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Formation, Stability and Structure of Radical Anions of Chloroform, Tetrachloromethane and Fluorotrichloromethane in the Gas Phase

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Formation, Stability and Structure of Radical Anions of Chloroform, Tetrachloromethane and Fluorotrichloromethane in the Gas Phase UvA-DARE (Digital Academic Repository) Formation, stability and structure of radical anions of chloroform, tetrachloromethane and fluorotrichloromethane in the gas phase Staneke, P.O.; Groothuis, G.; Ingemann Jorgensen, S.; Nibbering, N.M.M. DOI 10.1016/0168-1176(94)04127-S Publication date 1995 Document Version Final published version Published in International Journal of Mass Spectrometry and Ion Processes Link to publication Citation for published version (APA): Staneke, P. O., Groothuis, G., Ingemann Jorgensen, S., & Nibbering, N. M. M. (1995). Formation, stability and structure of radical anions of chloroform, tetrachloromethane and fluorotrichloromethane in the gas phase. International Journal of Mass Spectrometry and Ion Processes, 142, 83. https://doi.org/10.1016/0168-1176(94)04127-S General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:29 Sep 2021 and Ion Processes ELSEVIER International Journal of Mass Spectrometry and Ion Processes 142 (1995) 83-93 Formation, stability and structure of radical anions of chloroform, tetrachloromethane and fluorotrichloromethane in the gas phase P.O. Staneke, G. Groothuis, S. Ingemann, N.M.M. Nibbering* Institute of Mass Spectrometry, University of Amsterdam, Nieuwe Aehtergracht 129, 1018 WS Amsterdam, The Netherlands Received 23 September 1994; accepted 8 December 1994 Abstract The gas-phase reactions of the CH2S'- radical anion with chloroform, tetrachloromethane, and fluorotrichloro- methane (CXC13; X = H, D, C1 and F) have been studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The primary reactions lead to minor amounts of the molecular radical anions of the halomethanes in addition to CHzSC1- and CI-, which are formed as the major product ions. The initial step is suggested to be electron transfer with formation of a [CHzS q- CXCI;-] ion/molecule complex, which may dissociate to the radical anion of the halomethane or react further to yield a [CH2S + C1- + CXC1½-] complex prior to the generation of the C1- or CH2SC1- ions. The radical anions of CHC13 and CC14 react with the parent compounds to yield CHC14 and CCI;- ions, respectively, whereas CFClj transfer a C1 ion only to the CHsSH molecules also present in the FT-ICR cell. On the basis of the facile occurrence of CI- transfer, the CXCIs- ions are proposed to exist in a structure which can be described as a chloride ion weakly bonded to a CXCI2 radical. The formation of the CXCIs- ions in the reaction with the CHzS'- radical anion places a lower limit of 45kJmo1-1 for the electron affinities of the CXC13 molecules. The occurrence of CI- transfer from the CHCI~ ion to the parent compound leads to an indicated upper limit of 75 kJmo1-1 for the electron affinity of chloroform. Similarly, the occurrence of C1- transfer from the CC1U ion to the parent compound is used to derive an upper limit of 110 kJ mol 1 for the electron affinity of tetrachloromethane. These upper limits suggest that previously reported values overestimate the electron affinities of chloroform and tetra- chloromethane. Keywords. Electron affinities; Electron transfer; Fourier transform ion cyclotron resonance; Halomethanes; Radical anions I. Introduction electron affinities [3-9] and the kinetics of elec- tron transfer reactions involving stable radical Electron transfer reactions involving radical anions or anions [3,10,11]. Only a limited num- anions and anions have been studied exten- ber of studies have been concerned with the sively in condensed phase systems [1,2]. For possible occurrence of electron transfer to gas-phase systems, the majority of the reported haloalkanes in the gas phase [12-18] notwith- studies is concerned with the determination of standing that special attention has been paid to the interplay between dissociative electron * Corresponding author. transfer and SN 2 substitution in reactions with 0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0168-1176(94)04127-X 84 P.O. Staneke et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 83 93 these compounds in the condensed phase of tetrachloromethane [25]. Furthermore, [1,19]. Furthermore, the reactions of a CF3CI'- ions have been reported to be potential electron donor such as the super formed by electron ionization of chloro- oxide ion, O~-, with halomethanes contain- trifluoromethane clusters [26] and also by ing more than a single halogen atom (e.g. electron impact induced decarbonylation of freons) are known to play a role in the nega- CF3COC1 [27]. tive ion chemistry occurring in the upper The rare observations of stable molecular atmosphere [20,21 ]. radical anions of simple halomethanes appear The reported studies of gas-phase reactions to be in conflict with the large and positive with halomethanes indicate that radical anions adiabatic electron affinities which have been and anions react preferentially with these suggested for species such as chloroform, molecules by dissociative electron transfer tetrachloromethane and fluorotrichloro- (Eq. (1)) or nucleophilic processes, whereas methane on the basis of results obtained by resonance electron transfer with formation of experiments [22,28] and high level calcula- a stable molecular radical anion (Eq. (2)) is an tions [29,30]. These apparently inconsistent uncommon process: findings raise questions as to the true adia- batic electron affinities of these molecules as A'- + RX ---, [A+R" +X-]* --+ A + R'+X - well as to the intrinsic stability of the molecu- (1) lar radical anions with respect to dissociation into a halide ion and a halogen substituted A'- + RX + [A + RX'-]* + A + RX'- methyl radical. In an attempt to answer these (2) questions, we examined the gas phase reactions of various (radical) anions with In a few instances stable radical anions selected halomethanes and observed that the of halomethanes have been observed. For thioformaldehyde radical anion reacts example, minor amounts of the radical anions uniquely with CHC13, CDC13, CC14 and of CF2C12 and CFC13 are reported to arise by CFC13 to afford minor amounts of stable electron transfer from the O~- ion [12,17] and molecular radical anions. This allowed us to in a recent study, we observed that CBr4- ions derive upper limits for the adiabatic electron are formed by electron transfer from the affinities of these molecules and experimentally radical anion of azobenzene to tetrabromo- study the reactivity of the related radical methane [14]. Low but measurable yields of anions. the tetrachloromethane radical anion have been formed also in the collisions of CC14 molecules with neutral potassium atoms with 2. Experimental a controlled kinetic energy [22]. Likewise, attachment of electrons to free halomethane The experiments were performed with a molecules in the gas phase is referred to nor- Fourier transform ion cyclotron resonance mally as an inherent dissociative process (FT-ICR) [31] instrument designed and con- leading mostly to the predominant or exclu- structed at the University of Amsterdam sive formation of a halide ion [23,24]. Low [32-34]. In a typical experiment, the primary yields of CCI~- ions have been generated, negative ions were formed by a pulsed electron however, by attachment of electrons with an beam (duration 150 ms) and trapped in a mag- energy close to zero electronvolts to carbon netic field of 1.23 T by applying a small nega- dioxide clusters containing small amounts tive voltage (~ -1 V) to the trapping plates of P.O. Staneke et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 83-93 85 the cubic inch FT-ICR cell. The CH2S'- and 1.0 ortho-benzyne (C6H~-) radical anions were formed by reacting O'- with methanethiol 0.8 ~ a [35] and benzene [36,37] respectively. The O-" ions were formed by dissociative capture ~1~ o.6, of electrons with an energy of ~ 1.2eV by nitrous oxide [38]. The ions CH3NO 2- and C6F 6- were formed by electron transfer from 0,4 NO-, which was generated in a secondary ci- CHCI4 reaction between O'- and N20 [39,40]. The 0.2 reactant ions of interest were isolated by eject- - - -- ~. ;. a. ing all other ions from the cell as described 0.0 elsewhere [41,42]. The reactions of the various 0.0 0.2 0.4 0.6 0.8 ions with the halogenated methanes were Reaction time in s studied subsequently as a function of time by varying the delay between the selection of the o.os "] I CHCl,'/ b ions of interest and the start of the excitation pulse [31]. The relative abundances of the iso- topic chloride containing product ions were _~ measured with an accuracy of ~< 5% by follow- ing procedures described elsewhere [33,41,43]. 0.03 The total pressure was around 8 x 10 -5 Pa with a background pressure better than _~ 3 x 10 -TPa. The ratio between the partial pressure of N20 , the halomethane, and the 0.01 neutral precursor of the reactant radical anion was typically 1 : 1 : 1. The pressures O.OC 0.0 0.2 0.4 0.6 0.8 were measured with an uncalibrated ioniza- Reaction time in s tion gauge placed in the side arm of the main pumping line.
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