Ionization potential of substituted benzenes

Crable and Kearns, 1962) correlate with the a values of Brown and Okamoto (1958). Later studies also demonstrated correlations between the ionization potentials of substituted benzenes and a values (Buchs et al., 1964 Johnstone and Payling, 1968a). Appearance-potential measurements for the loss of methyl radicals from substituted acetophenones (Buchs et al., 1964) and substituted anisoles (Tait et al., 1962) correlated with a and cr+ values respectively. 

The ionization potentials of substituted benzenes are well correlated by a Hammett relationship. What would be the effect of the substituents in Problem 4.5 on the half-wave electrode potentials of a series of substituted benzenes ... 

As already mentioned in Section 2.3, donor ligands are expected to diminish the incorporation of a-olefins. For a proper selection of donor ligands we may compare the ionization potentials of substituted benzenes. Good donor substituents lift the energy level of the highest occupied molecular orbital of the aromatic molecule, i.e. they provoke a decrease of the first ionization potential as compared with that of unsubstituted benzene (see Table 3). 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

The ionization potentials of substituted cyclopropanes also show a significant correlation with eq. (2). The value of pr obtained is comparable to that observed for substituted ethylenes and 1-substituted propenes (section II.A.2.) and is considerably above that found for substituted benzenes (for which a value of Pr = 59 is obtained). This result confirms the existence of a large resonance interaction between the cyclopropane ring and substituents. The magnitude of a is considerably greater for substituted cyclopropanes than it is for substituted ethylenes or benzenes. 

In this section we shall examine the effects of n—n and n—n interactions on the ionization potentials of substituted ethylenes and benzenes. A theoretical analysis has already been given in section 1.1. In the space below we survey some pertinent data. 

Table 40. Ionization potentials of substituted ethylenes and benzenes (eV)...

It has been shown that the ionization potentials of substituted benzyl radicals (Harrison et al., 1961), and substituted benzenes and toluenes... 

As for the substituent effects cm the one-electron reduction potentials of substituted benzenes, the magnitude of the solvent effects increase linearly with the gas-phase ionization potential of the corresponding molecule. In conclusion, the more stable the radical cation is (i.e., the more delocalized the charge is), the less sensitive it is to its environment. 

Vertical ionization potentials of the rcs orbital in substituted benzenes were correlated with the LDRA equation (equation 70) to give the regression equation ... 

Fio. 18. Linear relationships between the ionization potentials of 4-substituted pyridines and the corresponding substituted benzenes (upper line —O—) and toluenes (lower line —X—). After Basila and Clancy (1963), electron impact data. 

Rafikov et al. [35] describe a correlation between the electron-donating capacity of substituted benzenes and the efficiency of adduct formation with maleic anhydride. This is only valid if similar compounds are compared. The ionization potentials of benzene and toluene are 9.246 and 8.820 eV, respectively the yields of adduct formation are 70% and 30%, respectively. In the series of halogenobenzenes, the ionization potentials are as follows fluorobenzene, 9.195 eV chlorobenzene, 9.080 eV bromobenzene, 9.030 eV the yields of adducts are 7%, 2%, and <1%, respectively. Anisole and diphenyl ether, with ionization potentials of 8.220 and 8.090 eV, respectively, do not give adducts with maleic anhydride. It thus seems that an increase in the electron-donating capacity of the benzene derivative leads to a decrease in the yield of photoadducts. 

Data on few substituted species are available. Monomethylation decreases the ionization potential from 9.47 to 9.15 eV, a change somewhat comparable to that for the acyclic analogs of CH2=CHCHO, MeCH=CHCHO and CH2=C (Me)CHO, 10.13,9.75 and 9.92 eV. Neither the considerable strain nor the significant aromaticity in cycloprope-nones, as shown by the earlier discussed thermochemistry of diphenylcyclopropenone, seem to have any effect on the ionization potential. As befits a compound with two benzene rings, the ionization potential of diphenylcyclopropenone is 8.1 eV. While comparison with (Z)-PhCH=C (Ph)CHO would be interesting, there are no data on the latter species. 

O A composite delocalized electrical-effect parameter of the (Tq type with r equal to 3.31. It is derived from ionization potentials of the lowest-energy 7t orbital in substituted benzenes. 

Substitution at the vinylic carbons can also be used to alter the electronic properties. For example, the substitution of electronegative -CN groups has been found to increase the electron affinity and ionization potential of PPV derivatives, thus improving the electron-injection characteristics.35,36 The synthesis is performed by the Knovenagel condensation polymerization of a terephthaldehyde and a benzene-1,4-diacetonitrile derivative, as shown in Fig. 5.10. 

FlO. 16. Ionization potential vs. log Aa/Jfei of substituted benzenes 1, Cells—CN 2, CeHs— NOs 3, CeHs—COCH3 4, CeHs—Cl 5, CeHs—Br 6, CeHs—ClCHsls 7, CeHs—8, CeHs— 9, CeHs—CsHs, 10, CeHs—CHlCHsls 11, CeHs—CH3 12, CeHs—OH 13, CeHs—OCH3. (Reproduced from Whittemore et al., 1962, with permission.)... 

The gas-phase ionization potentials have been measured for a large number of substituted benzenes. A comprehensive and continuously updated review can be found in ref. [37]. 

As for benzene radical cations in solution, substituents containing S and Se deviate from the general trend. Interestingly, also the halogenated benzenes (except CjHsF) follow the same trend as the S- and Se-substituted benzenes, i.e., the ionization potential of the substituent halogen atom rather than the substituent effect as described by the substituent constant governs the ionization potential. 

If we treat the substituent effects on the ionization potential of 1,4-substituted benzenes in the same way as the one-electron reduction potentials in aqueous solution we obtain equation (13) (based on data from ref. [37]). 

In a related example, the photolysis of cerium(IV) ammonium nitrate generates the NO radical (Eq. 10) which has recently been demonstrated to react with alkylbenzenes via an electron transfer reaction. This particular oxidant is able to generate radical cations of substituted benzenes with ionization potentials less than that of p-xylene (8.44 eV). 

The preference of this conformation (the barrier hindering the rotation of the p-XCgH — at X = Cl, CHj, CFj amounts to 8.5 kcal/mol., cf. also ° 0 is assumed to be due, at least partially, to the donor-acceptor interaction between the K-system of the aryl residue and the carbenium centre at Cjg in the electronic absorption spectra of the 9-p-X-phenyl-9,10-dimethylphenanthrenium ions the charge transfer bands have been revealed whose position correlates well with the ionization potentials of respective X-substituted benzenes. Judging by the NMR- C spectra, however, the extent of this interaction in the main state of ions is insignificant — the chemical shift of the Cjo atom nearly remains unchanged as the X substituent... 

Table 3. Ionization potentials of some substituted benzenes (25)...

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