Reactivity of the CHBr 2+ Dication toward Molecular Hydrogen Jana Roithova ´ ,* ,†,‡ Ja ´ nZ ˇ abka, ‡ Zdenek Herman, ‡ Roland Thissen, § Detlef Schro 1 der, †,| and Helmut Schwarz | Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, FlemingoVo n. 2, 166 10 Prague, Czech Republic, V. C ˇ erma ´ k Laboratory, J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkoVa 3, 182 23 Prague, Czech Republic, Laboratoire de Chimie Physique, Ba ˆ t. 350, UMR 8000, Centre UniVersitaire Paris-Sud, 91405 Orsay Cedex, France, and Institut fu ¨r Chemie der Technischen UniVersita ¨t Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany ReceiVed: February 20, 2006; In Final Form: March 23, 2006 Structural aspects as well as the stability and reactivity of the CHBr 2+ dication are studied both experimentally and theoretically. Translational energy distributions of the CHBr + products from charge transfer between CHBr 2+ and Kr indicate that the dication exists in two isomeric forms, H-C-Br 2+ and C-Br-H 2+ . In the reaction of CHBr 2+ with H 2 , the dominant channel corresponds to proton transfer leading to CBr + + H 3 + . Other reaction channels involve the formation of the intermediates CH 3 Br 2+ and CH 2 BrH 2+ , respectively. Both of the latter dications can either lose a proton to form CH 2 Br + or undergo a spin-isomerization followed by cleavage of the C-Br bond. The proposed mechanisms are supported by DFT calculations and deuterium labeling experiments. Introduction Small molecular dications often represent models for reactive intermediates under extreme conditions. The research concerning dications addresses chemical processes relevant to interstellar space, 1 planetary atmospheres, 2 etching technology, 3 or reactions in superacidic media, 4 just to mention a few examples. The accumulation of charge very often leads to thermodynamic instabilities of the molecular dications; i.e., there exists at least one exothermic fragmentation pathway, which yields two singly charged products. 5,6 Nevertheless, over the past decade an increasing number of small, thermodynamically stable dications has been reported. 6-8 A fundamental process occurring upon the encounter of a dication and a neutral reaction partner corresponds to electron transfer (reaction 1). This process leads to a pair of singly charged ions and is associated with a considerable translational energy release, often referred to as Coulomb explosion. 6 Electron transfer proceeds effectively at internuclear distances larger than those typically required for chemical reactions and therefore is kinetically favored. 9 The simplest chemical reaction corresponds to proton transfer from a hydrogen-containing dication to a neutral reaction partner (reaction 2, B ) H). Proton transfer, in comparison with electron transfer, often leads to thermochemi- cally more favored products. Nevertheless, proton transfer can only compete with the kinetically preferred electron transfer, if the exothermicity of electron transfer is lower than 2 eV 9-11 or if the neutral partner has a significantly large permanent dipole moment. 12 Here, we report an investigation of the reaction of CHBr 2+ with molecular hydrogen. For the closely related CHCl 2+ dication, a series of studies 12-16 has demonstrated that the CHCl 2+ dication is formed as two isomers, HCCl 2+ and CClH 2+ , upon electron ionization of methyl chloride. The reaction of CHCl 2+ with H 2 is dominated by proton transfer to form CCl + and H 3 + with a minor channel leading to the products CH 2 Cl + and H + . In addition to these, a novel process is presented here for CHBr 2+ /H 2 which does not occur in the related system CHCl 2+ /H 2 . Another difference between the dications involved here evolves from the fact that both isomers of the CHCl 2+ dication are metastable, whereas the HCBr 2+ dication belongs to a family of thermodynamically stable dications. 12 On the other hand, the role of excited states of CHBr 2+ might be enhanced due to a larger spin-orbit interaction associated with bromine compared to the lighter congener chlorine. The comparison of both systems thus affords new insights into the relationship between the stability and the reactivity of dicationic species. Experimental Section The electron-transfer experiments were performed using the crossed beam apparatus EVA II. 9 The CHBr 2+ dications were formed by dissociative electron ionization (130 eV) of methyl bromide. 17 Ions were extracted, mass-selected, and decelerated to a specific laboratory energy. The CHBr 2+ beam was then crossed at 90° with a beam of Kr emerging from a multichannel jet. The angular and energy spread of the CHBr 2+ beam was 1° and 0.5 eV (full width at half-maximum, fwhm), respectively. The angular spread of the neutral beam was about 6° (fwhm), and the energy distribution corresponded to the thermal distribu- tion at 300 K. Reactant and product ions were further analyzed behind a detection slit (2.5 cm from the scattering center). First, * Corresponding author. E-mail: jana.roithova@uochb.cas.cz. † Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic. ‡ V. C ˇ erma ´k Laboratory, J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic. § Laboratoire de Chimie Physique, Ba ˆt. 350, UMR 8000, Centre Universitaire Paris-Sud. | Institut fu ¨r Chemie der Technischen Universita ¨t Berlin. AB 2+ + C f [ABC] 2+ f AB + + C + (1) AB 2+ + C f [ABC] 2+ f A + + BC + (2) 6447 J. Phys. Chem. A 2006, 110, 6447-6453 10.1021/jp0610719 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006