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

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 n π* 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.

Reconstruction of Nuclear Ensemble Approach Electronic Spectra Using Probabilistic Machine Learning

“…A particular example that we found during this investigation was that of the U6OH radical. We reconstructed the electronic absorption cross-section spectra using the available cases reported previously 29 (100 geometries). Both GMM-NEA and auto-δ reconstructed spectra are distorted, specially the former ( Figure 6 a).…”

“…Nevertheless, for this species of relevance in the field of DNA damage mechanisms by reactive oxygen species, the most interesting range of wavelengths is that of the visible part of the spectrum (<3.3 eV). 29 , 60 …”

“…In addition, values for Δ E and f for the uracil nucleobase OH radical (U6OH radical) 29 and the HgBrOOH atmospheric compound, 58 both previously reported by our group, have been used to test the methodologies developed in the current work. For the U6OH radical, there are pairs {Δ E , f } for 9 different transitions and 100 geometries, whereas for HgBrOOH, there are data for 79 transitions and 200 geometries.…”

“…This method has attracted significant attention in the last decade, as it allows to predict reliable electronic absorption and emission spectra without a prohibitive computational burden. 15 − 29 …”

Multiscale Modeling of Electronic Spectra Including Nuclear Quantum Effects

“…Within the harmonic approximation, the Wigner sampling is also a suitable choice to represent the nuclear quantum distribution. 5,[18][19][20][21][22][23][24] This method, however, fails for highly flexible molecules because the harmonic approximation cannot be applied. González et al proposed two new sampling protocols to simulate absorption spectra called "local temperature adjustment" and "individual QM/MM-based relaxation."…”