Structural and electronic properties of reduced transition metal oxide clusters, M3O8 and M3O8- (M = Cr, W), from photoelectron spectroscopy and quantum chemical calculations.

We report a comparative study of reduced transition metal oxide clusters, M(3)O(8)(-) (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M(3)O(8)(-) (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. Extensive DFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr(3)O(8) and Cr(3)O(8)(-) are predicted to be the (3)B(2) and (4)B(2) states of a C(2v) structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W(3)O(8) and W(3)O(8)(-) are predicted to be the (1)A' state (C(s) symmetry) and the (2)A(1) state (C(2v) symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M(3)O(8) are calculated from the clustering energies and the heats of formation of MO(2) and MO(3). The energetics have been used to predict redox reaction thermochemistry.