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

Abstract

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{(2 π) X − (2 π) − X − (2 π) Y − (2 π) − Y −} n applied simultaneously to both high-γ ( I) and low-γ ( S) nuclei create an effective heteronuclear dipolar Hamiltonian H d ( 0 ) = d 2 ( 2 I Z S Z + I X S X + I Y S Y ) , 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.