Stoichiometric and Oxygen-Rich M₂O_n⁻and M₂O_n(M = Nb, Ta;n= 5−7) Clusters: Molecular Models for Oxygen Radicals, Diradicals, and Superoxides

Abstract

We investigated the structures and bonding of two series of early transition-metal oxide clusters, M(2)O(n)(-) and M(2)O(n) (M = Nb, Ta; n = 5-7) using photoelectron spectroscopy (PES) and density-functional theory (DFT). The stoichiometric M(2)O(5) clusters are found to be closed shell with large HOMO-LUMO gaps, and their electron affinities (EAs) are measured to be 3.33 and 3.71 eV for M = Nb and Ta, respectively; whereas EAs for the oxygen-rich clusters are found to be much higher: 5.35, 5.25, 5.28, and 5.15 eV for Nb(2)O(6), Nb(2)O(7), Ta(2)O(6), and Ta(2)O(7), respectively. Structural searches at the B3LYP level yield triplet and doublet ground states for the oxygen-rich neutral and anionic clusters, respectively. Spin density analyses reveal oxygen radical, diradical, and superoxide characters in the oxygen-rich clusters. The M(2)O(7)(-) and M(2)O(7) clusters, which can be viewed to be formed by M(2)O(5)(-/0) + O(2), are utilized as molecular models to understand dioxygen activation on M(2)O(5)(-) and M(2)O(5) clusters. The O(2) adsorption energies on the stoichiometric M(2)O(5) neutrals are shown to be surprisingly high (1.3-1.9 eV), suggesting strong capabilities to activate O(2) by structural defects in Nb and Ta oxides. The PES data also provides valuable benchmarks for various density functionals (B3LYP, BP86, and PW91) for the Nb and Ta oxides.

High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

“…The PES data also provided valuable benchmarks for density functionals, and B3LYP was found suitable for Nb and Ta oxides. 54 , and Nb 3 O 8 ) have the same geometric structures as the corresponding V counterparts do. The similarities between Nb and V oxide clusters of small cluster size in structure and reactivity motivate us to explore the possibility of HAA on nanosized Nb oxide clusters.…”

“…17,42,62,63,67,70,71,84,85 B3LYP was also adopted in many comparative studies on Group VB metals oxide clusters. 49,51,[53][54][55]86 Here, we performed a benchmark calculation on Nb and its monoxide clusters using B3LYP/TZVP/Def2-TZVP. All the calculated values compared well with available experimental values (Table I, and also the ionization potential (IP) of Nb atom: calc.…”

“…39 The formation, structure, stability, and reactivity of gas phase Nb oxide clusters have been extensively studied by both experimental and theoretical methods, [40][41][42] including some comparative studies with other Group VB metal (V or Ta) oxide clusters. 8,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] Most of these works were on small clusters containing Nb atoms less than 5, such as those on mononuclear NbO y ( y = 1-12), 41,[43][44][45]53,55,[59][60][61][62][63][64] on Nb 2 O y ( y = 1-12), 42,64 and on Nb 3 O. 46,57,[65][66][67] Several research groups have contributed to this field.…”

“…They concluded that different modes of attack in the initial phase of C-H bond activation together with quite different bond-dissociation energies of the M-O bonds caused the rather varying reactivities. Wang and coworkers 51,54,67,70 investigated the structures of anionic and neutral Nb oxide clusters by photoelectron spectroscopy (PES) and computational chemistry. The PES data also provided valuable benchmarks for density functionals, and B3LYP was found suitable for Nb and Ta oxides.…”

Generation and reactivity of putative support systems, Ce-Al neutral binary oxide nanoclusters: CO oxidation and C–H bond activation

“…[37][38][39] Gas phase nanoclusters in an "isolated" environment are ideal model systems with which to simulate real surface reactions and to discover surface reaction mechanisms. 37,39,40 Many excellent studies have been reported in the last decade with regard to the structure and reactivities of gas phase cationic 37,38, and anionic 42 [80][81][82] The groups of Wang and Jarrold performed detailed spectroscopic studies on aluminum oxide clusters. 76,[83][84][85][86][87] Rather interesting reactivity trends are reported by the Schwarz and Castleman groups for oxidation reactions generated by Al 2 O 3 + , 51,88,89 and methane activation by the oxygen-rich Al 2 O 7 + cluster suggests the importance and influence of chemically absorbed O 2 for the C-H bond activation reaction.…”