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Additional Gas Phase Ionic Reactions

In the second half of the twentieth century four experimental techniques were developed to measure Ea the equilibrium, the photon, the beam, and the thermal charge transfer methods [4, 18-29]. In addition to the above reactions observed in the ECD and NIMS, negative ions can be formed in other reactions and complementary energetics and kinetics determined. The ECD and NIMS results have been integrated and compared with data obtained from other studies [4]  [Pg.15]

Another pertinent energy quantity is the gas phase acidity (GPA) of a molecule. It is also called the deprotonation energy of a molecule. It is the energy for the reaction [Pg.15]

The deprotonation energy is related to the A—H bond dissociation energy, the electron affinity of the radical, and the ionization potential of the hydrogen atom as [Pg.15]

The temperature dependence of reactions 2.14 and 2.18 yields relative energetics. If the reactant negative ion is a radical, the hydrogen transfer reaction results in relative gas phase acidities. If the reactant ion is a molecule, the relative molecular electron affinities are obtained. If we assume similar entropy changes, the ratio of the ions and that of the concentration of the neutral reactants give a value of the equilibrium constant and hence a measure of the relative acidity or electron affinity. In some cases only the direction of the reaction is observed to yield limiting electron affinities or acidities. In order to obtain an absolute value, the value of the reference compound must be known. A better procedure is to determine the temperature dependence of the equilibrium constant in order to remove the assumptions [Pg.15]

Another method used alkali metal beams (AMB) to give a negative ion and a positive ion (equation 2.15). The threshold is the sum of the ionization potential of the alkali metal and the electron affinity of the molecule. A related procedure is the determination of the threshold for endothermic charge transfer (EnCT) from one anion to a neutral to form a second anion. The threshold is equal to the difference in the electron affinities of the species so that the electron affinity of the reactant must be known. These procedures yielded accurate electron affinities of the halogen molecules [18-19, 28, 29]. [Pg.16]


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