Proton and Electron Affinities

Some cations can be considered as neutral molecules that have accepted a hydrogen ion. For example, 

Ammonium compounds decompose in a number of ways when heated. For many of them, the heat of 

The decomposition of NH4HSO4 and the appropriate thermochemical cycle for determining the proton affinity of the HS04-(g) ion can be shown as follows  

By means of appropriate thermochemical cycles, it is possible to calculate proton affinities for species for which experimental values are not available. For example, using the procedure illustrated by the two foregoing examples, the proton affinities ofions such as HC03-(g) (1318 k J mol-1) and C032-(g) (2261 kj mol-1) have been evaluated. Studies of this type show that lattice energies are important in determining other chemical data and that the Kapustinskii equation is a very useful tool. 

In Chapter 1 we discussed the electron affinities of atoms and how they vary with position in the periodic table. It was also mentioned that no atom accepts two electrons with a release of energy. As a result, the only value available for the energy associated with adding a second electron to O- is one calculated by some means. One way in which the energy for this process can be estimated is by making use of a thermochemical cycle such as the one that follows, showing the steps that could lead to the formation of MgO. 

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McDonald et al. [26] used a flowing afterglow apparatus to measure the proton and electron affinities of hypovalent ion radicals in the gas phase. 

Barber, M. J., Siegel, L. M. Proton and electron affinities and magnetic interactions associated with the molybdenum flavin, and iron-sulfur centers of milk xanthine oxidase. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 796-804, New York, Amsterdam, Oxford, Elsevier/North Holland 1982... 

For a computational study of proton and electron affinities see Sauers, R.R. Tetrahedron 1999, 55, 10013. 

Proton and electron affinities have been calculated for a series of 28 carbanions and radicals using MP2/6-31 and B3LYP methods there was poor correlation between proton affinity and % -character of C-H bonds and anions the importance of C—H and C-C hyperconjugative effects is apparent. A review of anions in the gas phase has referred to stability, structure, and energetic peculiarities of a wide range of anion types. ... 

Acidity, Basicity, Proton and Electron Affinities 9.2.5.1 Acidity and Basicity... 

In principle, the equilibrium approach can be used to measure any of the thermochemical properties listed above. However, in practice, it is most commonly used for the determination of gas-phase acidities, proton affinities, and electron affinities. In addition, equilibrium measurements are used for measuring ion affinities, including halide (F, Cl ) and metal ion (alkali and transition metal) affinities. 

In addition to the concepts reviewed in the last two sections (appearance energy, ionization energy, and electron affinity), three others are relevant in gas-phase molecular energetics, namely, proton affinity, gas-phase basicity, and gas-phase acidity. 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

W1/W2 theory and their variants would appear to represent a valuable addition to the computational chemist s toolbox, both for applications that require high-accuracy energetics for small molecules and as a potential source of parameterization data for more approximate methods. The extra cost of W2 theory (compared to W1 theory) does appear to translate into better results for heats of formation and electron affinities, but does not appear to be justified for ionization potentials and proton affinities, for which the W1 approach yields basically converged results. Explicit calculation of anharmonic zero-point energies (as opposed to scaling of harmonic ones) does lead to a further improvement in the quality of W2 heats of formation at the W1 level, the improvement is not sufficiently noticeable to justify the extra expense and difficulty. 

Electrons are selectively captured by the pyrimidines and the holes formed on the base stack, or transferred to the base stack, are nearly all trapped by Gua, giving Gua +. This species is in a reversible equilibrium with its deprotonated form Gua(NI-H) (Fig. 2). While initial distributions between the pyrimidines are nearly equal upon annealing, or irradiation to high dose, the excess electron transfers to cytosine as the most stable site as a result of protonation of Cyt at N3 forming Cyt(N3+H) [10]. Molecular orbital calculations show that the major factor that affects the relative yields of DNA base ion radicals is the difference in both the ionization potential and electron affinity ofthe DNA bases [11]. In addition, experimental and theoretical work shows that variations in relative radical yields are also affected by the... 

Ionization potentials, proton affinities and electron affinities... 

A similar type of research took place also in nuclear physics during the thirties with a systematic characterization of different properties for a number of atomic nuclei [24]. As an example can be mentioned the studies of the neutron cross sections as a function of the number of neutrons or protons in the nuclei, which showed systematic variations with very small values at certain numbers corresponding to nuclei with 20, 50, 82 and 126 neutrons. This discovered periodicity was rather different compared with the periodicity of atomic properties as the first ionization potential and electron affinity for alkali and noble gas atoms. Speaking at a meeting of the Chemical Society on April 19, 1934, the centenary of the birth of Mendeleev, Rutherford concluded, /< may be that a Mendeleev of the future may address the Fellows of this Society on the Natural Order of Atomic Nuclei and history may repeat itself [25]. Measurements of for example nuclear spins for a number of isotopes also showed a similar type of periodicity as found in neutron cross sections. This kind of periodicity could not at that time be understood from the commonly used liquid drop model [26] but based on the single particle model formulated by Mayer, Haxel, Jensen and Suess in 1949 [27]. 

Determination of proton affinities (basicities) and acidities in the gas phase provides a means of systematically representing a large number of ion-molecule reactions and of the relationships between these quantities and bond strengths, ionization potentials and electron affinities without disturbance by solvation phenomena 47). 

O Malley, P.J. (1997). A density functional study of the effect of resuction on the geometry and electron affinity of hydrogen bonded 1,4-benzoquinone. Implications for quinone reduction and protonation in photosynthetic reaction centres. Chem. 

Three important classes of gas-phase ion-molecule reaction schemes demonstrate the value of thermochemical data deduced from equilibrium systems. The first of these concerns the derivation of extensive scales of relative proton affinities, gas-phase acidities, and electron affinities. These results derive primarily from measurements of enthalpy changes for proton or electron transfer reactions ... 

Properties calculated for the species include ionization energies and proton affinities [15], dipole moments and polarizibility functions [16], and electron affinities [17]. The ground state reduced potential curve obtained from measured spectra has been analyzed [18], and bond critical points and several electronic properties have been calculated for a series of molecules including BH using GAUSSIAN 80 [19]. 

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