ARTICLE pubs.acs.org/JPCC
Structural and Electronic Near Degeneracy of M3O9 (M = Cr, Mo, W) Shenggang Li and David A. Dixon* Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336, United States
bS Supporting Information ABSTRACT: Density functional and coupled cluster theories are used to study several near-degenerate structures/states of M3O9 (M = Cr, Mo, W). For M = Mo and W, triple-ζ basis sets are used in our CCSD(T) studies for calculating both the valence and corevalence contributions to the cluster energetics in an attempt to reach “chemical accuracy” of ∼1 kcal/ mol. Electron addition to M3O9 has a significant effect on the relative stability of the ring and the chain. For M3O9, the ring is much more stable than the chain, but for M3O9, the ring and the chain are nearly degenerate in energy. For W3O9, in addition to the structural near degeneracy, there is also an electronic near degeneracy for the ring. The presence of calculated harmonic frequencies of 7 eV, the transition for the chain is not likely to contribute to the anion photoelectron spectrum, even though the 2A1(C2v) state of the chain lies very close in energy to the 2A1(C2v) state of the ring (Table 1). The calculated ADEs for the two transitions of the ring are both lower than the experimental ADE for band X, whereas that of the chain is higher. The best agreement with the experimental ADE reported by Huang et al.7 is found for the 1A10 (D3h) r 2A1(C2v) transition of the ring, which is assigned to band X. However, possible contributions from the 1A10 (D3h) r 2A10 (D3h) transition of the ring cannot be eliminated because of the good agreement between its calculated VDE and the experimental VDE and the large uncertainty in the experimental ADE resulting from the broadness of band X. The calculated VEA for the Mo3O9 ring at the CCSD(T)/aT//B3LYP/aD level is ∼2.5 eV if
ARTICLE
Figure 4. Potential energy surface for the interconversion between the different structures of W3O9 and W3O9. Relative energies in kilocalories per mole calculated at the CCSD(T)/aD//B3LYP/aD level are shown for the neutral cluster in black and for the anionic cluster in red. Corresponding values for M = Mo are shown in italics.
the electron is added to the lowest doubly degenerate virtual orbital and ∼2.0 eV if it is added to the lowest totally symmetric virtual orbital. The calculated VEA for the chain is ∼3.7 eV, much higher than the VEA of the ring. For W3O9, the calculated VDEs for the same three electronic states/structures as those for Mo3O9 at the CCSD(T)/aT// B3LYP/aD level are within 0.3 eV of the experimental value of band X.6,7 Analysis of the anion photoelectron spectrum obtained by Huang et al.,7 which has a better signal-to-noise ratio than that of Sun et al.,6 shows that the most intense signal is located at 4.10 (5) eV, with two less intense peaks at 4.20 (5) and 4.40 (5) eV and two apparent shoulder peaks to the red of these peaks at ∼3.60 (5) and ∼3.85 (5) eV. It is of course possible that all five peaks are vibrational bands of the same electronic transition with a VDE of ∼4.10 (5) eV. The calculated VDE of 4.22 eV for the 1A10 (D3h) r 2A10 (D3h) transition of the ring is the closest to the experimental value of 4.10 (5) eV, although that of 3.89 eV calculated for the 1A10 (D3h) r 2A1(C2v) transition of the ring is only ∼0.2 eV lower. The calculated VDE of 4.45 eV for the 1A1(C2v) r 2A1(C2v) transition of the chain is close in energy to the peak at 4.40 (5) eV. The computational results suggest that transitions from all three low-lying states of W3O9 could be contributing to the observed band X in the photoelectron spectrum of W3O9. The calculated ADEs for the two states of the ring are both ∼3.0 eV, whereas the ADE for the chain is ∼3.9 eV. This implies that at least one transition from the ring must contribute to the photoelectron spectrum of W3O9, as the experimental signal first appears at ∼3.2 eV, and it already has appreciable intensity at ∼3.6 eV. The calculated VEA for the W3O9 ring at the CCSD(T)/aT//B3LYP/aD level is 1.8 to 2.0 eV depending on whether the electron is added to the lowest doubly degenerate or totally symmetric virtual orbital. The VEA calculated for the chain is ∼3.4 eV, also substantially higher than the VEA of the ring as in the case of M = Mo. These findings suggest that it can be quite difficult to assign experimental anion photoelectron spectra with a limited resolution for large TMO clusters when there is the possibility of more than one low-lying electronic state/structure for the anion. Our 19194
dx.doi.org/10.1021/jp2038703 |J. Phys. Chem. C 2011, 115, 19190–19196
The Journal of Physical Chemistry C results suggest that the presence of a number of low-lying structures/states is a likely occurrence. In such cases, spectra at a much higher resolution and calculations at sufficiently high levels of theory, such as CCSD(T) with triple-ζ quality basis sets for both the valence and corevalence corrections or an appropriately benchmarked DFT functional, are both necessary for definitive spectral assignments. Furthermore, the ground state of the neutral cluster may or may not be observed by anion PES depending on whether the ground state of the neutral cluster is structurally similar to that of the anion. Effects of Electron Addition. Our study shows that electron addition to a TMO cluster can significantly alter the relative energies of the different cluster structures. This controls how and what neutral or anionic clusters can be synthesized experimentally. For example, the M3O9 ring for M = Cr, Mo, and W is predicted to be more stable than the chain by >20 kcal/mol. If the anion is prepared from the neutral M3O9 cluster, then it will likely be a ring, assuming a substantial energy barrier between the different structures of the anion. We therefore searched for transition states connecting the different structures of the neutral and anionic clusters, and the potential energy surface calculated at the CCSD(T)/aD//B3LYP/ aD level is given in Figure 4 for M = Mo and W. The interconversion of the ring and the chain structures of the neutral cluster or the anion involves multiple coupled motions and is unlikely to occur in a single step. The calculated barrier heights for the interconversion between the terminal M = O and bridge MO bonds for the ring and the chain isomers for M = W are 14 to 24 kcal/mol for the anion and 35 to 45 kcal/mol for the neutral cluster. For M = Mo, the calculated barrier heights are 15 to 25 kcal/mol for the anion and 30 to 40 kcal/ mol for the neutral cluster. For both metals, the calculated reaction barriers are lower for the anion than for the neutral cluster. The imaginary frequencies are
