Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn 2 Si n , n = 10, 12, and 13, and Cationic [Mn 2 Si 13 ] .

A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn 2 unit. Mn 2 Si 12 and [Mn 2 Si 13 ] + are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a 1 ) and perpendicular (e 1 ) components. Information about the Mn-Mn and Mn-Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn 2 Si 13 ] + , where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D 6 d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn 2 Si 12 ) or the capping atom is present but displaced off the principal axis (Mn 2 Si 13 ), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.