Localization and Local Phenomena in Molecular Excited and Ionized Stationary States

Abstract

The localization of the ground state SCF MO’s gives bond and lone pair MO’s without changing the H—F determinant. The delocalized symmetry adapted MO’s allow the construction of symmetry-adapted determinants and remove the energetic degeneracies between them, while the degenerate, unsymmetrical locally excited determinants only may be considered as a basis for a CI (excitonic) treatment. One may express the delocalized MO’s as linear combinations of localized MO’s, and analyse a nπ* or ππ* delocalized Virtual Orbital Approximation description in terms of Local and Charge Transfer Excitations. It is shown that the π* delocalized MO’s are completely irrelevant for the description of an nπ* excited state in a polyenic aldehyde, since the ‘particle’ should remain in the region of the ‘hole’; the CI process is equivalent in this case to the relocalization of the π* MO in the CO bond. In linear polyenes the delocalized VO Approximation overestimates the role of the long range charge transfer components of the wave function. The comparison with the excitonic treatment shows that after the CI of singly excited determinants, the ππ* excited state is essentially composed of intrabond ethylenic excitations and charge transfers between adjacent bonds. The σπ coupling and mixture toward the Rydberg levels had been thought to have a vanishing effect (as N ^{− 1} ) when the dimension N of the polyene increases; the short range character of the hole-particle’ pair implies on the contrary that these effects, so important in ethylene, remain important whatever the dimension of the conjugated system. It is shown that in butadiene and hexatriene unsymmetrically distorted geometries may be prefered in the ππ* excited states; this Born-Oppenheimer geometry instability suggests one represent the adiabatic excited state as a linear combination of locally excited, locally distorted components. Most of these results are obtained through excitonic treatments using fully localized MO’s. It is shown however on the case of the transition energies of linear polyenes that the tails of the localized SCF-MO’s (i.e. the ground state delocalization) may have a qualitative influence upon spectroscopic features.