Methyl Cation Affinities

Article abstract of DOI:10.1021/ja00231a002

Measurement of the CH3⁺ (Me⁺) transfer equilibria, B'Me⁺ + B = B' + MeB⁺, with a pulsed high-pressure mass spectrometer and an ion cyclotron resonance mass spectrometer led to a scale of relative methyl cation affinities of bases B, MCA(B).This scale when calibrated to the literature value for the Me⁺ affinity of N2 led to absolute MCA values for the other 24 bases B that were part of the scale.Rate constants for several Me⁺ transfer reactions were determined also.These were equal to collision rates for large exothermicity of the reactions and decreased below collision rates at low exothermicities.The MCA(B) values follow in general the proton affinities PA(B).A plot of PA(B) vs.MCA(B) leads to a slope of ca. 3 at low MCA(B) which becomes close to ca. 1 at high MCA(B).It is shown that this behaviour can be rationalized.