The Journal of Physical Chemistry A最新文献

We report an atypical competition between fenchyl radical β-scission and peroxidation at low temperatures and unravel the impacts of strain energy and ring substituent location on their respective contributions. Our RRKM modeling reveals that radicals positioned on secondary carbons are the fastest-scission ones, exhibiting maximum local ring relief. Dimethyl substituents contribute to increased local strain compared to norbornane, hindering bridge scission and leading to cyclopentene and isoprene products. The dimethyl corset generates extra torsional strain during HO₂ elimination from QOOH, while ether formation is favored by electron donation from the carbonyl group. The falloff extent is also affected by steric hindrance, insofar as it increases bridge stiffness, leading to a lower vibrational partition function and low-pressure rate constant. Furthermore, methyl-induced restrictions on reactant reorganization are found to modulate an enthalpy-entropy compensation in the Korcek reaction of fenchyl hydroperoxide. Unlike in our previous stirred reactor experiments, the impact of fenchyl peroxidation on reactivity is notable under our new rapid compression machine (RCM) experiments. The present model predicts contrasted fenchyl selectivities with radical position, β-scission and peroxidation prevailing respectively for F₁/F₂/F₃/F₄ and F₅/F₆ radicals. The kinetic mechanism accurately predicts the experimental IDT but indicates a slight first-stage pressure inflection point at the lower experimental temperature, which could not be confirmed experimentally. This new insight into fenchone ring-opening and -closing mechanisms under high-pressure oxidation can be useful for other polycyclic ketones.

We report an atypical competition between fenchyl radical β-scission and peroxidation at low temperatures and unravel the impacts of strain energy and ring substituent location on their respective contributions. Our RRKM modeling reveals that radicals positioned on secondary carbons are the fastest-scission ones, exhibiting maximum local ring relief. Dimethyl substituents contribute to increased local strain compared to norbornane, hindering bridge scission and leading to cyclopentene and isoprene products. The dimethyl corset generates extra torsional strain during HO₂ elimination from QOOH, while ether formation is favored by electron donation from the carbonyl group. The falloff extent is also affected by steric hindrance, insofar as it increases bridge stiffness, leading to a lower vibrational partition function and low-pressure rate constant. Furthermore, methyl-induced restrictions on reactant reorganization are found to modulate an enthalpy–entropy compensation in the Korcek reaction of fenchyl hydroperoxide. Unlike in our previous stirred reactor experiments, the impact of fenchyl peroxidation on reactivity is notable under our new rapid compression machine (RCM) experiments. The present model predicts contrasted fenchyl selectivities with radical position, β-scission and peroxidation prevailing respectively for F₁/F₂/F₃/F₄ and F₅/F₆ radicals. The kinetic mechanism accurately predicts the experimental IDT but indicates a slight first-stage pressure inflection point at the lower experimental temperature, which could not be confirmed experimentally. This new insight into fenchone ring-opening and -closing mechanisms under high-pressure oxidation can be useful for other polycyclic ketones.

Halide ions in oceans and sea-spray aerosol particles are an important source of reactive halogen species in the atmosphere that impact the ozone budget and radiative balance. The multiphase cycling of halogen species is linked to the abundance of halide ions at the aqueous solution-air interface. Ubiquitously present surface-active organic compounds may affect the interfacial abundance of halide ions. Here, we use liquid jet X-ray photoelectron spectroscopy and molecular dynamics (MD) simulations to assess the impact of surfactants with different headgroups on the abundance of bromide and sodium ions at the interface. Core level spectra of Br 3d, Na 2s, and O 1s are reported for solutions containing tetrabutylammonium, hexylamine (HA), and propyl sulfate. We used a photoelectron attenuation model to retrieve the interfacial concentration of bromide in the presence of these different surfactants. The experimental results confirm the previously reported strong enhancement of bromide in the presence of tetrabutylammonium at the interface. In turn, propyl sulfate had a minor impact on the abundance of bromide but led to a significantly enhanced concentration of sodium cations. The MD simulations performed for bromide solutions containing hexylammonium and propyl sulfate show an enhancement of the interfacial bromide and sodium concentrations, respectively, comparable to the experimental results. The difference between the measured enhancement of bromide for HA and the nearly nonexistent effect of HA on bromide in the MD simulations is ascribed to the small amounts of hexylammonium present in the experimental solution. The present work suggests an important role of electrostatic interactions at the interface, which may guide the assessment of anion and cation abundances in atmospheric particles more generally.

Halide ions in oceans and sea-spray aerosol particles are an important source of reactive halogen species in the atmosphere that impact the ozone budget and radiative balance. The multiphase cycling of halogen species is linked to the abundance of halide ions at the aqueous solution–air interface. Ubiquitously present surface-active organic compounds may affect the interfacial abundance of halide ions. Here, we use liquid jet X-ray photoelectron spectroscopy and molecular dynamics (MD) simulations to assess the impact of surfactants with different headgroups on the abundance of bromide and sodium ions at the interface. Core level spectra of Br 3d, Na 2s, and O 1s are reported for solutions containing tetrabutylammonium, hexylamine (HA), and propyl sulfate. We used a photoelectron attenuation model to retrieve the interfacial concentration of bromide in the presence of these different surfactants. The experimental results confirm the previously reported strong enhancement of bromide in the presence of tetrabutylammonium at the interface. In turn, propyl sulfate had a minor impact on the abundance of bromide but led to a significantly enhanced concentration of sodium cations. The MD simulations performed for bromide solutions containing hexylammonium and propyl sulfate show an enhancement of the interfacial bromide and sodium concentrations, respectively, comparable to the experimental results. The difference between the measured enhancement of bromide for HA and the nearly nonexistent effect of HA on bromide in the MD simulations is ascribed to the small amounts of hexylammonium present in the experimental solution. The present work suggests an important role of electrostatic interactions at the interface, which may guide the assessment of anion and cation abundances in atmospheric particles more generally.

Current developments in X-ray absorption spectroscopy (XAS) for liquid samples in the water window demand a rigorous understanding of the interactions between molecules or solute-solvent interactions observed in the spectra. Meanwhile, a theoretical description of such effects, in addition to inner-shell excitations, remains controversial. The controversy is mainly over whether the orbitals should be optimized in the final states or whether the orbital optimizations can be expressed by dynamic electron correlation. In the present work, we measured the XAS spectra of indole in aqueous solution at the carbon and nitrogen K-edges to compare them with those measured in the gas phase. Obvious solvatochromism was observed only in the XAS spectrum measured at the nitrogen K-edge. We then interpreted the observed solvatochromism by simulating spectra with both ΔSCF, where the orbitals were optimized in the final states, and the algebraic-diagrammatic construction through second order [ADC(2)], where the molecular orbitals optimized in the ground state were used throughout. The present results indicate that covalent interactions, such as hydrogen bonds, are the dominant causes of the solvation effects observed in XAS spectra. The present simulations with ΔSCF and ADC(2), in addition to some other reports, highlight the importance of optimizing the orbitals in the final inner-shell excited states for general inner-shell calculations with predictive accuracy.

Current developments in X-ray absorption spectroscopy (XAS) for liquid samples in the water window demand a rigorous understanding of the interactions between molecules or solute–solvent interactions observed in the spectra. Meanwhile, a theoretical description of such effects, in addition to inner-shell excitations, remains controversial. The controversy is mainly over whether the orbitals should be optimized in the final states or whether the orbital optimizations can be expressed by dynamic electron correlation. In the present work, we measured the XAS spectra of indole in aqueous solution at the carbon and nitrogen K-edges to compare them with those measured in the gas phase. Obvious solvatochromism was observed only in the XAS spectrum measured at the nitrogen K-edge. We then interpreted the observed solvatochromism by simulating spectra with both ΔSCF, where the orbitals were optimized in the final states, and the algebraic-diagrammatic construction through second order [ADC(2)], where the molecular orbitals optimized in the ground state were used throughout. The present results indicate that covalent interactions, such as hydrogen bonds, are the dominant causes of the solvation effects observed in XAS spectra. The present simulations with ΔSCF and ADC(2), in addition to some other reports, highlight the importance of optimizing the orbitals in the final inner-shell excited states for general inner-shell calculations with predictive accuracy.

Mechanisms of photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerizations using zinc tetraphenylporphyrin (ZnTPP) or tetraphenylporphyrin (TPP) as photoredox catalysts (PCs) were studied using density functional theory calculations. To explain the selectivity of ZnTPP for trithiocarbonate compounds, the radical generation mechanisms of two chain transfer agents (CTAs), a trithiocarbonate (BTPA) and a dithiobenzoate (CPADB), were compared. The results suggest that the reaction mechanism (i.e., electron or energy transfer) depends on both the PC and CTA. For the most efficient combination, ZnTPP and BTPA, the reaction proceeds via an electron transfer mechanism. In contrast, TPP reacts with CPADB via an energy transfer mechanism. Furthermore, the formation of a stable complex between intermediates is identified for the reaction of ZnTPP and BTPA. These findings reveal the detailed mechanism and will offer insight into improving the yield and selectivity of PET-RAFT polymerization using porphyrins as PCs.

The unexpected tautomerism of methyl allophanates has been observed in the solid state. X-ray analysis, IR/UV spectroscopic data, and density functional theory (DFT) calculations showed that the molecule adopts an imidic form in the crystal, whereas the amide form, which is more stable in aqueous solution, is expected. The imidic form in the solid state is stabilized by a robust hydrogen-bond network, which facilitates the selective isolation of minor imidic species.

We report the first observation of triplet-singlet resonance energy transfer at room temperature without involving the intersystem crossing process. Triplet (T₁) to singlet (S₁) Förster resonance energy transfer (FRET) has been measured at room temperature with a long-wavelength direct donor's triplet-state excitation. The donor coumarin 106 (C106D) and acceptor rhodamine 101 (R101A) were embedded in thin poly(vinyl alcohol) (PVA) films. The direct excitation of the C106D triplet state was at 470 nm, well outside the absorption, which avoids the donor singlet-state excitation and its involvement in the FRET process. The intensity of C106D decreases in the presence of R101 and is accompanied by an increase of acceptor R101A emission. The observed FRET results in a red glow of the illuminated area lasting hundreds of milliseconds. The FRET measurements with direct triplet-state excitation can also be used to estimate the photophysical parameters of the donor triplet state.