Multiconfigurational Quantum Chemistry Determinations of Absorption Cross Sections (σ) in the Gas Phase and Molar Extinction Coefficients (ε) in Aqueous Solution and Air-Water Interface.

Ana Borrego-Sánchez,Madjid Zemmouche,Isabelle Navizet,Daniel Roca-Sanjuán,Pep Mulet,Antonio Francés-Monerris,Javier Carmona-García

doi:10.1021/acs.jctc.0c01083

Abstract

Theoretical determinations of absorption cross sections (σ) in the gas phase and molar extinction coefficients (ε) in condensed phases (water solution, interfaces or surfaces, protein or nucleic acids embeddings, etc.) are of interest when rates of photochemical processes, J = ∫ ϕ(λ) σ(λ) I(λ) dλ, are needed, where ϕ(λ) and I(λ) are the quantum yield of the process and the irradiance of the light source, respectively, as functions of the wavelength λ. Efficient computational strategies based on single-reference quantum-chemistry methods have been developed enabling determinations of line shapes or, in some cases, achieving rovibrational resolution. Developments are however lacking for strongly correlated problems, with many excited states, high-order excitations, and/or near degeneracies between states of the same and different spin multiplicities. In this work, we define and compare the performance of distinct computational strategies using multiconfigurational quantum chemistry, nuclear sampling of the chromophore (by means of molecular dynamics, ab initio molecular dynamics, or Wigner sampling), and conformational and statistical sampling of the environment (by means of molecular dynamics). A new mathematical approach revisiting previous absolute orientation algorithms is also developed to improve alignments of geometries. These approaches are benchmarked through the nπ* band of acrolein not only in the gas phase and water solution but also in a gas-phase/water interface, a common situation for instance in atmospheric chemistry. Subsequently, the best strategy is used to compute the absorption band for the adduct formed upon addition of an OH radical to the C6 position of uracil and compared with the available experimental data. Overall, quantum Wigner sampling of the chromophore with molecular dynamics sampling of the environment with CASPT2 electronic-structure determinations arise as a powerful methodology to predict meaningful σ(λ) and ε(λ) band line shapes with accurate absolute intensities.

Highlights

Theoretical molecular electronic spectroscopy is based on the characterization of the electronic structure of several electronic states and the determination of the electronic transition energies and intensities
Three distinct strategies were used to generate the ensemble of geometries of the chromophore in the gas phase: (i) molecular dynamics (MD), with parametrized molecular mechanics (MM) force fields; (ii) ab initio molecular dynamics (AIMD), in which quantum chemistry was used for describing the electrons of the chromophore and the nuclei moved according to classical equations; and (iii) Wigner sampling (WS), where frequencies were computed at the equilibrium ground state structure with quantum chemistry and a Wigner distribution was generated.[70]
For the chromophore-solvent macromolecular systems, solvent geometries were sampled with MD simulations, giving rise to the following three strategies, which are analogous to those for the gas phase, (i) MD; (ii) quantum mechanics/molecular mechanics (QM/MM) MD, where the chromophore is described at the QM level and the solvent with MM; and (iii) WS+MD, where the chromophore is sampled with WS and the solvent with MD

Summary

Introduction

Theoretical molecular electronic spectroscopy is based on the characterization of the electronic structure of several electronic states and the determination of the electronic transition energies and intensities. An optimal approach to address the mentioned cases (and others) is to obtain the rovibrational structure of the electronic bands. In this context, some interesting methodologies have been developed.

Methods

Results

Conclusion