Structural Properties of Gas-Phase Molybdenum Oxide Clusters [MoO], [HMoO], and [CHMoO] Studied by Collision-Induced Dissociation.

Molybdenum oxide-based catalysts are widely used for the ammoxidation of toluene, methanation of CO, or hydrodeoxygenation. As a first step towards a gas-phase model system, we investigate here structural properties of mass-selected [MoO], [HMoO], and [CHMoO] by a combination of collision-induced dissociation (CID) experiments and quantum chemical calculations. According to calculations, the common structural motif is an eight-membered ring composed of four MoO units and four O atoms. The 13th O atom is located above the center of the ring and connects two to four Mo centers. For [MoO] and [HMoO], dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [MoO], but energetically more demanding for [HMoO]. In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMoO] or [HMoO]. The doubly charged species [MoO] loses one MoO unit at low energies while Coulomb explosion into the complementary fragments [MoO] and [MoO] dominates at elevated collision energies. [CHMoO] affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CHMoO] also reacts efficiently with water, leading to methanol or formaldehyde elimination.