Substituted systems valence transition

The valence bond method has not been used as widely as the molecular orbital approach. With the inclusion of polar structures, however, the valence bond method gives correct orientation for electrophilic substitution and a calculated dipole moment close to the experimental value.100 An application of the one-center method of the 7r-electron system of pyrrole gives electron densities of 1.612, 1.167, and 1.028 on the nitrogen atom and the a- and /3-carbon atoms, respectively.101 Transition energies and the dipole moment by this method are in accord with the observed values. 

In two recent publications we have tried to characterize the excited state properties of 1 and 3 in order to facilitate their detection by LIF-spectroscopy. Our main tool in this effort has been equation of motion coupled cluster theory (EOM-CC). The EOM-CCSD method, which is equivalent to linear response CCSD, has been shown to provide an accurate description of both valence and excited states even in systems where electron correlation effects play an important role [39]. Computed transition energies for excitations that are of mainly single substitution character are generally accurate to within 0.1 eV. We have found the EOM-CCSD method to perform particularly well in combination with the doubly-augmented cc-pVDZ (d-aug-cc-pVDZ) basis set. This basis seems to provide equally balanced descriptions of ground and excited states,... 

Bimolecular nucleophilic substitution reactions (Sn2) involve pentacoordinate carbon in their transition state. Whereas pentacoordinate CHs -like intermediates of Se2 reactions represent only an eight-electron system around the carbocationic center involving 3c-2c bonding (octet rule is obeyed), an Sn2 reaction intermediate would represents a 10-electron system involving three-center, four electron (3c-4e) bonding (a lO-C-5 species). This, however, is an unstable situation since carbon cannot accommodate 10 electrons in its valence shell. A simple picture of typical molecular orbitals in 3c-4c bonding is shown in Figure 6.9. 

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