Unimolecular fragmentations of long-chain aliphatic iminium ions

The metastable ion characteristics of N-alkyl-N-methylmethyleneiminium ions ($ {rm H}_2 {rm C}=!=mathop {rm N}limits^ + ({rm CH}_3){rm R} $, R = n-C_mH_{2m + 1}, m = 3, 4, 5, 6, 8, 10, 14, 18) are reported and discussed. For R = n-propyl, alkene loss by onium reaction and alkene loss by McLafferty rearrangement occur, whereas for the higher homologues only the latter reaction is observed. As a result of ²H and ¹³C labelling experiments, the mechanism of alkene loss by γ-H transfer and β-cleavage does not change with increasing chain length and the iminium ions do not isomerize prior to decomposition, neither by H–D scrambling nor by carbon skeleton rearrangement. Whereas the sequence of elementary steps during fragmentation is not affected, the energetics of the reaction change as the chain length increases. Resulting from thermodynamic estimations, the enthalpy of reaction ΔH_r, critical energy E₀ and reverse critical energy E_0r diminish markedly as R increases from n-propyl to n-octadecyl. The knowledge of the reaction energetics including kinetic energy release data allows information about partitioning of excess energy into internal and translational degrees of freedom to be deduced.