Knowledge Bank

The $S0\leftrightarrow S1$ absorption and emission spectra of styrene have been analyzed with the help of an ab-initio calculation. It was found that out-of-plane vibrations undergo, in general, a red shift upon electronic excitation, while in-plane ones are not appreciably changed. An important exception to this rule is the $\nu 17$ mode, which is similar the $\nu 14(b2u)$ mode of benzene - it shows a considerable frequency increase upon excitation. The analysis led to some new assignments of certain vibronic transitions. In addition, the spectra of styrene-ammonia adducts were recorded. Cluster formation affects the frequencies of some normal modes of $S1$ to a small but easily measured extent. It was found that in general, out-of-plane modes were blue shifted with respect to the bare molecule, while in-plane modes were essentially unchanged. Vibronic bands leading to out-of-plane in the cluster appear to be less intense than in the bare molecule; in contrast, some bands at about $500cm-1$ are found to have a considerably more intense than the corresponding bare molecule ones. These bands are due to a mode similar to the $\nu 6(e1m)$ mode of benzene, responsible for the vibronic coupling that makes the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag\leftrightarrow 1B2u$ transition weakly allowed. The dissociation energy of the styrene-ammonia cluster was roughly determined, by recording the dispersed emission spectra following excitation of the cluster to different vibronic levels. It was found to be $650\pm 100$ for $S1$ and $600\pm 100$ for $S0$.