| Abstract: | A general method is described in which the mechanism of a reaction occurring in the ion source is inferred from the kinetic energy release accompanying further fragmentation of metastable product ions. In several cases the probe reaction occurred competitively by two mechanisms, and if high energy resolution is available this allows the detailed metastable peak shapes and not merely the average kinetic energy released, to be used to characterize the product ion formed in the fast (ion source) reaction. It is found that most substituted benzaldoxime O-methyl ethers undergo HCN elimination via a five-centered methoxyl transfer in the ion source, but that the p-methoxy substituted compound reacts through both a four- and a five-membered cyclic intermediate. The slow reactions of the corresponding metastable ions occur predominantly through a four-centered transition state in the p-methoxy compound and probably through both four- and five-membered intermediates for the less strongly electron donating substituents. The fraction of the excess energy of the products is higher than expected from a consideration of energy partitioning data for other systems involving activated complexes of comparable tightness. |
