Quantum-Classical Calculation of Vibronic Spectra along a Reaction Path: The Case of the ECD of Easily Interconvertible Conformers with Opposite Chiral Responses

We present a novel mixed method suitable for computing the
low-to-middle resolution spectra of systems characterized by a large-amplitude
motion s (defined by either a reaction or an internal path) coupled to a bath of
harmonic oscillators r, which change with s. The method is based on an adiabatic
approximation whereby s is considered much slower than r degrees of freedom
and is treated classically. We show that the spectrum can be obtained by a
suitable average of quantum vibronic spectra of the fast coordinates computed at
representative values of the slow coordinate along the path. By our method we
calculate the electronic circular dichroism (ECD) of 2,2,2-trifluoro-anthrylethanol,
which possesses two conformers with opposite chirality separated by a
low-energy barrier. Path-averaged spectra are significantly different from those
obtained only at the stable minima: Agreement with experiment is improved.
The method represents a generalization of those currently based on harmonic
approximation.