Antibonding σ* orbitals in molecular spectroscopy

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Abstract
This paper discusses the role of σ* antibonding valence MO's in the electronic spectra of large molecules. Using the term value and antishielding concepts, molecular absorption patterns terminating at σ* MO's are systematized and rationalized; the frequencies of such excitations correlate with the ground-state bond strengths of the bonds giving rise to the σ* MO's. In unsaturated systems, the π → σ* and np → σ* excitations can be distinguished on the basis of their term values. Confusion in the past regarding the existence of excitations to σ* relates to the fact that σ* MO's often can be totally mixed into the surrounding sea of Rydberg levels of the same symmetry, and for all practical purposes can cease to exist except as perturbations on the Rydberg levels. For nonpenetrating Rydberg manifolds there is little or no mixing with σ* MO's of the same symmetry, setting the stage for the appearance of valence-shell excitations to σ* in high-symmetry molecules which are analogous to the d → f inner-well giant resonances of atomic spectroscopy.

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