Aluminum nanoparticles (ANPs) are promising high-energy-density fuels for amphibious propulsion systems. However, differences in mass transfer and chemical properties between H2O and O2 have led to uncertainty regarding the oxidation kinetics of ANPs, thereby limiting their performance regulation in water–air transmedia environments. In this work, the nonequilibrium oxidation and combustion of ANPs in mixed H2O–air atmospheres with various H2O/air ratios (i.e., 1:9, 1:4, 1:2, 1:1, 2:1, 4:1, and 9:1, namely, M1:9, M1:4, M1:2, M1:1, M2:1, M4:1, and M9:1) and temperatures (1500–3000 K) are investigated by molecular dynamics simulations. The results reveal that H2O acted as a critical promoter of ANP oxidation: it adsorbed and dissociated on Lewis acid sites of the Al surface, supplying OH– and O2– reactive oxygen species, which dramatically accelerated Al–O bond formation and enhanced energy release. The synergistic effect of high temperature and high-humidity environment accelerated H2O consumption and drove vigorous Al–water reactions, releasing substantial amounts of H2. The formation of hollow nanostructures was highly dependent on the H2O/air ratio: at M4:1, the pressure generated by internal H2 ruptured the oxide shell, destabilizing the hollow structure; at M1:1, a stable hollow shell emerged due to outward diffusion of Al3+ dominating over inward O2– diffusion; while at M1:4, a dense Al2O3 layer rapidly formed, which inhibited hollowing. Crucially, the oxidation of ANPs in humid environments proceeds via proton-coupled pathways involving hydroxyl intermediates, whereas it follows direct electron transfer in air. This investigation provides atomic-level insights into the reactive dynamics of ANPs in complex oxidizing environments, offering critical guidance for the design of high-performance and environment-adaptive metallic fuels.
Abstract Arylpyrrolinium salts exhibit intriguing excited-state behavior that couples fluorescence with photochemical reactivity. While unsubstituted phenylpyrrolinium salts are nonfluorescent, methoxy-substituted derivatives display room-temperature fluorescence, providing an opportunity to investigate the structural and electronic factors governing their excited states. Here, we combine steady-state spectroscopy, time-resolved fluorescence measurements, and time-dependent density functional theory (TDDFT) calculations to establish a model for the photophysics of these compounds. The observed fluorescence originates from a predominantly planar S1 excited state that is in equilibrium with a lower-energy twisted conformer. The planar conformer corresponds to the bright emissive state, whereas the twisted conformer represents a dark state that was previously implicated as the reactive intermediate in photochemical [σ2+π2]-type fragmentative addition reactions with electron-deficient olefins. The biexponential fluorescence decay observed for selected derivatives is consistent with dynamic interconversion between these excited-state conformations. Further, an N-methyl analogue is nonfluorescent because steric effects destabilize the planar geometry required for emission, supporting the proposed model. Fluorescence quantum yields are low but strongly dependent on solvent and counteranion identity, demonstrating that environmental interactions modulate the equilibrium between emissive and nonemissive pathways. These results reveal how subtle structural modifications control the balance between fluorescence and photochemical reactivity in arylpyrrolinium salts and provide a framework for understanding excited-state conformational dynamics in this class of molecules.
Abstract The bonding characteristics of N-heterocyclic carbenes (NHCs) have long been the subject of debate, particularly regarding the relative importance of carbenic versus ylidic resonance contributions. In this work, we employ ab initio valence bond (VB) theory to systematically examine the electronic structure of a series of five-membered NHCs and closely related species. Our VB analyses reveal that carbenic and ylidic characters contribute comparably to the overall wave function, with ylidic character often more pronounced in NHCs, although their relative contributions depend sensitively on the atomic composition of the heterocyclic ring. The π framework of NHCs, arising from six electrons distributed over five pπ orbitals, exhibits substantial cyclic delocalization. In addition, our VB analysis provides insights into the bidirectional electron-delocalization processes between the carbene carbon and the adjacent atoms, namely π donation to the carbene carbon and σ donation from the carbene carbon, together with their associated energetic contributions. Taken together, these results offer a unified, quantitatively grounded description of NHC bonding, highlighting the distinctive roles of π and σ interactions in shaping the electronic structure of these chemically versatile systems.
Abstract Rapid cooling of multicomponent vapors produces oxide particles through coupled oxidation, nucleation, condensation, and aggregation occurring on millisecond time scales. In uranium-bearing systems, these pathways remain poorly understood when redox-active and refractory elements condense together. Here, we isolate the effect of initial composition by comparing binary uranium/aluminum (U/Al) and ternary uranium/aluminum/iron (U/Al/Fe) vapors condensed under a fixed temperature profile in a plasma flow reactor (PFR). Oxygen for oxide formation was supplied through the aqueous nitrate precursors introduced into the argon plasma. Transmission electron microscopy (TEM), electron diffraction, elemental mapping, and image-based particle statistics were used to relate particle morphology, crystal structure, elemental partitioning, and projected particle size. Binary U/Al condensates formed predominantly α-UO3 and η-Al2O3 as small, weakly aggregated particles. Increasing Fe relative to Al shifted the condensates toward larger aggregates containing UO2, η-Al2O3, Fe3O4, and mixed U–Fe oxides. Correlated high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) measurements show that U and Fe are spatially associated within particle aggregates, including mixed U–Fe oxide crystallites, whereas Al preferentially partitions into spatially distinct crystalline and amorphous Al-rich regions. These results demonstrate that initial composition controls both phase selection and nanoscale chemical partitioning during rapid condensation, providing particle-scale constraints for models of nonequilibrium multicomponent oxide formation.
Abstract Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were employed to systematically investigate the effects of substituent electronic characteristics, para-position heteroatom substitution, solvent polarity, and external electric fields (EEFs) on excited-state intramolecular proton transfer (ESIPT), emission photophysics, and antioxidant-related reactivity of 4′,5-dihydroxy-7-methoxyflavone (GKO) and its derivatives. Geometric, infrared vibrational, reduced density gradient (RDG), frontier molecular orbital (FMO), natural bond orbital (NBO), and potential energy curve (PEC) analyses reveal pronounced photoinduced reorganization of the intramolecular hydrogen-bonding environment. For GKO in dichloromethane (DCM), the H···O distance decreases from 1.670 Å in S0 to 0.987 Å in S1, accompanied by substantial elongation of the donor O–H bond, indicating pronounced proton displacement toward the acceptor upon excitation. The ground state proton transfer barrier of GKO is 6.41 kcal mol–1, decreasing to 4.57 kcal mol–1 with −CN substitution and increasing to 7.32 kcal mol–1 with −NH2 substitution. O → S substitution at the para position of the proton acceptor lowers the S0 barrier to 5.26 kcal mol–1, whereas increasing solvent polarity from DCM to DMSO produces only a minor decrease to 6.31 kcal mol–1. In the excited state, no apparent energy maximum is observed along the selected O–H proton transfer coordinate for any investigated system. Unconstrained S1 optimizations instead yield spontaneous proton migration without convergence to a stable normal excited-state (E*) minimum. The energy decrease toward the proton transferred region increases from 10.84 kcal mol–1 for GKO to 14.61 kcal mol–1 for GKO-NH2, demonstrating substantial substituent-dependent modulation of the S1 potential energy profile. EEFs induce pronounced nonlinear and direction-dependent changes in proton transfer energetics. A positive field of +0.01 au causes only minor perturbation of the S1 profile, whereas +0.02 au introduces finite barriers of 2.10 and 2.80 kcal mol–1 in DCM and DMSO, respectively. Negative fields produce substantially larger S1 barriers, reaching 10.07 kcal mol–1 in DCM and 11.79 kcal mol–1 in DMSO. Photophysical calculations show that all systems exhibit characteristic K* emission with Stokes shifts exceeding 110 nm, consistent with substantial excited-state structural relaxation associated with ESIPT. For GKO in DCM, the calculated keto emission is 482.67 nm with a Stokes shift of 148.89 nm. −CN substitution shifts the emission to 446.77 nm and increases the oscillator strength from 0.2719 to 0.4849, whereas −NH2 produces a pronounced red shift to 770.51 nm, a Stokes shift of 399.27 nm, and a reduced oscillator strength of 0.0169. EEFs further regulate emissive-state characteristics: +0.01 au increases the K* oscillator strength to 0.5691, while +0.02 au induces competition between E* and K* emission and changes the dominant emissive state. Negative fields strongly suppress the E* transition, yielding nearly zero oscillator strengths. Antioxidant-related calculations distinguish the roles of electron and hydrogen transfer descriptors. Photoexcitation decreases the IP of GKO from 7.52 to 6.71 eV, indicating enhanced electron transfer propensity, while −NH2 substitution lowers the O–H BDE from 97.662 to 78.960 kcal mol–1, favoring hydrogen atom transfer. Overall, substituent characteristics, para-position heteroatom substitution, solvent polarity, and EEFs exert distinct and state-dependent effects on ESIPT energetics, emission photophysics, and electron and hydrogen transfer-related reactivity, providing quantitative insights into the regulation of ESIPT and a theoretical basis for designing molecular systems with tunable optical and antioxidant-related properties.
