Theory for vibrationally resolved two-photon circular dichroism spectra. Application to (R)-(+)-3-methylcyclopentanone.

A harmonic adiabatic approach in combination with density functional response theory for computing two-photon vibronically resolved circular dichroism spectra of chiral molecules is presented. It includes both Franck-Condon and Herzberg-Teller contributions and it takes fully into account frequency changes and Duschinsky effects. Model calculations have been performed for two dominant conformers of (R)-(+)-3-methylcyclopentanone in the gas phase. It is found that the Herzberg-Teller contribution can introduce a sign change in two-photon circular dichroism of a single excited electronic state of a conformer. The change survives after Boltzmann averaging, and it might be amenable to experimental verification. Interesting interference effects between Franck-Condon and Herzberg-Teller contributions are revealed and analyzed in detail. Results obtained within the more approximate and less computationally intensive linear coupling vibronic model are also given for comparison.