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Frontier orbital positions LUMO

When you request an orbital, yon can use the cardinal number of the orbital (ordered by energy and starting with number=l) or an offset from either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LL MO). Offset from the HOMO are negative and from the LUMO are positive. Often these frontier orbitals are the ones of most chemical interest. [Pg.244]

The positively charged allyl cation would be expected to be the electron acceptor in any initial interaction with ethylene. Therefore, to consider this reaction in terms of frontier orbital theory, the question we need to answer is, do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another The orbitals that are involved are shown in Fig. 1.27. If we analyze a symmetrical approach, which would be necessary for the simultaneous formation of the two new bonds, we see that the symmetries of the two orbitals do not match. Any bonding interaction developing at one end would be canceled by an antibonding interaction at the other end. The conclusion that is drawn from this analysis is that this particular reaction process is not favorable. We would need to consider other modes of approach to analyze the problem more thoroughly, but this analysis indicates that simultaneous (concerted) bond formation between ethylene and an allyl cation to form a cyclopentyl cation is not possible. [Pg.51]

The amplitude of the frontier orbitals determines the selectivity. The most reactive atom in a molecule has the largest amplitude of the frontier orbitals. The frontier orbitals overlap each other to the greatest extent at the sites with the largest amphtudes. Reactions occur on the atoms in the electron donors and acceptors, where the HOMO and LUMO amplitudes are largest, respectively. Electrophiles prefer the a position of naphthalene, an electron donor, with the larger HOMO amplitude (Scheme 21). Nucleophiles attack the carbons of the carbonyl groups, an electron acceptor, with the larger LUMO amplitude (Scheme 7). [Pg.17]

Thus we find that the reaction is a syn (suprafacial) addition with respect to both the diene and dienophile. The frontier orbitals involved shows that the reaction occurs by interaction of HOMO and LUMO. So there is no possibility of substituents to change their position. Substituents which are on the same side of the diene or dienophile will be cis on the newly formed ring as is seen between the reaction of dimethyl maleate (a cis dienophile) with 1,3 butadiene. The product formed is cis 4,5 dicarbomethoxy cyclohexane. [Pg.46]

There have been discussions whether the amide addition at C-6 is charge-controlled or orbital-controlled. Charge density calculations in 4-phenylpyrimidine (MNDO method) predict that the addition of the amide ion would preferably take place at position 2 (95UP1) this, however, does not agree with the experimental results. Therefore, the conclusion seems justified that the addition is not charge-controlled. Frontier orbital calculations, using the SCF-PPP method, show that the frontier orbital densities in the LUMO of pyrimidine are zero at C-2 and C-5, making these positions... [Pg.30]

In homolytic substitution reactions, the 2-position of thiophene is the preferred site of attack. This is easily rationalized in terms of frontier orbital theory (B-76MI31401). Because of symmetry, both HOMO and LUMO of thiophene have the same absolute values for the coefficients (as shown in 216). Thus it is immaterial whether the [SOMO (radical)-HOMO (thiophene)] or the [SOMO (radical)-LUMO (thiophene)] interaction determines the site of attack only the 2-position is the point at which the radical would attack. The same conclusion is iso reached by consideration of product development control (74AHC(16)123). Attack at the 2-position would result in a transition state with an allylic radical, which would be stabilized to a greater extent than the one arising from attack at position 3 (Scheme 57). [Pg.779]

The preference for bridging at the meso positions is dictated by HOMO-LUMO coefficients in the exciplex. At high arene concentrations, the stereochemical preference is diminished. In general, however, the local symmetry of the frontier orbitals determines implicitly the favorable pathway for cycloadditions involving significant charge transfer but without the intervention of radical ions (55). [Pg.255]

However, the frontier orbital picture based on the free arene does not account for nearly exclusive meta selectivity in addition to [(anisole)Cr(CO)3] LUMO for anisole shows essentially the same pattern as for toluene.98-100 With a strong resonance electron donor the traditional electronic picture (deactivation of the ortho and para positions) is sufficient to account for the observed meta selectivity. In this case the balance of charge control and orbital control is pushed toward charge control by strong polarization. The same argument applies to the aniline and fluotobenzene complexes. [Pg.538]

In contrast to the [4+2]-additions of butadiene to ethene or acetylene (Figures 15.8 and 15.9), the two HOMO/LUMO interactions stabilize the transition state of the [2+2]- addition of ketenes to alkenes to a very different extent. Equation 15.2 reveals that the larger part of the stabilization is due to the LUMOketene/HOMOethene interaction. This circumstance greatly affects the geometry of the transition state. If there were only this one frontier orbital interaction in the transition state, the carbonyl carbon of the ketene would occupy a position in the transition state that would be perpendicular above the midpoint of the ethene double bond. The Newman projection of the transition state (Figure 15.11) shows that this is almost the case but... [Pg.653]


See other pages where Frontier orbital positions LUMO is mentioned: [Pg.232]    [Pg.307]    [Pg.204]    [Pg.76]    [Pg.63]    [Pg.68]    [Pg.79]    [Pg.41]    [Pg.84]    [Pg.97]    [Pg.133]    [Pg.28]    [Pg.227]    [Pg.192]    [Pg.65]    [Pg.20]    [Pg.89]    [Pg.480]    [Pg.493]    [Pg.65]    [Pg.111]    [Pg.34]    [Pg.229]    [Pg.1123]    [Pg.62]    [Pg.171]    [Pg.76]    [Pg.376]    [Pg.370]    [Pg.130]    [Pg.8]    [Pg.176]    [Pg.480]    [Pg.493]    [Pg.488]    [Pg.37]    [Pg.91]    [Pg.179]    [Pg.22]    [Pg.99]   
See also in sourсe #XX -- [ Pg.231 , Pg.235 , Pg.237 ]




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Frontier

Frontier orbitals

LUMO

LUMO orbital

LUMOs

Orbital, frontier

Orbitals LUMO)

Positive orbit

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