Broadband population inversion in solid state NMR

Abstract

We present theory, simulations and experimental demonstrations of composite π pulses for population inversion in coupled spin systems such as occur in solid state NMR. The composite π pulses are phase‐shifted rf pulse sequences designed to invert spins over a larger range of dipole or quadrupole couplings than a conventional π pulse, for a given rf power. We discuss a previously proposed theory for constructing composite pulses, in the specific context of solids. Two particular sequences 45₀180₉₀90₁₈₀180₉₀45₀ and 180₀180₁₂₀180₀ are examined in detail. Their performance in coupled spin systems of various sizes is evaluated in simulations. Experiments are performed on two solid compounds, Ba(ClO₃)₂⋅H₂O and C₄O₄H₂. The results reveal markedly less spectral distortion after composite pulse inversion than after conventional π pulse inversion at low rf powers.