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

The reactions of the radical cations of vinyl chloride (1) and vinyl b romide (2) with ammonia in the gas phase have been studied by FT-ICR s pectrometry and ab initio molecular orbital calculations. The FT-ICR e xperiments show that vinyl halide radical cations react mainly by subs titution of the halogen atom, yielding C2H6N+ product ions and moderat ely by formation of MH4+ with a total reaction efficiency of 57% (1(.)) and 35% (2(.+)), respectively. The initial formation of NH3D+ from 1-d(3)(.+) and 2-d(3)(.+) establishes a direct deprotonation of the ra dical cations by NH3. Ab initio calculations confirm an exothermic dep rotonation for all possible halovinyl radicals as the neutral product. The structure of the substitution product ions C2H6N+ was investigate d by CA mass spectrometry and gas-phase titration experiments. The res ults shaw the presence of vinylammonium ions 5(+) and acetaldiminium i ons (6(+)) in the product ion mixture, but additional experiments reve al that 6(+) is generated by a base catalyzed isomerization of initial ly formed 5(+) (''shuttle mechanism''). The reaction energy profiles o f the reactions of the vinyl halide radical cations 1(.+) and 2(.+) wi th ammonia were calculated by ab initio methods. The addition of NH3 i s very exothermic for both radical cations generating chemically activ ated distonic ammonium ions in the first reaction step. The addition p roceeds regioselectively (Markovnikov and anti-Markovnikov), yielding preferentially an unreactive distonic ion by the Markovnikov orientati on. This intermediate has to rearrange by a 1,2-NH3 shift before event ually decomposing into vinylammonium ions 5(+) by loss of the halogen atom. It is suggested that this rearrangement corresponds to the ''bot tle 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 nb 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 wit hin the intermediate distonic ion.