On the asymmetry in molybdenum-oxygen bonding in the $MoO_3$ structure : ETS–NOCV analysis

In the present study, the analysis of natural orbitals for chemical valence (NOCV) combined with the extended-transition-state (ETS) bond-energy decomposition method (ETS–NOCV) was applied to characterize an asymmetry in Mo–O bonding in MoO 3 crystal. Considered were three non-equivalent oxygen sites (O1, O2, O3) in the Mo 7 O 30 H 18 cluster model of (010) surface of MoO 3. The ETS–NOCV method leads to the conclusion that an increase in the Mo–O distances, from 1.68 Å (for Mo–O1), through 1.73 Å (for Mo–O 2), up to 1.94 Å (for Mo–O 3), is directly related to decrease in strength of both σ- and π-contributions of Mo–O bond. Further, Mo–O connection appeared to exhibit both ionic (the charge transfer from 2p orbital of oxygen to molybdenum) and the covalent (charge accumulation in the region of Mo–O) components. Finally, the trend in the orbital energy stabilization (ΔE orb) originating from the dominant σ- and π-bond contributions appeared to correlate very well with the oxygen-vacancy formation energies published earlier by Tokarz-Sobieraj et al. (Surf Sci 489:107, 2001).