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, CH 3O• 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 (CH3-O •⋯H-O(H)-CH3 +) and the CH 3O• → •CH2OH 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)-CH3 + can be viewed as an acid-catalyzed rearrangement. The ion CH3-O•⋯ H-O(H)-CH3 + represents the most stable form of the methanol dimer radical cation. The ion •CH2-O(H) ⋯H-O(H)-CH3 + can fragment directly to CH 3OH2 + + •CH2OH or it can rearrange further to produce the hydrogen-bridged radical cation •CH2-O+(CH3)-H⋯OH 2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or to CH2=O⋯H +⋯OH2 + CH3 • via an SN2 type reaction. Alternatively, •CH 2-O+(CH3)-H⋯OH2 may undergo "proton-transport catalysis" to produce the complex ion CH 3-O-CH3 •+⋯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.

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International Journal of Mass Spectrometry
Department of Neurology

Burgers, P., & Ruttink, P. (2005). The acid-catalyzed rearrangement CH3O• → •CH2OH and its involvement in the dissociation of the methanol dimer radical cation: A Quid pro Quo reaction. International Journal of Mass Spectrometry, 242(1), 49–56. doi:10.1016/j.ijms.2004.11.006