Alternant aromatic hydrocarbons

The replacement of the —CH= group in an alternant aromatic hydrocarbon with a heteroatom gives rise to the alternation of the 7r-electron density at the ring atoms. As a result, the aromaticity should, according to the Julg criterion, Eq. (21), be somewhat reduced. Indeed, the values of A found from the data of MNDO calculations on benzene (22), boraben-zene (33), and pyridine (34) are, respectively, 1.0,0.939, and 0.971. Homo-desmotic stabilization energy calculations indicate (Table II) that pyridine is, in its aromatic character, only slightly inferior to benzene. The values... 

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

Tab. 8.8 Oxidation potentials, reduction potentials, ionization potentials and electron affinities of alternant aromatic hydrocarbons (AAHs) ...

In the case of the alternant aromatic hydrocarbons, w, = w2 and a = b hence, the transition moment of the a band is zero. Since the oscillator strength is proportional to the square of the transition moment, the a band is forbidden, and its absorption intensity is very weak. For alternant aromatic nitrogen compounds, however, the values of w, and a are not equal to those of 2 and b, respectively hence, its transition moment does not vanish. Hence, the a band is allowed, and its absorption strength is stronger than that of the parent hydrocarbon. But it is still rather weak, because the value of a is near to that of b, and the transition moment (ML) is not so large. 

This mechanism does not constitute a major reaction pathway for the anodic oxidation of alternant aromatic hydrocarbons (AAH) or alkyl-substituted AAH under common experimental conditions [12,16-19], but has been found to be operative, for example, for tetra-arylethylenes [20-23]. The difference in the reaction pattern of AAH and tetraarylethylenes is related to the rotational freedom around the central C-C bond experienced by the radical cations of the latter [24]. This results in a small or even negative difference between the standard potentials for the first and second one-electron transfer, E and E2, and, accordingly, the value of AT(j,spr [Eq. (15)] may take relatively large values. 

As more experimental values of both electron affinities and ionization potentials were measured, this relationship was tested. For the alternate aromatic hydrocarbons the EN is approximately 4.02 eV, as opposed to the work function of graphite that is 4.39 eV. The EN for the smaller aromatic hydrocarbons is 4.1 eV. The EN for hydrocarbons with hve-membered rings, 4.4 eV, and Cwork function of graphite. Table 4.4 gives the Ea, IP, and EN values for several hydrocarbons. From a larger set of data the EN is not constant. If the values for styrene, fluoranthene, naphthalene, styrene, and azulene are not included, then EN = 4.02 0.02 eV can be used to calculate either the Ea or IP. The calculated Ea are compared to the ECD values in Table 4.4 [10]. 

The potentials of reversible one-electron oxidations (reductions) of alternant aromatic hydrocarbons have been found to give good straight lines when plotted against the calculated energies of the highest occupied molecular orbitals (HOMOs lowest unoccupied molecular orbitals = LUMOs) [Heilbronner (91)]. The results indicate two important trends ... 

Hartree-Fock eigenvalue of the highest occupied molecular orbital the electron affinity should approximate to the eigenvalue of the lowest unoccupied molecular orbital. According to Pople s theory, the mean of these two quantities should be the same for all alternant aromatic hydrocarbons and equal to the work function of graphite Pople and Hush showed that this relationship is verified by experiment. Brickstock and Pople1 extended the theory to radicals and ions. 

Cob is the nonbonding MO (NBMO) coefficient on the carbon atom where the oxirane ring is opened. It is calculated according to the Longuet-Higgins zero sum rule for odd alternant aromatic hydrocarbons [36,65], Incidentally, the first authors to try such NBMO coefficients of exocyclic atoms of odd-alternant PAH derivatives were Dipple, Lawley and Brookes [66] in 1968. 

The electronic spectra of the mono- and divalent negative ions in the 5,000 -45,000 cm region of various alternant aromatic hydrocarbons (51-55) and l,n-diphenylpolyenes (56, 33) have been reported. The transitions observed in this spectral range are excitations within the delocalized 7T-electron system. Hoijtink et al. (57-61) have given a satisfactory explanation for the experimental results in relatively simple quantum-mechanical terms. 

The experimental spectrum of alternant aromatic hydrocarbons will generally display a strong absorption band due to the transition to the plus state, a medium weak one due to the transition n+1 <- , and a very weak one. since the transition to the minus state is formally forbidden. It should be noted that Clar 40) has introduced for the transition n + l- w the symbol p and for the plus and minus states the notation jS" " and a. 

An interesting confirmation of these ideas is provided by the rates of solvolysis of jS-arylethyl esters, ArCH2CH2X, with ArH being an alternant aromatic hydrocarbon. Primary chlorides normally react by the 5 2 mech-... 

In an attempt to determine the factors which control the rate of protonation, we excunined the correlation of these rates with some basic properties of the molecules, such as electron affinity, ionization potential, and singlet energy, which are known to be interrelated for alternant aromatic hydrocarbons. 

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