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1. Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical

   https://doi.org/10.1039/D0CP05216G  

   Rana, Bhaskar ; Herbert, John M. ( December 2020 , Physical Chemistry Chemical Physics)

   The presence of a hemibond in the local solvation structure of the aqueous hydroxyl radical has long been debated, as its appearance in ab initio simulations based on density functional theory is sensitive to self-interaction error (favoring a two-center, three-electron hemibond) but also to finite-size effects. Simulations reported here use a mixed quantum mechanics/molecular mechanics (QM/MM) framework in a very large periodic simulation cell, in order to avoid finite-size artifacts and to facilitate testing of various density functionals, in order to probe the effects of delocalization error. The preponderance of hemibonded structures predicted by generalized gradient approximations persists in simulations using the hybrid functionals B3LYP and PBE0, but is reduced to a minor population if the fraction of exact exchange is increased to 50%. The hemibonded population is also small in simulations employing the long-range corrected functional LRC- ω PBE. Electronic spectra are computed using time-dependent density functional theory, and from these calculations emerges a consensus picture in which hemibonded configurations play an outsized role in the absorption spectrum, even when present as a minority species. An intense 1b 2 (H 2 O) → 2pπ(˙OH) charge-transfer transition in hemibonded configurations of the radical proves to be responsible for an absorptionmore »feature at 230 nm that is strongly shifted with respect to the gas-phase absorption at 307 nm, but this intense feature is substantially diminished in aqueous geometries where the hemibond is absent. Although not yet sufficient to quantitatively establish the population of hemibonded ˙OH(aq), these simulations do suggest that its presence is revealed by the strongly shifted ultraviolet absorption spectrum of the aqueous radical.« less
2. Delocalization of dark and bright excitons in flat-band materials and the optical properties of V2O5

   https://doi.org/10.1038/s41524-022-00754-2  

   Gorelov, Vitaly ; Reining, Lucia ; Feneberg, Martin ; Goldhahn, Rüdiger ; Schleife, André ; Lambrecht, Walter R. L. ; Gatti, Matteo ( April 2022 , npj Computational Materials)

   The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V₂O₅, we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.
3. Exciton energy transfer reveals spectral signatures of excited states in clusters

   https://doi.org/10.1039/D0CP02042G  

   Lu, Wenchao ; Metz, Ricardo B. ; Troy, Tyler P. ; Kostko, Oleg ; Ahmed, Musahid ( July 2020 , Physical Chemistry Chemical Physics)

   Electronic excitation and concomitant energy transfer leading to Penning ionization in argon–acetylene clusters generated in a supersonic expansion are investigated with synchrotron-based photoionization mass spectrometry and electronic structure calculations. Spectral features in the photoionization efficiency of the mixed argon–acetylene clusters reveal a blue shift from the 2 P 1/2 and 2 P 3/2 excited states of atomic argon. Analysis of this feature suggests that excited states of argon clusters transfer energy to acetylene, resulting in its ionization and successive evaporation of argon. Theoretically calculated Ar n ( n = 2–6) cluster spectra are in excellent agreement with experimental observations, and provide insight into the structure and ionization dynamics of the clusters. A comparison between argon–acetylene and argon–water clusters reveals that argon solvates water better, allowing for higher-order excitons and Rydberg states to be populated. These results are explained by theoretical calculations of respective binding energies and structures.
4. Absorption spectra of benzoic acid in water at different pH and in the presence of salts: insights from the integration of experimental data and theoretical cluster models

   https://doi.org/10.1039/C9CP06728K  

   Karimova, Natalia V. ; Luo, Man ; Grassian, Vicki H. ; Gerber, R. Benny ( March 2020 , Physical Chemistry Chemical Physics)

   The absorption spectra of molecular organic chromophores in aqueous media are of considerable importance in environmental chemistry. In this work, the UV-vis spectra of benzoic acid (BA), the simplest aromatic carboxylic acid, in aqueous solutions at varying pH and in the presence of salts are measured experimentally. The solutions of different pH provide insights into the contributions from both the non-dissociated acid molecule and the deprotonated anionic species. The microscopic interpretation of these spectra is then provided by quantum chemical calculations for small cluster models of benzoic species (benzoic acid and benzoate anion) with water molecules. Calculations of the UV-vis absorbance spectra are then carried out for different clusters such as C 6 H 5 COOH·(H 2 O) n and C 6 H 5 COO − ·(H 2 O) n , where n = 0–8. The following main conclusions from these calculations and the comparison to experimental results can be made: (i) the small water cluster yields good quantitative agreement with observed solution experiments; (ii) the main peak position is found to be very similar at different levels of theory and is in excellent agreement with the experimental value, however, a weaker feature about 1 eV to lower energy (redmore »shift) of the main peak is correctly reproduced only by using high level of theory, such as Algebraic Diagrammatic Construction (ADC); (iii) dissociation of the BA into ions is found to occur with a minimum of water molecules of n = 8; (iv) the deprotonation of BA has an influence on the computed spectrum and the energetics of the lowest energy electronic transitions; (v) the effect of the water on the spectra is much larger for the deprotonated species than for the non-dissociated acid. It was found that to reproduce experimental spectrum at pH 8.0, additional continuum representation for the extended solvent environment must be included in combination with explicit solvent molecules ( n ≥ 3); (vi) salts (NaCl and CaCl 2 ) have minimal effect on the absorption spectrum and; (vii) experimental results showed that B-band of neutral BA is not sensitive to the solvent effects whereas the effect of the water on the C-band is significant. The water effects blue-shift this band up to ∼0.2 eV. Overall, the results demonstrate the ability to further our understanding of the microscopic interpretation of the electronic structure and absorption spectra of BA in aqueous media through calculations restricted to small cluster models.« less
5. Single-active electron calculations of high-order harmonic generation from valence shells in atoms for quantitative comparison with TDDFT calculations

   https://doi.org/10.1088/2399-6528/ab9a68  

   Reiff, R. ; Joyce, T. ; Jaroń-Becker, A. ; Becker, A. ( June 2020 , Journal of Physics Communications)

   We present a reproducible ab-initio method to produce benchmark tests between time-dependent Schrödinger equation (TDSE) in the single-active-electron approximation (SAE) and time-dependent density functional theory (TDDFT) in the highly nonlinear multiphoton and tunneling regime of strong-field physics. To this end we compare results for high-order harmonic generation from valence shells in atoms using the SAE-TDSE approach and TDDFT calculations. As key to the benchmark comparison we obtain an analytic form of SAE potentials based on density functional theory, which we applied for different atoms and ions. The ionization energies of atomic ground and excited states, as well as the energies of inner shells, for the SAE potentials agree well with experimental data. Using these potentials we find remarkable agreement between the results of the two independent numerical approaches (TDDFT and SAE-TDSE) for the high-order harmonic yields in helium, demonstrating the accuracy of the SAE potentials as well as the predictive power of SAE-TDSE and TDDFT calculations for the nonperturbative and highly nonlinear strong-field process of high harmonic generation in the ultraviolet and visible wavelength regime. Finally, as another application of the SAE potentials, high harmonic spectra from outer and inner valence shells are calculated and it is shown thatmore »unphysical artifacts in the SAE-spectra from the individual shells are removed once all the amplitudes are considered.

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