Koller H, Meijer E L, van Santen R A
Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, Netherlands.
Solid State Nucl Magn Reson. 1997 Dec;9(2-4):165-75. doi: 10.1016/s0926-2040(97)00056-8.
The electric field gradient (EFG) has been calculated in zeolite clusters at the aluminium site surrounded by four SiO4 tetrahedra. Density functional theory (DFT) with the 6-31G** basis set has been employed. Formation of a Brønsted acid site by protonation of one oxygen atom of the AlO4 tetrahedron perturbs the coordination of aluminium, i.e., the corresponding Al-O bond is considerably weaker than in the unprotonated case. This leads to a large EFG, and the calculated quadrupole coupling constant (QCC) for 27Al is 18.2 MHz. Different probe molecules were adsorbed on the Brønsted site. The hydrogen bond formed between the acid proton and the probe molecule weakened the zeolitic O-H bond. For conservation of the overall bond order of the oxygen atom, its bonds to the neighboring tetrahedral atoms (Si, Al) become stronger. As a consequence, the perturbation of the AlO4 tetrahedron and the EFG at the aluminium position decrease depending on the strength of the hydrogen bond. Perturbation of an oxygen atom of the AlO4 tetrahedron by accepting a hydrogen bond from the base molecule also affects the corresponding Al-O bond order. A linear correlation is found between the calculated QCC constants for 27Al and the Al-O bond orders of the oxygen atoms which are perturbed by protonation or by hydrogen bonds. A geometrical shear strain parameter and a simple electrostatic point charge model are less successful at predicting the trends in EFG which clearly shows the importance of the chemical bonds.
已在由四个SiO₄四面体包围铝位点的沸石簇中计算了电场梯度（EFG）。采用了具有6 - 31G**基组的密度泛函理论（DFT）。通过AlO₄四面体的一个氧原子质子化形成布朗斯特酸位点会扰乱铝的配位，即相应的Al - O键比未质子化情况下弱得多。这导致了较大的EFG，并且计算得到的²⁷Al的四极耦合常数（QCC）为18.2 MHz。不同的探针分子吸附在布朗斯特位点上。酸质子与探针分子之间形成的氢键削弱了沸石的O - H键。为了保持氧原子的整体键级，它与相邻四面体原子（Si、Al）的键变得更强。因此，AlO₄四面体的扰动以及铝位置处的EFG根据氢键的强度而降低。AlO₄四面体的一个氧原子通过接受来自碱分子的氢键进行扰动也会影响相应的Al - O键级。在计算得到的²⁷Al的QCC常数与因质子化或氢键而受到扰动的氧原子的Al - O键级之间发现了线性相关性。几何剪切应变参数和简单的静电点电荷模型在预测EFG趋势方面不太成功，这清楚地表明了化学键的重要性。
