Molecules with CX3NO, CX3CHO, and related structures have eclipsed conformations in their ground electronic states, but adopt a staggered equilibrium conformation in theirnn*excited states. This dramatic difference in the torsional barrier in different electronic states of the same molecule is an important clue towards a fuller understanding of the relation between electronic structure and preferred conformation. Results to date are reviewed, and suggestions are made for further experimental and theoretical studies.