Highest lying occupied molecular orbital

Fig. 6 Plot of the highest-lying occupied molecular orbital (HOMO) of the borylene complex (BH) (CAAC)2 [90]...

Start from the two-highest singly occupied molecular orbitals triplet state of A-B. When A = B, and are given by [54] ... 

Figure 1 shows the electron attachment energies (AE) and ionization potentials (IP) of silyl substituted 7t-systems and related compounds [4], AE can be correlated with the energy level of the LUMO (lowest unoccupied molecular orbital) and IP can be correlated with the energy level of the HOMO (highest occupied molecular orbital). For a-substituted 7t-systems, the introduction of a silyl group produces a decrease in the tc -(LUMO) level. This effect is attributed to the interaction between a low-lying silicon-based unoccupied orbital such as the empty d orbital of silicon and the it orbital (d -p interaction) as shown in Fig. 2. Recent investigations on these systems, however, indicate that d orbitals on silicon are not necessarily required for interpreting this effect a-effects of SiR3 can also be explained by the interaction between Si-R a orbitals and the 7r-system.

TPP (45, 46), as well as data for octaalkyl and octathioalkyl porphyrazines. As with the phthalocyanines and porphyrins, the electronic spectra of porphyrazines can be rationalized using Gouterman s four-orbital model, shown in Fig. 5 (47, 48). All of these macrocycles, when symmetrically substituted and with a metal ion incorporated in the central hole, for example, the M[pz(A4)] or B4, have D4h symmetry, with a doubly degenerate lowest unoccupied molecular orbital (LUMO) (eg) and two highest lying highest occupied molecular orbitals (HOMOs) that complete the four Gouterman orbitals with alM and a2u symmetry. 

In the four-orbital model (1 ), low-lying ir-ir states of free-base porphyrins (symmetry D2h) are considered as resulting from single electron excitation from a pair of nondegenerate highest occupied molecular orbitals (bi, bo) to a pair of nondegenerate lowest unoccupied molecular orbitals (ci, cg). In the case of symmetry D2h mutually perpendicular electric transition dipoles X and Y are not equivalent and, therefore, in the visible absorption spectra of free-base porphyrins two different electronic bands Qx(0>0) and Qy(0,0) are observed (Table 1 and Fig. 10). 

Figure 22.9. Disrotatory ring opening of 1,1-difluorocyclopropane (9) to 2,2-difluorocyclo-propane-l,3-diyl (10). The in-phase comhination of 2p-n AOs in the highest occupied molecular orbital (HOMO) is stabilized by a bonding interaction with the 2p AO at C2 in the n combination of low-lying, C—F, antibonding orbitals.

Compared to the analogous reactions of the parent molecules, many radical cation reactions show a dramatic decrease in activation barriers, one of the most striking aspects of radical cation chemistry. Intuitively, this observation can be ascribed to the fact that the highest occupied molecular orbital (HOMO) of a radical cation is occupied by a single electron. As a result, the bond strength of one or more key bonds must be reduced and the bonds more easily decoupled. However, the barriers to some radical cation rearrangements appear to lie even lower than might be expected on the basis of this simple model. 

Photoelectron studies of (CH3)3P=CH2 have provided data on the energy of the highest occupied molecular orbital and some of the lower-lying states (4-9). The frontier orbital energy of 6.81 eV is very low and seems to illustrate the carbanionic nature of the carbon, where this orbital is largely localized. This is borne out by semiquantitative CNDO/2 calculations on this molecule. The orbital sequence obtained is satisfactory according to ab initio results. 

Most of these semiconductors have relatively high-lying HOMO (highest occupied molecular orbitals) and narrow band gaps. They are very susceptible to photoin-... 

The molecular orbitals that will be of greatest interest for reactions between molecules are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), collectively known as frontier orbitals because they lie at the occupied-unoccupied frontier. The MO diagram of CQ helps explain its... 

---