Li Shenggang, Dixon David A
Chemistry Department, The University of Alabama, Shelby Hall, Alabama 35487-0336, USA.
J Phys Chem A. 2006 May 18;110(19):6231-44. doi: 10.1021/jp060735b.
The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO(3))(n) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-zeta quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-zeta quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures (n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO(3))(n) were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M(6)O(18), two additional isomers were also studied, the cage and the inverted cage. The relative stability of the different conformations of M(6)O(18) depends on the transition metal as well as the level of theory. Normalized and differential clustering energies of the ring structures were calculated and were shown to vary with respect to the cluster size. Brönsted basicities and Lewis acidities based on a fluoride affinity scale were also calculated. The Brönsted basicities as well as the Lewis acidities depend on the size of the cluster and the site to which the proton or the fluoride anion binds. These clusters are fairly weak Brönsted bases with gas phase basicities comparable to those of H(2)O and NH(3). The clusters are, however, very strong Lewis acids and many of them are stronger than strong Lewis acids such as SbF(5). Brönsted acidities of M(6)O(19)H(2) and M(6)O(18)FH were calculated for M = Mo and W and these compounds were shown to be very strong acids in the gas phase. The acid/base properties of these TMO clusters are expected to play important roles in their catalytic activities.
利用密度泛函理论（DFT）方法研究了第VIB族金属（MO(3))(n)（M = Cr、Mo、W；n = 1 - 6）的过渡金属氧化物（TMO）团簇的结构和性质。在局部和非局部DFT水平上，使用极化价双ζ质量基组进行几何优化和频率计算，并在局部和非局部DFT几何结构下，使用极化价三ζ质量基组在非局部DFT水平上计算最终能量。使用有效核势来处理过渡金属原子。研究了两种类型的团簇，即环状和链状，环状团簇能量较低。大的环状结构（n > 3）在其平面外变形中表现出易变形性。在非局部DFT水平上，预测(CrO(3))(n)的长链结构（n > 3）是较小团簇的弱结合复合物。对于M(6)O(18)，还研究了另外两种异构体，笼状和倒笼状。M(6)O(18)不同构象的相对稳定性取决于过渡金属以及理论水平。计算了环状结构的归一化和差分聚类能量，并表明其随团簇大小而变化。还基于氟化物亲和标度计算了布朗斯特碱度和路易斯酸度。布朗斯特碱度以及路易斯酸度取决于团簇大小和质子或氟化物阴离子结合的位点。这些团簇是相当弱的布朗斯特碱，其气相碱度与H(2)O和NH(3)相当。然而，这些团簇是非常强的路易斯酸，其中许多比诸如SbF(5)等强路易斯酸更强。计算了M = Mo和W时M(6)O(19)H(2)和M(6)O(18)FH的布朗斯特酸度，这些化合物在气相中显示为非常强的酸。这些TMO团簇的酸碱性质预计在其催化活性中起重要作用。
