Qualitative theories frontier molecular orbital theory

It is important to realize that whenever qualitative or frontier molecular orbital theory is invoked, the description is within the orbital (Hartree-Fock or Density Functional) model for the electronic wave function. In other words, rationalizing a trend in computational results by qualitative MO theory is only valid if the effect is present at the HF or DFT level. If the majority of the variation is due to electron correlation, an explanation in terms of interacting orbitals is not appropriate. 

It is important to realize that whenever qualitative or frontier molecular orbital theory... 

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. 

Frontier Molecular Orbital Theory can be used to describe qualitatively the trajectory of a nucleophile when it attacks a jt centre. Two sets of first-order interactions are considered. Firstly the stabilizing interaction of the HOMO of the nucleophile with the LUMO (jt and a orbitals) of the jt system and secondly the destabilizing interaction of the HOMO of the nucleophile with the HOMO (jt and a molecular orbitals) of the jt system, as shown in Figure 6. 

A related method to interpret the diastereofecial selectivities of the reactions of double bonds has been proposed by Dannenberg and coworkers [8, 13, 14,]. Tins method also relies on the 7t frontier orbitals of non symmetrical molecules, and proposes breaking the symmetry of the n or it orbitals due to polarization induced by the substituents. Application of frontier molecular orbital theory, taking into account only the substrate MOs, gives a qualitative trend of stereoselection in a number of nucleophilic (reductions of carbonyl compounds) and electrophilic reactions. 

In contrast to the claim (10) that the ECW model "disguises the relationship between reactivity and periodic elemental properties", elementary application of frontier molecular orbital theory (H) can be used to understand the trends. Using qualitative trends in ionization energies, inductive effects, electronegativities and partial charge/size ratios, one can estimate trends in the HOMO-LUMO separation of the donor and acceptor. Increasing the separation decreases the covalent and increases the electrostatic nature of the interaction. Decreasing the separation has the opposite effect. Trends in the reported acid and base parameters as well as in the Ey E0 and C C0 products can be understood in this way. 

The molecular geometries and the frontier orbital energies of heterophospholes 28-31 were obtained from density functional theory (DFT) calculations at the B3LYP/6-311- -G, level. The 1,3-dipolar cycloaddition reactivity of these heterophospholes in reactions with diazo compounds was evaluated from frontier molecular orbital (FMO) theory. Among the different types of heterophospholes considered, the 2-acyl-l,2,3-diazaphosphole 28, 377-1,2,3,4-triazaphosphole 30, and 1,3,4-thiazaphosphole 31 were predicted to have the highest dipolarophilic reactivities. These conclusions are in qualitative agreement with available experimental results <2003JP0504>. 

The observed regioselectivities during these cycloadditions have been explained in terms of qualitative frontier molecular orbital perturbation theory <78CB2028>. An intramolecular cycloaddition has been observed for the 1,3-dithiolone (111) to afford the tricyclic thiophene derivative (112) (Equation (13)) <81LA347>. 

Frontier Molecular Orbital (FMO) theory may also be applied to provide qualitative understanding. The frontier orbital of the radical is that bearing the free spin (the SOMO) and during radical addition this will interact with both the jt antibonding orbital (the LUMO) and the n-orbital (the HOMO) of the olefin. Both the SOMO-HOMO and the SOMO-LUMO interactions lead to a net drop in energy i.e. E respectively - Figure 1.4J. The dominant interaction and... 

It is sometimes possible to use qualitative MO theory to explain the electronic absorption spectra of simple molecules. The UV/VIS spectrum of formaldehyde shows three peaks in the 300-360 nm range. These peaks (in decreasing energy) have been identified as n n y n energy electronic transitions will occur between the frontier molecular orbitals and... 

The reactions of naphthalenes properly mono- and disubstituted with an electron-withdrawing and a series of dienes in thermal conditions, were evaluated with the frontier molecular orbitals (FMO) theory which provide qualitative information about the feasibility of these DA reactions. Besides, the global electrophilicity index (co) is employed to estimate the electrophilic character of the dienophiles used in the cycloaddition reactions. 

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

Qualitative Theories 15.1 Frontier Molecular Orbital Theory 15.2 Concepts from Density Functional Theory 15.3 Qualitative Molecular Orbital Theory 34, 347 351 353 ... 

---