Assignments in the Ultraviolet Spectra of Olefins

Indirect SCF calculations in a Gaussian basis of the lower $\mathrm{\pi \to \sigma *}$ and $\mathrm{\sigma \to \pi *}$ states of ethylene demonstrate that the lower $\mathrm{\pi \to \sigma *}$ excitations are Rydberg and involve $\text{3s}\mathrm{and}\text{3p}$ orbitals on carbon, whereas the lower $\mathrm{\sigma \to \pi *}$ excitations are strictly valence shell. Calculations in an identical basis using the virtual‐orbital approximation instead lead to excited states which are mixtures of Rydberg and valence‐shell configurations. The calculations also show that the oscillator strengths of $\mathrm{\pi \leftrightarrow \sigma }$ transitions should not exceed 0.1, and that in twisted olefins, the rotational strengths of $\mathrm{\pi \to \sigma *}$ excitations, in general, are smaller than those for $\mathrm{\sigma \to \pi *}$ excitation. The e.ectronic spectrum of tricyclo[3.3.0.0^2,6]oct‐3‐ene(TCO) in the gas phase shows that an olefin can have up to four $\mathrm{\pi \leftrightarrow \sigma }$ transitions preceding the $\mathrm{\pi \to \pi *}$ absorption. Comparison of the spectra of ethylene in high‐pressure nitrogen and of TCO in rare‐gas matrices show that the lowest transition in the latter is a valence‐shell transition, but that the second and third are $\mathrm{\pi \to \sigma *}$ Rydberg transitions. It is concluded that Rydberg $\mathrm{\pi \to \sigma *}$ excitations will be lowest in simple olefins, but in the more complex polycyclic strained olefins, a low‐lying valence‐shell transition (probably $\mathrm{\pi \to \sigma *}$ ) is nearly degenerate with, or falls below, the lowest Rydberg $\mathrm{\pi \to \sigma *}$ transition.