IR spectroscopy of gaseous fluorocarbon ions: The perfluoroethyl anion Maria Elisa Crestoni a , Barbara Chiavarino a , Joel Lemaire b , Philippe Maitre b , Simonetta Fornarini a,⇑ a Dipartimento di Chimica e Tecnologie del Farmaco, Università di Roma ‘‘La Sapienza’’, P. le A. Moro 5, I-00185 Roma, Italy b Université Paris Sud, Laboratoire de Chimie Physique – UMR8000 CNRS, Faculté des Sciences – Bâtiment 350, 91405 Orsay Cedex, France article info Article history: Available online 12 March 2011 Keywords: IRMPD spectroscopy FT–ICR mass spectrometry Negative hyperconjugation Gas phase ion chemistry abstract The first IR spectrum of a perfluorinated carbanion has been obtained in the gas phase by IRMPD spec- troscopy. Quantum chemical calculations at the MP2/cc-pVTZ level were performed yielding the opti- mized geometries and IR spectra for a covalently bound C 2 F 5  species and for conceivable loosely bound F  (C 2 F 4 ) complexes. Both the computational results and the IR characterization point to a covalent structure for the assayed species in agreement with the reactivity pattern displayed with selected neutrals. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Fluorocarbon plasmas are widely used for the etching of silicon and silicon compounds in semiconductor device manufacturing industry. A notable amount of research has addressed the study of plasma conditions and components that contribute to the competing processes of deposition and etching in fluorocarbon plasmas, ultimately directed to optimize the features of microelec- tronic circuits [1]. Both gas and plasma-surface interactions affect the SiO 2 etch process and the mechanisms of selective SiO 2 to Si and Si 3 N 4 etching have been extensively studied [2]. In addition, fully dense fluorocarbon films deposited from plasmas have been considered for use as dielectric interlayers that can have dielectric constants of less than 2.0 [3]. A rich chemistry appears to be in- volved in the overall film growth and etching processes. Both charged and radical species are active in these etching and deposi- tion processes [4] and mass spectrometric methods have identified the presence of various fluorocarbon ions [5]. In the present study, that we wish to dedicate to Mario Capitelli in recognition of his paramount contribution to plasma science, the focus is on perfluoroethyl anion, C 2 F 5  , a prototypical perfluori- nated anion, which is characterized by its reactivity and IR spectro- scopic features. The study moves in particular from two specific questions. The first one arises from a very recent investigation of the gas phase ion–molecule reactions in C 2 F 4 where C 2 F 5  ions formed by the addition of F  to C 2 F 4 display a peculiar behaviour suggesting the existence of distinct isomeric species [6]. The Van’t Hoff plots for the clustering reaction F  +C 2 F 4 ? F  (C 2 F 4 ) show an increase of the equilibrium constant above room temperature, followed by a decrease at higher temperatures, which has been interpreted as due to the existence of both a loosely bound and a tightly bound complex. The second question attached to C 2 F 5  regards the operation of negative hyperconjugation which may affect its structure and bonding features. For some time the importance of negative hyper- conjugation has been under discussion. First introduced in 1950 by Roberts to explain certain electronic influences of the CF 3 group [7], it is now a well accepted concept particularly in cases where fluorine is involved in bonding [8]. Landmark features ascribed to this effect are the bond length and bond angle situation in CF 3 O  . Negative hyperconjugation is also held responsible for the compar- atively high acidity of CF 3 OH with respect to that of CF 3 SH [9]. Other related examples are the structures of OC 2 F 5  , OCF(CF 3 ) 2  , and OCFCF 2 O  anions investigated by single crystal X-ray cristal- lography, all indicative of a negative hyperconjugation effect [8]. Negative hyperconjugation has been shown to be a general phe- nomenon dictating conformational stabilities and controlling chemical reactivity. In negative hyperconjugation a p electron pair (a lone pair in the present case) interacts with an acceptor r / orbi- tal and their interaction is maximized when an antiperiplanar ori- entation is adopted and a small gap exists between the interacting orbitals [10,11]. The direction of ‘‘electron flow’’ is of p ? r / type, such as the p C ! r  CF interaction in FCH 2 CH 2  . It has been often la- beled ‘‘anionic hyperconjugation’’ but it is not at all restricted to anions and has been more generally termed ‘‘negative hyperconju- gation’’ [12]. In this way p-bond character is built into bonds that nominally have only r character at the expense of weakening adjoining C–F r bonds through the population of the correspond- ing r / orbital. Ab initio calculations including correlation corrections have been performed on three fluorine-substituted ethyl carbanions, namely CF 2 HCH 2  , CF 3 CH 2  , and CF 3 CHF  [13]. Three criteria were considered indicative of negative hyperconjugation based on (i) the 0301-0104/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2011.03.005 ⇑ Corresponding author. Tel.: +39 (0) 6 49913510. E-mail address: simonetta.fornarini@uniroma1.it (S. Fornarini). Chemical Physics 398 (2012) 118–123 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys