How the nature and charge of metal cations affect vibrations in acetone solvent molecules

Vibrational spectra of a series of gas-phase metal 1+ and 2+ ions solvated by acetone molecules are collected to investigate how the metal charge, number of solvent molecules and nature of the metal affect the acetone. The spectra of Cu⁺(Ace)(N₂)₂, Cu⁺(Ace)₄, and M²⁺(Ace)₄, where M = Co, Ni, Cu, and Zn are measured via photodissociation by monitoring fragment ion signal as a function of IR wavenumber. The spectra show a red shift of the CO stretch and a blue shift of the C-C antisymmetric stretch. DFT calculations are carried out to provide the simulated spectra of possible isomers to be compared with the observed vibrational spectra, and specific structures are proposed. The red shift of the CO stretch increases as the number of acetone molecules decreases. Higher charge on the metal leads to a larger red shift in the CO stretch. Although all of the M²⁺ complexes have very similar red shifts, they are predicted to have different geometries due to their different electron configurations. Unexpectedly, we find that the calculated red shift in the CO stretch in M^+/2+(Ace) is highly linearly correlated with the ionization energy of the metal for a wide range of metal cations and dications.