Unimolecular reactions of isolated organic ions: reactions of the immonium ions CH2N+(CH3)CH(CH3)2, CH2N+(CH3)CH2CH2CH3 and CH2N+(CH2CH2CH3)2

Abstract

The reactions of metastable CH₂N⁺(CH₃)C₃H₇ immonium ions have been investigated by means of ²H-labelling experiments and kinetic energy release measurements. Loss of C₃H₆, with specific β-H transfer, is the sole channel for dissociation of CH₂N⁺(CH₃)CH(CH₃)₂. This process gives rise to a Gaussian metastable peak. The isomeric ion, CH₂N⁺(CH₃)CH₂CH₂CH₃, also expels C₃H₆; however, both α-H and γ-H as well as β-H transfer occurs in this case, and the reaction proceeds with an increased kinetic energy release. The role of ion-neutral complexes in C₃H₆, loss from CH₂N⁺(CH₃)C₃H₇ ions is discussed. In addition, CH₂N⁺(CH₃)CH₂CH₂CH₃ eliminates C₂H₄. This fragmentation yields a broad dish-topped metastable peak, corresponding to a very large kinetic energy release (T_½∼ 73 kJ mol^–1), and it involves specific and unidirectional γ-H transfer. A potential energy profile summarising the reactions of CH₂N⁺(CH₃)CH₂CH₂CH₃ and CH₂N⁺(CH₃)CH(CH₃)₂ is constructed. The mechanisms by which immonium ions of this general class eliminate C₃H₆ and C₂H₄ have been further probed by studying the behaviour of the higher homologue, CH₂N⁺(CH₂CH₂CH₃)₂. The mechanistic conclusions derived from this work are found to be in excellent qualitative agreement with those of previous studies.