Reactions of gaseous mono- and dihaloethenes with methylamine radical cation: a study of mechanism by Fourier transform ion cyclotron resonance spectrometry and ab initio molecular orbital calculation

The gas-phase reactions of the methylamine radical cations A(.+) with chlor oethene 1, bromoethene 2, 1,2-dibromoethene 4 and 1,1-dibromoethene 6 have been studied by FT-ICR spectrometry to complete previous studies of the rea ction of A(.+) with 1,2- and 1,1-dichloroethenes 3 and 5. In every case goo d pseudo-1(st) order kinetics were observed for fast reactions with total e fficiencies between 21% and 42%. Reaction occurs by substitution of one hal ogen substituent of the halogenated ethene and by formal hydride abstractio n yielding the ion H2C=NH2+ and a halogenated ethyl radial as products. Thi s latter process is a minor reaction channel of the mono- and symmetrically dihalogenated ethenes 1-4 hut produces the main product of the reaction of asymmetrically substituted 1,1-dihaloethenes with A(.+). In every case the reaction efficiency of the chloro-substituted ethene is superior to that o f the corresponding bromo-substituted one. High-level ab initio calculation was used to establish the minimum reaction energy path of the reaction of the methylamine radical cation A(.+) and chloroethene 1 along both reaction channels. The substitution reaction and the formal H abstraction are calcu lated to be exothermic by 77 kJ mol(-1) and by 35 or 47 kJ mol(-1), respect ively, depending on the structure of the chloroethyl radical formed. Both r eaction pathways start by a very exothermic addition of the methylamine rad ical cation A(.+) to the double bond of 1. This gives rise to an excited be ta-distonic N-methyl ammonium icon which eventually eliminates time chlorin e or decomposes by a 1,4-H shift and elimination of a chloroethyl radical. Using this reaction model, the reduced reactivity of the brominated ethenes compared to the chloro derivatives is attributed to a less exothermic addi tion step. Although the structure and stability of the intermediate adducts depend on the substitution pattern of the halogenated ethene, this has no clear effect on the efficiency of the total reaction. However, the increase d abundance of the formal H abstraction channel in the case of the 1,1-diha loethenes 5 and 6 is obviously caused by the increased stability of the 1,1 -dihaloethyl radical which may be formed by this process.