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 chloroethene 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 reaction of A^•+ with 1,2- and 1,1-dichloroethenes 3 and 5. In every case good pseudo-1^st order kinetics were observed for fast reactions with total efficiencies between 21% and 42%. Reaction occurs by substitution of one halogen substituent of the halogenated ethene and by formal hydride abstraction yielding the ion H₂C=NH₂⁺ and a halogenated ethyl radial as products. This latter process is a minor reaction channel of the mono- and symmetrically dihalogenated ethenes 1–4 but 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 of 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 calculated to be exothermic by 77 kJ mol⁻¹ and by 35 or 47 kJ mol⁻¹, respectively, depending on the structure of the chloroethyl radical formed. Both reaction pathways start by a very exothermic addition of the methylamine radical cation A^•+ to the double bond of 1. This gives rise to an excited β-distonic N-methyl ammonium ion which eventually eliminates the chlorine 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 addition 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 increased abundance of the formal H abstraction channel in the case of the 1,1-dihaloethenes 5 and 6 is obviously caused by the increased stability of the 1,1-dihaloethyl radical which may be formed by this process.