Nuclear magnetic resonance heteronuclear correlation experiments

We present a solid-state nuclear magnetic resonance (NMR) experiment that allows the observation of a high-resolution two-dimensional heteronuclear correlation (2D HETCOR) spectrum between aluminum and phosphorous in aluminophosphate molecular sieve VPI-5. The experiment uses multiple quantum magic angle spinning (MQMAS) spectroscopy to remove the second order quadrupolar broadening in Al nuclei. The magnetization is then transferred to spin-1/2 nuclei of P via cross polarization (CP) to produce for the first time isotropic resolution in both dimensions. 

In many locations it is advantageous to mount the whole of the magnet assembly on a vibration damping system as floor vibrations (which may arise from a whole host of sources including natural floor resonances, air conditioners, movement in the laboratory and so on) can have deleterious effects on spectra, notably around the base of resonances (Fig. 3.3). Whilst such artefacts have lesser significance to routine ID observations, they may severely interfere with the detection of signals present at low levels, for example those in heteronuclear correlation or nuclear Overhauser effect experiments. 

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]. 

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