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Creators: Robin, M. B.; Basch, Harold; Kuebler, N. A.
Issue Date: 1967
Abstract: There is in the spectra of olefins, a weak absorption band preceding the $N\rightarrow V$ transition, the assignment of which has been the topic of recent debate. Though most agree that the olefin ``mystery band'' must involve sigma orbitals, it has not been made clear whether the assignment is $\pi \rightarrow \sigma^{\ast}$ or $\sigma \rightarrow \pi^{\ast}$, nor is it clear just what $\sigma^{\ast}$ really is. Recent spectroscopic experiments on selected olefins in high pressure $N_{2}$ gas and as solutes in rare gas matrices demonstrate that the question as to whether the mystery band is assigned as $\pi \rightarrow \sigma^{\ast}$ or $\sigma \rightarrow \pi^{\ast}$ is meaningless. This follows from the fact that olefins really have several low lying mystery bands, some $\pi \rightarrow \sigma^{\ast}$ and some $\sigma \rightarrow \pi^{\ast}$. All electron, indirect SCF calculations show that in ethylene, the four lowest $\pi \rightarrow \sigma^{\ast}$ states are big orbit, Rydberg-like, whereas the $\sigma \rightarrow \pi^{\ast}$ states are strictly valence shell. A similar situation has developed in the spectra of amides, where a mystery band has been found between the $n \rightarrow \pi^{\ast}$ and lowest $\pi \rightarrow\pi^{\ast}$ excitations. Calculations and experiments together suggest that the amide mystery band is a big orbit $n \rightarrow \sigma^{\ast}$ excitation.
URI: http://hdl.handle.net/1811/15231
Other Identifiers: 1967-H-3
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