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The purpose of this short review is critically to assess the important experiments in gas phase ion chemistq whose correct interpretation can lead to the assigning of structures to organic positive ions. The methods fall into two main categories, (i) the measurement of ion enthalpies and transition state energies for their fragmentations and ( i ) the detailed examination of the unimolecular and collision-induced fragmentation behaviour of cations, anions and neutral species. It is argued that in general, none of the above methods alone can suflice for an ion structure determination, but that in combination these techniques provide a powerful tool by means of which ion structures may confidently be assigned.

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This short review focuses attention upon the present status of metastable ion studies with emphasis upon the relationship between metastable peak shapes, ion structures and fragmentation mechanisms. Some recommendations are made concerning nomenclature and the reporting of observations on Gaussian-type metastable peaks. Experimental methods for recording relative abundances of metastable peaks are critically appraised. The relationship between metastable ion phenomena and isomerization of gaseous ions is reviewed with particular attention drawn to the effect of rate-determining isomerizations. The shapes of Gaussian-type metastable peaks are discussed in some detail and selected examples from recent studies are used to show that such peaks may, by appropriate experiments, be separated into two Gaussian-type components thus revealing new features of the fragmentation reaction. The magnitude and significance of released kinetic energies, T, are considered and it is stated that few conclusions can be drawn from the evaluation of T alone; the importance of accurate thermochemical data as an aid to understanding and interpreting kinetic energy release data is emphasized. Other topics discussed include composite metastable peaks, metastable peaks produced in chemical ionization and field ionization and the partitioning of internal energy of the fragmenting ion into translational degrees of freedom of the products, for reactions with and without a reverse energy barrier.

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