Structure and Bonding of Sc₃C₂ and La₃C₂ Clusters from MATI Spectroscopy and Theoretical Calculations

Understanding the fundamental properties of transition metal-carbide clusters is essential for advancing research in metal-carbide-based materials and devices. In this study, scandium and lanthanum trimer dicarbides (Sc₃C₂ and La₃C₂) are synthesized in laser ablation metal cluster beams and characterized by mass analyzed threshold ionization (MATI) spectroscopy. The MATI spectra yield precise ionization energies of the neutral clusters and vibrational frequencies for both neutral and ionic species. Spectroscopic measurements combined with theoretical calculations reveal that both clusters adopt trigonal bipyramid structures, with neutral species in C_2v symmetry and their corresponding ions in D_3h. The neutral ground states are doublets (²B₁), with an unpaired electron localized on two of the three metal atoms. Ionization removes this metal-based electron and yields singlet ions (¹A₁'). The metal-carbon bonding is predominantly polar covalent in character. Comparison between free Sc₃C₂ and the same cluster encapsulated within a C₈₀ fullerene highlights significant encapsulation effects on the cluster's geometric, electronic, and dynamic properties.