Abstract
A proton ENDOR investigation has been conducted on the lowest triplet state of benzil in bibenzyl. The triplet molecule was shown to possess a precise center of inversion. The dicarbonyl fragment therefore has to be precisely trans‐planar. The orientation of the two phenyl rings with respect to the dicarbonyl plane was semiquantitatively determined. These results, in comparison with a previous study on the geometry of triplet benzil in its own lattice, demonstrate the dominant role of lattice packing on determining the conformation and spectroscopy of flexible chromophores.