Calculation of electron affinities

Calculation of electron affinities of the cationic species using P3 method with 6-311G(2df,2p) basis set. 

Gua has the lowest reduction potential among the four nucleobases (Table 10.2), and hence it is preferentially oxidized to its radical cation (for the calculation of ionization potentials of the DNA bases see Close 2004 Crespo-Hernandez et al. 2004), and this property makes Gua and its derivatives to stick out of the other nucleobases with respect to its different free-radical chemistry. In contrast, Thy and Cyt are good electron acceptors, while the purines are only poor ones in comparison (for the calculation of electron affinities, see Richardson et al. 2004). This is of special importance in the effects caused by the absorption of ionizing radiation by DNA. 

Explicit separation of the center-of—mass motion in variational calculations of electron affinities of H D and T"... 

The correlation energy of a 2n electron system is in general larger in absolute value than that of the system with one less electron. Therefore, the quantity (Et — Ec) is positive and tends to compensate the error AEt. On the other hand, the same argument, applied to the calculation of electron affinities (the change in energy produced by the capture of an electron in an empty orbital (pt), suggests that the errors —AEt and E — Et) should cumulate rather than cancel. 

The reactions of organic molecules in solution are related to gas phase electron affinities and electronegativities. Anions are often intermediates in such reactions. The electron conduction of polymers is related to the electron affinities of the components. The theoretical calculations of electron affinities of aromatic hydrocarbons and the effect of substitution on electrons affinities and gas phase acidities are important to organic chemistry. Pseudo-two-dimensional Morse potentials have been used to represent the dissociation of organic molecules and their anions [18]. 

To describe a semi-empirical procedure for quantum mechanical calculations of electron affinities. 

For the calculation of electron affinities the laws are all known, but it is not possible to rigorously calculate the integrals with exact Hamiltonians and wave functions. It is possible to calculate the electron affinities of some atoms and molecules, but the... 

The electron affinity of the hydrogen atom was calculated to chemical accuracy in 1930 using the variational method. A value of 0.74(4) eV is compared with the EvV of 0.75419(2) eV. This was obtained by applying the variational principle to approximate wave functions for the neutral and anion. In 1962 C. L. Pekeris used 444 parameters and obtained a value of 0.75421 eV. Until 1991 this was the most accurate and precise value for the electron affinity of the H atom [82-85]. The calculation of electron affinities of atoms beyond hydrogen were challenges to theoretical chemists until recently. The earliest calculations gave negative electron affinities for the first row elements, except for F and C. The problem was that the Hartree Fock method only considered the correlation of electrons with parallel spins [85]. 

In spite of the development of new methods of measurements, fewer than 300 molecular electron affinities have been measured in the gas phase [5-7]. With the limited number of experimental Ea it is important to have a simple rigorous quantum mechanical procedure for the calculation of electron affinities of large molecules. Electron correlation is the crucial problem in the calculation of Ea. The correlation energies are significant because Ea represent a small difference between two large quantities. Since an electron is added to the system, the effects of geometry changes and correlation reinforce each other rather than cancel out, as in ionization potentials. 

A unique feature of the occupation number representation is that the number of electrons does not appear in the definition of the Hamiltonian operator in this form as it does in the wavefunction form. This is because all of the occupation information resides in the bras and kets. This is true for any operator in second quantized form. This feature is used to advantage in theories that allow the number of particles to change, and to a more limited extent in the calculation of electron affinities and ionization potentials. It is less important to the MCSCF method but it is useful to remember that the bras and kets contain all of the occupation information. Other details of the wavefunction, such as the AO and MO basis set information, are included in the integrals that are used as expansion coefficients in the second quantized representation of the operator. 

Unfortunately, we do not yet know how to calculate 3 with any accuracy, so that the constants of proportionality are not known. But this is an area where progress can be made, hopefully, by further study of Equation (4.35). Another problem also exists, in attempting to relate changes in 77 to changes in energy. The values of p and rj depend very much on the quality of the method used to calculate them. While the relative values that have been calculated may be quite good, as mentioned earlier, absolute values are not. The calculation of electron affinities is especially difficult. On the positive side, if we know Rq, then we also know (dEei/dR) without any further quantum mechanical calculations. It is simply equal to the force due to nuclear repulsion, ZaZb/T q. 

The self-interaction problem is responsible for some of the failures of the LDA and the GGA, namely (i) the too small ionization potentials when calculated from ehomo (ii) the non-existence of Rydberg series (iii) the incapacity to bind extra electrons, thus rendering almost impossible the calculation of electron-affinities (EA). 

Landau, A., Eliav, E., Ishikava, Y., Kaldor, U. Benchmark calculations of electron affinities of the alkali atoms sodium to eka-francium (element 119). J. Chem. Phys. 115, 2389-2392 (2001)... 

Compton, R. N., Yoshioka, Y.> Jordan, K. D. (1980). Comment on semi-empirical calculations of electron affinities. Theoretica Chimica Acta, 54, 259. 

This linear combination between two quantities of different dimensions stems from uncertainties in the calculation of electronic affinities. In fact, the exact calculation of the valence states allows a precise evaluation of the ionization potential, electronic affinity and hydridation s in the valence state, leading to [18]... 

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