The Journal of Physical Chemistry A最新文献

Hydrogen bonds dictate molecular conformation and are essential in pharmaceutical design, supramolecular chemistry, and catalysis, among others. The ability to manipulate a molecule's potential for forming molecular hydrogen bonds has attracted significant interest, as it can affect bioactivity and physicochemical properties. To clarify the dynamics of certain profen derivatives that influence the structure, activity, and interactions with biological targets, as well as to gain insights into their conformational dynamics within biological systems, the IR spectra of RS-ibuprofen and RS-ketoprofen were recorded at 293 K within the υ_S(O-H) band frequency range and analyzed theoretically from a quantum analysis perspective. The primary distinctions among the spectra of these two different systems lie in the corresponding bandshapes and the intricate structure that defines the bands. An integrated quantum model susceptible to clarifying the differences in the IR spectral density of RS-ibuprofen and RS-ketoprofen is proposed and can be extended to address other complex hydrogen-bonded systems. A satisfactory agreement is achieved between the simulated spectra and experimental results by utilizing a set of physical input parameters that are validated by theoretical and experimental grounds. The quantum approach emphasizes the significance of dynamic cooperative interactions among the vibrational modes, specifically the "Davydov coupling" and "strong anharmonic coupling" mechanisms, in conjunction with the damping mechanisms in the formation of the spectral characteristics of RS-ibuprofen and RS-ketoprofen. This suggests that the synergistic effects of these mechanisms, within the framework of linear response theory, can be regarded as the primary dependable cause of the unconventional IR spectral properties observed. It is anticipated that this innovative algorithm will minimize the discrepancies between the experimental and simulated spectra and may facilitate the computation of spectra in more intricate hydrogen-bonded systems.

We have reinvestigated the dynamics of excited-state proton transfer in [2,2'-bipyridyl]-3,3'-diamine, or BP(NH₂)₂. Femtosecond fluorescence upconversion spectroscopy for this molecule [Chem. Phys. Lett. 2005, 407, 487] identified two emission bands following photoexcitation, viz., a shorter wavelength band I identified as the emission from the (normal) diamine form and a longer wavelength band II attributed to the doubly proton transferred diimine form. Both bands were found to have low fluorescence quantum yields, and both decayed in about 250 fs. A subsequent computational investigation [ChemPhysChem 2007, 8, 1199] showed that only the formation of singly proton transferred monoimine is energetically feasible and hence would be the origin of band II. It was also suggested that, following the proton transfer, the timescale of the inter-ring twisting in the monoimine formed may correspond to that of the decay of band II. A recent study including excited-state trajectory simulations [New J. Chem. 2020, 44, 8018] showed that only the monoimine is formed and that the timescale of the proton transfer is commensurate with the experimental timescale. Revisiting BP(NH₂)₂ in the present work, we have used trajectory surface hopping simulations to study the proton transfer dynamics and decay rate of the experimental fluorescence signals. We find that the molecule shows both C₂ and C_i types of ground-state minima, while only a C_i form is present on the lowest bright state S₁. Initiating dynamics on S₁ from both ground-state minima, we also find that only single proton transfer takes place, with our proton transfer times in agreement with both experiments and prior simulation studies. Our key findings are about the dynamics after the proton transfer. The nascent monoimine twists to near perpendicularity in about 200-300 fs and also loses oscillator strength for the S₀ → S₁ transition en route. These offer a dynamical explanation of the band II decay timescale seen in the experiments and also agree with the aforementioned computational study.

A tetrel bond (TtB) is an attractive interaction between an electrophilic element of group 14 and a nucleophile. Experimental data and theoretical calculations show that if, in an intramolecular tetrel-bonded system, the electrophilic tetrel atom Tt is connected to the nucleophilic atom through alternating single and double bonds within a supramolecular ring, the resonance resulting from π-electron conjugation/delocalization strengthens the TtB. These TtBs with conjugation/delocalization tend to be shorter and closer to linearity than analogous TtBs in rings wherein conjugation/delocalization is not possible. The TtBs, wherein π-electron conjugation/delocalization is present, are called resonance-assisted tetrel bonds (RATtBs) in analogy to the well-known resonance-assisted hydrogen bond (RAHB) proposed by the Gilli group. This work discusses several crystal structures from the Cambridge Structural Database (CSD) wherein the five tetrel atoms form RATtBs. Experimental data and theoretical calculations (QTAIM and NBO) prove that the strength and directionality of RATtBs can be regulated by varying the involved tetrel atom, the nucleophile, and the substituents bonded to or close to the interacting atoms. Importantly, calculations reveal that the interaction weakens significantly when the conjugation-delocalization along the covalent bridge connecting the electrophilic tetrel and the nucleophile is interrupted.

This work presents a comprehensive theoretical investigation of magnesium-to-nitrogen (Mg···N) noncovalent interactions in substituted pyridine-MgH₂ (X-Pyr.MgH₂) complexes and their implications for single-molecule electron transport. Geometry optimization, binding-energy analysis, AIM, NBO, and SAPT calculations reveal electrostatically dominated Mg···N interactions with strengths ranging from -100.03 to -77.29 kJ/mol, modulated systematically by the substituent-dependent basicity of the pyridine ring. Complex formation induces measurable structural perturbations and a pronounced reduction (1-2.5 eV) in the HOMO-LUMO energy gap. DFT-NEGF simulations of Au-molecule-Au junctions show that Mg···N bonding significantly alters transmission characteristics, producing distinct quantum interference features and a substituent-dependent suppression of current near the Fermi level. The resulting I-V responses exhibit stepwise "Coulomb staircase'' behaviour, indicating quantized charge transport across the junction. These results establish Mg···N noncovalent interactions as tunable electronic perturbations that can modulate conductance in pyridine-based single-molecule junctions, providing a feasible molecular framework for exploring single-electron transistor-like behaviour.

Uranium oxides occur in a variety of phases that differ in their crystal structure and uranium oxidation states. Electron energy loss spectroscopy (EELS) is one of the few techniques that has sufficient spatial resolution and sensitivity to electronic structure to distinguish among phases at the nanoscale. However, beam-sensitive materials, such as uranium oxides, are subject to spectral modification due to interactions with the electron beam. Therefore, theory support is essential to reliably exclude the impact of beam damage and generate true reference data sets. Here, we use a comparison of theoretical and experimental spectra to probe the impact of beam damage on the O K-edge and U N-edge (N_6,7 and N_4,5) EELS spectra of various single-valent and mixed-valence uranium oxide bulk phases. Using a low-dose experimental setup, we show that the K-edge theoretical spectra are in excellent agreement with experiment for both peak positions and relative intensities of respective peaks. In contrast, U N-edge features are less distinguishing due to the partially localized nature of the U 5f orbitals and overlapping multiplet and spin-orbit coupling effects. This work demonstrates that O K-edge EELS is sufficiently diagnostic to distinguish a wide range of uranium oxides and that the experimental approach used here minimizes beam damage and allows valence state discrimination across the U(IV), U(V), and U(VI) series. When combined with imaging modes available in electron microscopy, this work enables a detailed investigation and characterization of uranium redox transformations at the nanoscale.

Employing density functional theory (DFT), we systematically investigate the structural, electronic, and magnetic spin-coupling properties of a series of homogeneous and heterogeneous dimers of reduced uracil radicals (U^•-, U6H^•, U5H^•, U4H^•). Different hydrogen-bonding (Watson-Crick, Hoogsteen, and minor-groove) and π-π stacking configurations are examined. DFT and complete active space self-consistent field calculations reveal that the double-electron reduced, hydrogen-bonded base pairs (U6H^•U6H^•, U5H^•U5H^•, U6H^•U5H^•, U4H^•U4H^•) exhibit diradical character with tunable ferromagnetic (FM) or antiferromagnetic (AFM) coupling. Hydrogen-bonded dimers linked through Watson-Crick sites typically form weakly coupled open-shell singlets, while Hoogsteen or minor-groove connections significantly enhance AFM coupling strength. These magnetic interactions are governed by a balance of hydrogen bonding, electrostatic repulsion, and radical coupling. Notably, the U6H^•U^•- and U5H^•U^•- pairs form stable, nonmagnetic closed-shell complexes under strong hydrogen bonding. For the U^•-U^•- dimer, double-electron reduction induces metastability, leading to a negative yet barrier-hindered dissociation energy, an unusual phenomenon arising from competing hydrogen-bond attraction and electrostatic repulsion. In contrast, π-π stacked systems exhibit significantly stronger magnetic coupling and richer magnetic behavior. This work provides the first theoretical prediction of the electronic properties of potentially doubly reduced uracil-uracil base pairs, offering new insights into their magnetic tunability.

Hydrocarbon pyrolysis at high pressures and temperatures is relevant to the decomposition of aviation fuel in advanced thermal management applications. To unravel the dynamics of hydrocarbon cracking and surface deposition, we have developed a novel experimental technique to characterize a neat, supercritical hydrocarbon fluid undergoing pyrolysis in a glass tube reactor (GTR). Using optical absorption spectroscopy, we sensitively measure the onset and rate of amorphous carbon deposition. Simultaneously, we unravel the chemical speciation of the fluid by online quadrupole mass spectrometry (MS). For n-hexane, we reveal four chemical regimes with increasing temperature: (1) no chemistry, (2) cracking with little-or-no deposition, (3) cracking with deposition, and (4) rapid, severe deposition. By modeling the GTR using computational fluid dynamics, we validate its representation as a simple plug flow reactor. The fluid phase decomposition of n-hexane, evident by MS, is consistent with an overall first-order process with an activation energy of 217.7 ± 2.4 kJ·mol^-1. The temperature-dependent deposition rate is analyzed by a crude two-step model, and we compare our findings to those previously reported [e.g., Pramanik, M., et al. Ind. Eng. Chem. Process Des. Dev. 1985, 24 (4), 1275-1281]. We anticipate that our experimental methods will provide a powerful means to quickly evaluate purported decomposition mechanisms of hydrocarbon fuel surrogates.

We present new experimental measurements of rotationally inelastic scattering of vibrationally excited NO(X²Π) with Ar and CH₄. A molecular beam of NO was prepared in a single rotational and parity-resolved state, j = 1.5 F₁e, in the v = 1 vibrational level using mid-infrared radiation from a distributed feedback quantum cascade laser. Following collision with a crossed molecular beam of Ar or CH₄, rotationally excited NO(X, v = 1) in the isolated final rotational states j' = 4.5 F₁f and j' = 10.5 F₁f was detected by 1 + 1' resonance-enhanced multiphoton ionization coupled with velocity-map imaging. Differential cross sections and rotational angular momentum polarization moments for inelastic scattering with Ar are in excellent, near-quantitative agreement with quantum scattering predictions on a literature potential energy surface. Images for scattering from CH₄ for both final states show clear evidence of significant rotational excitation in the CH₄. Overall, a negative correlation is observed in the NO-CH₄ rotational excitation, with higher average CH₄ rotational energy for final NO j' = 4.5 than for j' = 10.5. For NO j' = 10.5, higher rotational energies of CH₄ are surprisingly correlated with forward hemisphere scattering, while lower CH₄ rotation is correlated with backward hemisphere scattering. These measurements demonstrate the importance of the preparation of an initial rotational and parity-selected state, and the varied and surprising dynamics that remain underexplored in molecule-molecule inelastic scattering.

The properties of subnanosized metal clusters are of great interest, since they exhibit high activity in catalysis and can serve as an elemental base for nanoelectronics or quantum computing technologies. For such applications, a fundamentally important issue is the possibility of size- and structure-selective synthesis of clusters, which is largely determined by the number and energy distribution of their isomers, as well as the energy barriers to their rearrangements. Thus, the lifetimes and rates of rearrangement of isomers of subnanosized clusters is an urgent problem of both physicochemical theory and practical applications. Earlier, the complete set of magnesium cluster isomers was found in the course of global DFT optimization with subsequent optimization at the MP2/cc-pVQZ and CCSD(T)/cc-pVQZ levels. In this paper, we study the structural rearrangements of these isomers on the DFT PES by searching for transition states of their structural rearrangements and by the ADMP ab initio molecular dynamics simulation of their time evolution. The ADMP trajectory length was up to 50 ps with a step of 0.2 fs at temperatures from 300 to 700 K. The trajectories were analyzed using a specially developed algorithm for assigning trajectory points to the reference isomer structures. Based on this assignment, the average lifetimes, rearrangement probabilities, and average ratios of isomers in the equilibrium mixture were established. It is shown that the most kinetically stable structures are the Mg₉ and Mg₁₀ clusters. In contrast, other clusters at 300-400 K contain a noticeable admixture of isomeric structures.

Achieving dynamic control of light-matter coupling regimes in plasmonic nanocavities at room temperature is pivotal for quantum technologies but remains challenging due to limitations in polarization-selective excitation efficiency. Here, we demonstrate a polarization-driven reversible switch between weak and strong coupling at the vertical incidence. Leveraging radial vector beam (RVB's) cylindrical symmetry, we generate a confined longitudinal electric field that directly couples to nanoparticle-on-mirror plasmonic modes without sample tilting. This strategy enhances the local electric field by 327-fold (71% higher than linearly polarized beam, LPB) and compresses the mode field volume, amplifying the coupling strength to g = 107 meV, surpassing the strong coupling criterion. Using Rhodamine 800 as a quantum emitter, we demonstrate reversible all-optical switching between a Purcell-enhanced weak coupling regime (under LPB) and a strong coupling regime with 32.8 meV Rabi splitting (under RVB) within the molecule-nanocavity coupling system characterized by highly resolved Rabi splitting in the fluorescence spectra. Further optimization via Au nanoparticle size (R = 40 nm) and collective molecular coupling (N ≥ 5) establishes a ternary synergy for robust quantum control. This noninvasive, polarization-mediated platform enables on-demand manipulation of quantum states for reconfigurable nanophotonic devices.