Infrared spectroscopy of [XFeC24H12](+) (X = C5H5, C-5(CH3)(5)) complexes in the gas phase: Experimental and computational studies of astrophysical interest

We report the first experimental mid-infrared (700-1600 cm(-1)) multiple-photon dissociation (IRMPD) spectra of [XFeC24H12](+) (X = C5H5 or Cp, C-5(CH3)(5) or Cp*) complexes in the gas phase obtained using the free electron laser for infrared experiments. The experimental results are complemented with theoretical infrared (IR) absorption spectra calculated with methods based on density functional theory. The isomers in which the XFe unit is coordinated to an outer ring of C24H12+ (Out isomers) were calculated to be the most stable ones. From the comparison between the experimental and calculated spectra, we could derive that, (i) for [CpFeC24H12](+) complexes, the (1)A Out isomer appears to be the best candidate to be formed in the experiment but the presence of the (1)A In higher energy isomer in minor abundance is also plausible; and (ii) for [CpFeC24H12](+) complexes, the three calculated Out isomers of similar energy are likely to be present simultaneously, in qualitative agreement with the observed dissociation patterns. This study also emphasizes the threshold effect in the IRMPD spectrum below which IR bands cannot be observed and evidence strong mode coupling effects in the [XFeC24H12](+) species. The effect of the coordination of Fe in weakening the bands of C24H12+ in the 1000-1600 cm(-1) region is confirmed, which is of interest to search for such complexes in interstellar environments.