Estimation of electron affinities

The availability of laser photodetachment techniques has permitted more accurate experimental determinations of electron affinities. Even so, tables of electron affinities hst some calculated values, in particular for the formation of multiply charged ions. One method of estimation uses the Bom-Haber cycle, with a value for the lattice energy derived using an electrostatic model. Compounds for which this is valid are limited (see Section 6.15). 

We can apply the Bom-Haber cycle to a metal oxide having a stmcture type of known Madelung constant, and for which an electrostatic model is a reasonably valid approximation. Magnesium(II) oxide fits these criteria it has an NaCl stmcture type, fo has been accurately determined by X-ray diffraction methods, and compressibility data are available an electrostatic model gives AC/(0K) = —3975kJmoU. All other quantities in the appropriate Bom-Haber cycle are independently measurable and a value for (298 K) for reaction 6.21 can be evaluated. A series of similar values for S Aea 7 °(298 K) for reaction 6.21 can be obtained using different group 2 metal oxides. 

The attachment of two electrons to an O atom can be considered in terms of the consecutive processes in scheme 6.22, and accepted values for the associated enthalpy changes for the two steps are —141 and +798kJmol .  

The second step is highly endothermic. It appears that the only reason the ion exists is because of the high lattice energies of oxide salts, e.g. AU 0K) forNa20, K2O, MgO and CaO are -2481, -2238, -3795 and 3414kJmof 

The attachment of two electrons to an O atom can be considered in terms of the consecutive processes in scheme 

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S. W., Melton, C. M. Estimation of electron affinity based on structure acfivity relafionships. Quant. Struct.-Activ. Rd. 1993, 12, 389-396. 

Several pieces of information are available which tend to provide support for our estimated EA value. Formation of SF involves the addition of an electron to an antibonding orbital of SF (2) which is located primarily on the sulfur atom. It is reasonable, therefore, to assume that EA(SF) EA(S) = 2.0772 0.005 eV (3). O Hare (2) has pointed out that Koopmans theorem ( ) gives reliable estimates of electron affinities and ionization potentials provided that the parent molecule or ion have a closed-shell electronic configuration. Application of this theorem to SF which has the required closed-shell structure leads to EA = 1.7 eV (2). Other estimates of EA based on MO calculations have included (all in eV) 2.8 (5), 2.5 0.5 (6), and <3.2 (2). 

The substitution rules were originally described in 1963 for the estimation of electron affinities of organic charge transfer complexes. The origin of the rules and their uncertainty are reflected in the following passage ... 

The error for negative ions is due to the wrong asymptotic form of the LSD potential. An LSD electron far from a neutral atom still sees itself and the extra repulsion leads to instability. Reasonable estimates of electron affinities can be obtained by preventing the electron from entering this poorly described asymptotic region, either by imposing a potential barrier or through the use of a finite basis-set expansion with no very diffuse functions . 

D.1 and results in Koopmans theorem-level estimates of electron affinities and ionization potentials. The success of using Koopmans theorem to assign peaks in photoelectron spectra relies on the fact that corrections to Koopmans theorem first appear in second order. 

C08-0073. Repeat the calculation of Problem 8.37 for K and I, using 500 kJ/mol as the estimated second electron affinity of iodine and assuming no change in distance of closest approach. 

When estimating the energetics of excess electron transfer in DNA via differences of electron affinities (EA) of nucleobases B in WCP trimers 5 -XBY-3 [92], we found the EA values of bases to decrease in the order C T A>G. The destabihzing effect of the subsequent base Y is more pronounced than that of the preceding base X. As strongest electron traps, we predicted the sequences 5 -XCY-3 and 5 -XTY-3, where X and Y are pyrimidines C and T. These triads exhibit very similar EA values, and therefore, the corresponding anion radical states should be approximately in resonance, favoring efficient transport of excess electrons in DNA [92]. 

Table 1 Summary of best estimates of electron- transfer distances, tunneling constants, and calculated electron affinities [7aj. rReprinted with permission from the J. Phys. Chem. Copyright (2000) American Chemical Society...

The enthalpy of formation of sodium fluoride is —571 kj mol. Estimate the electron affinity of fluorine Compare your value with that giver in Table 23. 

Clearly, in the thermodynamic cycle above for the estimation of hydrogen affinities there must indeed exist, for a family of homologous compounds, a linear relationship between the basicity, defined as PA or as AG° (because the entropic correction would be constant within the family), and the IPV for the lone electron pair that undergoes the protonation. 

Use bond energies (Table 13.6), values of electron affinities (Table 12.8), and the ionization energy of hydrogen (1312 kj/mol) to estimate AH for each of the following reactions. 

Using Fig- 4.7 generate the first five terms of the senes for the Madelung constant for NnCL How close is the summation of these terms to the limiting value given in Table 4.1 The enthalpy of formation of sodium fluoride is —571 kj mol. Estimate Ihe electron affinity of fluorine Compare your value with that ven in Table 23. 

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]... 

The electron affinities of many of the molecules determined in the ECD or NIMS have been verified by half-wave reduction potentials and charge transfer complex data. These methods were developed in the 1960s but have been significantly improved. The relationship between the electronegativity and the electron affinities and ionization potentials for aromatic hydrocarbons can be used to support the Ea. The use of the ECD model and these techniques to estimate the electron affinities of aromatic hydrocarbons are illustrated for selected compounds. We will also describe the use of charge transfer complex data to obtain the electron affinities of acceptors. 

It was possible to obtain better resolution for these compounds because there are three independent experimental methods for estimating the electron affinities that could be iterated to consistency. These were the ECD measurements, half-wave reduction potential measurements, and electronegativity values. In addition, these electron affinities had been calculated with the MINDO/3 procedure. We also calculated the values using the CURES-EC procedure and obtained estimates of the charge densities. These procedures were extended to a larger set of 80 compounds, some without gas phase Ea that will be discussed in Chapter 10. 

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. 

The simple molecular orbital theory of bonding in homonuclear diatomic molecules can be used to estimate the electron affinities of clusters. In these cases, there can be different geometries. The Cn clusters have been studied most extensively. In the case of the triatomic molecules, there are now two distances and one angle that... 

Reduction potential data support this prediction. Also, the inclusion of multiple substitutions of CH by N in benzene is expected to increase electron affinity. On the basis of reduction potentials the Ea of pyridine is about zero and that for the diazines ranges from 0.2 to 0.36 eV. The triazines vary from 0.5 to 0.9 eV. The predicted value for hexazine is 2.8 eV. These substitution and replacement effects can be used to predict electron affinities. Indeed, the first attempt at estimating the electron affinities of AGCUT was made using such correlations. These will be discussed in more detail in Chapter 12. 

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