Ionization energy fragments

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

Stevenson s rule When a fragmentation takes place, the positive charge remains on the fragment with the lowest ionization energy. 

Now the molecule M = AB at large separation has an ionization energy /jf equal to the lowest ionization energy of one of the atomic fragments, in our case /m = I A- This means that the highest occupied molecular orbital <1) satisfies... 

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