Electron affinity selected values

The original paper defining the Gaussian-2 method by Curtiss, Raghavachari, Trucks and Pople tested the method s effectiveness by comparing its results to experimental thermochemical data for a set of 125 calculations 55 atomization energies, 38 ionization potentials, 25 electron affinities and 7 proton affinities. All compounds included only first and second-row heavy atoms. The specific calculations chosen were selected because of the availability of high accuracy experimental values for these thermochemical quantities. 

This chapter gives a selected compilation of the standard and other characteristic (formal, half-wave) potentials, as well as a compilation of the constant of solubility and/or complex equilibria. Mostly, data obtained by electrochemical measurements are given. In the cases when reliable equilibrium potential values cannot be determined, the calculated values (calcd) for the most important reactions are presented. The data have been taken extensively from previous compilations [5-13] where the original reports can be found, as well as from handbooks [13-16], but only new research papers are cited. The constant of solubility and complex equilibria were taken from Refs 6-11,13,17-21. The oxidation states (OSs), ionization energies (IBs) (first, second, etc.), and electron affinities (EAs) of the elements and the... 

Br- (g). The electron affinity of Br (g) is calculable by the method of lattice energies. Selecting the crystal RbBr, because Rb+ and Br have exactly the same nuclear structure, and taking the exponent of the repulsive term to be 10, we have computed, for the reaction, RbBr (c) = Rb+ (g)+Br g), Dz= —151.2 whence the electron affinity of Br (g) becomes 87.9. Using the lattice energies of the alkali bromides as calculated by Sherman,1 we have computed the values 89.6, 85.6, 84.6, 83.6, and 89.6, respectively. Butkow,1 from the spectra of gaseous TIBr, deduced the value 86.5. From data on the absorption spectra of the alkali halides, Lederle1 obtained the value 82. See also Lennard-Jones.2... 

Electron affinity, Eea — is the energy released when an additional electron (without excess energy) attaches itself to an atom in a gas phase to form a negatively charged ion. ea values for atoms of selected elements are shown in the Table below. 

Table 1. Electron affinities of selected atomic and molecular species (in eV). [Estimated values for O and S are in brackets.]...

We calculate the enthalpy of formation at 0 K as the difference between a H"(PF, g, OK) = -8.55 5.0 kcal mol" (1 ) and the selected value of 1.1 0.5 eV (25.366 kcal mol" ) for the electron affinity (EA) of PF. The value of EA refers to the vertical electron detachment process PF"(g) = PF(g) + e" and is taken from the molecular orbital study of O Hare 2). This value was obtained from Hartree-Fock energies and estimated corrections for correlation effects. The estimated uncertainty in EA is 0.5 eV which should be adequate to cover the possibility that the adiabatic value is lower than the vertical EA. Other theoretical predictions of EA include 2.55 eV ( ) and 1.4 eV (4). 

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. 

In 1966 the relative electron affinities of charge transfer complex acceptors were calculated from spectral data and half-wave reduction potentials. Unfortunately, at the time, no accurate electron affinities of typical n charge transfer complex acceptors existed so one could obtain absolute electron affinities from either half-wave reduction potentials or charge transfer complexes. Thus, the magnetron Ea of 1.40 eV for the electron affinity of benzoquinone was selected. This is now known to be about 0.5 eV too low, making all the values low. This emphasizes the difference between the determination of relative electron affinities that depend on the absolute electron affinity of a reference compound and absolute ones from experimental measurements and fundamental constants. 

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 objective of any review of experimental values is to evaluate the accuracy and precision of the results. The description of a procedure for the selection of the evaluated values (EvV) of electron affinities is one of the objectives of this book. The most recent precise values are taken as the EvV. However, this is not always valid. It is better to obtain estimates of the bias and random errors in the values and to compare their accuracy and precision. The reported values of a property are collected and examined in terms of the random errors. If the values agree within the error, the weighted average value is the most appropriate value. If the values do not agree within the random errors, then systematic errors must be investigated. In order to evaluate bias errors, at least two different procedures for measuring the same quantity must be available. 

Figure 8.2 Electron affinities of the elements in the form of a Periodic Table. The values in parentheses are the uncertainties in the last figure. The other statistics are given with their proper number of significant figures. The dates are those of the determination or selection as the evaluated values [10-17].

Symbols EA electron affinity thr threshold energy Q total cross sections a differential cross section. Specification parameters a absolute value of cross section E energy dependence m mass selection of negative ions T temperature dependence. 

Electron affinity (EA) is a measure of the energy change that occurs when an electron is added to the valence shell of an atom. Some selected values of Eea for nonmetals are listed in Table 2.5. The addition of the first electron is usually exothermic (e.g., oxygen, sulfur) further additions, when they occur, are by necessity endothermic since the second electron is now approaching a negatively charged entity. The electron affinity of Cl is - 348.7 kJ/mol. 

Figure 8.6b shows the resulting term diagram, and Table 8.1 contains experimental values for the ionisation energies, the electron affinities, and the energy gaps of selected molecules and molecular crystals [15,16]. 

In these equations, AG is the energy of formation of the indicated species, AGfat is the crystal lattice energy, / is the ionisation potential of the metal and A is the electron affinity of the halide. In sect. 2.11.2 the calculation of AG at and Ai/i t (at 298.15 K) is discussed and in Appendix 2.11.1 values are given for the alkali metal halides and a few other selected salts. 

Equilibrium in the ion source of a mass spectrometer cannot be achieved in certain ion-molecule systems because of rapid competitive reactions or because one of the species in the equilibrium of interest is an unstable species such as a free radical. In such cases, it is sometimes possible to obtain an experimental estimate of the enthalpy change of a particular reaction (charge transfer, proton transfer, etc.) by use of a technique known as bracketing, in which the ion of interest is reacted with a series of molecules selected to provide a range of values for the relevant thermochemical parameter of interest, e.g., ionization energy, electron affinity, gas-phase basicity, or acidity. Reaction is presumed to occur for exothermic processes and not to occur for endothermic processes. 

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