Toward a general mixed quantum/classical method for the calculation of the vibronic ECD of a flexible dye molecule with different stable conformers: Revisiting the case of 2,2,2‐trifluoro‐anthrylethanol

Abstract

We extend a recently proposed mixed quantum/classical method for computing the vibronic electronic circular dichroism (ECD) spectrum of molecules with different conformers, to cases where more than one hindered rotation is present. The method generalizes the standard procedure, based on the simple Boltzmann average of the vibronic spectra of the stable conformers, and includes the contribution of structures that sample all the accessible conformational space. It is applied to the simulation of the ECD spectrum of (S)-2,2,2-trifluoroanthrylethanol, a molecule with easily interconvertible conformers, whose spectrum exhibits a pattern of alternating positive and negative vibronic peaks. Results are in very good agreement with experiment and show that spectra averaged over all the sampled conformational space can deviate significantly from the simple average of the contributions of the stable conformers. The present mixed quantum/classical method is able to capture the effect of the nonlinear dependence of the rotatory strength on the molecular structure and of the anharmonic couplings among the modes responsible for molecular flexibility. Despite its computational cost, the procedure is still affordable and promises to be useful in all cases where the ECD shape arises from a subtle balance between vibronic effects and conformational variety.

Adiabatic-Molecular Dynamics Generalized Vertical Hessian Approach: A Mixed Quantum Classical Method To Compute Electronic Spectra of Flexible Molecules in the Condensed Phase

“…In this contribution we presented a general mqc method, that we named Ad − classical-coordinates models we presented in refs. 62,106 Its main innovation is the introduction of a coherent scheme to combine MD sampling and vibronic calculations which allows to treat all the flexible coordinates of the solute and all the environment explicitly, and a generalization of the projectors to separate soft and fast coordinates. In this way, while in its current applications we considered dyes solvated in simple solvents, the method is ready to be extended to deal also with heterogeneous media, like proteins and surfaces.…”

“…For larger ones, like flexible molecules in condensed phase, a number of approximated protocols to mix classical MD sampling and vibronic computations have been proposed. 7,37,48,[58][59][60][61][62] In the simplest approaches, the vibronic spectrum of the solute has been considered independent of the specific MD snapshot. [58][59][60][61] A way to go beyond this approximation, explicitly accounting for the coupling of intra-molecular and inter-molecular vibrations, is to use in vibronic calculations spectral densities extracted from classical MD trajectories.…”

“…In some cases, the latter can be computed with “on-the-fly” semiclassical (sc) approximations of the propagator, but these methodologies are, at the state of the art, only feasible for small systems. For larger ones, like flexible molecules in condensed phase, a number of approximated protocols to mix classical MD sampling and vibronic computations have been proposed. ,,,− In the simplest approaches, the vibronic spectrum of the solute has been considered independent of the specific MD snapshot. − A way to go beyond this approximation, explicitly accounting for the coupling of intramolecular and intermolecular vibrations, is to use in vibronic calculations spectral densities extracted from classical MD trajectories. ,,, The potential of such an approach has been recently illustrated by Loco and Cupellini; its partial limitation is that the spectrum is computed with reference to vibronic harmonic models that only account explicitly for linear couplings. The effects of quadratic differences in the initial and final state PES, which are responsible for shift of the maxima and contribute to the spectral width, are only implicitly reflected in the parameters of the spectral density.…”

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

“…Electronic structure calculations were performed adopting the Gaussian16 package of programs 26 with CAM-B3LYP/def2-TZVP [27][28][29] because previous studies on similar systems and using a similar level of theory provided very good agreement with the experiments. 20,23,24,30,31 Solvent effects were included with the Polarizable Continuum Model (PCM). 32 As a standard choice, for computational convenience, we used the equilibrium solvation (EqSolv) regime for which analytical Hessians are available.…”

“…However, classical MD reveals not adequate when, like in our case, vibronic peaks need to be described, because they arise from the quantum nature of molecular vibrations. In addition, investigating the ECD of chiral anthryl-ethanol, we recently showed 23,24 that, in principle, when the potential energy surface (PES) separating the different conformers is rather flat, even offequilibrium structures can modulate the ECD signal.…”