THE DISSOCIATION OF LOW-ENERGY 1,2-PROPANEDIOL IONS - AN INTRIGUING MECHANISM REVISITED

The fascinating unimolecular chemistry of ionized 1,2-propanediol, CH3 C(H)OHCH2OH.+, 1, has been re-examined using computational chemistry t ab initio MO and density functional theories) in conjunction with mode rn tandem mass spectrometric and C-13 labelling experiments. The calcu lations allow a considerable simplification of a previously proposed c omplex mechanism (Org. Mass Spectrom., 23 (1988) 355). Again, the cent ral intermediates are proposed to be stable hydrogen bridged ion-dipol e complexes, but our present calculations indicate that the key transf ormation now is the rearrangement CH3C(H)OH+... O(H)-CH2. --> CH3C(H)O H+... (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 pro cesses (loss of CH3., H2O, H2O + CH3., H2O + CH4) are interpreted in t erms of Bohme's 'methyl cation shuttle' (J. Am. Chem. Soc., 118 (1996) 4500) taking place in ion-dipole complexes. The most stable intermedi ate is the hydrogen bridged ion-dipole complex CH2=CHOH.+... O(H)CH3, which is the reacting configuration for loss of methanol. (C) 1997 Els evier Science B.V.