Electron affinities, gas phase

Since the values of for many organic acceptors are generally unobtainable (in organic solvents), an alternative measure of the electron-acceptor property is often based on the irreversible cathodic peak potential F (in cyclic voltammetry). Thus for a series of related compounds, Fig. 6 shows that the values of Fred are linearly related to gas-phase electron affinities (EA).70... 

Although the same limitations apply to the use of F as those described above for the anodic counterpart, the global trend in Fig. 7 shows that gas-phase electron affinities also generally reflect the trend in the reduction potentials measured in solution for the large variety of (uncharged) acceptor structures included in Table 4.71... 

Fig. 6 A linear correlation of the reduction potentials Ertd (V versus SCE) with the gas-phase electron affinities EA (eV) of various nitrobenzenes and quinones. Reproduced with permission from Ref. 70.

First, high-mobility anions occur both in liquids whose molecules have negative and positive EAg. The gas-phase electron affinity has no effect on the rate and the activation barrier of electron hopping in neat liquid solvents. 

Here fD is the first gas-phase ionization potential of D, and Aa is the first gas-phase electron affinity of A. If the term in braces remains approximately constant as one changes D, A, or solvents, then hvCT is approximately linear... 

Gas Phase Electron Affinities of DNA Bases and Base pairs (Valence and Diffuse States)... 

Table 20-6. Gas phase electron affinities (eV) derived from experiment and theory... 

Knowledge of the gas phase electron affinities for a series of molecules, such as the fluoroethylenes, provides new insight into substituent effects. The correlation between the trends in the singlet and triplet excitation energies of the neutral molecules and the quantity (IP - EA) is also examined. 

Measures gas-phase electron affinities corresponding to electron capture into low-lying unoccupied molecular orbitals (i.e., the energy of the anion state arising from electron capture)... 

Here Ip is the first gas-phase ionization potential of D, and A is the first gas-phase electron affinity of A. If the term in braces remains approximately constant as one changes D, A, or solvents, then hvcx is approximately linear with Ip - A. Ip is easily measured experimentally, A is difficult to measure. Hence A has been estimated approximately from CT bands, or from shifts in solution electrochemical half-wave reduction potentials E 2, relative to known compounds. Table 1 is a short list of Ip and A values [78]. 

The reactions of organic molecules in solution are related to gas phase electron affinities and electronegativities. Anions are often intermediates in such reactions. The electron conduction of polymers is related to the electron affinities of the components. The theoretical calculations of electron affinities of aromatic hydrocarbons and the effect of substitution on electrons affinities and gas phase acidities are important to organic chemistry. Pseudo-two-dimensional Morse potentials have been used to represent the dissociation of organic molecules and their anions [18]. 

When the ECD electron affinities were measured, the AAG for the aromatic hydrocarbons (2.0 eV) and the aromatic aldehydes and ketones (2.3 eV) were observed to be approximately constant within the class of molecules, but different from each other. The fiillerenes ( AAG = 1.76 eV) have a lower charge density and lower AAG than the majority of other compounds. With the determination of more gas phase electron affinities, the AAG values range from 1.7 eV for larger fullerenes to 2.7 eV for small anions [3, 10]. 

In this chapter the experimental ECD and NIMS procedures for studying the reactions of thermal electrons with molecules and negative ions are described. Gas phase electron affinities and rate constants for thermal electron attachment, electron detachment, anion dissociation, and bond dissociation energies are obtained from ECD and NIMS data. Techniques to test the validity of specific equipment and to identify problems are included. Examples of the data reduction procedure and a method to include other estimates of quantities and their uncertainties in a nonlinear least-squares analysis will be given. The nonlinear least-squares procedure for a simple two-parameter two-variable case is presented in the appendix. 

Figure 1 shows a plot of the gas-phase electron affinities of a number of anions plotted against the gas-phase proton affinities. The circles refer to anions where the donor atom is a second-row element, C,N, O, or F. The straight line is a least-squares fit to these values only. The slope is — 0.87 and the correlation coefficient is —0.923. In spite of the scatter, clearly a strong negative correlation between the electron affinity of a radical and the proton affinity of its anion occurs. A strong base, such as CH3-, loses its electron readily, whereas P03- (metaphosphate ion) is a weak base and is difficult to oxidize. 

The feasibility of electron transfer from alkoxides to acceptor species, attractive as it is for its simplicity, has been prospected as a real possibility only in a few instances (44, 45a) and, in some cases, questioned (9, 45b, 46a). Buncel and Menon (45b) estimated that electron transfer from t-BuO- to 4-nitrotoluene in f-BuOH is energetically unfavorable to the extent of about 3 eV. The estimate is based on calculations involving gas-phase electron affinity data for t-BuO (1.93 eV) instead of its oxidation potential in solution, which, as is the case for other alkoxides, is not known. The approximation is necessarily crude, because solvation effects could be of significant magnitude nevertheless, the estimated value has met wide acceptance. 

Electron transmission spectroscopy of 3 and 4, which measures gas-phase electron affinities, reveals an even larger splitting (ca. 1.6 eV) of the nonbonding orbitals in these pericyclynes [9]. The lowest unoccupied molecular orbitals (LUMOs) are stabilized by 0.4-0.7 eV relative to the LUMO of acetylene. 

Nonetheless, gas-phase acidities are valuable since when they are combined with electron affinities ( A) of the radical related to the conjugate base, they can be used to determine bond dissociation energies BDEf as shown in equations 7-10. This is particularly useful for species whose BDE may not be directly measured via traditional techniques l Unfortunately, little progress has been made in this area since there is also a dearth of data on gas-phase electron affinities of phosphorus species". Berger and Brauman" have commented on how both the EA and BDE can influence the gas-phase acidity of a species. 

Reactions of Sp6 have been studied extensively as a means of determining gas-phase electron affinities. At first, most reactions were found to be close to the diffusion-controlled limit, and slow reactions were deduced to be endoergic. Since then, a number of relatively slow exoergic reactions have been discovered.They imply an energy barrier associated with formation of the collision complex which has been explained as a reorganization energy due to structural differences between SF and... 

A bridging ligand reduction model vs. the outer-sphere mechanism for electron transfer has been tested using rate constants from Cr(II) systems. A correlation between the rate constant and the gas-phase electron affinity of the bridging group implies an inner-sphere mechanism. If such a correlation is absent an outer-sphere mechanism is assumed. ... 

Ong, S. P. Ceder, G., Investigation of the Effect of Functional Group Substitutions on the Gas-Phase Electron Affinities and Ionization Energies of Room-Temperature Ionic Liquids Ions Using Density Functional Theory. Electrochim. Acta 2010,55, 3804-3811. 

Figure 1 Four one-electron donors D (1-4) with their first gas-phase ionization potentials four one-electron acceptors A (5-8) with their gas-phase electron affinities Aa, and three metals and one semimetal with their bulk work functions p. The abbreviations is N, N, N, Af -tetramethyl-para-phenylenediamine (TMPD), bisethylenedithiolene (BEDT), p-benzoquinone (BQ), and 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ).

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