The acid-catalyzed rearrangement CH3O → CH2OH and its involvement in the dissociation of the methanol dimer radical cation

The barrier for the radical isomerization CH3O → CH2OH is calculated by CBS-QB3 to be 29.7 kcal mol−1 and lies higher (by 5.7 kcal mol−1) than the dissociation limit CH2O + H. Hence, CH3O does not isomerize to the more stable CH2OH on its own. However, this barrier is reduced to 15.8 kcal mol−1 when the CH3O radical is coordinated with protonated methanol (CH3O⋯HO(H)CH3+) and the CH3O → CH2OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation CH2O(H)⋯HO(H)CH3+ can be viewed as an acid-catalyzed rearrangement. The ion CH3O⋯HO(H)CH3+ represents the most stable form of the methanol dimer radical cation. The ion CH2O(H)⋯HO(H)CH3+ can fragment directly to CH3OH2+ + CH2OH or it can rearrange further to produce the hydrogen-bridged radical cation CH2O+(CH3)H⋯OH2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or to CH2O⋯H+⋯OH2 + CH3 via an SN2 type reaction. Alternatively, CH2O+(CH3)H⋯OH2 may undergo “proton-transport catalysis” to produce the complex ion CH3OCH3+⋯OH2 which then dissociates. Our calculations confirm for the most part recent experimental findings on the methanol dimer radical cation [Y.-P. Tu, J.L. Holmes, J. Am. Chem. Soc. 112 (2000) 3695] but they also provide a different mechanism for the key isomerization reaction observed in that study.