Rearrangement and dissociation processes of solitary ethane-1,Zdiol radical cations were investigated by ab in& MO calculations, executed at the SDCI//RHF/DZP level of theory, including Pople-type size-consistency corrections. In order to obtain an accurate description of the chemistry involved, part of the potential energy surface was investigated by using the multi-reference CI method and also by using the valence bond (VB) method followed by SDCI calculations using the natural orbitals of the VB wavefunction. The ethane-l,2diol radical cation is metastable with respect to CH30HZ+ + HCO'; it has been shown recently that the isotopologue DOCH,CH,OD loses (exclusively) H C O to produce CH,DOHD+, not the isotopomer CH30D,+ expected from earlier mechanistic proposals. We have traced a low-energy pathway which explains the observed label distribution and which takes place at the experimentally derived energy level. First, ionized ethane-1,Zdiol collapses to the one-electron bond species [ HOCH, . . + *. . CH,OH I +. which subsequently rearranges to the hydrogen-bonded species CH,=O.*-HO(H)CH:'.Next, transformation to the transient CH,=O.*.HCH,OH+' takes effect and this rearrangement can be viewed as the 1,Zhydrogen shift, CH,OHi' + CH30H+', catalyzed by formaldehyde. Following this, charge transfer takes place from the methanol cation to the formaldehyde molecule which thus becomes charged; because it is now charged, the formaldehyde unit can rotate and donate a proton to the methanol molecule, after which dissociation follows. Our calculations and experimental results can be interpreted in terms of proton shifts rather than hydrogen shifts taking place in ion-molecule (proton-bound) complexes.
Ab inilio calculations indicated that these intermediate ion-dipole complexes (hydrogen-bridged structures)enjoy considerable stability (they lie below ions CH20HCH20H+'), making them attractive intermediates. However, attempts to locate the corresponding transition state failed.5A different mechanistic proposal came from the Radom group.7 They showed from ab initio calculations that ethane-1,2-diol ions, 1, produced by vertical ionization (la) spontaneously undergo an intramolecular proton migration to produce the drstonic ion 'OCH2CH20H2+, 2, which was calculated to lie 69 kJ mol-below ions la. The adiabatic ionization energy (IE,) of ethane-1,2-diol' was proposed to correspond to the formation of structure 2.' The latter ions were proposed to dissociate via a 1,2-hydrogen shift to CH,OH,+ + HCO'. Note that both mechanisms