Highest occupied molecular orbital hydrocarbons

Figure 2. Minimum atomic localization energy in benzenoid hydrocarbons plotted against energy of the highest occupied molecular orbital (HOMO).

These interactions may usefully be described as an acid-base type interaction, in which the cyclopropane ring acts as a base (electron donor) and the proton or cationic center acts as the acid (electron acceptor). One of the factors that controls the basicity of a hydrocarbon is the energy of the highest occupied molecular orbital (HOMO).60 The 6-31 G HOMO energies of some cycloalkanes and cycloalkenes are given in Table 4.61... 

In solvents of weak basicity (e.g. AN), aromatic hydrocarbons (AH) are oxidized, at least in principle, in two steps. In the first step, the compound gives an electron in its highest occupied molecular orbital (HOMO) to the electrode to form a radical cation (AH +), whereas, in the second step, it gives another electron to the electrode to form dication AH2+ ... 

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. 

The PE studies on small ring hydrocarbons reveal a considerable interaction between their highest occupied molecular orbitals and those of a tt system or another strained ring system. 

Photoelectron spectral studies on pyridine, and on 2- and 3-fluoro-, 2,4-and 2,6-difluoro-, 2,4,6-trifluoro-, and pentafluoro-pyridine have enabled deductions to be made about the highest occupied molecular orbitals of the pyridine nucleus, and pentafluoropyridine and cyanuric fluoride have been compared with their hydrocarbon analogues. ... 

Another example that illustrates the effect of the overlap density upon the magnitude of K j involves two conjugated hydrocarbons. Listed in Table 8.1 are the experimental values of the first IP, the EA, the first singlet excitation energy ( — g) and the first triplet excitation energy (Ej — Ec) for azulene 8.7 and anthracene 8.8 [6]. For simplicity of notation, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 8.7 and 8.8 are denoted by the... 

Conversely, nucleophilic molecules Nu) [Lewis bases e.g., catechols, hydroquinones, phenols, alcohols, and thiols (and their anions) aromatic hydrocarbons and amines (benzene, toluene, pyridine, bipyridine, etc.)] can be oxidized (1) by direct electron-transfer oxidation [Eq. (161)] [with the electron coming from the Highest-Occupied-Molecular-Orbital (HOMO)] or (2) by coupling with the oxidation product of H2O (or HO ), hydroxyl radical (HO ) [Eq. (162)]. 

Originally invoked for electroluminescent reactions, this idea has now been developed for the reaction of peroxides with fluorescent compounds of low ionisation potential [51]. Many of these reactions are discussed in Chap. XI, but Fig. 2 can be most succintly exemplified by the radical ion annihilation shown, where Ar is a fluorescent aromatic hydrocarbon such as diphenylanthracene, LUMO is the lowest, normally unoccupied molecular orbital and HOMO is the highest occupied molecular orbital. 

The first paper of the frontier-electron theory pointed out that the electrophilic aromatic substitution in aromatic hydrocarbons should take place at the position of the greatest density of electrons in the highest occupied (HO) molecular orbital (MO). The second paper disclosed that the nucleophilic replacement should occur at the carbon atom where the lowest unoccupied (LU) MO exhibited the maximum density of extension. These particular MO s were called "frontier MO s . In homolytic replacements, both HO and LU.were shown to serve as the frontier MO s. In these papers the "partial" density of 2 pn electron, in the HO (or LU) MO, at a certain carbon atom was simply interpreted by the square of the atomic orbital (AO) coefficient in these particular MO s which were represented by a linear combination (LC) of 2 pn AO s in the frame of the Huckel approximation. These partial densities were named frontier-electron densities . 

The following quantities are presented in Table IX total 77-elec-tronic energy (W), its deviation from that of the parent hydrocarbon (AW), energies of the four frontier 7r-molecular orbitals (k2, klt and Jfc 2 k is the highest occupied orbital, HOMO is the lowest free orbital, LFMO), Fj transition energy [ (A - Ft)] and the atom localization energy for the parent hydrocarbon in the position in which the heterocycle has a nitrogen atom. 

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