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)CH . Next, transformation to the transient CH2O…︁HCH2OH+˙ takes effect and this rearrangement can be viewed as the 1,2‐hydrogen shift, CH2OH → 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.