RESUMEN
Recoupling, decoupling, and multidimensional correlation experiments in magic-angle-spinning (MAS) solid-state NMR can be designed by exploiting the symmetry of internal spin interactions. One such scheme, namely, C521, and its supercycled version SPC521, notated as a five-fold symmetry sequence, is widely used for double-quantum dipole-dipole recoupling. Such schemes are generally rotor synchronised by design. We demonstrate an asynchronous implementation of the SPC521 sequence leading to higher double-quantum homonuclear polarisation transfer efficiency compared to the normal synchronous implementation. Rotor-synchronisation is broken in two different ways: lengthening the duration of one of the pulses, denoted as pulse-width variation (PWV), and mismatching the MAS frequency denoted as MAS variation (MASV). The application of this asynchronous sequence is shown on three different samples, namely, U-13C-alanine and 1,4-13C-labelled ammonium phthalate which include 13Cα-13Cß, 13Cα-13Co, and 13Co-13Co spin systems, and adenosine 5'- triphosphate disodium salt trihydrate (ATPâ 3H2O). We show that the asynchronous version performs better for spin pairs with small dipole-dipole couplings and large chemical-shift anisotropies, for example, 13Co-13Co. Simulations and experiments are shown to corroborate the results.
RESUMEN
The power and versatility of NMR spectroscopy is strongly related to the ability to manipulate NMR interactions by the application of radio-frequency (rf) pulse sequences. Unfortunately, the rf fields seen by the spins differ from the ones programmed by the experimentalist. Pulse transients, i.e., deviations of the amplitude and phase of the rf fields from the desired values, can have a severe impact on the performance of pulse sequences and can lead to inconsistent results. Here, we demonstrate how transient-compensated pulses can greatly improve the efficiency and reproducibility of NMR experiments. The implementation is based on a measurement of the characteristics of the resonance circuit and does not rely on an experimental optimization of the NMR signal. We show how the pulse sequence has to be modified to use it with transient-compensated pulses. The efficiency and reproducibility of the transient-compensated sequence is greatly superior to the original POST-C7 sequence.