Knowledge Bank

In comparing the vacuum ultraviolet spectra of bromo- and iodoethylene with the spectra of their perfluoro analogues, it is noted that many of the absorptions are shifted in energy. Fluorine substitution has been found to stabilize the $\sigma$ systems and increase the term values of $(\Pi ,ns)$ Rydberg transition, but to have little effect on $\Pi$ ionization potentials or p Rydberg term $values.1$ However, in these compounds, there are two $(\Pi ,np)$ Rydberg series assigned and the term values of one series, believed to be associated with in-plane p Rydberg orbitals, showed a marked increase with fluorination. Additional substituent effects on band intensities and halogen-$\Pi$ system interaction parameters have also been observed and are discussed.