Methane loss from (CH3)3O+: An asynchronous, concerted 1,2-alkane elimination

To advance our understanding of 1,2-eliminations, ab initio and density functional theories are used to characterize the elimination of CH4 from (CH3)3O+. The reaction begins with the extension of a CO bond. Near the transition state, the carbon of the moving methyl (CMe) begins to approach a hydrogen (Ht) in another methyl, and the original CHt bond begins to stretch. Finishing the transfer of Ht after the transition state is passed completes the elimination. At the transition state, more than half of the positive charge is on the moving methyl, so the initial phase of methane elimination resembles the beginning of heterolytic dissociation to +CH3 + CH3OCH3, a reaction that also occurs. There is no electron density overlap between the C and O at the transition state, but there is significant overlap density between CMe and Ht. This and the charge distribution lead to classification of methane elimination as a concerted but highly asynchronous process rather than as being ion-neutral complex-mediated, in contrast to many alkane eliminations. The concertedness of this process probably arises from the threshold for partial dissociation of this system to an ion-neutral complex being much higher than the endothermicity for methane elimination.