二维多量子MAS NMR的计算机模拟研究

COMPUTER SIMULATION OF THE 2D MULTIPLE-QUANTUM MAGIC-ANGLE SPINNING NMR

最近Frydman等人报道了联合MAS及多量子方法获取半整数四极核各向同性谱的新途径，本文以自旋3／2的四极核为例，采用步进式时序积分手续，数值计算了魔角样品旋转条件下的传播子，从而真实地模拟了非对易含时Hamiltonian情况下的密度矩阵演化；对Fryman等人采用的双脉冲序列实验应采取的最佳条件进行了详尽的理论分析；并对Na2C2O4样品的2D－MQ－MAS实验进行拟谱。

Very recently, Frydman et al. have shown that the combined use of MAS and two - dimensionalmultiple - quantum (MQ) spectroscopy can refocus the second - order anisotropic effects of half - in-teger quadrupoles. The present paper discusses theoretical aspect of this novel 2D MQ MAS methodthrough evaluating the propagation of density matrices evolving under the effects of noncommutingtime - dependent Hamiltonians. The influence of radiofrequency pulse excitation and conversion on theintensities in the spectra is analyzed and the conditions that maximize the MQ MAS signal for a spin- 3/2 are obtained. Numerical simulation of the 2D experimental spectrum of 23Na of Na2C2O4 showsthat the quadrupolar coupling parameters and the isotropic chemical shift can be determined.