A general, most basic rule for ion dissociation: Ionized molecules.

Herein we revisit a basic rule for the interpretation of ion chemistry of ionized molecules, first proposed by the pioneers of MS spectra interpretation, but somewhat overlooked over the years. This rule states that, when rationalizing or predicting the dissociation chemistry of an ionized molecule (M+. ), a model analog to the "mobile proton model," that is, a "mobile electron model" via "e- -jumping" should be considered. Ground-state M+. is indeed the first species to be considered, but "e- -jumping" may eventually lead to other more energetic electromers-ionized molecules that differ only in the location of the missing electron-and each one of these electromers may dissociate via distinctive routes. In such a scenario, the route involving not necessarily the ground-state M+. , but the most labile electromer could become predominant or even exclusive. We argue that this "most labile electromer" rule, as well as an analogous "most labile protomer" rule that we have proposed for protonated molecules in an accompanying article, with the application of our conventional toolbox of a few cleavages and rearrangements, greatly simplifies the interpretation and prediction of ion chemistry.