Chain-Amplified Photochemical Fragmentation of <i>N</i>‑Alkoxypyridinium Salts: Proposed Reaction of Alkoxyl Radicals with Pyridine Bases To Give Pyridinyl Radicals

Photoinduced electron transfer to <i>N</i>-alkoxypyridiniums, which leads to N–O bond cleavage and alkoxyl radical formation, is highly chain amplified in the presence of a pyridine base such as lutidine. Density functional theory calculations support a mechanism in which the alkoxyl radicals react with lutidine via proton-coupled electron transfer (PCET) to produce lutidinyl radicals (BH^•). A strong electron donor, BH^• is proposed to reduce another alkoxypyridinium cation, leading to chain amplification, with quantum yields approaching 200. Kinetic data and calculations support the formation of a second, stronger reducing agent: a hydrogen-bonded complex between BH^• and another base molecule (BH^•···B). Global fitting of the quantum yield data for the reactions of four pyridinium salts (4-phenyl and 4-cyano with <i>N</i>-methoxy and <i>N</i>-ethoxy substituents) led to a consistent set of kinetic parameters. The chain nature of the reaction allowed rate constants to be determined from steady-state kinetics and independently determined chain-termination rate constants. The rate constant of the reaction of CH₃O^• with lutidine to form BH^•, <i>k</i>₁, is ∼6 × 10⁶ M^–1 s^–1; that of CH₃CH₂O^• is ∼9 times larger. Reaction of CD₃O^• showed a deuterium isotope effect of ∼6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases <i>k</i>₁ by a factor of ∼400

MoS₂ Quantum Dots: Effect of Hydrogenation on Surface Stability and H₂S Release

We employ density functional theory to investigate effects of hydrogenation on the energetic stability and electronic properties of triangular MoS₂ nanoclusters with S-edges. Excess edge sulfur atoms relative to the bulk stoichiometry along the edges are passivated by hydrogen atoms. We find that the hydrogen coverage for maximum stability can be calculated by (<i>n</i> – 2)/2­(<i>n</i> – 1), where <i>n</i> is the number of S atoms along an edge. The energetics reveal a preference for the zigzag arrangement of adsorbed hydrogen atoms on the edges. Our calculations show vanishing HOMO–LUMO gaps mainly due to the presence of dangling bonds at the edges and can be considered metal-like. We find that the activation energy required to release H₂S lies in between 0.47 and 0.62 eV, and this value is in good agreement with the recently reported experimental value

Empirical Relationship between Chemical Structure and Redox Properties: Mathematical Expressions Connecting Structural Features to Energies of Frontier Orbitals and Redox Potentials for Organic Molecules

A set of mathematical expressions to predict redox potentials and frontier orbital energy levels for organic molecules as a function of structural features is proposed. This is achieved by using the principal component regression method on reduction potential (<i>E</i>_red), oxidation potential (<i>E</i>_ox), highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) values calculated using density functional theory (DFT) on a training set consisting of 77 547 molecules from PubChem database. The first set of expressions allows prediction of <i>E</i>_red, <i>E</i>_ox, HOMO, and LUMO values using molecular fingerprints alone with <i>R</i>² of ca. 0.74, 0.82, 0.92, and 0.85, respectively, which can be used for preliminary screening of molecules before performing DFT calculations. In the second set of expressions, when we include DFT-calculated HOMO and LUMO values as additional descriptors, the <i>R</i>² values of <i>E</i>_ox and <i>E</i>_red predictions increase to 0.91 and 0.90, respectively. This more accurate approach for redox potential predictions is still significantly more computationally efficient compared to DFT calculations of redox potentials. The potential of these approaches is demonstrated by using the examples of polyacenes and quinoxaline family of molecules. These empirical relations are ideally suited for high-throughput screening for a variety of optoelectronic applications. The resultant tool, QSROAR, is made available at https://github.com/piyushtagade/qsroar_version2