Jt orbitals

From the standard orientation, we see that the plane of the molecule is the XY plane. Both orbitals are composed of only p components, indicating that they are Jt orbitals. Thus, this excited state corresponds to the Jt— Jt transition. 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

The excited state of the carbonyl compound is the (n, it ) state where one electron is excited from the HOMO to the LUMO. The SOMO is the n-orbital on the carbonyl oxygen atom. The SOMO is the antibonding jt -orbital. 

Bending of unsaturated bonds reduces the overlap between the p-orbitals and weakens the interaction. The n orbital hes high in energy and the Jt orbital lies low. Bent unsaturated bonds are electron acceptors as well as donors. The energy gap between Jt and Jt is small. Bent unsaturated bonds are readily pseudoexcited to undergo [2+2] cycloaddition reactions. 

Because of the better jt-type overlapping of the carbonyl jt orbital with the o bonds of the ethano bridge as compared with that of the methano bridge in 27 (i.e., 9 (dihedral angle, or ZCfff. ) < 9 (dihedral angle or... 

On the other hand, when the jt orbital lies low enough to interact with the jt-HOMO, the participation of the jt orbital needs to be taken into account. 

On the other hand, when n lies close to Jt-HOMO, the interaction between 7t-HOMO and n is strong. Both orbitals contribute considerably to FMO. The combined orbital, Jt-HOMO - n is a component of FMO. The Jt orbital interacts with the n more strongly than with the jt-HOMO dne to the spatial proximity. The phase of is determined by the relation with n rather than jt-HOMO so as to be out-of-phase with n (viz. in phase with Jt-HOMO). As a result, is the opposite... 

Mataka and coworkers reported the studies of the Diels-Alder reactions of [3.3] orthoanthracenophanes 96 and 97, of which anthraceno unit, the potential diene, has two nonequivalent faces, inside and outside. The reactions of 96 with dien-ophiles gave the mixtures of inside and outside adducts with the ratios between 1 1 and 1 1.5. However, the ratio changes drastically, in favor of the inside adducts, when 97 reacts with dienophiles such as maleic anhydride, maleimide and naphto-quinone [55] (Scheme 46). Mataka suggested that the Jt-facial selectivity is controlled by an orbital interaction between the electron-poor dienophiles and the Jt-orbital of the facing aromatics, which would lead to a stabilization of the transition state, while Nishio suggested that the selectivity is due to the attractive k/k or CH/jt interaction [53]. 

Based on experimental results and complementary calculations, an out-of-plane n-delocalization is suggested for thiirene dioxides . As far as the thiirene oxide is concerned, theoretical calculations predict possible spiroconjugative-type interaction between the n c=c orbital of the ring and the Jt-orbitals of the SO (which leads to aromatic stabilization and a n charge transfer backward from the SO to the C=C). There exists, however, a rather strong destabilization effect, due to the jt so(dxz)-orbital. 

Furthermore, it was shown the unpaired spin S = 1/2, which is delocalized over the two Pc rings, still remained in the Jt-orbitals after absorption on Au(lll). Consequently, STS measurements also provided direct observation ofthe S = 1/2 radical on the TbPc2 molecules on Au(lll) whereby the indicative Kondo-peak could be switched off by tunnelling current pulses [215]. Indeed the tunnelling conductance (dl/dV) was analysed from STS experiments of TbPc2 on Au(lll) near the Fermi level showed a zero-bias peak (ZBP) in the spectra, which could be assigned as a Kondo resonance. Clear Kondo features for the molecules with 9 = 45° were observed when the tip was positioned over one ofthe lobes of TbPc2. 

The relatively high aromaticity of the parent 1,2,5-thiadiazole renders it good thermal stability (stable up to 220 °C) despite this, 3,4-diphenyl-l,2,5-thiadiazole 8 suffers slow photochemical degradation to give benzonitrile and sulfur. The low basicity of 1,2,5-thiadiazole indicates a relatively high electron density in the Jt-orbital and corresponding low electron density of the nitrogen lone pairs. Addition reactions such as Walkylation do not occur readily. A-Oxidation is... 

Figure 19. Interactions between the jt MOs of the two double bonds and the pseudo-jt orbitals of the methano group in 20 and the methane molecule in 21.

Figure 29. Relative orientations of magnetic orbitals of the Cu(II) ion (d c2 2) in the x,y plane relative to that of the half-occupied Jt orbital of the phenoxyl radical. Here a is the Cu-O-C bond angle and P the dihedral angle between the x,y plane and the plane of the phenyl ring of the coordinated phenoxyl and St is the expected electronic ground state (204).

This leads to modifications of the localized it orbitals. In benzene, for example, a Kekule localization which mixes the a and ir orbitals to form double banana bonds is preferred over the other equivalent ir localizations discussed. 60) In naphthalene a Kekule type structure is found similar to the one presently discussed, but different in that the (jtE2) are hybridized with corresponding o-CC bonding orbitals to form banana bonds, whereas the (ttC2 ) remains a pure jt orbital. 61 > While this is of interest in the discussion of the whole molecule, it is clear that certain intrinsic properties of the ir-electrons are more readily recognized by the localization which has been discussed here. We hope to discuss elsewhere localized orbitals involving a bonds in organic molecules. 

Figure 2.7 The ways in which Jt-bonding interactions with a ligand can influence the value of the energy difference, A for an octahedral complex. High energy, poorly populated jr-orbitals in the ligand increase the splitting (i.e. are Jt-acceptors), whereas filled, low-energy Jt-orbitals decrease the splitting (they are Tt-donors).

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

The double degeneracy of NBMOs in m-[8] has nothing to do with the geometrical symmetry of the molecule, but, rather, with the connectivity of the two radical centres, or the phase relationship of the atomic orbitals in the conjugated system. Therefore, the term topological symmetry has been proposed to describe the connectivity of the carbon atoms carrying the n-electrons and the periodicity of the Jt-orbitals in this class of non-Kekule hydrocarbons. 

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