Antibonding 7t* orbital

The aromatic character of mono-, di-, tri-, and tetraphosphatriafulvenes has recently been calculated [122, 123] using DFT methods (Scheme 16). The energy of the donor-acceptor interaction between the Lewis-type Ji-orbital of the ring and the antibonding 7t -orbital of the etobond was computed by second-order... 

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

It should be noted that no direct transformation from the superoxo-like state to the peroxo-like state was observed in this experiment. From the EEL spectra results it was not possible for these investigators to separate the contribution from the superoxo- and peroxo-like peaks to the rise in the atomically adsorbed oxygen peak. Therefore, a direct pathway from the superoxo-like 02 state to the atomically adsorbed state cannot be dismissed as a possibility. Thus, the assertion by Nolan et al. that the molecularly bound 02 progresses sequentially from the superoxo-like state to the peroxo-like state is a hypothesis. However, this sequential progression does appear to be a very attractive explanation, as a superoxo-like species arises from the contribution of one electron to the antibonding p orbital of the 02 molecule and the peroxo-like species arises from the contribution of two electrons to this antibonding 7T orbital. 

In thiophene, the 7t-orbital manifold comprises the HOMO la2 (tts) describing C(2)-C(3) bonds, the 2bi(7t2) orbital related to the 3p sulfur lone pair, and the deep Ibi(iti) orbital describing the bonding of all the ring atoms. Among the unoccupied orbitals are two antibonding 7t-orbitals rt 4(bi) and p s(a2). The rt-orbital system of the chlorothio-phenes is closely analogous there are three occupied molecular orbitals, 4a"(%), 3a"(Jt2), and la"(Jti), and two unoccupied 7t (a") orbitals. 

The LUMO of an electrophile is usually a (antibonding) 7t -orbital. (This is lower in energy than a o -orbital.)... 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

Figure Bl.6.11 Electron transmission spectrum of 1,3-cyclohexadiene presented as the derivative of transmitted electron current as a function of the incident electron energy [17]. The prominent resonances correspond to electron capture into the two unoccupied, antibonding 7t -orbitals. The negative ion state is sufficiently long lived that discrete vibronic components can be resolved.

In general, semiconductors can have different types of valence band and conduction band structures. These differences can affect the chemical reactivity of the various types of semiconducting solids. For example, in a covalent solid such as Si, the valence and conduction bands can be considered as crystal orbitals that are either bonding or antibonding combinations of hybridized Si atomic orbitals. This situation is closely related to our polyene example, where the valence band consisted of bonding tt-orbitals and the conduction band consisted of antibonding 7T -orbitals. However, in an ionic crystal such as Ti02, the valence band is composed of crystal orbitals that are derived from the filled 2p orbitals, while the conduction band is composed of crystal orbitals that are... 

There is a second interaction that involves the empty antibonding 7t orbital and the metal d orbital with the same symmetry xz), which is lower in energy than the n orbital (3-37b). If this latter orbital is doubly occupied, this interaction is stabilizing, and it leads to a transfer of electron density from the metal to the ligand. This is therefore a hack-donation interaction, where ethylene plays the role of a itt acceptor, using its empty tt orbital. The doubly occupied orbital, mainly concentrated on the metal, is part of the d block of the complex it can be described as a metal d orbital that is stabilized by a bonding interaction with the n orbital on ethylene. 

Figure 5.9 (See colour insert following page 142.) p-Orbitals interacting to form two bonding and two antibonding 7t-orbitals.

It is seen that as compared to the LUMO density (antibonding 7t orbital) orbital relaxation mixes the frontier orbital with the other occupied MO s includii o orbitals, a feature present both in the finite difference and differential method. In Figure 4 a more detailed comparison between these methods is given along a line parallel to the CO bond in the plane of Figure 3. It is cleariy seen that the differential method approaches the finite difference results ones upon decreasing AN. 

CO can also act as a Lewis acid by accepting electrons into its vacant antibonding 7t-orbitals. 

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