The Journal of Physical Chemistry A最新文献

Mass spectrometry (MS) is a fundamental tool for chemical identification. The current in-silico prediction tools can handle broad instrument conditions, large molecular libraries or fragment structures only on a very limited level. In this work, we propose a dual-model machine learning strategy that can solve this problem by jointly a classification model for fragment identification and noise filtering, and a regression model for spectral prediction. With the help of attention mechanism, our method outperforms other algorithms in accuracy and efficiency, providing a deeper understanding of the molecular fragmentation behavior in mass spectra. Our method can facilitate the large-scale in-silico spectra calculations and the analysis of unknown molecular structures, which may promote wider applications for MS.

Real-time propagation methods for chemistry and physics are invariably formulated using variational techniques. The time-dependent bivariational principle (TD-BIVP) is known to be the proper framework for coupled-cluster type methods, and is here studied from a differential geometric point of view. It is demonstrated how two distinct classical Hamilton's equations of motion arise from considering the real and imaginary parts of the action integral. This in turn leads to two distinct bivariational principles for real bivariational approximation submanifolds. Conservation laws and Poisson brackets are introduced, completing the analogy with classical mechanics. Furthermore, the time-dependent univariational principles (the time-dependent variational principle, the McLachlan principle, and the Dirac-Frenkel principle) are reconstructed using the TD-BIVP and a bivariational submanifold on product form. An overview of established real-time propagation methods is given in the context of our formulation of the TD-BIVP, namely time-dependent traditional coupled-cluster theory, orbital-adaptive coupled-cluster theory, time-dependent orthogonal optimized coupled-cluster theory, Brueckner coupled-cluster theory, and equation-of-motion coupled cluster theory.

The bonding and spectroscopic properties of ULi^+/0/- and UBe^+/0/- to complete the series for UX^+/0/- for X = Li to F were investigated by high-level ab initio SO-CASPT2 and CCSD(T) electronic structure calculations. The low-lying spin-orbit states were obtained at the SA-CASPT2/aQ-PP level; bond dissociation energies (BDEs), ionization energies (IEs), adiabatic electronic affinities (AEAs), and vertical detachment energies (VDEs) were calculated at the Feller-Peterson-Dixon (FPD) level. A dense manifold of low-lying states was predicted for ULi^+/0/- and UBe^+/0/-. The calculated BDEs for ULi (37.7 kJ/mol) and UBe (8.0 kJ/mol) show that UBe is weakly bound. For redox processes, the BDEs increased for ULi⁺ (109.3 kJ/mol), ULi^- (47.4 kJ/mol), UBe⁺ (35.6 kJ/mol), and UBe^- (72.3 kJ/mol). The IE(ULi) = 4.650 eV is lower than IE(Li); the IE(UBe) = 5.901 eV is close to the IE(U) and to the IEs of UB, UC, UN, UO, and UF. The AEAs of ULi (0.708 eV) and UBe (0.989 eV) are lower than those for UB, UC, UN, and UO but higher than that for EA(UF). Natural bond orbital (NBO) calculations show that ULi has the 5f³6d¹7s² configuration for U and 2s¹ for Li, with a small partial negative charge slightly delocalized on U. UBe arises from the U(5f³6d¹7s²) and Be(2s²) electron configurations with no charge separation. The same calculations were made for WX (X = Li, Be, C-F) to enable detailed comparisons of the properties for UX with WX (X = Li-F). For WX, BDE(WX) is higher than that for UX for X = Li to N and lower than BDE(UX) for X = O and F, mostly due to the higher IE of W than U as ionic character becomes more important going from Li to F.

Recent advances in numerically exact quantum dynamics methods have brought the dream of accurately modeling the dynamics of chemically complex open systems within reach. Path-integral-based methods, hierarchical equations of motion, and quantum analog simulators all require the spectral density (SD) of the environment to describe its effect on the system. Here, we focus on the decoherence dynamics of electronically excited species in solution in the common case where nonradiative electronic relaxation dominates and is much slower than electronic dephasing. We show that the computed relaxation rate is highly sensitive to the choice of SD representation─such as the Drude-Lorentz or Brownian modes─or strategy used to capture the main SD features, even when early-time dephasing dynamics remains robust. The key reason is that electronic relaxation is dominated by the resonant contribution from the high-frequency tails of the SD, which are orders of magnitude weaker than the main features of the SD and can vary significantly between strategies. This finding highlights an important, yet overlooked, numerical challenge: obtaining an accurate SD requires capturing its structure over several orders of magnitude to ensure correct decoherence dynamics at both early and late times. To address this, we provide a simple transformation that recovers the correct relaxation rates in quantum simulations constrained by algorithmic or physical limitations on the shape of the SD. Our findings enable a comparison of different numerically exact simulation methods and expand the capabilities of analog simulations of open quantum dynamics.

High-accuracy ab initio thermochemical predictions for the ionization energy of NF₃, the barrier height (to inversion) of NF₃⁺, and the dissociative ionization threshold of NF₃ to NF₂⁺ + F are presented and incorporated into Active Thermochemical Tables. The adiabatic ionization energy of the first ionization band of NF₃, calculated at 12.647 ± 0.010 eV, is at odds with previous experimental interpretations by nearly 0.36 eV due to unfavorable Franck-Condon factors associated with this transition. The barrier (to inversion) height is calculated to be about 0.6 eV lower in energy than the prior interpretation, which instigates a discussion of the supposed vibrational structure of the first ionization band of NF₃. Updated assignments of the photoelectron spectrum are proposed, and the loss in vibrational spacing on the high-energy side of the experimental ionization band is discussed. Rudimentary anharmonic Franck-Condon simulations qualitatively reproduce the broad spectral features observed in experiment.

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.