Energies affecting orbital interactions

Now, examine the orbital on cyclohexanone lithium enolate most able to donate electrons. This is the highest-occupied molecular orbital (HOMO). Identify where the best HOMO-electrophile overlap can occur. Is this also the most electron-rich site An electrophile will choose the best HOMO overlap site if it is not strongly affected by electrostatic effects, and if it contains a good electron-acceptor orbital (this is the lowest-unoccupied molecular orbital or LUMO). Examine the LUMO of methyl iodide and trimethylsilyl chloride. Is backside overlap likely to be successful for each The LUMO energies of methyl iodide and trimethylsilyl chloride are 0.11 and 0.21 au, respectively. Assuming that the lower the LUMO energy the more effective the interaction, which reaction, methylation or silylation, appears to be guided by favorable orbital interactions Explain. 

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

Substituent Effects and Reactivity. If the SOMO is relatively low in energy, the principal interaction with other molecules will be with the occupied MOs (three-electron, two-orbital type, Figure 3.8). In this case the radical is described as electrophilic. If the SOMO is relatively high in energy, the principal interaction with other molecules may be with the unoccupied MOs (one-electron, two-orbital type, Figure 3.10). In this case the radical is described as nucleophilic. Substituents on the radical center will affect the electrophilicity or nucleophilicity of free radicals, as shown below. 

The second order perturbations again fail to affect the total energy of the interacting orbitals. If the orbitals y>mi, y>nj are occupied in the unperturbed system, their mutual interactions have no effect on the total energy of the electrons occupying them. 

The orbital interactions discussed above not only govern the energy of ground state conformations or configurations but can also modulate the energy of transition states and, therefore, the reactivity of compounds. In conformationally constrained systems it has been observed that orbital overlap can affect the nucleophilicity and basicity of unshared electron pairs. The basicity differences of the amines shown in Scheme 2.11 [39] can, for instance, be interpreted as a result of a more or less efficient overlap between vicinal rrC-N and rr c x orbitals, where X represents an electron-withdrawing group. 

The rank k can take values 0, 1 and 2 by the triangle rule. Of these, the scalar term with k = 0 has no A dependence and hence does not affect the relative positions of the ro-vibrational energy levels. It just makes a small contribution to the electronic energy of the state r], A). The first-rank term produces a second-order contribution to the spin orbit interaction because it is directly proportional to the quantum number A from the 3-j symbol in the first line of (7.119). The contribution to the spin-orbit parameter A(R) which arises in this way is given (in cm-1) by... 

As also shown in Table 3, the energy difference between the ground Aj and first singlet excited B state is 48.0kcalmol in AsHj, decreases to 47.2kcalmol" in SbHj", but increases to 47.6 kcal mol in BiHj. This trend is a little affected by the spin-orbit interaction because of the closed-shell character of these states. 

In the next sections we describe briefly the main interactions, which are in charge of splitting of the 3d ions energy levels in crystals. These interactions include the Coulomb interaction, the crystal field interaction, the spin-orbit interaction and the JT interaction. As it was pointed out by Ham [13], the observed spin-orbit and trigonal field splittings of the orbital triplet states are significantly affected by the dynamic JT effect. 

For the reverse reaction, namely the removal of Nu, the important frontier orbital interaction is between the C-Nu cr-orbital (the HOMO) and the C-Nu a -or-bital (the LUMO) (Fig. 6-12). A frontier orbital explanation requires a knowledge of how proximate bonds affect the energy of these orbitals. The orientation of... 

There are several properties of luminescent materials that need to be controlled in order to make efficient LEDs and lasers. The first is the colour of the emission, which is primarily determined by the energy difference (band-gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), but in the solid state is also affected by interactions between the molecules or polymer chains which can lead to red-shifts in the emission due to formation of aggregates. This can be controlled by manipulating both the polymer backbone and the substituents. Polyphenylenes are intrinsically blue-emitting materials with large HOMO-LUMO gaps, but as we will show, by copolymerisation with other materials it is possible to tune the emission colour across the entire visible spectrum. Even without the incorporation of comonomers it is possible to tune the... 

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