O orbitals

Metal Weak Strong Ligand interaction interaction o orbits... 

First, try to draw resonance contributors for both ground state and triplet anthrone. Then display a spin density surface for the triplet state of anthrone. (Note that the spin density surface shows the location of both unpaired electrons, one of which may be in a 7t orbital and one of which may be in a o orbital.) Where are the two unpaired electrons Are they localized or delocalized Given that spin delocalization generally leads to stabilization, would you expect the triplet state of anthrone to be stable ... 

Another important polyatomic molecule is benzene, C6f I6, the parent of the aromatic compounds. In the molecular orbital description of benzene, all thirty C2s-, C2p-, and Hls-orbitals contribute to molecular orbitals spreading over all twelve atoms (six C plus six H). The orbitals in the plane of the ring (the C2s-, C2px, and ( 2/ -orbitals on each carbon atom and all six Hls-orbitals) form delocalized o-orbitals that bind the C atoms together and link the H atoms to the C atoms. The six C2pz-orbitals, which are perpendicular to the ring, contribute to six delocalized tt-orbitals that spread all the way around the ring. However, chemists... 

An s-orbital and a p-orbital on different atoms may overlap to form molecular orbitals. One of these interactions forms a bonding o-orbital and the other is nonbonding. Draw diagrams to represent the two types of orbital overlap that give rise to tr-bonding and nonbonding orbitals. 

For planar unsaturated and aromatic molecules, many MO calculations have been made by treating the a and n electrons separately. It is assumed that the o orbitals can be treated as localized bonds and the calculations involve only the tt electrons. The first such calculations were made by Hiickel such calculations are often called Hiickel molecular orbital (HMO) calculations Because electron-electron repulsions are either neglected or averaged out in the HMO method, another approach, the self-consistent field (SCF), or Hartree-Fock (HF), method, was devised. Although these methods give many useful results for planar unsaturated and aromatic molecules, they are often unsuccessful for other molecules it would obviously be better if all electrons, both a and it, could be included in the calculations. The development of modem computers has now made this possible. Many such calculations have been made" using a number of methods, among them an extension of the Hiickel method (EHMO) and the application of the SCF method to all valence electrons. ... 

Interaction of o Orbitals at the P-Position with the Olefin It Orbitals. 147... 

Fukui applied the orbital mixing rule [1,2, 59] to the orbital hybridization or the deformation of the LUMO of cyclohexanone to explain the origin of the Jt-facial selectivity in the reduction of cyclohexanone. Cieplak [60] proposed that electron delocalization occurs from the bonds into the o orbital of the incipient bonds at the transition state. 

In the present case, the Walsh orbital will overlap with the k orbital of the carbonyl group more efficiently than the (3-o orbitals because of agreement of orbital symmetry and the efficient overlapping. This out-of-phase motif (13) is consistent with retardation of syn addition with respect to the cyclopropyl group, that is, anti preference. 

The HOMOs of unsubstituted vinylidenenorbomane 47a and unsubstituted bicyclo[2.2.2]octene 48a are intrinsically comprised of similar components, i.e., the n orbital of the ethylene and the o-orbitals of the ethano bridges, the coupling being in an out-of-phase fashion (50 and 51), though the arrangement of the components is different. The contour plot (49) of the HOMO of 48a is consistent with these orbital interactions (50). 

As discussed in connection with the facial selectivities of 7-methylidenenorbom-ane 46 and bicyclo[2.2.2]octene 48, the components of the molecules, i.e., n functionality and two interacting o orbitals at the two P positions, are the same, but the connectivity of these fragments, i.e., the topology of the n systems, is different (A and B, Fig. 9). A similar situation was found in the case of spiro[cyclopentane-l,9 -fluorene] 68 [96, 97] and 11-isopropylidenedibenzo-norbomadienes 71 (see 3.4.1 and 3.4.2) [123]. In these systems, the n faces of the olefins are subject to unsymmetrization due to the difference of the interacting orbitals at the P positions. In principle, consistent facial selectivities were observed in these systems. 

Combination of the it-HOMO with the high-lying orbital in phase, followed by mixing of the o orbital out-of-phase with the n orbital gives the FMO, which distorts to favor the reaction at anti side of the substiment (HOMO = Jt-HOMO + sub - (Scheme 10). 

We pointed out that these results can be attributable to the a-n interaction. At the transition state, the o orbital at C5 on the anti side of the dienophile is parallel with the n orbital, the o bond electrons are able to delocalize much more effectively than that on the syn side. Since the electron donating o bond on the anti side stabilizes the transition state, the a-n interaction can contribute to rr-facial selectivity. These results suggested that the bond lengthening cannot necessarily be convincing evidence for the Cieplak effect, but can be explained in terms of the a-n interaction without assuming the incipient a bonds at the transition state (Scheme 30). 

The delocalization of excessive a- (or P-) spins and the bond polarization can take place among radical orbitals, p and q, and the central n (or o) and n (or o ) orbitals, resulting in the electron transferred configurations (T) and locally excited configurations (E), respectively (Fig. 5a). The delocalization-polarization mechanisms are different between singlet and triplet states, as addressed in the following subsections. 

In the singlet state of Jt-type 1,3-diradical (e.g., TM, 2), there may also exist the through-space interaction between radical centers, i.e., p...q interaction (Fig. 9), in addition to the previously addressed cyclic -p-o -q-o- orbital interactions (Fig. 6). The through-space interaction is indispensable for the bond formation between the radical centers. The corresponding delocalization of the a-spin electron is shown in Fig. 9a. Clearly, the involvement of the through-space p... q interaction gives rise to two cyclic orbital interactions, -p-o -q- and -p-o-q-. From Fig. 9, one can find that the cyclic -p-o -q- orbital interaction can satisfy the phase continuity requirements for the a-spin electron the electron-donating radical orbital, p (D) can... 

Interaction of the donbly occnpied bonding orbital of a o bond with the vacant antibonding o -orbital of another bond at the geminal position was theoretically shown to be bonding (Scheme 4a) and antibonding (Scheme 4b) when the s-character... 

Scheme 5 The hybrids and interbond population (IBP) between the geminal o and o orbitals and the SEs...

In the ring molecules containing a n bond, delocalization of tt electrons occurs through the interaction with o orbitals [19]. 

Scheme 6 Phase continuity of the n, a, and O orbitals for cyclic delocalization of a lone pair...

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