Electron affinities, of elements, molecules

Table 4.4 Electron Affinities of Elements, Molecules, and Radicals...

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

EA = electron affinity of element or molecule (eV) k = Boltzmann s constant (eV/K)... 

Electron affinity (EA) describes the energy required to add an electron to a neutral ground state free atom or molecule in the gas phase, or in other words the minimum energy needed to form the negatively charged ion of -1 charge. Although electron affinities are element/molecule specific, these follow less systematic trends with electronic structure than 7p values. Electron affinities of the elements are listed in Appendix A.3. 

Several portions of Section 4, Properties of Atoms, Radicals, and Bonds, have been significantly enlarged. For example, the entries under Ionization Energy of Molecular and Radical Species now number 740 and have an additional column with the enthalpy of formation of the ions. Likewise, the table on Electron Affinities of the Elements, Molecules, and Radicals now contains about 225 entries. The Table of Nuclides has material on additional radionuclides, their radiations, and the neutron capture cross sections. 

The determination of further electron affinities is not an easy matter it is possible that the electronic equilibrium method could be extended to a few more elements, but at the temperatures involved, molecules and radicals would be decomposed. The only reasonable hope of estimating the electron affinities of radicals would seem to lie in a study of the appearance potential of negative ions, and the determination of their kinetic energies, although it must be borne in mind that a careful search of the mass spectrum of methane has failed to reveal the existence of a CH3(-)ion. [4]... 

Inorganic chemistry concerns molecules of all the atoms. The electron affinities of atoms, small molecules, and radicals and their relationship with the Periodic Table, electronegativities of elements, Morse curves of diatomic anions, and the energies of ion molecule reactions and bond energies are inorganic problems we have considered. Ionic radii can be estimated using potential energy curves. 

The concepts of precision and accuracy, P and A plots, and timelines are applied to the evaluation of the accuracy and precision of the electron affinities of selected atoms and molecules. The adiabatic electron affinities of the elements have been measured with a variety of techniques. Thus, the most accurate and precise values will be the weighted averages, which is also the least-squares solution. 

An application of the electron affinities of the elements and the experimental work functions involves the prediction of the electron affinities of clusters. The Cn molecules are an important type of cluster studied experimentally and theoretically. With experimental data the CURES-EC method of calculating electron affinities can be evaluated. The READS-TCT procedure can also be used to determine relative electron affinities. The clusters of C, Si, and Ge involve covalent bonds, while the bonds in the Sn and Pb clusters are partially metallic. With available electron affinities the relationship between the electron affinities and work functions of these anion clusters can be investigated. 

The homonuclear diatomic molecules are the simplest closed set of molecules. Many of the electron affinities of the main group diatomic molecules have been measured by anion photoelectron spectroscopy (PES), but only a few have been confirmed. These Ea can be examined by their systematic variation in the Periodic Table. Calculating Morse potential energy curves for the anions and comparing them with curves for isoelectronic species confirm experimental values. The homo-nuclear diatomic anions of Group IA, IB, VI, VII, and 3d elements and NO are examined first. 

The electron affinities of the main group homonuclear diatomic anions have been measured by PES. A few experimental values for the transition metal dimers are also available. The electron affinities of all the 3d homonuclear diatomic molecules have been calculated using density functional methods [1-4], Only the AEa of I2, 2.524 eV C2, 3.27 Si2, 2.2o S2, 1.67 F2, 3.0g Cl2, 2.4s Br2, 2.5, and 02, 1.07 have been measured by more than one method [1-3]. CURES-EC calculations confirm these to within 0.1 eV. Positive excited states Ea have been measured for 02, C2, and I2 and are inferred for other X2 [5-8]. Just as in the case of the atomic Ea, the trends in the Periodic Table can support the assignments of AEa for the other elements. 

Figure 9.1 Electron affinities of the elements, electron affinities, and bond dissociation energies of the homonuclear diatomic molecules in the form of a Periodic Table [1],...

In this method molecular orbitals are constructed from a set of basis orbitals represented by the valence orbitals of the atoms in the molecule, i.e. 2 s and 2p for first TOW elements, 3 d, 4 s, and 4p functions for the transition elements, and the 1 s orbital for hydrogen. The one-electron elements are represented by an average of the ionisation energy and the electron affinity of the orbital, while all two-electron integrals are n ected with the exception of one- and two-centre coulomb type terms and one-centre exchange integrals. Calculations were performed in the unrestricted Hartree-Fock formalism using the matrix elements —... 

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