Gee Becky A
Department of Chemistry and Biochemistry, Long Island University-Brooklyn Campus, Brooklyn, New York 11201, USA.
Magn Reson Chem. 2004 Jan;42(1):30-8. doi: 10.1002/mrc.1303.
The spatial ordering of aluminum atoms in CsAl(SiO3)2 and 3Al2O3.2SiO2 was probed by 27Al dipolar solid-state NMR spectroscopy. The 27Al response to a Hahn spin-echo pulse sequence in a series of aluminum-containing model crystalline compounds demonstrates that quantitative 27Al homonuclear dipolar second moments can be obtained to within +/-20% of the theoretical values, if evaluation of the spin-echo response curve is limited to short evolution periods (2t1 < or = 0.10 ms). Additionally, selective excitation of the central transition m = 1/2 --> -1/2 is necessary in order to ensure quantitative results. Restriction of spin exchange affecting the dephasing of the magnetization may decelerate the spin-echo decay at longer evolution periods. Considering these restraints, the method was used to probe the spatial distribution of aluminum atoms among the tetrahedral sites in two aluminosilicate materials. Experimental 27Al spin-echo response data for the aluminosilicates CsAl(SiO3)2 (synthetic pollucite) and 3Al2O3.2SiO2 (mullite) are compared with theoretical data based on (I) various degrees of aluminum-oxygen-aluminum bond formation among tetrahedrally coordinated aluminum atoms (Al(T(d) )-O-Al(T(d) )) and (II) the maximum avoidance of Al(T(d) )-O-Al(T(d) ) bonding. Analysis of the second moment values and resulting echo decay responses suggests that partial suppression of spin exchange among aluminum atoms in crystallographically distinct sites may contribute to the 27Al spin echo decay in 3Al2O3.2SiO2, thus complicating quantitative analysis of the data. Silicon-29 and aluminum-27 magic angle spinning (MAS) NMR spectra of 3Al2O3.2SiO2 are consistent with those previously reported. The experimental 27Al spin-echo response behavior of CsAl(SiO3)2 differs from the theoretical response behavior based on the maximum avoidance of Al-O-Al bonding between tetrahedral aluminum sites in CsAl(SiO3)2. A single unresolved resonance is observed in both the silicon-29 and aluminum-27 MAS spectra of CsAl(SiO3)2.
通过²⁷Al偶极固态核磁共振光谱法探究了CsAl(SiO₃)₂和3Al₂O₃·2SiO₂中铝原子的空间排列。在一系列含铝模型晶体化合物中，²⁷Al对哈恩自旋回波脉冲序列的响应表明，如果将自旋回波响应曲线的评估限制在短演化期（2t₁≤0.10毫秒）内，定量的²⁷Al同核偶极二阶矩可在理论值的±20%范围内获得。此外，为确保定量结果，有必要选择性激发中心跃迁m = 1/2→ -1/2。影响磁化去相的自旋交换限制可能会在较长演化期使自旋回波衰减变慢。考虑到这些限制因素，该方法被用于探究两种铝硅酸盐材料中四面体位置间铝原子的空间分布。将铝硅酸盐CsAl(SiO₃)₂（合成白榴石）和3Al₂O₃·2SiO₂（莫来石）的实验²⁷Al自旋回波响应数据与基于以下情况的理论数据进行了比较：(I) 四面体配位铝原子（Al(T(d) )-O-Al(T(d) )）之间不同程度的铝-氧-铝键形成；(II) Al(T(d) )-O-Al(T(d) )键的最大避免情况。二阶矩值及由此产生的回波衰减响应分析表明，晶体学上不同位置的铝原子间自旋交换的部分抑制可能导致3Al₂O₃·2SiO₂中²⁷Al自旋回波衰减，从而使数据的定量分析变得复杂。3Al₂O₃·2SiO₂的²⁹Si和²⁷Al魔角旋转（MAS）核磁共振谱与先前报道的一致。CsAl(SiO₃)₂的实验²⁷Al自旋回波响应行为不同于基于CsAl(SiO₃)₂中四面体铝位置间最大程度避免Al-O-Al键的理论响应行为。在CsAl(SiO₃)₂的²⁹Si和²⁷Al MAS谱中均观察到一个未分辨的单一共振峰。
