Predictions of Electron Affinities

2 ELECTRON AFFINITIES OF PURINES AND PYRIMIDINES 12.2.1 Predictions of Electron Affinities 

Before 1990 there were no accurate experimental or theoretical values for the Ea of AGCUT. It was recognized that ionization potentials and electron affinities were important to charge transfer in biological processes, but there were three potential measures of the Ea. These were donor acceptor complex data, reduction potentials, and theoretical calculations, each of which resulted in different measures of the Ea. Much of our work during the past decade has attempted to reconcile these differences. 

In 1975 the anion of T was observed in a mass spectrometer, indicating a positive valence-state Ea for T. In 1990 the Ea of AGCUT were predicted using substitution, replacement, and conjugation effects [10-14], In order to estimate the Ea of substituted compounds, that of the parent compounds is required. In 1974 I. Nenner and G. J. Schulz estimated the AEa of quinoline (0.36 eV), pyradazine (0.40 eV), pyrimidine (0.00 eV), pyrazine (0.40 eV), and s-triazine (0.45 eV) from electron transmission spectra and half-wave reduction potentials [15]. No adiabatic electron affinities of aromatic nitrogen heterocyclic compounds were measured in the gas phase before 1989 [16]. 

In 1990 the AEa for C, U, and T were predicted to range from 0.6 eV to 0.75 eV, while the AEa of A and G were predicted to be higher. These estimates were based on an Ea of 0.3 eV for pyrimidine and the observation that C, U, and T are hydroxyl-, methyl-, and amino-substituted pyrimidines. From the data for pairs of molecules shown in Table 12.1, quantitative measures of the substitution, conjugation, and replacement effects were postulated. The replacement of a CH by N in an aromatic system increases the Ea by 0.2 eV to 0.8 eV. The substitution of a hydroxyl 

TABLE 12.1 Substitution and Replacement Effects on Electron Affinities [ 10 12,15 20] 

---

The eigenvalues of F are the orbital energies e. As we discussed when describing Koopmans theorem, the occupied orbital energies constitute a prediction of ionization potentials and the virtual orbital energies constitute a prediction of electron affinities. The values of are commonly a reasonable approximation to the observed ionization potentials, but — is usually of little use, even for a qualitative understanding of electron affinities. 

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]. 

In one of the most extensive studies of metal chloride catalysts,1 twenty of them supported on carbon were investigated, and a correlation was proposed between their activity and the electron affinity of the metal cation divided by the metal valence. Since the correlation consisted of two straight lines, it cannot be used predictively. However, electron affinity is necessarily a one-electron process, whereas hydrochlorination is more likely to be a two-electron process, involving the 2ir electrons of ethyne. Because many of the cations investigated are divalent, standard electrode potential was suggested1 as a more suitable parameter for correlating with activity. 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

Fig. 9. Size dependence of electron affinities (EA) of vanadium clusters. The abscissa is proportional to the reciprocal of the cluster radius. The solid line shows the predicted values by the spherical conducting drop model BA = Woa — c /(2fl), where IVoo is the work function of a bulk metal. The model well explains the experimental data above n 10. ...

A review of electron affinity and ionization potential prediction are... 

A Hbasis functions provides K molecular orbitals, but lUJiW of these will not be occupied by smy electrons they are the virtual spin orbitals. If u c were to add an electron to one of these virtual orbitals then this should provide a means of calculating the electron affinity of the system. Electron affinities predicted by Konpman s theorem are always positive when Hartree-Fock calculations are used, because fhe irtucil orbitals always have a positive energy. However, it is observed experimentally that many neutral molecules will accept an electron to form a stable anion and so have negative electron affinities. This can be understood if one realises that electron correlation uDiild be expected to add to the error due to the frozen orbital approximation, rather ihan to counteract it as for ionisation potentials. 

The semi-empirical methods have better MAD s than th Hartree-Fock-based methods, indicating that their parametrization ha accounted for some of the effects of electron correlation. However, thei maximum errors are very large. Semi-empirical methods are especiall poor at predicting ionization potentials and proton affinities. 

---