In solution, the differentiation by NMR methods between both structural motifs relies in most cases on chemical shift interpretations and, if possible, on heteronuclear Overhauser (NOE) measurements. The latter method is especially powerful in the case of lithium organic compounds, where H, Li or even H, Li NOE can be detected by one- and two-dimensional experiments. ... 

Selecting the C-bound protons before performing a homonuclear two-dimensional experiment enables to measure small heteronuclear coupling constants [16]. Such an experiment with a sample of natural isotopic abundance was first published by Otting and Wuthrich in 1990, where the half-filter element with spin-lock purge pulse was used to select the C-bound protons in a small protein in aqueous solution [6]. Later applications illustrated the usefulness of the same half-filter element with smaller molecules [17, 18]. 

The carbon and DEPT (distortionless enhanced polarization transfer) spectra are shown in Figure 10. The HETCOR (heteronuclear two-dimensional proton-carbon correlation) spectrum is shown in Figure 11. The carbon assignments are listed in Table 5. Long-range HETCOR experiments were used to make the assignments for the thiophene carbons. 

Fig. 31. Stacked plot of the heteronuclear two-dimensional J-resolved spectrum of cured, carbon black filled, natural rubber. The proton flip experiment was used with high-power proton decoupling during the detection time. The experiment was performed with the sample spinning at the magic angle (reprinted from Ref. 1911 with permission)...

Correlation between proton and carbon chemical-shifts and coupling-constants may also be obtained through heteronuclear, two-dimensional, n.m.r. experiments.73,74... 

Modern NMR experiments are used extensively for structure elucidation. A summary of the most important one- and two-dimensional methods for small molecules is given below. The experiments, which can be fully automated and used routinely in academic and industrial laboratories, are subdivided into homo- and heteronuclear one-dimensional methods and homo- and heteronuclear two-dimensional methods (see Friebolin, as above Sanders, J. K. M., and Hunter, B. K., Modern NMR Spectroscopy A Guide for Chemists (Oxford Oxford University Press, 1993)). 

Cicero DO, Barbato G, Bazzo R. Sensitivity enhancement of a two-dimensional experiment for the measurement of heteronuclear long-range coupling constants, by a new scheme of co- 81. herence selection by gradients. J. Magnet. Reson. 2001 148 209-213. 

Selective excitation of a resolved resonance followed by homonuclear or heteronuclear Hartmann-Hahn transfer can also be advantageous in the preparation period of two-dimensional experiments. For example, two-dimensional COSY, NOESY, TOCSY, and two-dimensional /-resolved subspectra of individual spin systems can be acquired based on this principle (Homans, 1990 Sklenaf and Feigon, 1990 Nuzillard and Massiot, 1991 Gardner and Coleman, 1994). In selective two-dimensional experiments like soft COSY (Briischweiler et al., 1987 Cavanagh et al., 1987),... 

Two dimensional experiments can also show correlations between different types of nuclei. These hetero-nuclear experiments have the advantage that nuclei such as and N have much wider chemical shift ranges, and therefore the 2D experiments achieve a tremendous reduction in spectral crowding. The HETCOR (HETeronuclear CORrelation) experiment was the first 2D experiment developed to provide... 

In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring spin diffusion indirectly from its effects on H spin-lattice relaxation. The spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section... 

Of the many types of two-dimensional experiments, two find the most frequent application. One of these is H—H Correlation Spectroscopy, better known by its acronym, COSY. In a COSY experiment, the chemical shift range of the proton spectrum is plotted on both axes. The second important technique is Heteronuclear Correlation Spectroscopy, better known as the HETCOR technique. In a HETCOR experiment, the chemical shift range of the proton spectrum is plotted on one axis, while the chemical shift range of the C spectrum for the same sample is plotted on the second axis. 

The principle purpose of correlation experiments is to establish a one-to-one mapping from the signal to its source i.e. to the particular atomic nucleus in the molecule. This assignment task involves identification of the members in the coupling network, referred to as the spin system. In addition, correlation experiments, as such or with modifications, are suitable for measurements of scalar and dipolar couplings. Correlation in the two dimensions is the most natural dimensionality because the spin-spin interactions are pair wise. Three-dimensional or experiments of higher dimensionality are constructed from concatenated two-dimensional experiments. Homonuclear three-dimensional experiments, such as TOCSY-NOESY, are not considered here because in many cases the multidimensional heteronuclear experiments are superior. 

The basic heteronuclear experiments are easy to combine with the two-dimensional homonuclear experiments to produce three- or four-dimensional edited spectra. In this terminology editing means selection of the protons that are attached to the heteronucleus. The main purpose of these experiments is to reduce the signal overlap of the homonuclear two-dimensional experiments. 

Frey MH, Wagner G, Vasak M, S0rensen OW, Neuhaus D, Worgotter, Kagi JHR, Ernst RR, Wiithrich K (1985) Polypeptide-metal cluster connectivities in metallothionein 2 by novel proton-cadium 113 heteronuclear two-dimensional NMR experiments. J Am Chem Soc 107 6847-6851... 

A recent example from the literature which is particularly illustrative of this approach is the so-called WISE experiment [137], in which the lineshape is detected as a function of chemical shift in a two-dimensional experiment. This sequence has also been modified to permit observation of the effects of spin diffusion in what is essentially a heteronuclear two-dimensional variant of the Goldman-Shen type experiment. This has allowed clear observation of the effects of micro heterogeneity in compatible blends of PS/PVME. 

They are most often combined with traditional two-dimensional experiments such as NOESY and TOC-SY to yield a three-dimensional experiment. For example, in the case of an HSQC-NOESY spectrum of a protein, two of the axes represent the heteronuclei such as and the protons which are directly attached to the nitrogen nuclei, while the third axis contains chemical shifts of protons which share an NOE effect with the amide proton. This offers a significant increase in resolution compared to a traditional two-dimensional NOESY. A large array of these types of three-dimensional, heteronuclear-edit-ed experiments have been designed to extract structural information in various situations. 

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