Polarization transfer dynamics in Lee-Goldburg cross polarization nuclear magnetic resonance experiments on rotating solids

This paper presents a theoretical description of continuous wave (CW) high frequency Lee-Goldburg cross polarization magic angle spinning (LG-CPMAS) nuclear magnetic resonance experiments. The full time-dependent LG-CPMAS Hamiltonian is replaced by its zero order time-independent Hamiltonian in the interaction representation. Carbon signal enhancements of LG-CPMAS experiments are calculated for spin systems consisting of six H-1 nuclei coupled to one C-13 nucleus. These simulations are based on Floquet theory calculations, explicitly taking into account the time dependence because of magic angle spinning, and calculations based on the zero-order Hamiltonian. The good agreement between these calculations justifies the use of the zero-order Hamiltonian. The time-dependent intensities of the cross peaks in heteronuclear C-13-H-1 correlation spectra, extracted from 3D LG-CPMAS experiments on a natural abundant DL-alanine sample with increasing CP mixing times, are in good agreement with the theoretical intensities simulated by using the zero-order Hamiltonian. The approximated LG-CPMAS Hamiltonian can be used to obtain structural information about a proton coupled to a single carbon. The simulated intensities of the carbon signals of an isolated C-13-H-1 group and a C-13-H-1 group that is coupled to additional protons, measured by LG-CPMAS experiments with increasing CP mixing times, are compared. This study suggests that the buildup curve of each LG-CPMAS carbon signal and its Fourier transformed CP spectrum can be interpreted in terms of a single distance between the observed C-13 and its nearest proton, if the additional protons are removed from this carbon by at least 1.2 times this distance. (C) 2000 American Institute of Physics. [S0021-9606(00)00116-1].