Spectroscopy and Conformer-Selective Photoelectron Circular Dichroism of Chiral Molecules.

Like most chiroptical spectroscopy techniques, photoelectron circular dichroism (PECD), that is, the forward-backward asymmetry in the angular distribution of the electron resulting from the ionization of a chiral molecule by a circularly polarized light, is very sensitive to structural changes, in particular conformational isomerism. Moreover, because of the large associated asymmetries (up to a few 10's of %), PECD is uniquely adapted to gas-phase studies of flexible chiral molecules and exists, for the case of valence-shell, in several variants relying upon one vaccum-ulraviolet (VUV) photon or multiple-photon ionization. We describe here recent advances in PECD spectroscopy that make this method conformer-selective. In addition to partial population selection in one-photon PECD, two-photon PECD resting on a resonance-enhanced ionization scheme allows complete conformer selection, as long as the studied system displays an electronic transition in the near UV. These experiments thus provide a direct connection between structures (conformations) and chirality (PECD), which both influence phenomena such as molecular recognition. The low temperature achieved under supersonic expansion conditions and the narrow-band nanosecond laser used allow deciphering conformers with very similar structures, like ring-inversion conformers or those related to a hydrogen bond, or even distinguishing the lone electron pairs in a diastereotopic molecule.