Raman optical activity of a purely sigma-bonded helical chromophore: (-)-(M)-sigma-[4]helicene

The recent synthesis of enantiomerically pure (-)-(M)-sigma-[4]helicene has provided an archetype helical model system for vibrational optical activit y; comparable to what pi -helicenes represent in the field of electronic op tical activity. We present the first measurements and the first calculation s of the Raman optical activity (ROA) of this interesting molecule. Observe d and calculated ROA is large throughout the vibrational spectrum, in agree ment with expectations, and spectacular effects, with triangle values close to 0.5%, occur in the 900cm(-1) region. Agreement between the experimental spectrum and the theoretical one, calculated with density-functional theor y for the vibrational part and Hartree-Fock linear response theory for the molecular electronic tensors, is excellent, clearly the best that has been achieved to date in the field. This allows us to place confidence in the re sults of an analysis of Raman and ROA scattering generation in the molecule , obtained by a newly developed graphical procedure for extracting this kin d of information from ab initio calculations. One finds that relative contr ibutions made by carbon and hydrogen atoms can be comparable in size, but c an also vary considerably, even between closely lying vibrations, and that, for most vibrations, the generation of ROA difference intensity is distrib uted rather differently than that of Raman intensity over the shape of the molecule. The sign of the ROA is, for the set of vibrations in the 900-cm(- 1) region, which we analyze in detail, determined by coupling terms between the two halves of the molecule, while Raman intensity is primarily generat ed within the two fragments, with coupling terms between them only adding t o or substracting from it.