Structural characterization of selectively prepared cationic iron complexes bearing monodentate and bidentate ether ligands using infrared photodissociation spectroscopy S. Le Ca€ er, M. Heninger, J. Lemaire, P. Boissel, P. Ma ^ ıtre, H. Mestdagh * Laboratoire de Chimie Physique (UMR 8000, associated to CNRS), Universit e Paris-Sud, B^ at. 350, 91405 Orsay Cedex, France Received 14 November 2003; in final form 18 December 2003 Published online: 20 January 2004 Abstract The infrared spectroscopy of gaseous ions Fe(CH 3 OCH 3 ) þ 2 and Fe(CH 3 OCH 2 CH 2 OCH 3 ) þ n (n ¼ 1–2) has been studied in the 800–2000 cm 1 energy range using the coupling of the free electron laser CLIO and an FTICR mass spectrometer, and compared to spectra calculated by ab initio quantum chemistry. The match between experimental and theoretical infrared spectra appears to be very good, the experimental spectra corresponding to the most stable structure predicted by the calculations. Characteristic ab- sorption bands of functional groups have been evidenced, allowing this coupling to be a very powerful tool for structure elucidation in the gas phase. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Infrared spectroscopy is a powerful tool to charac- terize the structure of chemical species. This is particu- larly true in the gas phase, since the experimental infrared spectra can be directly compared with those calculated by ab initio quantum chemistry. However, in the case of ionic species, the low density available in the gas phase prevents usual absorption measurements. In recent years much effort has been put into the develop- ment of experimental methods to study the infrared spectroscopy of gas phase ions, most of them being based on ion photodissociation coupled with mass spectrometric detection. Dissociation induced by light absorption requires either the presence of a low energy dissociation pathway which can be reached upon ab- sorption of one IR photon, or a high irradiation power so that absorption of several photons may occur and induce fragmentation. The former way has been used to get structural information upon complexes presenting weak electrostatic interactions [1]. The scope of this method can be broadened by using the Ômessenger atomÕ technique [2–5], the complex of interest being bound to a rare gas atom which is ejected upon absorption of one photon in the mid-IR. The latter way, known as infrared multiphoton dissociation (IRMPD), requires a high fluence, initially available only with CO 2 lasers. These lasers provide knowledge of the photofragmentation processes [6–8], but their narrow wavelength range prevents spectroscopic studies. In the recent years, the application of free electron lasers (FEL) which provide high peak power and wide tunability has enabled to record infrared spectra in a large energy range [9–13]. Under these conditions, infrared absorption can be probed by monitoring IRMPD by mass spectrometry. In this respect, Fourier transform ion cyclotron reso- nance (FT-ICR) mass spectrometers are ideal tools, al- lowing to selectively prepare ions and to distinguish fragmentation pathways yielding ions of close masses such as H or H 2 loss, because of their high resolution. Coupling of an FT-ICR mass spectrometer with an in- frared FEL, recently performed in our group [14–16], opened the way to the direct characterization of selec- tively prepared ions, including reactive intermediates [17]. * Corresponding author. Fax: +33-16-9153053. E-mail address: helene.mestdagh@lcp.u-psud.fr (H. Mestdagh). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.080 Chemical Physics Letters 385 (2004) 273–279 www.elsevier.com/locate/cplett