Electron attachment temperature dependence

A number of poly(arylene vinylene) (PAV) derivatives have been prepared. Attachment of electron-donating substituents, such as dimethoxy groups (structure 19.3), acts to stabilize the doped cationic form and thus lower the ionization potential. These polymers exhibit both solvatochromism (color changes as solvent is changed) and thermochromism (color is temperature-dependent). 

Figure 2 Dependence on C2H4 density of the effective two-body rate constant of thermal electron attachment in O2-C2H4 mixtures at room temperature. (From Ref. 52.) The dashed curve represents the expected contribution from the BB mechanism.

Figure 3 The temperature dependence of the three-body rate constant of O2. (From Ref. 58.) The broken curve shows the temperature dependence of the rate constant calculated from Herzenberg s theory. The solid curve shows a calculated rate constant, which involves both the contributions from the broken curve and the rate constant due to electron attachment to van der Waals molecule (02)2-...

Electron attachment to solutes in nonpolar liquids has been studied by such techniques as pulse radiolysis, pulse conductivity, microwave absorption, and flash (laser) photolysis. A considerable amount of data is now available on how rates depend on temperature, pressure, and other factors. Although further work is needed, some recent experimental and theoretical studies have provided new insight into the mechanism of these reactions. To begin, we consider those reactions that show reversible attachment-detachment equilibria and therefore provide both free energy and volume change information. 

An interesting example of a diffusion-controlled reaction is electron attachment to SFg. Early studies showed that in -alkanes, k increases linearly with over a wide range of mobilities from 10 to 1 cm /Vs [119]. Another study of the effect of electric field E) showed that in ethane and propane, k is independent of E up to approximately 90 kV/cm, but increases at higher fields [105]. This field is also the onset of the supralinear field dependence of the electron mobility [120]. Thus over a wide range of temperature and electric field, the rate of attachment to SFg remains linearly dependent on the mobility of the electron, as required by Eq. (15). 

When rotation occurs about a bond there are two sources of strain energy. The first arises from the nonbonded interactions between the atoms attached to the two atoms of the bond (1,4-interactions) and these interactions are automatically included in most molecular mechanics models. The second source arises from reorganization of the electron density about the bonded atoms, which alters the degree of orbital overlap. The values for the force constants can be determined if a frequency for rotation about a bond in a model compound can be determined. For instance, the bond rotation frequencies of ethane and ethylamine have been determined by microwave spectroscopy. From the temperature dependence of the frequencies, the barriers to rotation have been determined as 12.1 and 8.28 kJ mol-1, respectively1243. The contribution to this barrier that arises from the nonbonded 1,4-interactions is then calculated using the potential functions to be employed in the force field. 

The temperature dependence of the rate constant of electron transfer over large distance from the first triplet state of Zn porphyrin to Rum(NH3)5 covalently attached to histidine-33 in Zn-substituted cytc was studied in Ref. [318]. A temperature independent triplet quenching process with the rate constant 3.6 s-1, was observed at 10-100 K and tentatively attributed to electron transfer facilitated by nuclear tunneling. 

A major difficulty with this analysis, however, is that the assumption AS t % 0 requires that the solvation environment of the transition state is unaffected by its proximity to the electrode surface (Sect. 3.4). Stated equivalently, it is often expected that the temperature-dependent work terms required to extract kscorr from k ob contain large components from short-range solvation and other factors in addition to the usual "electrostatic doublelayer effects (Sect. 2.4 and 4.6). As noted in Sect. 2.3, the situation is somewhat more straightforward for surface-attached reactants since then the effects of work terms at least partly disappear. This question underscores the inevitable difficulties involved in extracting quantitative information on electron-transfer barriers from rate measurements. 

Warman JM, Sauer MC. (1971) The temperature dependence of electron attachment to CCI4, CHCI3 and CjHjCHjCl. Int JRadiat Phys Chem 3 273-282. 

To test the ECD hypothesis, E. C. M. Chen measured the temperature dependence of the molar response. This entailed a detailed study of all parameters associated with the pulse sampling method. For these molecules the most important reactions were postulated to be electron generation and collection, electron and ion recombination, electron attachment and detachment. It was discovered that the simple thermodynamic model was not applicable and that a kinetic model was necessary to explain the change in temperature dependence. If we assume a steady state exists, an expression can be obtained that relates the ECD molar response to kinetic rate constants for the above reactions [24, 25],... 

In 1967 B. H. Mahan and C. E. Young used a new microwave method to determine the rate constant for thermal electron attachment to molecules. These quantities were determined for SF6 and C7F14 using the ECD and agreed with the values reported using the microwave method at room temperature within the experimental error [37, 38]. In addition, the temperature dependence was determined so that activation energies were obtained. This was especially important in the case of strained molecules such as cyclooctatetrene [34],... 

This expression is obtained by assuming equilibrium between the two states. If we assume nondissociative electron attachment, this equation can reproduce the temperature dependence for O2, NO, CS2, C,I flCI. tetracene, and anthracene where excited state electron affinities have been measured in the gas phase [29, 103, 104]. The extension of the ECD model to two negative-ion states explains the structure in the data. 

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

The electron attachment reactions for inorganic molecules were reviewed in 1974. Those for organic molecules were summarized in 1984 [12, 13]. In many cases activation energies were not measured. If a nominal value for A and A is assumed, the activation energies can be estimated. Recently, the flowing afterglow procedure has been extended to include electron and gas heating so that the dependence of the rate constants on thermal electron attachment can be examined for bulk temperature and electron temperature [14]. 

The azide radical is linear, as is its anion. Ozone, sulfur dioxide, and N02 are bent in the neutral form and in the anion. The neutrals of C02, CS2, COS, and N20 are linear and the anions bent, as predicted by simple molecular orbital theory. The electron affinities of these species are uncertain [90-102], In Figure 9.17 the ECD data for these compounds are shown. The diversity in the temperature dependence of the isoelectronic molecules is remarkable. The ECD response of C02 is unexplained. The temperature dependence for CS2 is typical of nondissociative electron attachment to two states. The low-temperature dependence of COS is unusual, but can be attributed to attachment to two states with a low A. Dissociative electron attachment and molecular ion formation are observed in the ECD data for N20. Consequently, only an upper limit to the electron affinity may be obtained [90-93],... 

The temperature dependence of the alkyl halides was one of the first subjects to be studied using the ECD. These are the simplest to analyze because often there is only one temperature region when dissociative thermal electron attachment is exothermic. This means that the EDEA, the energy of dissociative electron attachment, is positive EDEA = a(X) - D(R —X). The alkyl bromides, iodides, and chlorides are among the few organic compounds that have positive EDEA. Like the homono-nuclear diatomic halogen ions, the ground-state anionic curves for these molecules are M(3), with positive values for all three Herschbach metrics—EDEA, Ea, and VEa. 

Some of the above alkyl halides contain fluorine. In the case of CF3C1 the activation energy indicated that two negative-ion curves contribute to the dissociative thermal electron attachment. In 1997 S. R. Sousa and S. E. Bialkowski carried out a classic study of the ECD temperature dependence of alternative fluorocarbon freon replacements [16]. The study used a commercial ECD and the fundamental concepts discussed in this book to obtain rate constants and energies for compounds. One of these, CF2C12, was used as an internal standard in all the measurements. The... 

The values of the electron affinities and rate constants for thermal electron attachment to the purines, pyrimidines, and heterocyclic compounds can be used to predict the temperature dependence of the ECD and NIMS response. These are similar to those made for the chlorinated biphenyls and naphthalenes in... 

As already mentioned, Ballhausen was much interested in the coupling between electronic and nuclear motions. Spin-orbit coupling and magnetic interactions also had his interest. These couplings express themselves in the fine structure of spectra and in the temperature dependence of the fine structure. The form of the fine structure is too intricate to be detailed here. But as the attached list of publications shows, it is treated in several of his articles. 

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