The acid-catalyzed rearrangement CH3Oo --> oCH2OH and its involvement in the dissociation of the methanol dimer radical cation; A Quid pro Quo reaction

Abstract

The barrier for the radical isomerization CH3Oo --> oCH2OH 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+Ho. Hence, CH3Oo does not isomerize to the more stable oCH2OH on its own. However, this barrier is reduced to 15.8 kcal mol-1 when the CH3Oo radical is coordinated with protonated methanol (CH3-Oo...H-O(H)-CH3+) and the CH3Oo -->oCH2OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation oCH2-O(H)...H-O(H)-CH3+ can be viewed as an acid catalyzed rearrangement. The ion CH3-Oo...H-O(H)-CH3+ represents the most stable form of the methanol dimer radical cation. The ion oCH2-O(H)...H-O(H)-CH3+ can fragment directly to CH3OH2+ + oCH2OH or it can rearrange further to produce the hydrogen-bridged radical cation oCH2-O+(CH3)-H...OH2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or tho CH2=O...H+...OH2+CH3o via an SN2 type reaction. Alternatively, oCH2-O+(CH3)-H...OH2 may undergo "proton-transport catalysis" to produce the complex ion CH3-O-CH3o+...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.