The Journal of Physical Chemistry A最新文献

Polymeric materials containing weak sacrificial bonds can be designed to engineer self-healing and higher toughness, improve melt-processing, or facilitate recycling. However, they usually exhibit a lower mechanical strength and are subject to creep and fatigue. For improving their design, it is of interest to investigate their mechanical response on the molecular scale. We report on a computational study of the response to a mechanical external force of a Zinc(II) bis-methyl phenyl-terpyridine ([Zn-bis-Terpy]²⁺) complex included in a cyclic poly(ethylene glycol) (PEG) tether designed to maintain the two partners of the metal–ligand bonds in close proximity after the rupture of the complex. The mechanical response is studied as a function of the pulling distortion by using the CoGEF isometric protocol, including interactions with a polar solvent (DMSO). We show that tethering favors recombination but destabilizes the complex before bond rupture because of the interactions of the PEG units with Terpy ligands. Similar effects occur between the DMSO molecules and the complex. Our results on the molecular scale are relevant for single-molecule force spectroscopy experiments. Interactions of the complex with solvent molecules and/or with the tether lead to a dispersion of the rupture force values, which could obscure the interpretation of the results.

Polymeric materials containing weak sacrificial bonds can be designed to engineer self-healing and higher toughness, improve melt-processing, or facilitate recycling. However, they usually exhibit a lower mechanical strength and are subject to creep and fatigue. For improving their design, it is of interest to investigate their mechanical response on the molecular scale. We report on a computational study of the response to a mechanical external force of a Zinc(II) bis-methyl phenyl-terpyridine ([Zn-bis-Terpy]²⁺) complex included in a cyclic poly(ethylene glycol) (PEG) tether designed to maintain the two partners of the metal-ligand bonds in close proximity after the rupture of the complex. The mechanical response is studied as a function of the pulling distortion by using the CoGEF isometric protocol, including interactions with a polar solvent (DMSO). We show that tethering favors recombination but destabilizes the complex before bond rupture because of the interactions of the PEG units with Terpy ligands. Similar effects occur between the DMSO molecules and the complex. Our results on the molecular scale are relevant for single-molecule force spectroscopy experiments. Interactions of the complex with solvent molecules and/or with the tether lead to a dispersion of the rupture force values, which could obscure the interpretation of the results.

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 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 improved the potential energy surfaces for 14 coupled ³A' states of O₃ by using parametrically managed diabatization by deep neural network (PM-DDNN) with three improvements: (1) We used a new functional form for the parametrically managed activation function, which ensures the continuity of the coordinates used in the parametric management. (2) We used higher weighting for low-lying states to achieve smoother potential energy surfaces. (3) The asymptotic behavior of the coupled potential energy surfaces was further refined by utilizing a better low-dimensional potential. As a result of these improvements, we obtained significantly smoother potentials that are better suited for dynamics calculations. For the new version of 14 coupled ³A' surfaces, the entire set of 532,560 adiabatic energies are fit with a mean unsigned error (MUE) of 45 meV, which is only 0.7% of the mean energy in the data set, which is 6.24 eV.

Meteoric material injected into the atmosphere of Titan, Saturn's moon, can react with nitriles and other organic compounds that constitute Titan's atmosphere. However, specific chemical outcomes have not been fully explored. To understand the fates of meteoric metal ions in the Titan environment, reactions of Mg⁺ and Al⁺ with CH₃CN (acetonitrile) and C₂H₅CN (propionitrile) were carried out using a drift cell ion reactor at room temperatures (300 K) and reduced temperatures (∼193 K) and modeled using density functional theory and coupled-cluster theory. Analysis of reactant ion electronic state distributions via electronic state chromatography revealed that Mg⁺ was produced in our instrument exclusively in its ground (²S) state, whereas Al⁺ was produced in both its ¹S ground state and ³P first excited state. Mg⁺(²S) and Al⁺(¹S) produce association products exclusively with both CH₃CN and C₂H₅CN. Primary association reactions with C₂H₅CN occurred with higher reaction efficiencies than those with CH₃CN. Mg⁺(²S) sequentially associates up to four nitrile ligands, and Al⁺(¹S) associates up to three, each via the nitrile nitrogen. Computed binding energies are strongest for the first ligand and diminish with subsequent nitriles. Mg⁺(²S) exhibits a stronger preference for binding nitriles than Al⁺(¹S) because its unpaired electron delocalizes to the nitrile ligands through back-bonding, whereas the lone pair on Al⁺(¹S) remains localized on the metal center. Al⁺(³P) exhibited evidence of bimolecular product formation with both nitriles. Computational modeling of Al⁺(³P) with CH₃CN suggests that the major product, AlCH₃⁺, is kinetically favored over the more energetically stable product, Al⁺(HCN).

Photolysis of the α-dicarbonyls biacetyl (BiAc, CH₃COCOCH₃) and acetylpropionyl (AcPr, CH₃COCOC₂H₅) following UV excitation to the S₂ state at 280 nm was studied using velocity-map ion imaging. Single-photon VUV ionization at 118 nm was used to detect alkyl and acyl radical photoproducts. Photolysis of BiAc at 280 nm yields the expected Norrish Type I photofragments CH₃ and CH₃CO in a 1.0:1.3 ratio. The CH₃CO speed distribution is bimodal; the fast component is assigned to formation of a CH₃CO fragment pair on the T₁ surface while the slow component most likely results from prompt secondary dissociation of energized CH₃COCO radicals initially produced in conjunction with CH₃, tentatively assigned to dissociation on T₂. AcPr photolysis at 280 nm produces CH₃, CH₃CO and additionally C₂H₅ and C₂H₅CO radicals, with a total alkyl to acyl ratio of 1.0:0.7. Both types of acyl radicals have bimodal speed distributions, which are momentum-matched only for the fast tails. By analogy with BiAc, the fast component is attributed to formation of the CH₃CO + C₂H₅CO pair on the T₁ surface. The slower components are attributed to secondary dissociation of the corresponding energized RCOCO radicals formed in conjunction with the detected alkyl radicals. The results highlight the role that characterization of the detailed partitioning of the available energy can play in identifying mechanisms and quantifying branching between competitive pathways.