Institute "Jožef Stefan," Jamova 39, 1000 Ljubljana, Slovenia.
J Chem Phys. 2017 May 21;146(19):194306. doi: 10.1063/1.4983484.
N nuclear quadrupole resonance (NQR) lineshapes mostly contain information of low interest, although in nanocrystals they may display some unexpected behaviour. In this work, we present an ab initio computational study of the N NQR lineshapes in urea nanocrystals as a function of the nanocrystal size and geometry, focusing on the surface induced broadening of the lineshapes. The lineshapes were obtained through a calculation of the electric field gradient for each nitrogen site in the nanocrystal separately, taking into account the individual crystal field by embedding the molecule of interest in a suitable lattice of point multipoles representing other urea molecules in the nanocrystal. The small influence of distant molecules is found with a series expansion, using the in-crystal Sternheimer shieldings which we also calculated ab initio. We have considered nanocrystals with two geometries: a sphere and a cube, with characteristic sizes between 5 and 100 nm. Our calculations suggest that there is a dramatic difference between the linewidths for the two geometries. For spheres, we find a steep drop in linewidths at ∼10 nm; at 5 nm the linewidth is ∼11 kHz, whereas for sizes above 20 nm the linewidth is practically negligible (<100 Hz). For cubes, on the other hand, we find a steady 1/size decrease, from 12 kHz at 10 nm to 1.2 kHz at 100 nm. This analysis is important for N NQR spectroscopy of crystalline pharmaceuticals, where nanoparticles are increasingly more often embedded in some sort of matrix. Although this is only a theoretical analysis, we believe that this work can serve as a guidance for the forthcoming experimental analysis.
N 核四极矩共振（NQR）谱线形状主要包含低兴趣的信息，尽管在纳米晶体中它们可能显示出一些意想不到的行为。在这项工作中，我们对尿素纳米晶体中 N NQR 谱线形状作为纳米晶体尺寸和几何形状的函数进行了从头算计算研究，重点关注表面诱导的谱线形状展宽。通过分别计算纳米晶体中每个氮位点的电场梯度来获得谱线形状，通过将感兴趣的分子嵌入合适的点偶极子晶格中来考虑单个晶体场，该晶格代表纳米晶体中的其他尿素分子。通过使用我们也从头算计算的晶内 Sternheimer 屏蔽进行级数展开，发现远处分子的影响很小。我们考虑了两种几何形状的纳米晶体：球体和立方体，特征尺寸在 5nm 到 100nm 之间。我们的计算表明，两种几何形状的线宽有很大的差异。对于球体，我们发现线宽在∼10nm 处急剧下降；在 5nm 时，线宽约为 11kHz，而在尺寸大于 20nm 时，线宽实际上可以忽略不计（<100Hz）。另一方面，对于立方体，我们发现线宽以 1/size 的速度稳定下降，从 10nm 处的 12kHz 下降到 100nm 处的 1.2kHz。这种分析对于结晶药物的 N NQR 光谱学很重要，其中纳米颗粒越来越多地嵌入某种基质中。虽然这只是理论分析，但我们相信这项工作可以为即将到来的实验分析提供指导。
