The dissociation of low energy 1,2-propanediol ions: an intriguing mechanism revisited

The fascinating unimolecular chemistry of ionized 1,2-propanediol, CH3C(H)OHCH2OH·+, 1, has been re-examined using computational chemistry (ab initio MO and density functional theories) in conjunction with modern tandem mass spectrometric and 13C labelling experiments. The calculations allow a considerable simplification of a previously proposed complex mechanism (Org. Mass Spectrom., 23 (1988) 355). Again, the central intermediates are proposed to be stable hydrogen bridged ion—dipole complexes, but our present calculations indicate that the key transformation now is the rearrangement CH3C(H)OH+···O(H)-CH2. → CH3C(H)OH+···.OCH3, which can best be viewed as the cation-catalyzed 1,2-hydrogen shift .CH2OH → CH3O., a rearrangement which does not occur so easily in the unassisted system. Another important process is the electron transfer CH3C(H)O···CH3OH·+ → OCH(CH3)·+···O(H)CH3 which allows proton transfer to generate CH3OH2+ + CH3CO.. Other dissociation processes (loss of CH3., H2O, H2O + CH3., H2O + CH4) are interpreted in terms of Bohme's ‘methyl cation shuttle’ (J. Am. Chem. Soc., 118 (1996) 4500) taking place in ion-dipole complexes. The most stable intermediate is the hydrogen bridged ion-dipole complex CH2CHOH.+···O(H)CH3, which is the reacting configuration for loss of methanol.