Basic Heteronuclear Correlation Experiments

From Check it 5.4.1.15 it is possible to understand the phase cycling of the X-H-H relay COSY sequence and hence the basic phase cycling of other heteronuclear correlation experiments. The results of Check it 5.4.1.15 can be summarized ... 

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... 

Many heteronuclear multidimensional correlation experiments appear superficially complicated but actually comprise simple building blocks for coherence transfer and for recording chemical shift and/or coupling evolution. These basic elements... 

The basic aim of heteronuclear correlation 2D NMR is, as in the homonuclear experiment, to indicate which nuclei are coupled to one another. In heteronuclear NMR the correlation information is doubly useful since it provides chemical shifts for two different nuclear species in a single experiment. In the discussion which follows, the nuclei concerned will be assumed to be protons and carbon-13, although similar experiments have been reported for 31p 1h,53,54 15n-1h55 and correlation. 

On a system equipped with multiple RF channels and receivers several such schemes can be executed in parallel. Let s consider one of the simplest NMR experiments - the two-dimensional COSY experiment. The basic homonuclear COSY pulse sequence consists of an excitation pulse followed by the evolution period, t, a read pulse and an acquisition period, t2 (see Fig. 2a). The same scheme can be executed in parallel on two or more RF channels (Fig. 2b). If the cross-talk between the different nuclear species could be avoided, such an experiment would produce two independent 2D COSY spectra. However, in practise the magnetically active and in particular spin 1/2 nuclei from the same molecule are usually coupled via the scalar spin-spin couplings and in such a simple pulse scheme cross-talk is unavoidable. Therefore we should also observe heteronuclear correlations arising ffran coherence transfer A X and X A, i.e. in total four two-dimensional spectra in a single measurement Indeed, all the correlations can be observed in instances where the gyrranagnetic ratios of the nuclei and their natural abundances are similar. In fact, as a cmisequence of the close proximity of the H-1 and F-19 resonance frequencies at the Earth magnetic field, their two-dimensional correlation spectra can be observed even with a single receiver [29]. 

There are basically three main types of 2D NMR experiments J-resolved, shift correlation through bonds (e.g., COSY), and shift correlations through space e.g., NOESY). These spectra may be of homonuclear or heteronuclear type involving interactions between similar nuclei (e.g., protons) or between different nuclear species (e.g., H with C). 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

The focus of the chapter then shifts to heteronuclear 2D correlation (HETCOR) experiments. The evolution of single quantum coherence (SQC) of two different nuclei in these experiments with a coherence transfer step provides the basic formula for these correlations. In WISE (wide line separation), a wide dipolar... 

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