CH4 Loss from (CH3)4N+ Revisited: How Does This High Energy Elimination Compete with •CH3 Loss?

The rate of CH4 elimination from the tetramethylammonium ion increases faster than simple •CH3 loss with increasing internal energy at both high and low energies. Improved understanding of this highly unusual competition is sought by ab initio theory and RRKM calculations. Geometries and energies of stationary points and pathways as traced by intrinsic reaction coordinate calculations are given. A transition state at an energy much higher than those of both the reactant (414 kJ mol−1 above) and the products (361 kJ mol−1 above) was found for methane elimination from (CH3)4N+. More importantly, this transition state was also 14 kJ mol−1 above one found for methyl loss. However, according to results obtained and presented by RRKM theory, methane elimination through this transition state would be too slow to compete with methyl loss. This transition state may be for a concerted or a complex-mediated process. It is unlikely that CH4 is actually lost by a concerted elimination because only a small fraction of the reverse activation energy becomes translational energy, whereas concerted eliminations usually convert substantial fractions of their reverse activation energies into translational energy. Also, complex-mediated CH4 loss is unlikely because such eliminations are usually very quickly overwhelmed by simple dissociation of the partners just above threshold with increasing energy, opposite to the behavior of the system studied. Thus it is concluded that CH4 elimination from (CH3)4N+ occurs by loss of •CH3 followed by loss of H• at all energies, even though that is a higher energy process than methane elimination. Kinetic shifts and a reverse activation energy for •CH3 loss appear to raise the energy in the ions dissociating in the field free regions high enough to dissociate (CH3)4N+• + to (CH3)2N=CH2+• + •CH3 + H•.