Low-Lying Excited States of Mono-olefins

Semiempirical calculations of energies, oscillator strengths, and rotational strengths for a number of singlet–singlet transitions of ethylene and of the twisted olefin trans‐cyclo‐octene are compared with experiments. The analysis shows that a minimum of three low‐lying singlet states are required to explain the data considered: a ${}^1{\mathrm{B}}_{\mathrm{xu}}{\mathrm{(\pi }}_z{\mathrm{\hspace{0.17em}\to \hspace{0.17em}\pi }}_z\mathrm{*)}$ responsible for the strong, broad olefinic absorption, a ${}^1{B}_{\mathrm{zu}}{\mathrm{(\pi }}_z\text{\hspace{0.17em}→\hspace{0.17em}σ*)3s}$ Rydberg state responsible for the olefin band with $\text{ε≅1000}$ , and a ${}^1{B}_{\mathrm{xg}}{\mathrm{(\pi }}_z{\mathrm{\hspace{0.17em}\to \hspace{0.17em}\pi }}_y{\text{*)3p}}_y$ Rydberg like state accounting for the low‐lying electric quadrupole‐allowed transition of ethylene and dominating the optical rotatory properties of the twisted double bond.