Infrared Spectroscopy of Gas-Phase MnxOy(CO2)z+ Complexes.

The interaction of manganese oxide clusters MnxOy+ (x = 2-5, y ≥ x) with CO2 is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630-1860 cm-1. Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200-1450 cm-1 and they are assigned to the characteristic Fermi resonance of CO2 arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO2 molecules, whereas the higher frequency band is largely unaffected. Despite these effects, the observation of the Fermi dyad indicates only a small perturbation of the CO2 molecule. This finding is confirmed by the theoretical investigation of Mn2O2(CO2)+ revealing only small orbital mixing between the dimanganese oxide cluster and CO2, indicative of mainly electrostatic interaction.