Evaluation of Atomic Electron Affinities

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 precision of a metric is determined by the random uncertainties of a method and the number of replications. The equipment, ability of the investigator, and material investigated affect the random uncertainties. It is important to know the best precision that has been attained and the number of replications used to attain that precision. In establishing the precision, it is assumed there are no systematic uncertainties. In the case of atomic electron affinities the largest systematic uncertainty is the state assignment. 

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1-13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1-3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4-7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9-13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14-17], 

In 1971 all the experimental atomic Ea in the literature were evaluated and compared with extrapolated values [3]. None of the experimental values were eliminated. Considered were 123 values for 23 elements. All but four of the elements 

TABLE 8.1 Atomic Electron Affinities Current Best Averages to 1970, Weighted Averages to 1970, and Weighted Average of Photon Values to 1975 [3-5] 

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Recent reviews on alkali metal beam studies, theoretical and experimental determinations of electron affinities using photon methods, and atomic electron affinities and an Internet source for electron affinities all give compilations [113-117]. The evaluation of molecular electron affinities is a major objective of this book. 

Much of what is presently known about atomic and molecular EAs can be found in Refs. [3,4] as well as in the following (a) G.I. Gutsev and A.l. Boldyrev, The theoretical investigation of the electron affinity of chemical compounds, Adv. Chem. Phys., 61 (1985) 169-221 (b) J. Baker, R.H. Nobes andL. Radom, The evaluation of molecular electron affinities, J. Comp. Chem., 7 (1986) 349-358 (c) J. Simons and... 

The general approximate method of theoretical calculation of the polarities of valence bonds has been developed only as applied to the simplest molecules and is based on a quantitative evaluation of the electron affinity of the atoms contained in the molecule, which can be defined as the energy of attraction by a given atom of the electrons bonded to it. The... 

It is shown that Density Functional Theory offers both a conceptual and a computational tool for chemists in relating electronic structure of atoms and molecules to their properties both as isolated systems and upon interaction. The computational performance of DFT in the calculation of typical DFT quantities such as electronegativity and hardness and in the ev uation of atomic electronic affinities and molecular dipole and quadrupole momCTits is assessed. DFT concepts are discussed as such (a non finite difference evaluation of the electronic Fukui function, local softness and its use in similarity analysis of peptideisosteres and the nuclear Fukui function as a indicator of nuclear rearrangemCTits upon reaction) and in the context of principles (EEM, MHP, HSAB) for a variety of reactions involving the influence of solvent on the acidity of alcohols and the addition of HNC to dipolarophiles. 

M.F. Herman, K.F. Freed, D.L. Yeager, in I. Prigogine, S.A. Rice (Eds.), Theoretical Studies of the Equations of Motion - Green s Function Methods for the Evaluation of Atomic and Molecular Ionization Potentials, Electron Affinities, and Excitation Energies, Advances in Chemical Physics, Vol. 48, Wiley, New York, 1981, p. 1. 

A major objective of this book is to evaluate the reported values of molecular electron affinities and their errors and to assign them to specific states. Prior to 1970 the magnetron and ECD methods were used to measure the majority of gas phase molecular electron affinities. An extensive compilation of unevaluated experimental, empirical, and theoretical electron affinities of atoms, molecules, and radicals was published before 1990 [9]. The electron affinities measured in the gas phase are now available on the Internet but have not been evaluated [26]. The molecular Ea in this list is defined and evaluated in Appendix IV. Values that are significantly lower than the selected values will be assigned to excited states. Semi-empirical calculations and the CURES-EC technique support these assignments. Unpublished electron affinities and updated electron affinities from charge transfer complex data and half-wave reduction potentials are given in Appendix IV. 

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. 

The theoretical methods for calculating the electron affinities of atoms, diatomic molecules, and polyatomic molecules have been summarized and compared with the CURES-EC method for molecules. The density functional calculations of the electron affinities of substituted benzoquinones and scaled ab initio LUMO agree with the evaluated values for nitrobenzenes. 

The electron affinities Ea of the main group atoms are the most precisely measured values. Recall that the Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. It was once believed that only one bound anion state of atoms and molecules could exist. However, multiple bound states for atomic and molecular anions have been observed. This makes it necessary to assign the experimental values to the proper state. The random uncertainties of some atomic Ea determined from photodetachment thresholds occur in parts per million. These are confirmed by photoelectron spectroscopy, surface ionization, ion pair formation, and the Born Haber cycle. Atomic electron affinities illustrate the procedure for evaluating experimental Ea. 

The tables in Appendix IV summarize the evaluated values of the electron affinities given in this book. The electron affinities of the atoms and homonuclear diatomic molecules are given in two tables, Al.l and A1.2. The references for both tables are combined. The electron affinities of the hydrocarbons are given in Tables A2.1 and Al.l. Tables A2.3 and A2.4 provide the electron affinities of the halogenated hydrocarbons. The odd-numbered tables are ordered by value and the even-numbered tables are ordered by molecular weight. The references for the hydrocarbons are given separately from those of the CHX compounds. Tables A3.1 and A3.2 list the values for the CHNX molecules. These were combined because there are so few halogenated compounds. Tables A4.1 and A4.2 contain the electron affinities of the CHO and CHOX compounds, while Tables A5.1 and A5.2 contain those of the CHON and CHONX compounds. 

The free electron interacts with all atoms and molecules that have finite electron affinities to produce anions, and thus is unstable in all except the most inert liquids. Electrochemistry attests to this general axiom and provides a convenient means for evaluation of the energetics for the addition of an electron to solvent molecules and to species at the electrode-solution interface, for example ... 

We will investigate the stability of the anionic lithium clusters in Section 5. A relevant quantity is the dependence of the binding energy per atom on the number of atoms, as well as the electron affinity of neutral Lin clusters. From the latter, we evaluate whether these neutral clusters are able to receive an extra electron and to form an anionic system. 

Electron affinity is a measure of the tendency of an atom to accept an electron. Excluding noble gases, electron affinity increases as the atomic number increases within a given period and decreases with an increase in atomic number within a group. The scale of electronegativities allows a chemist to evaluate the electron affinity of specific atoms when they are incorporated into a compound. Recall from Chapter 6 that electronegativity indicates the relative ability of an atom to attract electrons in a chemical bond. 

The electron affinities of the aromatic hydrocarbons have been calculated using Huckel theory and MINDO/3 procedures. The electron affinities of benzene, naphthalene, anthracene, and tetracene have been calculated by density functional and ab initio procedures [8]. The relationship between the experimental and calculated values is examined. The electron affinities of other organic compounds have been calculated using MNDO, density functional, and ab initio procedures [9]. A more thorough discussion of these experimental and theoretical methods can be found in Electron and Molecule Interactions and Their Applications, Volume 2, Chapter 6. The experimental and theoretical electron affinities of atoms, molecules, and radicals up to 1984 are listed but not evaluated [10]. The NIST site briefly discusses the various methods for determining electron affinities and gives an... 

The problem of calculating the electron affinity of an atom consists of evaluating the total energy of the negative ion and the total energy of the neutral atom ... 

In Chapter 8 the electron affinities of atoms were evaluated. In this chapter the electron affinities of diatomic and triatomic molecules and SF (n= 1 to 6) will be considered. The ECD has been used to study CI2, Br2, I2, NO, 02, C02, COS, CS2, N20, N02, SO2, SF6. All the Ea for these molecules have been calculated by the CURES-EC method. The comparison of the relative electron affinities of COS, CS2, and N20 will be illustrated by READS-TCT calculations. 

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