7r-orbital

The new orbital of the TT-electron in the 7T -state is larger than it was in the unexcited state. The two adjacent atoms with which the electron was associated in the ground state may be partially held by the electron in its expanded 7r -orbital. The atoms, in adjusting to the new binding condition, must move farther apart. They absorb the energy necessary to do this from the electron in its 7r -orbital. An additional vibrational ampHtude is attained by the two atoms as a system. Some added energy is transmitted to other atoms of the conjugated molecule. These atomic vibrational adjustments take place very quickly, in 10 to 10 s. 

C-Rh-Cl 150.7°) and some bending of the Rh—C-Obond (162.3°). Some short O—H and Cl-H intramolecular contacts may be responsible for the distortion though it has been suggested that in a distorted tetrahedral environment there may be an interaction between Rh dxz electrons and the CO 7r -orbital causing bending. 

Formation of a <7-bond by donation from the 7r-orbital of ethene into a vacant metal dsp2 hybrid orbital... 

This involves partial occupation of the 7r -orbital and hence a lengthening of the C—C bond moreover, as the bonding at carbon changes, acquiring some sp3 character, so the bond angle at carbon will decrease below 120°. 

The asymmetric induction that has been observed in this reaction can be explained in terms of the model shown in Scheme 9. In the most stable conformation the appropriately positioned phenyl group shields selectively the Re,Re face of the chromadiene by 7r,7r-orbital overlap forcing the nucleophile to attack preferentially on the opposite side. 

FIGURE 2 In a TT-to-ir transition, an electron in a bonding 7i--orliit.il is excited into an empty antibonding 7r -orbital. 

The complex contains 72 atoms with 244 valence electrons distributed in 226 valence atomic orbitals. In order to reduce the computational effort, and to assess the contribution of the ligand 7r-orbitals to the overall spectrum, we examined a "reduced" model, see Figure 2, in which the benzene rings of the ligands are replaced by -HC=CH- groups. This model compound consists of... 

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

Very recently, photoelectron spectra have been recorded for disilenes 1,3, and 25.54 Consistent with the electronic spectra, the lowest ionizations were assigned to the Si=Si 7r-orbital. The He(I) photoelectron spectrum for 1 is shown in Fig. 4. The low-energy ionization appears at 7. 53 eV. The more intense, higher energy bands were shown to be due to ionizations from mesityl 7r-orbitals by comparison with the photoelectron spectra of model compounds without a Si=Si bond. 

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