Ionization potentials of molecular species

TABLE 6.66A Ionization Potentials of Molecular Species ([Pg.765]

Another definition of aromaticity that does not depend explicitly on either experimental results or on comparison with reference compounds is derived from the concept of absolute hardness, 17, which is defined as one-half of the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a system (equation 4.70). According to Koopmans theorem, Ehomo is related to the ionization potential of the species, while Elumo is related to its electron affinity. A large gap between these two orbitals implies resistance to both oxidation and reduction, and low chemical reactivity is one of the defining characteristics of aromaticity. 

Even in reactions involving excited states or in reactions between two radicals, the primary interaction which determines the reactivity is thought to proceed adiabatically. The probability of nonadiabatic charge transfer also may not be ignored between a molecular specie with small ionization potential and a specie with large electron affinity, in particular in the form of free, gaseous, or nonsolvated state. In that... 

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

In several cases shown in Table XV, the ionization potential of the molecule is 1-1.5 eV lower than that of the metal atom, suggesting a high stability of the molecular ion with respect to neutral species, and this is more noticeable with the second- and third-row transition metals such as molybdenum, rhenium, tungsten, and osmium. 

The spontaneous oxidation of molecular oxygen and of atomic xenon, each of which has a first ionization potential of 281 kcal. mole , by platinum hexafluoride, has estabhshed the remarkable oxidizing power of the hexafluoride. None of the other third-transition-series hexafluorides will oxidize these species. Since it is well established - that the platinum hexafluoride-oxygen adduct is the salt Oj+[PtFe] , the lattice energy of which is estimated, employing Kapustinskii s second equation to be 125 kcal. moIe, the electron affinity of the PtF, molecule is required to be >156 kcal. mole, to ar count for the observed exothermic reaction [Pg.244]

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