固态NMR中优化的多量子MAS线型模拟
Optimized Multiple Quantum MAS Lineshape Simulations in Solid State NMR
摘要：
在固态核磁共振中可用于研究的大多数核具有半整数自旋$I > 1/2 $，具有相应的电四极矩。因此，它们可以与周围的电场梯度耦合。这种效应将各向异性谱线展宽引入光谱，这是由多晶固体中不同的化学物质引起的。在多量子魔角自旋（MQMAS）实验中，创建了第二个频率维度，没有四极各向异性。结果，高分辨率维度中峰的重心是各向同性二阶四极和化学位移单独的函数。然而，对于复杂的材料，这些参数采取随机性质，由于反过来结构和化学无序。在各向同性维度上，线形可能仍然重叠，这使得分配和解释的任务变得复杂。这里提出了一种分布式计算方法，它允许模拟的二维MQMAS光谱，产生的随机变量从模型分布的各向同性化学和四极位移。由于实验和模拟光谱之间的残差平方和（RSS）函数的非凸性质，模拟退火被用来优化模拟参数。以这种方式，可以表征无序材料的局部化学环境，并且经由重新采样方法，可以产生参数的误差估计。
英文摘要：
The majority of nuclei available for study in solid state Nuclear Magnetic Resonance have half-integer spin $I > 1/2 $, with corresponding electric quadrupole moment. As such, they may couple with a surrounding electric field gradient. This effect introduces anisotropic line broadening to spectra, arising from distinct chemical species within polycrystalline solids. In Multiple Quantum Magic Angle Spinning (MQMAS) experiments, a second frequency dimension is created, devoid of quadrupolar anisotropy. As a result, the center of gravity of peaks in the high resolution dimension is a function of isotropic second order quadrupole and chemical shift alone. However, for complex materials, these parameters take on a stochastic nature due in turn to structural and chemical disorder. Lineshapes may still overlap in the isotropic dimension, complicating the task of assignment and interpretation. A distributed computational approach is presented here which permits simulation of the two-dimensional MQMAS spectrum, generated by random variates from model distributions of isotropic chemical and quadrupole shifts. Owing to the non-convex nature of the residual sum of squares (RSS) function between experimental and simulated spectra, simulated annealing is used to optimize the simulation parameters. In this manner, local chemical environments for disordered materials may be characterized, and via a re-sampling approach, error estimates for parameters produced.
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