Vibrationally-resolved chiroptical spectra and chiral signatures of organic ketones with TD-DFT

We report a systematic computational investigation of the (chiro)ptical properties of a series of intrinsically chiral organic ketones, considering the prediction of one-photon absorption, electronic circular dichroism, fluorescence, and circularly polarized luminescence spectra, together with the absorption and luminescence dissymmetry factors, g_abs and g_lum, typically used to quantify chiral responses. A set of TD-DFT functionals, namely B3LYP, MN15, M06-2X, CAM-B3LYP, and ωB97X-D, is first assessed within the vertical approximation. Such an approach qualitatively reproduces the main trends in dipole and rotatory strengths, but provides rather unreliable g_abs and g_lum factors. For the same set of functionals, vibrationally resolved spectra were then simulated using a panel of vibronic models. For absorption and electronic circular dichroism spectra, vertical hessian and vertical gradient vibronic models reproduce the experimental vibronic structure more accurately than the adiabatic hessian model, which often yields overly broad and blueshifted bands. The inclusion of Herzberg-Teller effects significantly improves the prediction of g_abs, mainly by increasing the computed absorption dipole strength associated with the weak n → π* transition, with the range separated functionals, ωB97X-D and CAM-B3LYP, providing the best overall results. In contrast, the fluorescence and circularly polarized luminescence spectra are less sensitive to the vibronic model, as they are dominated by a single broad emission band. Nevertheless, Herzberg-Teller effects allow improving g_lum values for all the tested functionals. Overall, this work shows that reliable predictions of dissymmetry factors in chiral ketones require a balanced treatment of electronic-structure and vibronic effects. While non-Condon contributions are essential for accurately describing g_abs, the calculation of g_lum is comparatively more dependent on the electronic-structure method for the investigated set of compounds.