Lanthanides as Catalysts: Guided Ion Beam and Theoretical Studies of Sm⁺ + COS

Reactions of samarium cations with carbonyl sulfide are examined using a guided ion beam tandem mass spectrometer and a variable temperature selected ion flow tube apparatus. Formation of SmS⁺ + CO is observed in both instruments with a kinetic energy and temperature dependence demonstrating a barrierless reaction occurring with an efficiency of 26 ± 9%. Formation of SmO⁺ + CS is also observed at high kinetic energies and exhibits a threshold determined as 2.81 ± 0.32 eV, substantially higher than expected from known thermochemistry. The potential energy surfaces for these reactions along sextet and octet spin surfaces are also examined theoretically at the MP2 and CCSD(T) levels. The observed barrier for oxidation is shown to likely correspond to the energy of the crossing between surfaces corresponding to the ground state electronic configuration of Sm⁺ (⁸F,4f⁶6s¹) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO⁺ bond. In contrast, the S-CO bond is activated much more readily because this crossing occurs at much lower energies. This result is attributed to the much weaker S-CO bond energy as well as the ability of sulfur to bind effectively at different angles. Although both reactions are spin-forbidden, evidence for a more efficient spin-allowed process is also observed in the SmS⁺ + CO cross section.