Hetero-correlation experiments

H/ C One-Bond Shift Correlation by Hetero-COSY Experiment... 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

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... 

Evidently, the first step in any NMR study of carbohydrates involves assigning proton and carbon resonances. This can be done on the basis of scalar and through space connectivities, using 2D/3D homo- and hetero-nuclear correlation experiments.1,4-6 10... 

Hetero- and Homonuclear Correlation Experiments Involving MQMAS... 

HETERO- AND HOMONUCLEAR CORRELATION EXPERIMENTS INVOLVING MQMAS... 

MQMAS has further opened up vistas by making it possible to design high-resolution hetero- and homonuclear correlation experiments. These lead to connectivity information, distance measurements, and spectral editing. 

Fig. 3. The RDSQC (Random Direct Single Quantum Correlation) experiment developed by Hadden et al employ.s a randomized table of delays to sample the range of potential couplings specified by the accordion-optimization range of the experiment. As shown in the four comparison panels, the ADSQC experiment has artefact responses when only the reverse INEPT portion of the experiment is accordion-optimized which worsen when both INEPT steps are accordion-optimized. In contrast, the RDSQC experiment optimized for the same range of potential one-bond couplings affords data that are comparable to HSQC data and free of artefacts. (Reproduced with permission, Hetero Corp. 2001.)...

Nearly simultaneously with the report of the BIRD-HMBC experiment, Meissner and Sprensen described another modification of the basic long-range hetero-nuclear shift correlation experiment that they refer to as broadband HMBC. The authors employ the gradient dual-stage low-pass J-filter used in the accordion-optimized experiments described above (Section 3.3.5) followed by a delay. A, which replaces the accordion-optimized delay or a normal fixed delay for the evolution of long-range heteronuclear components of magnetization. Instead, Meissner and Sprensen acquire a series of several experiments with different... 

Figure 6.46. (a) The ID-refocused INEPT experiment, shown in simplified form with the refocusing pulses at the midpoints of Ai and A2 removed for clarity. The 2D shift correlation experiment, hetero-COSY (HETCOR) in (b) is derived from INEPT by the addition of the ti evolution period to encode proton chemical shifts prior to polarisation transfer. The 180 carbon pulse at the midpoint of ti refocuses heteronuclear coupling evolution and thus provides carbon decoupling in/i. 

For natural abundance H, correlation experiments, the 2D HMQC pulse sequence as described by Bax et al. 3 s used. A proton spectral width of 5(X)0 Hz and a carbon-13 spectral width of 25000 Hz were used, with 2K X 512 data points in the t2 and tl dimensions respectively. H, P correlation experiments were obtained using a hetero-TOCSY sequence, with an isotropic mixing time of 67 ms. The spectral width was set to 4000 Hz for protons and 607 Hz for phosphorus, with typically 2K X 256 data points in the two dimensions respectively. The proton 2D DQF-COSY and TOCSY experiments were recorded with standard pulse sequences the data size was 2K X 512, with spectral width of 5500 Hz in both dimensions. The NOESY sequence with a jump-return excitation pulse was used for optimal imino proton detection . Mixing times from 100 to 300 ms were used. The spectral widths were 11000 Hz in both dimensions. 

MQ MAS techniques for high-resolution solid-state NMR of half-integer spin quadrupolar nuclei have been reviewed by Goldbourt and Madhu. The combination of MQ MAS with other solid-state NMR techniques, such as hetero- and homo-nuclear correlation experiments has also been considered. Other aspects, such as basic theory, experimental implementation, signal optimisation, data analysis and interpretation, signal enhancement schemes and applications have also been presented. 

The NMR techniques discussed so far provide information about proton-proton interactions (e.g., COSY, NOESY, SECSY, 2D y-resolved), or they allow the correlation of protons with carbons or other hetero atoms (e.g., hetero COSY, COLOC, hetero /resolved). The resulting information is very useful for structure elucidation, but it does not reveal the carbon framework of the organic molecule directly. One interesting 2D NMR experiment, INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment), allows the entire carbon skeleton to be deduced directly via the measurement of C- C couplings. 

Like the HMBC, the COLOC experiment provides long-range hetero-nuclear chemical shift correlations. The COLOC spectrum, H-NMR, and C-NMR data of 7-hydroxyfrullanolide are presented here. Use the data to assign the quaternary carbons. 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

The most powerful techniques of all are undoubtedly the 2-D proton-carbon experiments (Hetero-nuclear Multiple Quantum Coherence///eteronuclear Single Quantum Coherence, or HMQC/HSQC and //ctcronuclcar Multiple Bond Correlation, or HMBC) as they provide an opportunity to dovetail proton and carbon NMR data directly. 

The double resonance experiment can be used to simplify a spectrum as discussed in Section 3.4.4, or to probe correlations between different nuclei. Two types of double resonance experiments are described. In the homonuclear double resonance experiment the nuclei irradiated are the same isotope as those observed Shorthand notation for this is, for example, In hetero-... 

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