CH4 Loss from (CH)4N+ revisited: how does this high energy elimination compete with ·CH3 loss?

Eur. J. Mass Spectrom. 10, 767–774 (2004)
DOI: 10.1255/ejms.708

CH₄ Loss from (CH)₄N⁺ revisited: how does this high energy elimination compete with ^·CH₃ loss?

Charles E. Hudson,^a Lawrence L. Griffin^b and David J. McAdoo^a,*
^aDepartment of Neuroscience and Cell Biology, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555–1043, USA. E-mail: djmcadoo@utmb.edu
^bDepartment of Marine Sciences, Texas A&M; University at Galveston, Galveston, TX 77553, USA

ABSTRACT:

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·.

Keywords: ab initio theory, alkane elimination, competition, concerted elimination, ion—neutral complexes, kinetic shift, Rice—Rampsberger—Kassel—Marcus theory, tetramethyl ammonium cation, translational energy release

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