The acid-catalyzed rearrangement CH 3O [rad] → [rad]CH 2OH and its involvement in the dissociation of the methanol dimer radical cation : A Quid pro Quo reaction

Abstract

The barrier for the radical isomerization CH 3O → CH 2OH 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 CH 2O + H . Hence, CH 3O does not isomerize to the more stable CH 2OH on its own. However, this barrier is reduced to 15.8 kcal mol −1 when the CH 3O radical is coordinated with protonated methanol (CH 3 O ⋯H O(H) CH 3 +) and the CH 3O → CH 2OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation CH 2 O(H)⋯H O(H) CH 3 + can be viewed as an acid-catalyzed rearrangement. The ion CH 3 O ⋯H O(H) CH 3 + represents the most stable form of the methanol dimer radical cation. The ion CH 2 O(H)⋯H O(H) CH 3 + can fragment directly to CH 3OH 2 + + CH 2OH or it can rearrange further to produce the hydrogen-bridged radical cation CH 2 O +(CH 3) H⋯OH 2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or to CH 2 O⋯H +⋯OH 2 + CH 3 via an S N2 type reaction. Alternatively, CH 2 O +(CH 3) H⋯OH 2 may undergo “proton-transport catalysis” to produce the complex ion CH 3 O CH 3 +⋯OH 2 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.