Practical treatment of singlet oxygen with density-functional theory and the multiplet-sum method

Theoretical Study of Reactivity Indices and Rough Potential Energy Curves for the Dissociation of 59 Fullerendiols in the Gas Phase and in Aqueous Solution with an Implicit Solvent Model

Buckminsterfullerene, C60, has not only a beautiful truncated icosahedral (soccer ball) shape but also simple Hückel calculations that predict a 3-fold degenerate lowest unoccupied molecular orbital, which can accommodate up to six electrons, making it a good electron acceptor. Experiments have confirmed that C60 is a radical sponge, and it is now sold for use in topical cosmetics. Further medical uses require functionalization of C60 to make it soluble, and one of the simplest functionalizations is to make C60(OH)n fullerenols. A previous article [Adv. Quantum Chem. 88, 351 (2023)] studied reactivity indices for the successive addition of the •OH radical to (•)C60(OH)n in the gas phase [(•)C60(OH)n is a radical only when n is an odd number]. This present article extends this previous work by examining various aspects of how the reaction, •C60OH + •OH → C60(OH)2 (R1) changes in aqueous solution. One obvious difference between C60 and their various isomers of C60(OH)2 is the presence of a dipole. As fullerendiols are nearly spherical, their change in dipole moment in going from the gas to aqueous phase may be estimated using back-of-the-envelope calculations with the Onsager model. The result is remarkably similar to what is obtained using density functional theory (DFT) with an implicit solvation model (surface molecular density, SMD). Calculation of fullerendiol C-O bond energies and reactivity indices using the SMD approach confirms that the general conclusions from the earlier work regarding gas-phase reactivity still hold in the aqueous phase. A major difference between the present work and the earlier work is the calculation of potential energy curves (PECs) for reaction R1 in the gas and aqueous phases. This is done in exploratory work for all 59 possible fullerendiols in both the gas phase and in aqueous solution with the SMD approach using spin-unrestricted DFT calculations with symmetry breaking. Surprisingly little change is found between the gas- and aqueous-phase PECs. However, it was discovered that the majority of C60(OH)2 shows radicaloid character, as might have been expected from trying to draw resonance structures. Spin-contamination curves are also remarkably similar for gas- and aqueous-phase results. Although our calculations do not include a dispersion correction, it was noticed that all calculated PECs have a 1/R6 behavior over a significant R = R(C-O) distance, underlying the need to be careful of double counting when including dispersion corrections in DFT. A shortcoming of our SMD approach is the lack of explicit water molecules, which can form hydrogen bonds with the OH groups and dissociating radicals.

Spin-Restricted Descriptions of Singlet Oxygen Reactions from XMS-CASPT2 Benchmarks

Reactions of singlet oxygen are numerous, some of which are desired but many are unwanted. Therefore, the ability to correctly predict and interpret this reactivity for complex molecular systems is essential to our understanding of singlet oxygen reactions. DFT is widely used for predicting many reactions but is not suited to degenerate electronic structures; application to isolated singlet oxygen often uses the spin-unrestricted formalism, which results in severe spin contamination. In this work, we demonstrate that spin-restricted DFT can correctly describe the reaction pathway for four prototypical singlet oxygen reactions. By careful benchmarking with XMS-CASPT2, we show that, from the first transition state onward, the degeneracy of the 1Δg state is broken due to differing interactions of the (degenerate) π* orbitals with the organic substrate; this result is well replicated with DFT. These findings demonstrate the utility of using spin-restricted DFT to explore reactions, opening the way to confidently use this computationally efficient method for molecular systems of medium to large organic molecules.

Diagrammatic multiplet sum method (MSM) density functional theory (DFT): Investigation of the transferability of integrals in "simple" DFT-based approaches to multideterminantal problems

Kohn-Sham density functional theory (DFT) typically works well for describing dynamic correlation. Two other types of correlation, arising in the cases of degenerate (static) or quasidegenerate (nondynamic) zero-order states, represent a difficult problem for DFT. When symmetry is present, multiplet sum method (MSM) DFT [Ziegler et al., Theor. Chim. Acta 4, 877 (1977)] provides one of the earliest and simplest ways to include static correlation in DFT. MSM-DFT assumes that DFT provides a good description of single-determinant energies and uses symmetry and simple ansätze to include the effects of static correlation. This is equivalent to determining the off-diagonal matrix elements in a small configuration interaction (CI) eigenvalue problem. Our ultimate goal, however, is nondynamic correlation in cases where symmetry is inadequate for fixing the dynamic-correlation limitation of DFT. To this end, we have developed a diagrammatic approach to MSM-DFT, which does not, by itself, solve the nondynamic correlation problem in DFT but which facilitates comparison with wave function CI and so allows educated guesses of off-diagonal CI matrix elements even in the absence of symmetry. In every case, an additional exchange-only ansatz (EXAN) allows the MSM-DFT formulas to be transformed into wave function formulas. This EXAN also works for transforming time-dependent DFT into time-dependent Hartree-Fock. Although not enough to uniquely guess DFT formulas from wave function formulas, the diagrammatic approach and the EXAN provide important constraints on any guesses that might be used. We illustrate how diagrammatic MSM-DFT may be used to guess a nondynamic correlation correction for the dissociation of H2 and how diagrammatic MSM-DFT may be used to guess a nonsymmetry-based coupling element in the O2 multiplet problem, which is reasonably close to a previous symmetry-derived result.

Proton transfer triggered in-situ construction of C=N active site to activate PMS for efficient autocatalytic degradation of low-carbon fatty amine

Removal of low-carbon fatty amines (LCFAs) in wastewater treatment poses a significant technical challenge due to their small molecular size, high polarity, high bond dissociation energy, electron deficiency, and poor biodegradability. Moreover, their low Brønsted acidity deteriorates this issue. To address this problem, we have developed a novel base-induced autocatalytic technique for the highly efficient removal of a model pollutant, dimethylamine (DMA), in a homogeneous peroxymonosulfate (PMS) system. A high reaction rate constant of 0.32 min^-1 and almost complete removal of DMA within 12 min are achieved. Multi-scaled characterizations and theoretical calculations reveal that the in situ constructed C=N bond as the crucial active site activates PMS to produce abundant ¹O₂. Subsequently, ¹O₂ oxidizes DMA through multiple H-abstractions, accompanied by the generation of another C=N structure, thus achieving the autocatalytic cycle of pollutant. During this process, base-induced proton transfers of pollutant and oxidant are essential prerequisites for C=N fabrication. A relevant mechanism of autocatalytic degradation is unraveled and further supported by DFT calculations at the molecular level. Various assessments indicate that this self-catalytic technique exhibits a reduced toxicity and volatility process, and a low treatment cost (0.47 $/m³). This technology has strong environmental tolerance, especially for the high concentrations of chlorine ion (1775 ppm) and humic acid (50 ppm). Moreover, it not only exhibits excellent degradation performance for different amine organics but also for the coexisting common pollutants including ofloxacin, phenol, and sulforaphane. These results fully demonstrate the superiority of the proposed strategy for practical application in wastewater treatment. Overall, this autocatalysis technology based on the in-situ construction of metal-free active site by regulating proton transfer will provide a brand-new strategy for environmental remediation.

A new freely-downloadable hands-on density-functional theory workbook using a freely-downloadable version of deMon2k

A hands-on workbook for density-functional theory (DFT) has been developed that can be used to provide practical teaching for students at the Masters or advanced undergraduate level that is free, can be used on a student’s own personal computer, and complements formal course work. The workbook is also very much intended to encourage students to explore program options, discover theory limitations, puzzle out what to do when the program does not work as expected, and to help students transition to thinking and using quantum chemistry programs as a researcher might do. After describing the structure of the workbook, we describe how the workbook has been used thus far as a teaching tool and as a useful step towards research-level problems.

Platinum–Dysprosium Alloys as Oxygen Electrodes in Alkaline Media: An Experimental and Theoretical Study

Platinum–dysprosium (Pt–Dy) alloys prepared by the arc melting technique are assessed as potential electrodes for the oxygen reduction reaction (ORR) using voltammetry and chronoamperometry in alkaline media. A relatively small change (10 at.%) in the alloy composition brought a notable difference in the alloys’ performance for the ORR. Pt₄₀Dy₆₀ electrode, i.e., the electrode with a lower amount of Pt, was identified to have a higher activity towards ORR as evidenced by lower overpotential and higher current densities under identical experimental conditions. Furthermore, DFT calculations point out the unique single-atom-like coordination and electronic structure of Pt atoms in the Pt₄₀Dy₆₀ surface as responsible for enhanced ORR activity compared to the alloy with a higher Pt content. Additionally, Pt–Dy alloys showed activity in the oxygen evolution reaction (OER), with the OER current density lower than that of pure Pt.

Insights into the Mechanism of Ozone Activation and Singlet Oxygen Generation on N-Doped Defective Nanocarbons: A DFT and Machine Learning Study

N-doped defective nanocarbon (N-DNC) catalysts have been widely studied due to their exceptional catalytic activity in many applications, but the O₃ activation mechanism in catalytic ozonation of N-DNCs has yet to be established. In this study, we systematically mapped out the detailed reaction pathways of O₃ activation on 10 potential active sites of 8 representative configurations of N-DNCs, including the pyridinic N, pyrrolic N, N on edge, and porphyrinic N, based on the results of density functional theory (DFT) calculations. The DFT results indicate that O₃ decomposes into an adsorbed atomic oxygen species (O_ads) and an ³O₂ on the active sites. The atomic charge and spin population on the O_ads species indicate that it may not only act as an initiator for generating reactive oxygen species (ROS) but also directly attack the organics on the pyrrolic N. On the N site and C site of the N₄V₂ system (quadri-pyridinic N with two vacancies) and the pyridinic N site at edge, O₃ could be activated into ¹O₂ in addition to ³O₂. The N₄V₂ system was predicted to have the best activity among the N-DNCs studied. Based on the DFT results, machine learning models were utilized to correlate the O₃ activation activity with the local and global properties of the catalyst surfaces. Among the models, XGBoost performed the best, with the condensed dual descriptor being the most important feature.