30 - J. Phys. Chem. A 2008, 112, 85518560 8551 Infrared...

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Infrared Spectroscopy of [XFeC 24 H 12 ] + (X ) C 5 H 5 ,C 5 (CH 3 ) 5 ) Complexes in the Gas Phase: Experimental and Computational Studies of Astrophysical Interest Aude Simon* and Christine Joblin Centre d’E ´ tude Spatiale des Rayonnements, Uni V ersite ´ Toulouse III s CNRS, Obser V atoire Midi-Pyre ´ne ´es, 9A V enue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France Nick Polfer and Jos Oomens FOM Institute for Plasma Physics “Rijnhuizen”, Edisonbaan 14, NL-3439MN Nieuwegein, The Netherlands Recei V ed: February 20, 2008; Re V ised Manuscript Recei V ed: June 17, 2008 We report the frst experimental mid-inFrared (700 - 1600 cm - 1 ) multiple-photon dissociation (IRMPD) spectra oF [X±eC 24 H 12 ] + (X ) C 5 H 5 or Cp, C 5 (CH 3 ) 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 X±e unit is coordinated to an outer ring oF C 24 H 12 + (Out isomers) were calculated to be the most stable ones. ±rom the comparison between the experimental and calculated spectra, we could derive that, (i) For [Cp±eC 24 H 12 ] + 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 [Cp*±eC 24 H 12 ] + 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 [X±eC 24 H 12 ] + species. The eFFect oF the coordination oF ±e in weakening the bands oF C 24 H 12 + in the 1000 - 1600 cm - 1 region is confrmed, which is oF interest to search For such complexes in interstellar environments. I. Introduction Le ´ger and Puget 1 and Allamandola et al. 2 suggested in the early 1980s that polycyclic aromatic hydrocarbons (PAHs) and PAH-related molecules are the major carriers oF the discrete emission inFrared (IR) spectrum that is observed ubiquitously in the interstellar medium (ISM) in the 3 - 13 µ m (770 - 3100 cm - 1 ) range. Since then, many studies, laboratory experiments 3 - 14 and theoretical calculations, 15 - 25 have been dedicated to the identifcation oF the carriers oF these Features, the so-called aromatic inFrared bands (AIBs) at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µ m (3030, 1613, 1299, 1663, 885, and 787 cm - 1 ). There is now compelling evidence that these Features are due to a complex mixture oF ionized and neutral, possibly substituted or complexed, PAHs oF large sizes, 26 - 36 but no satisFactory match to the observed spectra has been obtained yet.
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30 - J. Phys. Chem. A 2008, 112, 85518560 8551 Infrared...

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