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 tab initio MO and density functional theories) in conjunction with modern tandem mass spectrometric and C-13 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)-O-., which can best be viewed as the cation-catalyzed 1,2-hydrogen shift (CH2OH)-C-. --> 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(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. (C) 1997 Elsevier Science B.V.