Ground-state reactivity

A photochemical reaction coordinate has two branches an excited state branch and a ground state branch that is reached after decay at a conical intersection. Thus a conical intersection between ground and excited states of a molecule is a precursor to ground state reactivity, and conforms to the above definition of a reactive intermediate. The main focus of our article will be to develop this idea. In Figure 9.1b, we show the energy profile for a photochemical reaction with a conical intersection... 

When such structural or static information is not sufficient (i.e., the excited state may not decay at the minimum of the conical intersection line, or the momentum developed on the excited state branch of the reaction coordinate may be sufficient to drive the ground state reactive trajectory along paths that are far from the ground state valleys), a dynamics treatment of the excited state/ ground state motion is required.53 54 These techniques also are illustrated in the next subsection. 

Many aromatic compounds undergo heterolytic photosubstitution on irradiation. Nucleophilic aromatic substitutions are particularly frequent. The orientation rules are reversed compared to those known for ground-state reactivity that is, electron-withdrawing substituents orient incoming groups... 

Whereas the ground state reactivity of arenas is characterised by attack of electrophiles upon the ring, this mode of reactivity is comparatively uncommon for the arena excited state. During the year of coverage of this chapter two reports of such a process have appeared.The photolysis of the sodium salt of metanilic acid (186) in water yields aniline and the ortho and para isomers of (186) (i.e. orthanilic acid and sulphanilic acid,... 

With both the substituent effect and relative quenching order different from the ground state reactivities, the possibility that excited state cations may be quenched by water, alcohols, and ethers through an electron transfer process was considered [28,29], Application of the Rehm-Weller equation [43,44] shows that for an electron transfer mechanism to be operating, cation reduction potentials must account for the observed substituent effect on reactivity [28,29], In qualitative terms, an ease in reduction of the cations should parallel an increase in the quenching rate constants. Reduction potentials for the xanthyl cations [45] demonstrate that the trend for ease of reduction of the cations decreases with a concomitant increase in the magnitude of the quench-... 

Finally, some spectroscopic applications for pseudopotentials within SOCI methods are presented in section 3. We focus our attention on applications related to relativistic averaged and spin-orbit pseudopotentials (other effective core potentials applications are presented in chapters 6 and 7 in this book). Due to the large number of theoretical studies carried out so far, we have chosen to illustrate the different SOCI methods and discuss a few results, rather than to present an extensive review of the whole set of pseudopotential spectroscopic applications which would be less informative. Concerning the works not reported here, we refer to the exhaustive and up-to-date bibliography on relativistic molecular studies by Pyykko [21-24]. The choice of an application is made on the basis of its ability to illustrate the performances on both the pseudopotential and the SOCI methods. One has to keep in mind that it is not easy to compare objectively different pseudopotentials in use since this would require the same conditions in calculations (core definition, atomic basis set, SOCI method). The applications are separated into gas phase (section 3.1) and embedded (section 3.2) molecular applications. Even if the main purpose of this chapter is to deal with applications to molecular spectroscopy, it is of great interest to underline the importance of the spin-orbit coupling on the ground state reactivity of open-shell systems. A case study is presented in section 3.1.4. 

In this communication we have so far performed ground state reactivity index calculation, excited state calculation with Cl method whose credibility is vaUdated... 

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