Ionization potentials substituent effect

Interestingly, it has been observed that the ionization potentials (determined by the electron-impact technique) of several substituted five-membered heterocycles correlate with the substituent effects is larger than for benzene (p = —14.7) and larger than that measured for the most sensitive electrophilic substitution (bromination in acetic acid, which gives p = —10.0 for thiophene). As pointed out by Linda et al. [12], the sensitivity to substituent effects... 

Use PMO theory to describe the effect of the substituents on the ionization potential. Use an MO diagram to explain the interaction of the substituents with the n bonds. Explicitly take into account the fact that the two orbitals interact and therefore cannot be treated as separate entities (see Problem 10). 

Eq. (1) has potential application to other types of measurements of substituent effects besides those specifically considered in this paper e.g., nmr coupling constants and shifts for other nuclei, ir and uv spectral shifts and intensities. We caution (with emphasis) in these applications the needed use of data sets of high quality, both with respect to the precision of the measurement and substituents considered (i.e., a full complement of substituent o/ and Or properties must be encompassed for a meaningful correlation to be obtained). There is, of course, no requirement that all data sets will be uniquely fitted by eq. (1) using one of the four or scales of Table V. For example, the data for the ionization of the conjugate acids of pyridine-N-oxides (30), HjO, 25° is found to fit equally well the or(ba.) or Or scales (SD=. 14 /=. 072). The data (31) for the rates of alkaline ("OMe) cleavage of ArSnMea are not fitted to acceptable precision (fs >. 23) by any of the Or parameters. This data set is nevertheless indicated... 

Incidentally, oxidation data of the pyrrole monomers show an interesting increase in oxidation potentials when containing heavier substituents (Table 25). However, the ionization potential of N -methylpyrrole (7.95 V) is smaller than that of pyrrole (8.21 V). The accepted linear relationship between ionization potential and oxidation potential210 would have it the other way round. Considering, however, that trimethylsilyl and trimethylgermyl groups are weak electron donors211, it is plausible that a nonelectronic effect is responsible for the observed trend and the potential shifts are associated with steric effects. 

With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic colour changes are manifested in the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (IP) in the order benzene 9.23 eV, naphthalene 8.12eV, and anthracene 7.55 eV, much in the same manner as that observed with the tropylium acceptor (Takahashi et al.,... 

The effect of substituent groupings upon the first ionization potential of, at least, the aliphatic aldehydes and ketones does not seem to provide clear evidence for a choice between loss of a 7r-electron from the C=0 double bond or one of the oxygen lone-pair electrons. Concordant data for the homologous series of aliphatic ketones have been obtained by... 

On the assumption that all the reported aldehyde first ionization potentials refer to the oxygen lone-pair electrons Cook (1958) has classified the effect of substituents into two classes, A and B, according to whether inductive or resonance effects predominate. Two different linear correlations (Fig. 14) were found between ionization potential and the carbonyl stretching frequency. Anomalies were noted for diacetyl, benzaldehyde and mesityl oxide, ascribed in the last instance to noncoplanarity interfering with resonance. It seems more likely, however, that in these cases the first ionization potential refers to 7r-electrons and higher values for the lone-pair electrons (as yet undetermined) might remove the anomalies. 

These effects of electron withdrawal from and release to the ring also find quantitative expression in the <7+ substituent constants (Brown and Okamoto, 1967), and linear correlations between cr+ values and ionization potentials have been reported (Crable and Kearns, 1962). More recently, the very strongly electrophilic reagent CF3 has been shown to attack the benzene ring at a rate, k, determined by an activation energy linearly related to the ionization potential of the benzene electrons in... 

The results of the alkylbenzene series may also be readily explained in terms of ir complex adsorption. In this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors IT complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36). From previous data (Tables II and III) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of tt complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. 

This result is similar to that obtained by Widing and Levitt for the normal alkanes and to that observed for alkyl-substituted ethylenes [171] in the latter case, linear correlations were obtained between the IPs and the sum of charges of the unsaturated carbon atoms, whereby any increase of their electron density due to substituent effects leads to a lowering of the molecular ionization potential. 

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Ostoja Starzewski and Bock83 reported the photoelectron spectra of an extensive series of phosphonium ylides, focusing on the substituent effects. Their lowest ionization potential (IP) is associated with the carbanion orbital. Their main results are given in Table 9. A number of important trends can be identified and interpreted. Replacement of the methyl groups on P with phenyl groups reduces the IP of the carbanion. The phenyl group is able to stabilize the P+ charge, which reduces the ability of the phosphonium to stabilize the... 

Tolman notes that the high sensitivity of the Ni(0) equilibrium constants to structural modifications of the alkene is due to the low ionization potential of Ni(0) and the resulting small energy separation between the HOMO of the metal and the pi orbital of the alkene. Steric effects of substituents... 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

DiLabio, G. A. Pratt, D. A. Wright, J. S. Theoretical calculation of ionization potentials for disubstituted benzenes additivity vs non-additivity of substituent effects./. Org. Chem. 2000, 65, 2195-2203. 

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