Reactivity of (TiO₂)_N Clusters (N = 1−10): Probing Gas-Phase Acidity and Basicity Properties

Abstract

We present B3LYP calculations on a selection of small-size clusters with (TiO 2 ) N stoichiometry (N ) 1-10) built on previous works on TiO 2 and SiO 2 or derived by kinship with stable clusters of different sizes. Their reactivity is analyzed as a function of size and electronic structure. Gas-phase acidity is probed by H + interaction with the oxygen sites, while basicity is tested by interaction of molecular NH 3 with titanium sites. Correlation with size, topology, or electronic properties is observed for some systems. In general, the correlation with electronic levels (highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO) is good criteria of reactivity, although this is not always observed. The calculated values generally decrease with the size. The HOMO-LUMO gaps show oscillations and a general decrease with the size. Coordination of the active site influences both the levels of the frontier orbitals and their effect upon reactivity. The protonation testing the cluster basicity is found to be higher for clusters with N ) 3 and N ) 9 in accordance with the high HOMO values. A remarkable exception is found for N ) 4 for which the most stable protonated structure is different from the most stable naked cluster. We have not tested here the flexibility of the naked cluster; however, this case means that structure reorganization should be considered for reactivity. Regarding ammonia adsorption testing cluster acidity, clusters with N ) 3 and N ) 8 present the highest adsorption energy toward NH 3 in accordance with a low LUMO value for the former but because of the local topology of the adsorption site for the latter. Acidic and basic character decreases for N ) 8-10 probably because of the increase in cohesive energy. A structure with N ) 9 emerges as the strongest base with the largest protonation energy. A tetrahedral structure with N ) 10 is remarkably stable and presents the lowest adsorption energy values.

Dust in brown dwarfs and extra-solar planets

“…Geometry of the calculated (TiO 2 ) N structures. Molecules labelled "a" are the molecules calculated by Jeong et al (2000) and those labelled "b" or unlabelled are the current most stable cluster geometries (Calatayud et al 2008;Syzgantseva et al 2011). Grey balls represent Ti atoms while red represent O atoms.…”

“…This level of theory was used for its mix of accuracy and computational speed and to keep in line the previous investigations on the same molecules (Jeong et al 2000;Calatayud et al 2008;Syzgantseva et al 2011). B3LYP is a popular and well regarded density functional theory for metal oxides and other inorganic compounds.…”

“…2.3). Previous studies of these TiO 2 cluster geometries (Calatayud et al 2008;Syzgantseva et al 2011) have focused on the reactivity and electronic structure of the clusters and not specifically on the thermodynamics of the formation of the clusters themselves.…”

MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

“…There is a considerable amount of literature on the global minima structures of TiO 2 clusters. 22,42,43,44,45 Furthermore, in order to check the rigorousness of this approach we also considered candidate structures derived from different materials, namely SiO 2 , 46 and MgF 2 . 47,48 In order to survey the potential energy surface, especially of the smaller nanostructures, we relaxed many (59 in total) candidate structures.…”

How a Quantum Chemical Topology Analysis Enables Prediction of Electron Density Transfers in Chemical Reactions. The Degenerated Cope Rearrangement of Semibullvalene

“…20 While chemical reactions of the bare transition metal oxide cations with H 2 have received a significant interest, [24][25][26][27][28][29][30][31][32][33][34][35][36][37] neutral metal oxides have gained much less attention. [38][39][40][41][42][43][44][45][46][47][48][49][50] Recently Zhou et al 51 have reported a combined experimental and theoretical study on the hydrogenation reactions by group V metal dioxides using matrix isolation spectroscopy. These results show a thermodynamically favorable reaction for the dihydrogen cleavage process with small barrier for VO 2 In this work, we present a theoretical study on the reaction mechanism for the dihydrogen activation by neutral group IV metal dioxides (MO 2 , M = Ti, Zr, and Hf) according to the following reaction:…”