Oxidation frontier molecular orbital

An interpretation based on frontier molecular orbital theory of the regiochemistry of Diels Alder and 1,3-dipolar cycloaddition reactions of the triazepine 3 is available.343 2,4,6-Trimethyl-benzonitrile oxide, for example, yields initially the adduct 6.344... 

Prehminary AMI calculations carried out with the MOPAC program on 18 and related molecules suggest that there are atomic orbital contributions from the heteroatom (e.g., S in 18) to the frontier molecular orbitals. It is conceivable, therefore, that there is negative hyper conjugation involving specific orbitals of S and the P centers in 18. This electronic effect may explain the unusual stabiUty towards oxidation of 18 and other heteroatom functionaUzed primary bisphosphines as described above [51]. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

This chapter considers the oxidation of iodide in seawater by natural oxidants (02, H202, and 03). The oxidation of iodide to iodate is considered slow, yet the six-electron T-IOj redox couple normally used to represent the process (or predict stability) is thermodynamically favorable (2). We will discuss both one- and two-electron-transfer processes with these oxidants, focusing on the first step of electron transfer and using the frontier molecular orbital theory approach in conjunction with available thermodynamic and kinetic data. The analysis shows that the chemical oxidation of I to I03 is not a very important process in seawater, except perhaps at the surface microlayer. 

Fe(III) mineral reduction occur at roughly similar rates. This reactivity feature of the oxidation and reduction reactions for each system suggests a common pathway in the electron-transfer steps for the forward and reverse reactions in each system. However, the pathways for the iron and manganese reactions are discretely different from each other, which can be shown from the frontier-molecular-orbital approach. 

There are several major reaction types available to radicals coupling, addition, substitution, fragmentation, rearrangement, and oxidation. The frontier molecular orbital treatment shown here for addition to alkenes can, with modification, be used to predict reactivity in virtually all radical reactions, if the molecular orbitals for the reactive species are known. The following sections will describe the various radical reaction types. 

The question still remained could a trace amount of bis(p-oxo) species be the oxidizing agent Based on the upper concentration limit of bis(p-oxo) present, (0.0013 1 [Cu 02] [Cu2 (02)] ), its rate of reactivity would have to be > 10 times faster than that of the side-on peroxo in order to coincide with the kinetics observed. This would require the bis(p-oxo) core to be significantly more electrophilic than the side-on peroxo isomer. Frontier molecular orbital (FMO) theory was employed to assess the electrophilic character of the bis(p-oxo) species relative to that of the side-on peroxo isomer. The molecular orbital descriptions (specifically the percentage of ti character) and relative energies of the relevant LUMO s of the side-on peroxo... 

Figure 12.5. Plot of the frontier molecular orbitals involved in the oxidation mechanism (a) HOMO of the mercaptoethanol anion (b) LUMO of the Co(II) phthalocyanine.

This chapter has only scratched the surface of the opportunities and possibilities to demonstrate the natural relationship between chemistry and toxicology. Some of the topics that were not discussed include oxidation and oxidative stress, the influence of electrolytes on calcium homeostasis, immunotoxicology, focusing on biochemistry and applying frontier molecular orbital theory to predicting adverse outcomes. A discussion of catalysis may include the role of enzymes in toxication and detoxication, for example the mechanisms of action of organophosphate pesticides on the function of acetylcholinesterase. 

The adsorption of water on most metal surfaces is typically rather weak and controlled by a balance between the strength of the metal-water bond and the water waterl interactions. Molecular water adsorbs on metal and metal oxide substrates through the donation and back-donation of electrons between the frontier molecular orbitals of water and the states of the metal near the Fermi level. 

This process is very fast with = 1, and has been observed in the photolysis of nitrite esters X = NO and the oxidation of cyclopropanols, with a regiochemistry of ring opening which has been compared in terms of frontier molecular orbital theory to the one observed with cyclopropylcarbinyl radicals (Section V.2.B). It is also a fast process with n = 2 as in the oxidation of cyclobutanols, although photolysis of a steroidal cyclobutyl nitrite gives the cyclobutanol in substantial amounts (unexpected), along with products resulting from the )5-scission of the cyclobutanoxyl radical to tertiary and also... 

In Eq. 48.1, the value of the reaction constant p reflects the extent of interaction (electronic communication) between the substituent and the reaction center, that is, how much the measured potential depends on electron pushing/withdrawing ability of the substituent. In this way, a level of tt-delocalization along the molecule or displacement of oxidation and reduction centers, respectively, can be experimentally followed and correlated with quantum chemical calculations. Since the energy (potential) needed for oxidation or reduction of the studied molecule is proportional to the energy of the respective frontier molecular orbitals (and can be determined experimentally as and by an appropriate electrochemical method), their difference (E - E ) correlates well with the HOMO-LUMO gap obtained spectrophotometrically or using calculations. 

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