Frontier molecular orbital closing

Frontier Molecular Orbital theory is closely related to various schemes of qualitative orbital theory where interactions between fragment MOs are considered. Ligand field theory, as commonly used in systems involving coordination to metal atoms, can be considered as a special case where only the d-orbitals on the metal and selected orbitals of the ligands are considered. 

As a possible source for new long-wavelength dye systems, biradicaloid dyes possessing energetically close-lying frontier molecular orbitals could be used. 146 An example of such dye systems already in extensive use in imaging are squaric acid derivatives like SQ. 147, 14S ... 

A similar polar cyclization of an enamine and a thioketene derivative is shown in equation (87) (76TL4283), but electron-deficient alkenes and alkynes react in a concerted fashion. Concerted cyclizations may be subdivided into those in which the sulfur atom is part of the enophile or the dienophile. Into the first category fall the dimerizations of a,( - unsaturated thioaldehydes (equation 88) which may be shown to closely follow frontier molecular orbital predictions of regioselectivity (79JOC486). Related to this are the thiochalcones (equation... 

The sensitivity criteria proposed in the litearture fall mainly into the singlemolecule criteria [3, 7,8], although some preliminary analysis of the two-reactant concepts have also been reported [7,8]. The molecular responses to the (d/V, d v)-perturbations are determined to a large extent by the properties of the frontier molecular orbitals, i.e., the lowest unoccupied (LUMO) and the highest occupied (HOMO) orbitals of both reactants. Therefore, many sensitivity criteria, e.g., FF indices, are closely related to frontier orbital theory [3,11]. 

Theoretical studies indicate that these transition structures are probably influenced by frontier molecular orbitals (in addition to steric effects), as indicated in Scheme 5.34c [182]. For the reaction of aminoethylene (a primary enamine) and acrolein, the enamine HOMO and the enone LUMO have the most attractive interactions when aligned in the chair configuration shown, which has the enone in an 5-cis conformation. Note that this orientation places the NH and the electrophile a-carbon in close proximity for proton transfer via the ene transition structure. 

We close with some additional words on the significance of the Fukui function. The Fukui function shores up the theoretical foundations of frontier molecular orbital theory. Equations (43)-(46) reveal that the site reactivity indices of frontier molecular orbital theory (Eqs. 47-49) may be regarded as the frozen orbital approximation to the Fukui function. The Fukui function is the zeroth-order index for site reactivity various functional derivatives of the Fukui function represent higher-order corrections to the zeroth-order site reactivity map provided by the Fukui function. For instance, the first-order correction to the Fukui function is... 

Figure 14 shows the contour plots of the frontier molecular orbitals for the bis(ferrocenylethynyl) species 7. The HOMOs are very close to each other in orbital energies... 

In these molecules, the process of electron transfer takes place through the backbone of fully delocalized 7i-bridges, and consequently energetically closely spaced frontier molecular orbitals... 

Due to steric strain, the double bonds in N are pulled close together, which leads to the through-space interaction of their Jt-orbitals. This results in a considerable convergence of the energy levels of the frontier molecular orbitals.This interaction manifests itself in the notably longer wavelength... 

A radical has a singly occupied molecular orbital (SOMO). This is the frontier orbital. The SOMO interacts with HOMO and the LUMO of closed-shell molecules to stabilize the transition state (Scheme 27). The radical can be a donor toward a monomer with low LUMO or an acceptor toward one with high HOMO. 

Roald Hoffmann, a former coworker of R.B. Woodward and Nobel Prize as well for his contribution to the frontier orbital theory (the famous Woodward-Hoffmann rules concerning the conservation of molecular orbital symmetry), has also emphasised the artistic aspects of organic synthesis "The making of molecules puts chemistry very close to the arts. We create the objects that we or others then study or appreciate. That s exactly what writers, visual artists and composers do" [15a]. Nevertheless, Hoffmann also recognises the logic content of synthesis that "has inspired people to write computer programs to emulate the mind of a synthetic chemist, to suggest new syntheses". 

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