Modifying Homonuclear Dipolar Recoupling Pulse Sequences in Magic-Angle Spinning Nuclear Magnetic Resonance.

A numerical simulation study of recoupling pulse sequences in magic angle spinning solid-state NMR is carried out, using a model spin system. One particular pulse sequence, part of a family of symmetry-based pulse sequences, C72 1 is considered in this study. The C72 1 pulse sequence shows high double-quantum filtration efficiency for 13C spin pairs that have a large dipolar coupling interaction. However, the C72 1 pulse sequence shows very low double-quantum filtration efficiency for 13C spin pairs that have a small dipolar coupling interaction. The double-quantum filtration efficiency for spin pairs with large dipolar coupling interactions is found to be further improved by reducing the Larmor frequency. The double-quantum filtration efficiency for spin pairs with a small dipolar coupling interaction is found to be improved by increasing the duration or the amplitude of the radiofrequency pulses applied during the experiment. The modified pulse sequence depends on isotropic chemical shifts and chemical shift anisotropies.