HOMO-LUMO excitation

In a few instances the technique of magnetic circular dichroism (MCD) spectroscopy has been used to corroborate assignments based on UV-visible spectroscopy. For example, the assignment of the intense 360 nm band for [S,N,Y to a r (2e") r (2a2") (HOMO LUMO) excitation has been confirmed by the measurement of the MCD spectrum of The MCD spectrum of [S4N3] indicates that each of the... 

Figure 1. Hiickel n-MO correlation diagrams for planar- perp twisting of pcntamethine cyanine about the 2-3 and 3-4 bonds (a), and simultaneous twisting about 2-3 and 4-5 bonds (b). For the perp forms the orbitals belonging to the different n fragments are indicated and the CT nature of the HOMO-LUMO excitation is emphasized.

The latter feature leads to the expectation that, in cases where the HOMO —> LUMO excitation in the neutral occurs in the visible, the corresponding state of the radical cation should turn out to be the lowest one. There are indeed several cases where this was... 

The HOMO-LUMO excitation (tt, n ) of vinylcyclopropanes gives rise to a strong electronic band near 210 nm. If the cyclopropane ring or the double bond (or both) are substituted by alkyl groups, the lowest energy Cotton effect will be observed near 225 nm. ... 

For anthr ene, for the state that is characterized by the HOMO->LUMO excitation, we have... 

Table 2.3 HOMO-LUMO Excitation Energies of Condensed Aromatic Hydrocarbons. Comparison of Values (in cm ) Calculated from the PMO Model with Experimental Data for the L Band (Adapted from Heilbronner and Bock, 1968)...

In the ground configuration of a perimeter with AN+ 2 n electrons all orbitals up to and including doubly occupied. Thus, the degenerate MOs nd 0 jv are the HOMOs and the degenerate MOs -n- are the LUMOs of the Ji system. If only HOMO LUMO excitations are considered, four singly excited configurations... 

Figure 2.11. The perimeter model of an (4N+2)-electron [njannulene, geometry on the left, energies of the MOs on the right. The angular momentum quantum number is given for each MO. The sense and magnitude of electron circulation and the resulting orbital magnetic moment are shown schematically in a perspective view. Orbital occupancy in the ground configuration and the four one-electron HOMO->LUMO excitations are indicated (by permission from Michl, 1978).

In the case of many aromatics, the lowest singlet excited state is not represented by the HOMO-LUMO excited configuration ( La) but rather by a mixture of +, and ( Lb, cf. Section 2.2.2), and a barrier is ex-... 

Figure 6.18. Excited-state barriers a) orbital correlation diagram for a thermally forbidden conversion of an alternant hydrocarbon b) the corresponding configuration and state correlation diagram for the case that Ihe HOMO— LUMO excitation does not represent the longesl-wavelength absorption.

It has become evident only fairly recently that photochemical electron-transfer processes (PET) play an important role in many reactions. In this connection it is of great importance that in an excited state a molecule can be a better oxidant as well as a better reductant than in the ground state. For instance, after HOMO- LUMO excitation the half-occupied HOMO can readily accept another electron, while the single electron from the LUMO can readily be transferred to an acceptor. (Cf. Figure 5.25.)... 

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