Efficient Cross-Polarization Using a Composite 0° Pulse for NMR Studies on Static Solids

摘要

In most solid-state NMR experiments, cross-polarization is an essential step to detect low-γ nuclei such as ¹³C and ¹⁵N. In this study, we present a new cross-polarization scheme using spin-locks composed of composite 0° pulses in the RF channels of high-γ and low-γ nuclei to establish the Hartmann-Hahn match. The composite 0° pulses with no net nutation angle {(2π)X −(2π)−X −(2π)Y −(2π)−Y−}n applied simultaneously to both high-γ(I) and low-γ(S) nuclei create an effective heteronuclear dipolar Hamiltonian Hd(0)=d2(2IZSZ+IXSX+IYSY), which 2 is capable of transferring the Z-component of the I spin magnetization to the Z-component of the S spin magnetization. It also retains a homonuclear dipolar coupling Hamiltonian that enables the flip-flop transfer among abundant spins. While our experimental results indicate that the new pulse sequence, called composite zero cross-polarization (COMPOZER-CP) performs well on adamantane, it is expected to be more valuable to study semi-solids like liquid crystalline materials and model lipid membranes. Theoretical analysis of COMPOZER-CP is presented along with experimental results. Our experimental results demonstrate that COMPOZER-CP overcomes the RF field inhomogeneity and Hartmann-Hahn mismatch for static solids. Experimental results comparing the performance of COMPOZER-CP with that of the traditional constant-amplitude CP and rampCP sequences are also presented in this paper.