Nature of Electronic Levels in Ultraviolet Spectra of Hydrogen and Alkyl Halides

Abstract

The nature of the excited electronic levels involved in the ultraviolet absorption spectra of the alkyl and hydrogen halides, also ICl, IBr, BrCl, is discussed. On the basis of band structures in the hydrogen halides, and other facts, it is concluded that the excited orbital is of $\sigma$ type ($s\sigma ?$) in the $B$, $C$ bands, but of $p\pi$ type in the $D$, $E$ bands. For the upper level of the $B$ and $C$ bands, the coupling changes from nearly $J$, $j$-like in the iodides to predominantly $L$, $S$-like in the chlorides. The tendency toward $L$, $S$-like coupling is greater in the hydrogen than in the alkyl halides. For the $B$, $C$ "satellites" in HCl, a $4p\sigma$ excited orbital is indicated. The $D$, $E$ bands show nearly $J$, $j$-like coupling for all halides. The natures of the excited orbitals in the various observed Rydberg series are also discussed. According to Price, two of the most prominent Rydberg series start with the $A$ continuum and with the $D$, $E$ bands, and so, according to the present study, should have $\sigma$ and $p\pi$ excited orbitals, respectively. It is also concluded that the $\mathrm{np}\pi$ "non-bonding" orbital in the normal states of the hydrogen halides and other diatomic hydrides probably is slightly bonding.