The vibrational spectroscopy of indigo: A reassessment

We report the neutron vibrational spectra of indigo and its model compounds thioindigo and isatin. The neutron data extend the low energy range of the vibrational spectra of these molecules. The assignments, made with the help of ab-initio calculations, give convincing fits between the observed and scaled calculated results, and correct errors in the published literature. The indigo eigenvectors are described in terms of those of its model compound isatin. Finally, candidate modes, that could be used to study indigoids in matices (e.g. 'Maya Blue'), are selected.