Excited state, formation molecular geometry

The molecular geometry of the lepidopterene skeleton remains a feature of unique importance for the observed photochemical reaction. The photolytic An + 27i cycloreversion generates an electronically excited product in which the diene and dienophile moieties are bound to face each other in an arrangement which, subsequent to deactivation to the Franck-Condon ground state, is an ideal one for bond formation. 

The fluorescence quantum yields of pyrene-1-carboxaldehyde in water and methanol are 0.98 and 0.07/ an effect attributed to solvent effects on 7c,n and n,n states. Cycloaddition reactions of 1-naphthonitrile to 1,2-dimethyl-cyclopentene are attributed to both and Lj, states.It is pointed out that although dual fluorescence is known, this is the first example of divergent reaction from two nearly isoenergetic singlet states. An analysis of the u.v. spectra of some acyl pyridines, including a theoretical examination of the molecular geometry, and excited states of bipyrimidine compounds have also been made. Photo tautomerism and the fluorescence of the cation of 4-amino-pyrazole[3,4-iflpyrimidine, an analogue of adenine, has been published by Wierzchowski et Intramolecular heteroexcimer formation in... 

For a closed-shell ground-state molecule, those properties that involve only the ground-state wave function can be calculated just as well with either the localized of the delocalized MO description. Such properties include electron probability density, dipole moment, geometry, and heat of formation. Properties of a molecule that involve the wave function of the ground state and also the wave function of an open-shell excited state or the wave function of an open-shell ion cannot be calculated using a localized MO description. Such properties include the electronic absorption spectrum and molecular ionization energies. 

There are currently three commonly employed theoretical methods for the study of the properties of molecules. Here we will look at semiempirical methods, which occupy a position between the ah initio and molecular mechanics methods. High level ab initio methods have an accuracy comparable with experiments for heats of formation and are at least as accurate as experiments for the determination of molecular geometries. They are also versatile enough to allow transition states and excited states to be calculated. The only drawback to high level cUj initio work is the cost in terms of computer resources. 

---