Unimolecular reactions of isolated organic ions: The chemistry of the unsaturated oxonium ion documentclass{article}pagestyle{empty}begin{document}$ {rm CH}_{rm 2} = {rm CHCH =}mathop {{rm OCH}_{rm 3}}limits^{rm +} $end{document}

The reactions of metastable $ {rm CH}_{rm 2} = {rm CHCH =}mathop {{rm OCH}_{rm 3}}limits^{rm +} $ oxonium ions generated by alkyl radical loss from ionized allylic alkenyl methyl ethers are reported and discussed. Three main reactions occur, corresponding to expulsion of H₂O, C₂H₄/CO and CH₂O. There is also a very minor amount of C₃H₆ elimination. The mechanisms of these processes have been probed by ²H- and ¹³C-labelling experiments. Special attention is given to the influence of isotope effects on the kinetic energy release accompanying loss of formaldehyde from ²H-labelled analogues of $ {rm CH}_{rm 2} = {rm CHCH =}mathop {{rm OCH}_{rm 3}}limits^{rm + } $. Suggestions for interpreting these reactions in terms of routes involving ion–neutral complexes are put forward.