Ethylene Rydberg orbitals

Figure 1.25. Rydberg orbitals of ethylene. Shown is an ns orbital (bottom) and an nda orbital (middle) as compared to the n orbital (top) (by permission from Peyer-imhoff and Buenker, 1973).

The Rydberg state of alkene is designated as n, R(3s), to specify that one electron from the n bond remains in the n orbital and the other has been shifted to the Rydberg orbital, which resembles the 3s orbital of a helium atom. The Rydberg state of an alkene is considered to have a partially ionic stmcmre such that a net positive charge remains on a carbon atom. The Rydberg state of ethylene is shown below. 

Ethylene has been chosen for our second case study in far-ultraviolet Rydberg-valence spectroscopy. While more details are known about ethylene than about ethane, the main points can be treated more succinctly. The complications in the case of ethane were due to the existence of close-lying initiating o-orbitals. Yet, for ethylene too, problems remain. 

We caution that this level of theory was chosen in part for its computational efficiency CASSCF does have some deficiencies, and in particular is unable to describe dynamic electron correlation. The PESs for ethylene from SA-3-CAS(2/2) are qualitatively similar to the PESs from higher level calculations, but dynamics calculations with higher levels of theory (e.g., including perturbation theory corrections with MS-CASPT2 [27] and/or including diffuse orbitals to represent the Rydberg states [28]) suggest that there is a tendency for SA-3-CAS(2/2) dynamics to overestimate the importance of the ethylidene conical intersection. 

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