International Journal of Mass Spectrometry 354–355 (2013) 378–390 Contents lists available at ScienceDirect International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms Fragmentation reactions of Si 2 Cl 6 +• in the gas phase—A quantum-chemical and mass-spectrometric assessment  Elie M.L. Fink a , Alexander Schießer b , Robert Berger b,c,∗∗ , Max C. Holthausen a,∗ a Institute of Inorganic Chemistry, Goethe Universität, Max-von-Laue-Straße 7, 60438 Frankfurt am Main, Germany b Clemens-Schöpf Institute, Technische Universität Darmstadt, Petersenstraße 22, 64287 Darmstadt, Germany c Frankfurt Institute for Advanced Studies, Goethe University, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main, Germany article info Article history: Received 5 June 2013 Received in revised form 17 July 2013 Accepted 5 August 2013 Available online 19 August 2013 Keywords: Hexachlorodisilane Reaction pathways Fragmentation channels Ab initio calculations Mass spectrometry abstract The gas-phase ion chemistry of the hexachlorodisilane radical cation Si 2 Cl 6 +• has been reinvestigated by a combined electron ionization (EI) mass spectrometry (MS) and high-level quantum chemistry approach. The computational investigation of isomerization and fragmentation pathways of [Si 2 Cl n ] +( •) (n = 1–6) species reveals (a) the coexistence of structures of the type Cl 3 Si–SiCl m+1 +( •) and chlorobridged SiCl 4 –SiCl m +( •) isomers for m = 1,2, and (b) a wealth of nearly isoenthalpic structures[Si 2 Cl n ] +( •) for each n with n = 2–4, which can result from degenerate rearrangements. This structural diversity appears to be a general feature of the disilicon (radical) cations with low chlorine content. The ionization energy of Si 2 Cl 6 determined by EI-MS (10.1 ± 0.2 stat ± 0.2 syst eV) with argon (15.759 ± 0.001 eV) used as reference, is slightly lower than previously reported experimental values, but in good agreement with the CBS-QB3 (9.87 eV) and W1U (9.97 eV) adiabatic ionization energy. Measured appearance energies for the lowest two fragmentation channels, which afford SiCl 2 +• and SiCl 3 + , are well in line with CBS-QB3 values and with the literature value determined for SiCl 3 + in threshold photoelectron–photoion coincidence experi- ments. Using the experimental heats of formation ( f H ◦ 298 ) for SiCl 4 and Cl · as anchor points, W1U theory was employed to derive  f H ◦ 298 values for Si 2 Cl n +( •) with n = 0–6 and for SiCl m +( •) with m = 1–4 based on the lowest energy isomer identified in our quantum chemical investigation of reaction pathways for all species investigated. W1U heats of formations obtained for select neutrals lead to a more consistent description of kinetic aspects previously inferred from metastable ion mass spectra. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The gas-phase chemistry of chlorinated silanes, their radicals and ions, is of fundamental interest for silicon-based industries where techniques, such as plasma-enhanced chemical-vapor depo- sition [1–3] or chlorine-based surface etching of silicon [4–10] are in customary use for the fabrication of semiconductor and light-emitting devices, optical fibers, or photovoltaic solar cells, to name a few. While SiCl 4 , a major side-product on a multi-kiloton scale of state-of-the-art silicon production facilities [1], is used in most of these processes, the higher perchlorosilanes have found  This work is dedicated to the memory of Detlef Schröder, a colleague and friend, who has greatly impressed and inspired us by his scientific enthusiasm, sharp and critical analyses and winking humor – demonstrated at its best in the marvelous ‘late-night ion show’ on phonomer research at Blankensee castle in 2003. ∗ Corresponding author. Tel.: +49 69 798 29430; fax: +49 69 798 29260. ∗∗ Corresponding author at: Clemens-Schöpf Institute, Technische Universität Darmstadt Petersenstraße 22, 64287 Darmstadt, Germany. E-mail addresses: Robert.Berger@tu-darmstadt.de (R. Berger), Max.Holthausen@chemie.uni-frankfurt.de (M.C. Holthausen). implementation only in very specific fields of application. Although a jungle of patents, put forward in past decades, relating to the technical use of hexachlorodisilane (Si 2 Cl 6 ) indicates broad per- spectives [11–20], comparatively little chemical insights can be gained from the information disclosed. A substantial amount of work has been invested in the characterization of silicon films produced by chemical vapor deposition (CVD) of, e.g., Si 2 Cl 6 and SiCl 4 . Yet, only a scarce body of recent literature on efforts to char- acterize relevant precursors, reactive intermediates, or associated deposition-mechanisms exists [21–31]. To the best of our knowl- edge, the chemistry of perchloropolysilanes has not systematically been evaluated in recent years, and no modern unifying view has been framed, such that the interested chemist is left with little more than old reports [32–33] and rather general text-book knowledge [34]. Our own interest in this field has been spurred by an indus- try cooperation with the CitySolar AG, Bad Kreuznach (Germany), which successfully established a radio-frequency enhanced non- thermal plasma process to produce perchlorinated polysilanes Cl–(SiCl 2 ) x –Cl on a multi-ton per year scale, using SiCl 4 and H 2 as feedstock [35]. These polysilanes represent a quantitative source of 1387-3806/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2013.08.002