Unimolecular dissociation of doubly ionized toluene and electron transfer between neutral toluene and its dication Christopher Shaffer a , Detlef Schröder a,⇑ , Emilie-Laure Zins b , Christian Alcaraz c,d , Jan Z ˇ abka e,f , Jana Roithová e a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10 Prague, Czech Republic b Laboratoire de Dynamique, Interactions et Réactivité, UMR 7075 CNRS/UPMC, Université Pierre et Marie Curie, 4 Place Jussieu 75252 Paris, France c Laboratoire de Chimie Physique, Bât. 350, UMR 8000, CNRS-Univ. Paris-Sud 11, Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France d Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48, 91192 Gif-sur-Yvette, France e Department of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague 2, Czech Republic f J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic article info Article history: Received 9 January 2012 In final form 2 March 2012 Available online 10 March 2012 abstract Dissociative double ionization of toluene using synchrotron radiation in conjunction with theoretical studies leads to a determination of the thermochemistry of the dication c-C 7 H 2þ 6 . Further, photon-energy dependent reactivity studies reveal that the asymmetric energy deposition upon single-electron transfer in the C 7 H 2þ 8 =C 7 D 8 collision system is not influenced by the internal energy content of the dication pre- cursors. This finding agrees with the previous assignment of the asymmetric energy deposition as a con- sequence of the electronic states accessible upon recombination of a dication with an electron compared to the removal of an electron from a neutral compound, respectively. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Ever since Pauling’s prediction of the He 2þ 2 dication [1], the chemistry and physics of multiply-charged molecular ions have at- tracted the attention of basic research [2–5]; for a recent special is- sue of PCCP on multiply charged ions, see Ref. [6]. However, the chemistry of multiply charged ions often is dominated by electron transfer processes and still in 2003 had been stated that ‘very few studies exist for real chemical reactions involving multiply charged ions’ [7]. Recently, however, a series of bond-forming reactions of molecular dications [8,9] have been discovered in which the dipos- itive charge of the precursor ions is maintained in the products, opening a new kind of chemistry of gaseous dications [10,11]. The majority of these recently described reactions involve med- ium-sized hydrocarbon dications [12–16] with a particular focus on the C 7 H 2þ n dications (n = 6–8) derived from (dissociative) double ionization of toluene [17–20] or the isomeric cycloheptatriene [21]. In addition to the formation of new chemical bonds in the dicationic state, an interesting asymmetry in energy partitioning was also observed in that in degenerate reactions of a dication M 2+ with its neutral counterpart M (labeled in bold for clarity) leads to an internally ‘hot’ monocation M +/ from the dication and an internally ‘cold’ monocation M + from the neutral [12]; throughout the text, ‘hot’ may stand for rovibrational and/or elec- tronic excitation. Extensive work devoted to this particular prob- lem in smaller model systems has revealed that the differences in the amount of internal energy deposited in the monocations M +/ and M + depend on the electronic states accessible upon elec- tron transfer from the dication or the neutral compound to the monocation state [22–27]. These studies included detailed analysis of the kinetic energy dependences of the product ions, kinetic en- ergy releases and isotope distributions as well as complementary theoretical calculations. What has not been addressed so far, how- ever, is the role of the internal energy content of the incident dications. As a matter of fact, in most of the experiments referred to above, the precursor dications were generated by means of elec- tron ionization, which is a very efficient, but ‘hard’ ionization method which might lead to a substantial deposition of internal energy in the parent ions, also involving contributions from elec- tronically excited states [28,29]. Here, we address the particular case of the toluene dication [30] in more detail with regard to both the energetics of the loss of H 2 from C 7 H 2þ 8 to afford the C 7 H 2þ 6 dication [31] and the reaction of C 7 H 2þ 8 with neutral, perdeuterated toluene C 7 D 8 . Specifically, we apply a technique to which we refer as reactive monitoring with synchrotron radiation [32,33] in order to probe the role of internal energy in the single-electron-transfer processes of the C 7 H 2þ 8 =C 7 D 8 collision system. In a reactive monitoring experiment, the bimolec- ular reactivity of a mass-selected gaseous ion generated via photo- ionization is probed as a function of the energy of the ionizing 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.03.014 ⇑ Corresponding author. Fax: +420 220 183 462. E-mail addresses: schroeder@uochb.cas.cz, detlef.schroeder@uochb.cas.cz (D. Schröder). Chemical Physics Letters 534 (2012) 8–12 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett