自然键轨道
硅酸铝
化学键
自然(考古学)
电磁屏蔽
化学位移
计算化学
材料科学
化学
物理化学
密度泛函理论
有机化学
地质学
复合材料
古生物学
催化作用
作者
Yun Liu,H. Nekvasil,J. A. Tossell
摘要
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.
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