Unimolecular reactions of isolated organic ions: reactions of the immonium ions CH₂N⁺(CH₃)CH(CH₃)₂, CH₂N⁺(CH₃)CH₂CH₂CH₃and 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 (T½∼ 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.