Efficient cross-polarization using a composite 0° pulse for NMR studies on static solids

In most solid-state NMR experiments, cross-polarization is an essential step to detect low-gamma nuclei such as C-13 and N-15. In this study, we present a new cross-polarization scheme using spin-locks composed of composite 0 degrees Pulses in the RF channels of high-gamma and low-gamma nuclei to establish the Hartmann-Hahn match. The composite 0 degrees pulses with no net nutation-angle{(2 pi)(X) - (2 pi)(-X) - (2 pi)(Y) - (2 pi)(-Y) -}(n) applied Simultaneously to both high-gamma (I) and low-gamma (S) nuclei create an effective heteronuclear dipolar Hamiltonian H-d((0)) = d/2 (2I(Z)S(Z) + IXSX + IYSY), which 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. (C) 2008 Elsevier Inc. All rights reserved.