Structure and NMR assignment in calcined and as-synthesized forms of AlPO-14:: a combined study by first-principles calculations and high- resolution 27Al-31P MAS NMR correlation
Structure and NMR assignment in calcined and as-synthesized forms of AlPO-14:: a combined study by first-principles calculations and high- resolution 27Al-31P MAS NMR correlation
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DOI：
10.1039/b805681a
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发表时间：
2008-01-01
影响因子：
3.3
通讯作者：
Wimperis, Stephen
中科院分区：
文献类型：
--
作者：
Ashbrook, Sharon E.;Cutajar, Marica;Wimperis, Stephen
利用NMR参数的“第一性原理”计算，对两种合成形式的微孔磷酸铝AlPO-14和相应的煅烧-脱水形式的高分辨Al-27和P-31 NMR谱进行了归属（GIPAW，如在NMR-CASTEP中实施的）和Al-27-P-31异序相关NMR实验（MQ-J-HETCOR），其利用Al-27多量子相干和J耦合来鉴定Al-O-P键。从公布的AlPO-14晶体结构，这是来自粉末X-射线衍射（XRD）数据计算的NMR参数，是在与实验的协议差，它是必要的，以优化结构的几何形状，使用能量最小化之前，获得令人满意的协议。从实验和能量最小化结构的模拟粉末XRD图案的比较表明，在优化的结构中的相对原子位置的变化是相对较小的，产生布拉格峰强度的微小调整。这些结果表明，NMR光谱和NMR参数的第一性原理计算的组合可能很快被认为是一个普遍有用的步骤，从粉末衍射数据得到的微孔材料的结构的细化。
The high-resolution Al-27 and P-31 NMR spectra of two as-synthesized forms of the microporous aluminophosphate AlPO-14 and the corresponding calcined -dehydrated form were assigned using both "first-principles'' calculations of NMR parameters (GIPAW, as implemented in NMR-CASTEP) and a Al-27-P-31 heteronuclear correlation NMR experiment (MQ-J-HETCOR) that exploits Al-27 multiple-quantum coherences and J couplings to identify Al-O-P linkages. NMR parameters calculated from published AlPO-14 crystal structures, which are derived from powder X-ray diffraction (XRD) data, are in poor agreement with experiment and it was necessary to optimize the structure geometry using energy minimization before satisfactory agreement was obtained. Comparison of simulated powder XRD patterns from the experimental and the energy-minimized structures shows that the changes in relative atomic positions in the optimized structure are relatively small and yield only minor adjustments in the Bragg peak intensities. These results indicate that a combination of NMR spectroscopy and first-principles calculation of NMR parameters may soon be considered a generally useful step in the refinement of the structures of microporous materials derived from powder diffraction data.