Reactions of Gaseous Vinyl Halide Radical Cations with Ammonia. A Study of Mechanism by Fourier-Transform Ion Cyclotron Resonance Spectrometry and ab Initio Molecular Orbital Calculation

Abstract

The reactions of the radical cations of vinyl chloride (1) and vinyl bromide (2) with ammonia in the gas phase have been studied by FT-ICR spectrometry and ab initio molecular orbital calculations. The FT-ICR experiments show that vinyl halide radical cations react mainly by substitution of the halogen atom, yielding C2H6N+ product ions and moderately by formation of NH4+ with a total reaction efficiency of 57% (1•+) and 35% (2•+), respectively. The initial formation of NH3D+ from 1-d3•+ and 2-d3•+ establishes a direct deprotonation of the radical cations by NH3. Ab initio calculations confirm an exothermic deprotonation for all possible halovinyl radicals as the neutral product. The structure of the substitution product ions C2H6N+ was investigated by CA mass spectrometry and gas-phase titration experiments. The results show the presence of vinylammonium ions 5+ and acetaldiminium ions (6+) in the product ion mixture, but additional experiments reveal that 6+ is generated by a base catalyzed isomerization of initially formed 5+ (“shuttle mechanism”). The reaction energy profiles of the reactions of the vinyl halide radical cations 1•+ and 2•+ with ammonia were calculated by ab initio methods. The addition of NH3 is very exothermic for both radical cations generating chemically activated distonic ammonium ions in the first reaction step. The addition proceeds regioselectively (Markovnikov and anti-Markovnikov), yielding preferentially an unreactive distonic ion by the Markovnikov orientation. This intermediate has to rearrange by a 1,2-NH3 shift before eventually decomposing into vinylammonium ions 5+ by loss of the halogen atom. It is suggested that this rearrangement corresponds to the “bottle neck” of the substitution process. The activation energy of the 1,2-NH3 shift in the chloro- and bromo-substituted distonic ion is not significantly different, but the increased chemical activation of the chloro derivative results in an increased reaction efficiency for the total substitution process for the vinyl chloride radical cation. The ab initio calculations reveal further that the generation of the more stable substitution product acetaldiminium ion 6+ is inhibited by a large activation barrier for the hydrogen rearrangements necessary within the intermediate distonic ion.