Delocalized HOMO

The redox properties of cyclic polysilanes are interesting because they resemble those of aromatic hydrocarbons. For instance, cyclic polysilanes can be reduced to anion radicals or oxidized to cation radicals. ESR spectra for both the cation and anion radicals indicate that the unpaired electron is fully delocalized over the ring [17,19,20]. The aromatic properties of the cyclic polysilanes are ascribed to a high energy delocalized HOMO and a relatively low energy LUMO. Because the HOMO and LUMO levels lie at similar level to those of benzene, cyclic polysilanes can serve either as electron donors or electron acceptors. 

Dewar and Rzepa found that the MNDO (Section 6.2.5.3) electron affinities of 26 molecules with delocalized HOMOs (mostly radicals and conjugated organic molecules) had an absolute mean error of 0.43 eV for ten molecules with the... 

The interpretation of deuterium kinetic isotope effects, for substitution of D for H on a carbon bonded to the sulfur, invoked the change in C-H stretching force constants upon removing an electron from the delocalized HOMO of the sulfide, a molecular orbital with significant contributions from the hydrogen Is orbitals. 

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

The HOMO is quite delocalized and has contributions from the phenyl rings. Nitrogen p orbitals figure prominately at the center of this orbit.il. and these are all in phase with one another (the positive lobes all line up). 

Beeause the anion acts as an electron donor, we can find clues to its reactivity preferences by examining the shape of its HOMO. The HOMO is delocalized over several sites, but the largest contribution to the HOMO clearly comes from the terminal carbon atom. Therefore, we expect electron movement and bond formation to occur at this carbon, and lead to the product shown on the left. 

Finally, display the highest-occupied molecular orbital (HOMO) of triphenylphosphinemethylidene. Is it primarily concentrated on the methylene carbon as would be expected of a fully-developed anion, or is it delocalized over both phosphorous and carbon Does this suggest that the molecule incorporates a n bond ... 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 proto noted pyrrole) is lowest in energy Examine electrostatic potential maps to see if the lowest-energy form is also that in which the positive charged is best delocalized. Rationalize your result using resonance arguments. What should be the favored substitution product ... 

Consider now the rr-system in benzene. The MO approach will generate linear combinations of the atomic p-orbitals, producing six rr-orbitals delocalized over the whole molecule with four different orbital energies (two sets of degenerate orbitals). Figure 7.3. The stability of benzene can be attributed to the large gap between the HOMO and LUMO orbitals. 

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

The frontier orbital theory [7-9] assumes that the stabihzation by the electron delocalization could control chemical reactions. The stabilization comes from the interactions between the occupied molecular orbitals of one molecule and the unoccupied molecular orbitals of another (Sect. 1.4). The strong interaction occurs when the energy gap is small (Sect. 1.3). The HOMO and the LUMO are the closest in energy to each other. The HOMO-LUMO interaction, especially the interaction between the HOMO of electron donors and the LUMO of electron acceptors, controls the chemical reactions (Scheme 20). The HOMO and the LUMO are termed the frontier orbitals. ... 

Molecules have some occupied and some unoccupied orbitals. There occur diverse interactions (Scheme 1) when molecules undergo reactions. According to the frontier orbital theory (Sect 3 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume), the HOMO d) of an electron donor (D) and the LUMO (fl ) of an electron acceptor (A) play a predominant role in the chemical reactions (delocalization band in Scheme 2). The electron configuration D A where one electron transfers from dio a significantly mixes into the ground configuration DA where... 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

The primary delocalization occurs from tz of alkenes to of ketenes (Scheme 25). The pseudoexcitations occur through the HOMO-HOMO and LUMO-LUMO interactions (Scheme 4). The HOMO of the donors is n as usual, whereas the HOMO of the acceptors is not but The HOMO-HOMO interaction occurs between the C=C bonds of alkenes and ketenes and promotes the reaction accross the C=C bond of ketenes. The important DA configuration is the intramolecular... 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

Strong donor-acceptor interaction shifts the reaction from the pseudoexcitation band to the transfer band. Electrons delocalize from the HOMO of propene to the LUMO of X=Y too much to form a bond between the double bonds. One electron transfers and a radical ion pair forms. The negatively charged X=Y... 

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