Mechanism of CO₂ Elimination from Ionized Methyl Haloacetates in the Gas Phase. Formation of CH₃XCH₂⁺ and CH₃XCHX⁺. (X = Cl, Br) Halonium Radical Ions

The molecular ions of the methyl esters of chloro^-, bromo^-, dichloro^-, and dibromoacetic acids undergo unimolecular elimination of CO₂ in the gas phase. Collisional activation (CA) mass spectrometry suggests that the resulting [M - CO₂]⁺-ions possess novel types of hypervalent structures, i.e., CH₃XCH₂^+- and CH₃XCHX^+- (X = Cl, Br). MNDO and ab initio calculations show that the methylmethylenechloronium radical ion CH₃C1CH₂^+- and the isomeric chloroethane cation radical CH₃CH₂C1^+- exist in a potential mínimum. MNDO predicts that CH₃C₁CH₂^+- is more stable than CH₃CH₂C₁^+- by 10.1 kcal mol^-1, whereas according to 6-31G* and UMP2/6-31G* CH₃CH₂C₁^+- is more stable by 9.1 and 6.4 kcal mol^-1, respectively. Several possible mechanisms for the dissociative rearrangement ClCH₂COOCH₃^+- - CO₂ + CH₃C₁CH₂^+- were investigated computationally by MNDO. Selected intermediates and transition states were also calculated at the 4-31G level. Three competing processes for the unimolecular loss of CO2 from ionized methyl chloroacetate are examined in detail. According to the calculations the energetically most favorable pathway for the formation of CH₃C₁CH₂^+- from ClCH₂COOCH₃^+- commences with the migration of the ester methyl group to chlorine, followed by the elimination of CO₂ (i.e., C₁CH₂COOCH₃^+- – H₃CClCH₂COO^+- – CO₂ + CH₃C₁CH₂^+-).