CHEMICAL AND STRUCTURAL IMPLICATIONS OF HIGH- AND LOW-ENERGY ION/MOLECULE INTERACTIONS
Abstract
One of the most analytically useful consequences of the collisional interaction of a kilovolt energy ion with a gaseous target is excitation with subsequent decomposition. This process, termed collision-induced dissociation, has found widespread application in the structural elucidation of organic ions. Another important area of investigation in mass spectrometry is that of low-energy ion/molecule reactions. These processes have long been of interest because of their relationship to conventional chemical reactions. Individually these two areas account for a large amount of information about fundamental gas-phase processes. In tandem, however, they represent a dynamic new area of importance to many aspects of chemistry. This work deals with the combination of these two techniques for the investigation of eight classical organic reactions. Examination of the gas-phase analogs of the Dieckmann condensation, barbituric acid synthesis, benzoin condensation, Claisen-Schmidt condensation, lactam/lactone synthesis, cyclization of phenyl alkanols, methylation of substituted benzenes, and Schiff base synthesis reveals a wide range of behavior. The results of these investigations ranged from completely analogous behavior of the gas-phase reaction to that of its condensed phase counterpart, as in the case of the Schiff base synthesis, to instances where new and interesting variations of the reaction were discovered, as was observed for the gas-phase Dieckmann condensation. Another example of the utility of using individual ion/molecule reactions in a sequential manner is demonstrated by consecutive collision-induced dissociation reactions in a triple analyzer mass spectrometer. This modification of the standard CID technique exhibits increased selectivity for the analysis of mixtures of isobaric (oxonium vs. hydrocarbon ions) and isomeric ions (substituted benzenes). Increased selectivity is also displayed by another variant of collision-induced dissociation. Angle-resolved mass spectrometry is capable of selectively detecting ionic decomposition products resulting from high-energy collisions. Thus, by varying the observation angle it is possible to obtain a highly resolved view of unimolecular decomposition processes such as the elimination of methyl radical from isomeric unsaturated alcohols, previously attainable only through the time-resolved technique of field ionization kinetics.
Degree
Ph.D.
Subject Area
Analytical chemistry
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