Ionization potentials electron donors

Charge-transfer complexes are specific associations resulting from the interaction between molecules of low ionization potential (electron donor D) on one hand and molecules of high electron affinity (electron acceptor A) on the other hand. Following the electronic structure of the interacting species the complexes may be of n — n,n — n etc. type [11]. 

It should be mentioned that one can detect two types of equilibrium in the model of charge transfer in the absorbate - adsorbent system (i) complete transition of chemisorbed particles into the charged form and (ii) flattening of Fermi level of adsorbent and energy level of chemisorbed particles. The former type takes place in the case of substantially low concentration of adsorbed particles characterized by high affinity to electron compared to the work function of semiconductor (for acceptor adsorbates) or small value of ionization potential (for donor adsorbates). The latter type can take place for sufficiently large concentration of chemisorbed particles. 

In principle, the behaviour of any molecular species in forming donor-acceptor complexes depends on its ionization potential, electron affinity and polarizability. However, the donor (or acceptor) ability of a substance depends strongly on the requirements and properties of its partners. The same compound may act as a donor towards strong acceptor compounds or as an acceptor towards donor compounds. This is the case of the TT-amphoteric p-tricyanovinyl-AA/V-dimcthylaniline (41) which is a donor towards 2,4,7-trinitrofluorenone and an acceptor towards /V,/V-dirnclhy Ian Mine138. 

The ability of molecules to form donor-acceptor complexes depends not only on their ionization potential, electron affinity and polarizability, but also on the requirements and properties of partners. 

The unsubstituted thiopyrylium ion (2) has been found to form CT complexes in CHjCN with both olefins and aromatic hydrocarbons (72CL17 75BCJ1519). Two CT absorption bands have been observed in the former case, and one in the latter. The slope obtained by the plot of the CT transition energies vs the ionization potentials of donors is 0.27 for the olefin complexes and 1.04 for the aromatic hydrocarbon complexes. These slopes suggest that 2 interacts with olefins more strongly than with aromatic hydrocarbons. Strong interactions in the olefin complexes would manifest themselves also in the appearance of two CT bands. These have been ascribed to electronic transitions from the HOMO of the olefin donor to the lowest and the second lowest vacant orbital of 2. The CT absorption frequencies of the complexes of 2 with olefins and aromatic hydrocarbons have been used to calculate their heat of formation by an empirical relation (81MI4). 

By using 7r-donor solvents it was observed that the rate constants could be well correlated with specific solvent effect parameter (7 ionization potential of donor) when the substrate is an electron acceptor. No correlation with Ijy was obtained for donor substrates. 

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Let us discuss now the conditions required for the electron transfer process. This reaction requires, of course, a suitable electron donor (a species characterized by a low ionization potential) and a proper electron acceptor, e.g., a monomer characterized by a high electron affinity. Furthermore, the nature of the solvent is often critical for such a reaction. The solvation energy of ions contributes substantially to the heat of reaction, hence the reaction might occur in a strong solvating solvent, but its course may be reversed in a poorly solvating medium. A good example of this behavior is provided by the reaction Na -f- naphthalene -> Na+ + naphthalene". This reaction proceeds rapidly in tetrahydrofuran or in dimethoxy... 

Since the energy of the transfer band is determined by the difference between the donor ionization potential and the acceptor electron affinity, this fact points to the increase of the PCS ionization potential with decreasing conjugation efficiency. Therefore, the location of the transfer band of the molecular complexes of an acceptor and various PCSs can serve as a criterion for the conjugation efficiency in the latter. In Refs.267 - 272) the data for a number of molecular complexes are given, and the comparison with the electrical properties of the complexes is made. 

Reactions between much stronger donors and acceptors belong to the electron tranter band. Such olefins do not form cyclobutanes but ion radical pairs or salts of olefins. refrato(dimethylamino)elhylene has an ionization potential as low as Na. The olefin with extraordinary strong electron-donating power is known not to undergo [2+2]cycloaddition reaction, but to give 1 2 complex with TCNE (transfer band in Schane 3) [23]. 

Radicals can be prepared from closed-shell systems by adding or removing one electron or by a dissociative fission. Generally speaking, the electron addition or abstraction can be performed with any system, the ionization potential and electron affinity being thermodynamic measures of the probability with which these processes should proceed. Thus, to accomplish this electron transfer, a sufficiently powerful electron donor or acceptor (low ionization potential and high electron affinity, respectively) is required. If the process does not proceed in the gas phase, a suitable solvent may succeed. 

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