Numerical design of RNnν symmetry-based RF pulse schemes for recoupling and decoupling of nuclear spin interactions at high MAS frequencies

An approach for the efficient implementation of RNnν symmetry-based pulse schemes that are often employed for recoupling and decoupling of nuclear spin interactions in biological solid state NMR investigations is demonstrated at high magic-angle spinning frequencies. RF pulse sequences belonging to the RNnν symmetry involve the repeated application of the pulse sandwich {RϕR−ϕ}, corresponding to a propagator URF = exp(−i4ϕIz), where ϕ = πν/N and R is typically a pulse that rotates the nuclear spins through 180° about the x-axis. In this study, broadband, phase-modulated 180° pulses of constant amplitude were employed as the initial ‘R’ element and the phase-modulation profile of this ‘R’ element was numerically optimised for generating RNnν symmetry-based pulse schemes with satisfactory magnetisation transfer characteristics. At representative MAS frequencies, RF pulse sequences were implemented for achieving 13C–13C double-quantum dipolar recoupling and through bond scalar coupling mediated chemical shift correlation and evaluated via numerical simulations and experimental measurements. The results from these investigations are presented here.