Knowledge Bank

There has been considerable work on the first Rydberg state ($R1s$) of ethylene, which is elegantly summarized in a review by Merer and $Mulliken,1$ According to a careful analysis of the intensities of vibronic transitions involving the torsional mode $\nu 4$, Merer and $Schoonveld2$ have derived an upper state potential which indicates a nearly planar structure for ethylene in the $R1s$ state. Foo and $Innes3$ have essentially confirmed the results of the vibronic analysis by a study of the rotational envelopes of some of the $R1s$ vibronic transitions for the series of deuterated ethylenes. Thus, the state of affairs seemed settled. However, Watson and $Nycum4$ recently proposed a different explanation. In order to retain the same isotopic ratios ($C2H4:C2D4$) for the ground and Rydberg states, they suggested that the first of the two prominent peaks (57 336 $cm-1$ in $C2H4$) is only vibronically allowed by coupling with $\nu 8$ (Herzberg’s notation), and that the second peak (57 808 $cm-1$ in $C2H4$) is built on a very weak origin (at 56 900 $cm-1$ in $C2H4$) and becomes allowed by coupling with $\nu 4+\nu 10$. Their argument is based on the non-constancy of $\Delta /\nu 4\prime \prime$ and the constancy of $\Delta /(\nu 4\prime \prime +\nu 10\prime \prime -\nu 8\prime \prime )$, where $\Delta$ is the energy spacing between the two prominent peaks. This suggestion prompted us to reinvestigate the VUV spectrum of $1,1-C2H2D2$. In addition, we have extended the theoretical treatment of $C2H4$ and $C2D4$ by Merer and Schoonveld to $1,1-C2H2D2$ and approximately so to cis- and trans-l,2-$C2H2D2,C2H3D$ and $C2HD3$. Briefly, the results indicate that Merer and Schoonveld’s potential provides an excellent description of the $R1s$ torsional potential in the mono-, di-, and tri-deuterated ethylenes.