Assignments in the Ultraviolet Spectra of Olefins

Indirect SCF calculations in a Gaussian basis of the lower π→σ* and σ→π* states of ethylene demonstrate that the lower π→σ* excitations are Rydberg and involve 3s and 3p orbitals on carbon, whereas the lower σ→π* 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 π↔σ transitions should not exceed 0.1, and that in twisted olefins, the rotational strengths of π→σ* excitations, in general, are smaller than those for σ→π* excitation. The e.ectronic spectrum of tricyclo[3.3.0.02,6]oct-3-ene(TCO) in the gas phase shows that an olefin can have up to four π↔σ transitions preceding the π→π* 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 π→σ* Rydberg transitions. It is concluded that Rydberg π→σ* excitations will be lowest in simple olefins, but in the more complex polycyclic strained olefins, a low-lying valence-shell transition (probably π→σ*) is nearly degenerate with, or falls below, the lowest Rydberg π→σ* transition.