PUB - Publikationen an der Universität Bielefeld

The ion/molecule reactions of the phenylarsandiyl radical cation C6H5As center dot+, 1(.+), with methyl-halides CH3X, X = Cl, Br, I, have been investigated using Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The radical cation 1(.+) exhibits in its valence electron shell a vacant molecular orbital as well as a single occupied orbital and an orbital containing an electron pair. Consequently, reactivity of an electrophilic carbenoid and/or an electrophilic radical is expected. No reaction is observed with CH3Cl, while CH3Br undergoes a slow reaction (reaction efficiency 1.2%) and CH3I a fast reaction (reaction efficiency 21.6%) with 1(.+). In both reaction systems, the main product ion C6H5As+X is produced by radical substitution with transfer of the halogen atom X to 1(.+). The methylphenylarsenium ion C6H5As+CH3 is found as a second product ion in both systems but with distinctly less intensity. Unexpectedly, an ion C7H7+ is observed as a third product only of the reaction system 1(.+)/CH3Br. Information about reaction enthalpies was obtained by theoretical studies which show that the reaction with CH3Cl would be only slightly exothermic, if at all. Further, generation of the less abundant product C6H5As+CH3 is the more exothermic pathway for CH3Br and CH3I. This proves that reaction mechanisms and not reaction exothermicity determines the total rate constant and the branching ratio of the ion/molecule reactions observed. It is suggested that the mechanism for generation of G(6)H(5)As(+)X is electrophilically assisted radical substitution via formation of an intermediate adduct of electrophilic 1(.+) to an electron lone pair of the halogen of CH3X. In contrast, C(6)H5As(+)CH(3) is generated via insertion of the carbenoid 1(.+) into the C-X bond which yields a halogenomethylphenylarsane radical cation C6H5As(X)CH3.+ as intermediate. According to the calculations, this insertion is strongly exothermic but requires obviously a substantial activation energy. The investigation of the fragmentations of C6H5As(X)CH3 by tandem mass spectrometry corroborates these suggestions. Notably, only C6H5As(Br)CH3 center dot+, but not C6H5As(I)CH3 center dot+, generates abundant C7H7+ ions on fragmentation. (C) 2005 Elsevier B.V. All rights reserved.