The role of chlorine position in the electronic circular dichroism of chlorophenyl-ethanol investigated by vibronic calculations

(R)-1-phenyl-ethanol (PhEtOH) and the different isomers of (R)-1-(chlorophenyl)ethanol (ClPhEtOH) exhibit very interesting electronic circular dichroism (ECD) in methanol. In all cases, the spectrum shows clear vibronic features, but it is monosignated and negative for PhEtOH and meta-ClPhEtOH, positive for the ortho isomer and bisignated for the para isomer. We used computational chemistry to rationalise this behaviour adopting CAM-B3LYP/def2-TZVP, describing the bulk solvent effects with polarizable continuum models and solute-solvent specific interactions with clusters comprising the solute and two solvent molecules. We adopted harmonic vibronic models to compute the ECD spectral shapes of all stable conformers, and we obtained the room-temperature spectra by Boltzmann average. Simulated spectra are in very good agreement with experiment and allow us to rationalise their difference in terms of the relevance of Franck-Condon (FC) and Herzberg-Teller (HT) intensity-borrowing contributions, modulated by the substituent effect. The bisignated shape of the spectrum of para-ClPhEtOH arises from the competition of opposite-sign FC and HT bands, promoted by different vibrational modes. Due to the challenges we document in computing its ECD spectrum, para-ClPhEtOH represents a good test case to help the development of novel methodologies for an improved description of weak vibronic ECD spectra of flexible systems in explicit solvents.