Mechanistic studies

Abstract

A short description is given of some modern physical methods in mass spectrometry like field ionization kinetics, kinetic energy release measurements, collisional activation and ion cyclotron resonance. With these methods it is possible to study the mechanisms of fragmentation and the structures of ions in the gas phase over a time interval of 10 -11 to 10 -3 s. They are shown to full advantage in such studies if they are used in combination with isotopic labelling. For example, on the basis of field ionization kinetics and deuterium labelling experiments, evidence is presented that the molecular ions of isobutyl alcohol generate, at 10 -11 s, CH 5 O+ ions via a 1, 4- shift of a hydrogen atom from one of the methyl groups to the oxygen atom, followed by a 1, 2-elimination of protonated methanol with a hydrogen atom of the other methyl group. At times longer than 10 -11 s two distinct interchange processes between hydrogen atoms appear to compete with this reaction, as shown from field ionization kinetic experiments in the time interval of 10 -11 to 10 -9 s and from decompositions of metastable ions in the time interval of 10 -6 to 10 -5 s. Another example concerns the loss of formaldehyde from the [M —methyl]+ ions of methoxymethyl isopropyl ether. Combined labelling with 18 O and deuterium has shown that these ions eliminate formaldehyde via two reaction channels, i.e. by methyl migration and by methoxyl migration. Both channels lead to the same products, but the methoxyl migration has the higher activation energy and the higher frequency factor. An attempt has been made to prepare the [M - methyl]+ ions of methoxymethyl isopropyl ether as collision complexes in the ion-molecule reactions of methoxymethyl cations and acetaldehyde by using an ion cyclotron resonance spectrometer. On the basis of labelling experiments with 18 O and deuterium it is shown that such collision complexes are not involved. A simple methyl cation transfer from the methoxymethyl cations to acetaldehyde molecules through a nucleophilic displacement is found instead. Furthermore, from this study, evidence is presented that the threshold energy required for 1, 3-hydrogen shifts in long-lived ( ca. 10 -3 s) methoxymethyl cations is 2.3 eV.