REARRANGEMENT OF ETHANE-1,2-DIOL RADICAL CATIONS - PROCESSES INVOLVING DIPOLE-CATALYZED PROTON SHIFTS AND CHARGE-TRANSFER

Rearrangement and dissociation processes of solitary ethane-1,2-diol radical cations were investigated by ab initio 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-1,2-diol radical cation is metastable with respect to CH3OH2+ + HCO.; it has been shown recently that the isotopologue DOCH2CH2OD loses (exclusively) HCO. to produce CH2DOHD+, not the isotopomer CH3OD2+ 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-1,2-diol collapses to the one-electron bond species [HOCH2..+...CH2OH]+. which subsequently rearranges to the hydrogen-bonded species CH2=O...HO(H)CH2+. Next, transformation to the transient CH2=O...HCH2OH+. takes effect and this rearrangement can be viewed as the 1,2-hydrogen shift, CH2OH2+. --> CH3OH+., 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.