Electron affinities of molecules

In the late 1960s several major advances were made in the study of thermal electron reactions. These were based on the ECD, the extension of the magnetron method of studying electron molecule reactions to determine equilibrium constants for electron molecule reactions, and the invention of high-pressure thermal electron negative-ion sources for mass spectrometry [5-7], Electron swarms were also used to determine rate constants for thermal electron reactions [8, 9]. The electron affinities of molecules were measured using electron and alkali metal beams [10, 11]. Relative electron affinities were obtained from the direction of the reaction of a negative ion with a molecule [12, 13], Other major advances were photodetachment and photoelectron spectroscopy [14—17],... 

These remarks clearly previewed our studies on half-wave reduction potentials, semi-empirical calculations, and the direct measurement of electron affinities of molecules in the gas phase [15]. 

Two important methods for verifying the relative values of the electron affinities obtained from the ECD method were introduced in an article cautiously entitled, Potential Method for the Determination of Electron Affinities of Molecules Application to Some Aromatic Hydrocarbons, with comparisons to half-wave reduction potentials and SCF calculations [18, 21]. The relative ECD values agreed with the half-wave reduction potential order from two independent sets of measurements. From this correlation the relative values had an error of 10 to 15%, or for a value of 0.6 eV an absolute error of 0.1 eV, because the electron affinity is logarithmically related to the K value. The agreement was within the experimental and calculation error. It was suggested that electronic absorption spectra, ionization potentials (through the constant electronegativity concept), and... 

The theoretical calculation of the electron affinities of aromatic hydrocarbons was advanced by the development of the MINDO/3, MNDO, AMI, and PM3 semi-empirical techniques. These procedures gave the adiabatic electron affinities of molecules obtained from the ECD and from half-wave reduction potentials that agreed with the experimental values to within the experimental error. A different semi-empirical procedure yielded consistently lower values than the experimental values partially because they were adjusted to the lower values [67-69]. 

The experimental procedures for obtaining ECD and NIMS data have been described. Examples of the calculations are given for the various classes of molecules. For each group specific test molecules are provided. The aromatic hydrocarbons and aldehydes are Eql(l/lor 1/2) molecules, CS2 is a Eql(2/2) molecule, haloalkanes are DEC(l) molecules, and the halobenzenes and nitromethane are DEC(2) molecules that dissociate via a molecular ion. A graphical procedure for obtaining parameters from ECD data and the calibration of NIMS data using SF6 and nitrobenzene is presented. The use of multiple electron affinities of O2 to define negative-ion states from ECD data is illustrated. A method for the analysis of published NIMS spectra measured at two temperatures reveals the electron affinities of molecules when combined with substitution effects. We then explored the use of precision and accuracy plots and timelines for the evaluation of electron affinities. 

The absorption, emission, photodetachment, and photoelectron spectroscopy experiments are capable of providing accurate and precise values for the electron affinities of atoms. The best precision is about 1 part per million, more than precise enough for chemical purposes. The state of the ion must be identified and some excited-state electron affinities of atoms have been reported. The photoelectron spectroscopy and photodetachment procedure can give the accurate and precise electron affinities of molecules and radicals when the state of the anion is assigned. 

One use of the electron affinities of molecules is to predict the sensitivity and temperature dependence of the ECD to compounds that might be analyzed. Many environmental pollutants have different multiple substituents. Pesticides are highly chlorinated organic compounds. The chlorinated biphenyls, naphthalenes, and dioxanes are among the most toxic compounds. The temperature dependence of these compounds in the ECD is important, but has not been extensively studied. When the electron affinities and bond dissociation energies are known, the temperature dependence can be calculated from the kinetic model. This is done for the chlorinated biphenyls and naphthalenes, and the calculated temperature dependence is then compared with experiment. These calculations offer clues about the best conditions for analysis. 

Electron affinities of molecules are of interest not only in gas-phase reactions, e.g. in negative chemical ionization mass spectrometry, but also in the field of condensed-phase chemistry. It is characteristic that negative ions are by far not studied to the same level of detail as the corresponding positive ions. However, during the last decade a large number of EA determinations based on measurements of electron transfer equilibria utilizing pulsed high-pressure mass spectrometry have been reported . ... 

Similar experiments, involving electron transfer between an anion and a neutral molecule, yield relative or absolute EAs. The method has been used to determine relative free energies for electron attachment for a variety of metallocenes and /3-diketonate molecules. Electron photodetachment spectroscopy of negatively charged ions is another source for obtaining electron affinities of molecules. These data provide an important component of thermochemical cycles involving oxidation/reduction of metal complexes, and serve as a basis for obtaining other thermochemical values. 

There have been only a few calculations of the electron affinities of molecules with the correlation consistent basis sets. The EAs of O2 and CN from RCCSD(T) calculations with the aug-cc-pVnZ basis sets are listed in Table 27 and the basis set convergence errors are also plotted in Figure 13. As... 

---