Gas-phase interaction of thiophene with the Ti₈C ₁₂⁺ and Ti₈C₁₂ met-car clusters

The reactivity of the Ti₈C₁₂⁺ met-car cation toward thiophene was investigated using density functional theory (DFT) and mass selective ion chemistry. It is shown that the experimentally observed mass spectrum can be well described by the DFT calculations. In contrast to the weak bonding interactions seen for thiophene on a TiC(001) surface, the Ti ₈C₁₂⁺ met-car cation is able to interact strongly with up to four thiophene molecules with the cluster staying intact. In the most stable conformation, the thiophene molecules bond to the four low-coordinated Ti⁰ sites of Ti₈C₁₂⁺ via a η⁵-C,S coordination. The stability and the activity of the Ti₈C₁₂⁺ met-car is observed to increase with an increasing number of attached thiophene molecules at the Ti⁰ sites, which is associated with a significant transfer of electron density from thiophene to the cluster. The additional electron density on the Ti ₈C₁₂⁺ cation cluster, however, is not sufficient to cleave the C-S bonds of thiophene and the dissociation reaction of thiophene is predicted to be a highly activated process. By contrast, DFT calculations for the neutral Ti₈C₁₂ met-car predict that the dissociation reaction leading to adsorbed S and C₄H₄ fragments is energetically favorable for the first thiophene molecule. The binding behavior for subsequent addition of thiophene molecules to the neutral met-car is also presented and compared to that of the cation.