The Unimolecular Chemistry of [1, 2-Propanediol]+.: A Rationale in Terms of Hydrogen Bridged Radical Cations

The low-energy dissociations of radical cations in the gas phase often only occur from isomeric ions generated via extensive rearrangement processes. This is especially true for, but by no means limited to, those ions which have half-lives of > 10”5 s, i.e., the metastable ions which decompose in the drift regions of a mass spectrometer [1]. The quasi-equilibrium theory provides a rationale in that it stipulates that dissociation of metastable ions is strongly dependent on the overall activation energy rather than on the mechanistic complexity of the reaction. Among the many well-documented cases, ionized methyl isobutyrate, (CH3)2CHCOOCH3, provides a classical example [2]. The loss of CH3. from these ions occurs only by C-C cleavage in the isomeric radical cation [CH3CH2CH=C(OH)(OCH3)]+., whose formation involves inter alia a 1, 2 shift of the protonated ester moiety.