Fig. 8.2 Frontier-orbital coefficients and energies (eV) for acrolein and protonated acrolein [2 |

Fig. 15.26. Frontier orbital coefficients and energy difference of the H0M0-LUM0 gaps in orientation-selective Diels— Alder reactions (cf. Figure 15.25, X = H). |

Figure 5 Calculated (AMI) heats of formation AHf), dipole moments (/r), and frontier orbital coefficient and energies (e) for methyl esters 31a and 8a. |

Indole and benzofuran combine two problems they are nonalternant and they contain heteroatoms. The indole frontier orbital coefficients in the 8- and 9-positions78 are very similar, 0.491 and 0.493, respectively. The subjacent orbital lies only 0.256/1 below the HOMO, but it has very small coefficients in these positions. The overriding factor seems to be the net charge (—0.12 at position 9, essentially neutral at position 8), which strongly favors attack at position 9. The case of benzofuran is still more complicated. There is a difference between the frontier orbital coefficients at position 8 (0.54) and 9 (0.47), but charge control still prefers the 9-position (—0.10 versus —0.03 units for the 8-position). The experimental results show that the frontier orbital terms dominate. [Pg.136]

Fig. 5. Calculations for 10,9-borazarophenanthrene. (a) PMO localization energies (x /S-1 ) (b) -electron densities (c) frontier orbital coefficients. |

In these Hiickel calculations,273 the intramolecular distance is presumed to be the same for each transition state. Therefore, the atomic overlaps are identical and the V- values are proportional to the frontier orbital coefficients. [Pg.95]

The reactions of 521 with 1,3-dienes were found to proceed exclusively in an [8 + 2] addition mode. The reactions were completely site and regioselective, as exemplified by the reaction between 521 and 2-methyl-l,3-pentadiene (525) which gave 526 after loss of CO2 (equation 152). The regiochemistry observed was in agreement with the frontier orbital coefficients calculated with semi-empirical methods. [Pg.451]

Condition (2) is only properly satisfied if the two sites under comparison involve the same chemical element. If the elements are different, their orbitals are not of the same size and their interactions with the incoming reagent will depend on overlaps in addition to on frontier orbital coefficients. FO analysis is then much more complicated. Great care must be taken if the two elements are from different rows of the periodic table, C and S, for example. [Pg.130]

In comparison to other aromatic hydrocarbons, t-1 forms a more stable charge-transfer complex with tetracyanoethylene than would be expected on the basis of its ionization potential (79). That is, when EgT for a series of hydrocarbon donors is plotted against IPj), the point for t-1 falls well below the line (Fig. 3). The unusual stability of t-1 complexes may be related to their large frontier orbital coefficients which allow effective interaction with a small acceptor such as tetracyanoethylene. [Pg.182]

© 2019 chempedia.info