PUB - Publikationen an der Universität Bielefeld

The gas-phase reactions of the three isomeric dichloroethene radical cations 1 + -3 + with NH 3 and of the neutral species 1-3 and the isomeric 1,2-dibromoethenes 4 and 5 with CH 3-NH 2 +. and (CH 3) 2NH + , respectively, are studied by Fourier-Transform Ion-Cyclotron-Resonance (FT-ICR) mass spectrometry. A chloro substituent in 1 + -3 + is replaced by NH 3 in a fast reaction, yielding C 2H 5NCl + ions. These substitution reactions are very fast and occur nearly at the collision limit. The 1,1-dichloroethene radical cations 3 + exchange successively both Cl substituents, yielding C 2H 7N 2 + ions and eventually cluster ions N 2H 7 +. Substitution takes place also by the reaction of neutral haloethenes 1-5 with amine radical cations, although at very different rates. The efficiencies of the reactions of CH 3HN 2 + with 1-5 are quite large, but those of the reactions of (CH 3) 2NH + with 1 and 2 are < 1% and reach only 10% even in the case of the bromoethenes 4 and 5. 1,1-Dichlorethene (3) does not react at all. An addition-elimination mechanism is proposed for all these reactions, giving rise to the formation of intermediate -distonic ammonium ions. This reaction path and the stability of the intermediates are supported by MNDO calculations. However, the intermediate -distonic ions have to isomerize further by hydrogen shifts before the product ions are eventually formed. MNDO calculations support the assumption that the slow reactions of (CH 3) 2NH + with the haloethenes are due to slow rearrangements within the initially formed distonic ions, which have to precede the loss of Cl.