Background: Chiroptical Phenomena

The history of studies of chiroptical phenomena — that is the interaction of chiral molecules with polarized light — goes back nearly two centuries to the work of Biot between 1812–1838 demonstrating optical rotation of the plane of linearly polarized light by certain organic substances such as tartaric acid. Pasteur's work (1848) demonstrated the connection between this optical rotation and three dimensional molecular stereochemistry by separating distinct forms – enantiomers – of active sodium ammonium tartrate as chiral crystals. In 1895 Aime Cotton demonstrated circular dichroism (CD), the differential absorption of circularly polarized light by a spectroscopic transition occuring in an optically active enantiomers.

Although the differential absorption is typically only 10^-5–10^-4, CD has become very well established, particularly as a means for identifying structural motifs in chiral proteins. Its intrinsic weakness, however, means that it is almost always performed in solution. Solvent absorption and light source limitations limit the lower wavelength range observed in CD, while chirality induced in the solvation layer can complicate detailed modelling, so that the usual interpretation of CD spectra is somewhat empirically based.

PECD

Rather more recently (this century!) we initiated a theoretical investigation of the photoionization of randomly orineted chiral molecules by circularly polarized light. Numerical photoionization dynamics calculations predicted massive chiral asymmetry factors of up to 4×10^-1 would be observed in the angle-resolved photoelectron spectra of a molecular enantiomer. Since this exceeds normal chiral asymmetries by many orders of magnitude it was a very exciting result which naturally provoked follow up experimental investigations.

Using circularly polarized synchrotron radiation sources in Paris, Berlin and Trieste these expectations have been confirmed by experiments made using natural product odour molecules etc., randomly oriented in the gas phase. Work by ourselves and others has additionally revealed just how richly structured and detailed PECD spectra are, and how it is unexpectedly sensitive to conformation (shape) in small bio-molecules.

PECD has been rapidly developed, both experimentally and theoretically, to the point where it can:

• Distinguish, identify, and order different electron orbital components in a photoelectron spectrum — even when these contributions only partially resolve in the normal spectrum
• Determine absolute configuration of an enantiomer in dilute, solvent free environment
• Infer molecular conformation (shapes) present in a gaseous sample
• Distinguish subtle differences in molecular substituents, electronic structure or geometrical arrangement even when these are remotely situated in the molecule