Assignment of the Ultraviolet Mystery Band of Olefins

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Abstract
The weak mystery band found in the mono‐olefins at 5–6 eV is here reassigned as a symmetry‐allowed π→CH*(1Ag→1B2u) transition in which the CH* sigma upper orbital has considerable ns Rydberg character. Evidence supporting this assignment and militating against the recent CH→π*(1Ag→1B3g) assignment of Berry [J. Chem. Phys. 38, 1934 (1963)], the only other plausible assignment, is drawn from (a) ab initio calculations of the electronic spectrum and orbital energies of ethylene in large Gaussian bases, (b) a study of the intensities and vibronic band shapes of the mystery bands of cis—trans pairs of dialkylethylenes, (c) the observed effect of ring strain on the mystery‐band intensity, (d) a measurement of the Cotton effect and rotational strength of the mystery‐band transition, and (e) the spectra of olefins in condensed phases which demonstrate the Rydberg nature of the mystery‐band upper state. Evidence is also presented that shows that the first electron affinity of ethylene places an electron in the CH* orbital, rather than in the π* orbital, as previously assumed. The implication of this low‐lying π→σ* transition for the common assumption of π—σ separability is mentioned, and the importance of the almost certain presence of relatively low‐lying π→σ* transitions with out‐of‐plane polarization in hydrocarbons of all sizes is stressed.