Knowledge Bank

The polarized electronic absorption spectrum of single crystal $4,41$-dichlorobenzophenone has been measured at room temperature and $4.2\circ K$. Three singlet-singlet absorption bands have been characterized, the $n\pi \ast$ band in the $25,000-32,000cm-1$ region and the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u$-analog and intramolecular charge transfer bands in the $32,000-41,000cm-1$ range. From the polarization behavior, band shape, and vibrational analysis of the $4.2\circ K$ spectrum of the $n\pi \ast$ band, we have been able 1) to identify and assign the phenyl ring torsional oscillations, 2) to determine unambiguously the symmetry assignment for the excited singlet $n\pi \ast$ state, and 3) to show that the $n\pi \ast$ transition is electronically allowed and that the predominant mode of intensity gain is not via a vibronic coupling mechanism.