高岭土结构的多磁场^(27)Al NMR波谱和ab initio量子理论计算研究

Calculation study of ^(27)Al NMR Spectrum in Muti-magietic Field with Kaolinite Structure and ab initio Quantum Theory

自上世纪80年代以来,试图使用魔角旋转固体核磁共振(MAS NMR)波谱来分辨高岭土结构两个不同^(27)Al结晶位置的多次尝试均未获成功,所以本次研究试图通过结合量子理论计算和多场MAS NMR和MQMAS波谱再次尝试分辨出高岭土结构中的不同^(27)Al位置。超高场如21 T所引起的高各向异性化学位移作用(CSA)可能会超过四极核作用(QI),这对于具较小C_q的^(27)Al位置(如高岭土结构)而言尤为如此。而低场如4.68 T中,QI则是主宰的核磁共振作用,但是,由于高岭土结构中两个^(27)Al位置的环境很相似,所以,通常^(27)Al MAS NMR也无法分辨出这两个^(27)Al位置。结合完全势能线型增广平面波(FP LAPW)+局域轨道法(lo)、分子轨道(MO)的ab initio量子理论计算,从多量子魔角旋转波谱、卫星跃迁波谱中可得到高岭土结构中两个^(27)Al位置的准确核磁共振参数如C_q、η、CSA和δ_(iso)等。

Many unsuccessful attempts have been made to resolve the two different Al sites in kaolinite by magic angle spinning nuclear magnetic resonance（MAS NMR） spectroscopy since the early 1980’s.This study attempted to resolve these sites and explain the reasons for the lack of separation by combining NMR spectroscopy at various fields with ab initio quantum modeling.Ata high field such as 21.06 T,large chemical shift anisotropy（CSA） and dipolar effects（DD）can overwhelm the quadrupolar（QI） effects for sites with relatively small Cqvalues such as the two Al sites in kaolinite,so the spectra display featureless line- shapes lacking the typical QI singularities and shoulders.At lower fields such as 4.68 T,the QI effects become dominant but due to the similarity of the two Al environments in kaolinite,the 27Al MAS NMR peaks could not be separated either.Theoretical ab initio quantum modeling using Full Potential Linearized Augmented Plane Wave（ FP LAPW） and Molecular Orbital（MO） allowed experimental 27Al MAS NMR,multiple quantum（MQ） MAS and satellite transition（SATRAS） spectra to be interpreted.Cq,η,CSA and δiso for the two Al sites were obtained with high accuracy by simulating these spectra,while constraining the QI parameters.