Molecular Origin of the Optical Rotatory Dispersion of the Benzil Crystal

Abstract

The absorption spectrum and its relative polarization, the optical-rotatory-dispersion (ORD) spectrum, and the circular-dichroism spectrum (CD) of thin benzil single crystals grown perpendicular to the optic axis, are obtained and compared in the 3000–7000- Å region. The CD spectrum shows two main bands that correspond to the absorption bands with maxima at 3325 and 3850 Å. These two bands result from transitions that are found to have similar signs for the Cotton effect, but opposite to that of the strong optically active transition at 2400 Å and responsible for the plain curve on the longer-wavelength side of the absorption edge. The bands at 3325 and 3850 Å have similar values for the dissymmetry factor g, comparable absorption intensity, and similar relative orientation of their transition moments. From the observed value of the g factor, the two bands seem to result from magnetic-dipole-allowed transitions. These observations suggest the assignment of these two bands to the two split components of the n,π* transitions resulting from the intramolecular interaction between the two carbonyl groups of the benzoyl units in the benzil molecule. The CD spectrum assists in understanding the shape of the ORD spectrum, including the minimum on the longer-wavelength side of the absorption edge, that was previously observed, but not explained. An extremely weak band is observed in the CD spectrum of thick single benzil crystals at above 4250 Å, whose origin is briefly discussed. A comparison is made between predicted polarization characteristics, using an independent-model approach, and the observed properties of the split components of the molecular n, π* transition in the crystal. The disagreement is discussed in terms of mixing between the two molecular n, π* states with each other and with the other molecular (or crystal) states.