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解释铝硅酸盐中T-O-T键角对核磁共振化学位移的影响:自然键轨道(NBO)和自然化学屏蔽(NCS)分析。

Explaining the effects of T-O-T bond angles on NMR chemical shifts in aluminosilicates: A natural bonding orbital (NBO) and natural chemical shielding (NCS) analysis.

作者信息

Liu Yun, Nekvasil Hanna, Tossell John

机构信息

Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.

出版信息

J Phys Chem A. 2005 Apr 7;109(13):3060-6. doi: 10.1021/jp046103b.

Abstract

It has long been recognized that the 29Si and 27Al NMR chemical shifts for aluminosilicate crystals and glasses correlate to some extent with the T-O-T bond angle (where T is the tetrahedral atom Si or Al). With increasing T-O-T bond angle, the 29Si and 27Al NMR shieldings increase and the shifts thus become more negative. This result has been demonstrated both experimentally and through quantum computations. However, no simple qualitative explanation has ever been given for what appears to be a simple qualitative trend. We here provide such an explanation based upon quantum calculations. We have used high level ab initio NMR shielding calculations, natural bonding orbital (NBO) analysis, and natural chemical shielding (NCS) analysis, performed on model clusters with different T-O-T angles, to obtain an explanation for this trend from an electronic structure point of view. On the basis of both NBO populations and the NCS analysis, the following factors account for the correlation of shift with T-O-T angle: (1) a slight increase in population of the Al-O and Si-O bond orbital electrons and a dramatic change in bond orbital shapes and hybridization (with more s character and less bond bending as the T-O-T angle increases), (2) a movement of one of the lone pairs on O toward the vicinity of the Si or Al as the T-O-T angle increases, and (3) a change in the shielding contribution from the core 2p electrons of Al or Si. The changes in the 17O NMR shift with T-O-T angle are more complex, and the shifts are also more strongly influenced by distant atoms, but some systematic changes in O lone pair contributions can be identified.

摘要

长期以来,人们已经认识到,铝硅酸盐晶体和玻璃的29Si和27Al核磁共振化学位移在一定程度上与T-O-T键角相关(其中T是四面体原子Si或Al)。随着T-O-T键角的增加,29Si和27Al核磁共振屏蔽增加,因此位移变得更负。这一结果已通过实验和量子计算得到证明。然而,对于这一看似简单的定性趋势,从未给出过简单的定性解释。我们在此基于量子计算给出这样一种解释。我们对具有不同T-O-T角的模型簇进行了高水平的从头算核磁共振屏蔽计算、自然键轨道(NBO)分析和自然化学屏蔽(NCS)分析,以便从电子结构的角度对这一趋势作出解释。基于NBO布居数和NCS分析,以下因素解释了位移与T-O-T角的相关性:(1)Al-O和Si-O键轨道电子的布居数略有增加,且键轨道形状和杂化发生显著变化(随着T-O-T角增大,s特征更多,键弯曲更少),(2)随着T-O-T角增大,O上的一对孤对电子之一向Si或Al附近移动,以及(3)Al或Si的核心2p电子的屏蔽贡献发生变化。17O核磁共振位移随T-O-T角的变化更为复杂,且位移也受远处原子的影响更大,但可以识别出O孤对电子贡献的一些系统性变化。

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