Aromatic Carbonyl Spectra. I. The Polarized Absorption Spectrum of Single-Crystal 4,4′-Dichlorobenzophenone

The polarized spin‐allowed electronic absorption spectrum of single‐crystal 4,4′‐dichlorobenzophenone has been measured at room temperature and liquid‐helium temperature. Three absorption bands have been observed and characterized: the $\mathrm{n\pi *}$ band in the 25 000–32 000‐cm⁻¹ region and the ${}^1{B}_{\text{2u}}$ analog and intramolecular charge‐transfer bands in the 32 000–41 000‐cm⁻¹ range. In the $\mathrm{n\pi *}$ band, we have been able (1) to determine the polarization of the $\mathrm{n\pi *}$ transition moment as exclusively parallel to the carbonyl axis; (2) to observe the 0–0 transition at 25 552 cm⁻¹ indicating the electronic‐allowed character of the transition; (3) to definitely exclude the possibility of a magnetic dipole transition or vibronic coupling via non‐totally symmetric b‐type vibrations as sources of intensity; (4) to observe and assign the torsional oscillations of the substituent phenyl rings; and, further (5) to observe and analyze the torsional doubling of the totally symmetric (in C₂) vibration, ${\nu }_3\prime$ , due to the nonbonded interaction between the phenyl rings. We have found no evidence to indicate that the excited state is nonplanar (i.e., pyramidal carbonyl group). The most probable source of intensity gain of the normally symmetry‐forbidden $\mathrm{n\pi *}$ transition is shown to be the breakdown of local carbonyl symmetry (i.e., a delocalization of the oxygen nonbonding orbital) such that there is significant mixing between the nonbonding orbital and the phenyl‐ring $\pi$ orbitals.