THE ELECTRONIC SPECTRUM OF NORBORNADIENE.

Abstract

Numerous calculations on norbornadiene, [FIGURE] have demonstrated that the nominally unconjugated double bonds in fact do interact appreciably, yielding a forbidden $N \rightarrow V_{1}$ band at about 6 ev. followed by an allowed $N \rightarrow V_{2}$ band In support of this prediction, an absorption tail of unknown intensity observed in the 6 ev. region has been cited repeatedly. We have now extended the electronic spectral data on norbornadiene to 1200 {\AA} and through an analysis of the vibronic pattern of the first and second bands conclude that though the first band is of low intensity, it is electronically allowed. However it appears to cover another weak transition which may well be the predicted $N \rightarrow V_{1}$ band. We also note that whereas the gas phase spectrum of the first band displays a long series of totally symmetric ring flapping vibrations, the prominent vibration in the low temperature matrix spectrum is the totally symmetric $C=C$ stretch, suggesting that the excited state geometry is different in the gas and condensed phases. It appears that the first band in norbornadiene is the 6.5 ev. olefin mystery band, most recently assigned by Berry as $C-H$ bonding to antibonding $\pi$. Our tentative conclusions as to the symmetry of the first excited state, its intensity, polarization and non-Rydberg nature are in accord with Berry's suggestion. Our work on the remainder of the spectrum is inconclusive, at present.