The Journal of Physical Chemistry A最新文献

Determination of torsion angles via recoupling of backbone HC and HN dipolar interactions is a well-known method in magic-angle spinning NMR spectroscopy. Torsion angle values can be obtained by comparing simulated and experimental signals, either in the frequency or time domains. Typically, all molecular orientations are assumed to have identical detected amplitudes at zero recoupling time. The changes in these amplitudes during the recoupling period define the dipolar coupling values and the torsion angles. Experimentally, however, orientations may exhibit different detected amplitudes due to additional cross-polarization (CP) blocks that connect different spins in multidimensional experiments. We numerically and experimentally investigate how CP blocks bias backbone φ torsion angle determination and propose CP conditions that minimize this effect, thereby improving accuracy. Applying these conditions in pseudo-4D (H)CANH experiments yields improved agreement of the extracted angles with X-ray crystallographic data for microcrystalline chicken α-spectrin SH3. For the influenza A M2 membrane protein, we identify an unexpected backbone dihedral angle for the I32 residue, which is consistent with TALOS-N predictions but deviates from ideal α-helical transmembrane geometry.

Symmetry-breaking charge separation (SB-CS) shows enormous potential applications in solar energy conversion. Nonetheless, the realization of fast CS coupled with slow charge recombination (CR), which is crucial to achieving its application, remains a great challenge. To address this challenge, we have synthesized an anthracene dimer and a trimer by increasing the number of structural units to achieve fast generation and slow recombination of the SB-CS state. Transient absorption spectra show that these two oligomers could undergo the SB-CS process, even in low-polarity solvents. In the same solvent, the SB-CS rate of the trimer is 1.5-fold faster than that of the dimer, while its recombination rate is slowed down (from 1/17 ns^–1 to 1/20 ns^–1). The rate ratio between SB-CS formation and recombination in biph-trimer reaches an impressive ∼4000 in DMF, which is the highest record observed in the anthracene derivative system. These results suggest that increasing the structural unit number in oligomers may be an effective method to achieve fast SB-CS and slow CR.

A definitive answer to the existence and magnitude of the negative thermal expansion (NTE) and the ¹³C nuclear magnetic resonance (NMR) signature in C₆₀ fullerene has been previously demonstrated using quantum-mechanical treatments of thermal rovibrational motion. This approach, while accurate, is computationally expensive, lacks the implementation of dispersion corrections, and is fundamentally limited to systems with well-defined equilibrium geometries and sufficiently strong restoring forces, making it inapplicable to weakly bound van der Waals complexes. Alternative methods, such as ab initio path integral molecular dynamics (PIMD), are more flexible but remain computationally expensive, especially when combined with calculations of the ¹³C NMR parameters. To overcome these limitations, we introduce an accurate and efficient neural network-based approach that combines machine learning interatomic potentials (MLIPs) with an NMR machine learning (NMR-ML) model. MLIPs enable machine learning PIMD (MLPIMD) simulations, while the NMR-ML model computes ¹³C isotropic magnetic shielding, σ_iso, directly from MLPIMD snapshots. We perform temperature-dependent MLPIMD simulations with MLIPs trained at different levels of theory. In all cases, NTE is observed, and the results reveal how both dispersion effects and atomic basis set choices influence its magnitude. Furthermore, we confirm that NTE is a quantum-mechanical phenomenon, and hence, classical MD simulations cannot reproduce it. To further test our approach, we investigate fully quantum-mechanical secondary isotope shifts of ¹³C NMR magnetic shielding due to the isotope change from ¹²C to ¹³C of the immediate neighbor with hexagon–hexagon or hexagon–pentagon bonds with the observed nucleus. The results show good agreement with the experimental data, highlighting the accuracy of our approach. This work demonstrates that ML-accelerated simulations enable accurate and efficient modeling of thermally activated quantum mechanical phenomena.

We present a novel and efficient implementation of coupled-cluster with singles and doubles (CCSD) analytic gradients that combines the Cholesky decomposition (CD) of electron-repulsion integrals with the exploitation of Abelian point-group symmetry. This approach is particularly effective for medium-sized and large symmetric molecular systems. The CD of two-electron integrals is performed by using a symmetry-adapted two-step algorithm, while the derivatives of the Cholesky vectors are computed with respect to symmetry-adapted nuclear displacements and contracted on-the-fly with the CCSD density matrices. Geometry optimizations of symmetric systems with several hundreds of basis functions have been carried out to assess the efficiency of our implementation and quantify the computational gain provided by the exploitation of point-group symmetry.

The rational design of stimuli-responsive organic room-temperature phosphorescence (RTP) materials is often hindered by an incomplete understanding of the intricate interplay between molecular structure, crystal packing, and excited-state dynamics, particularly in polymorphic systems. Clarifying how subtle structural variations govern photophysical properties is crucial for advancing tunable luminescent materials. Herein, we systematically investigate the dual-emission mechanism and pressure-responsive behavior of a polymorphic RTP material, BrTA-F, in its two crystalline phases (Cry-A and Cry-B), using density functional theory (DFT) and time-dependent density functional theory (TDDFT) combined with quantum mechanics and molecular mechanics methods (QM/MM) and thermal vibration correlation function (TVCF) methods. The results reveal that the distinct spatial distribution of fluorine (F) atoms modulates intermolecular interactions and molecular planarity, leading to different hydrogen bond strengths and excited-state characteristics between the two polymorphs. The dual-RTP emission in Cry-B is attributed to competitive radiative decay from the monomeric first (T₁) and second (T₂) triplet excited state, which is facilitated by enhanced spin orbit coupling (SOC) resulting from variations in n-π*/ππ* transition proportions. Furthermore, Cry-A demonstrates high sensitivity to hydrostatic pressure, which tunes the emission wavelength and decay rates by compressing the lattice and altering intermolecular force balances. This work provides fundamental insights into the structure–property relationships in polymorphic RTP systems and offers guidance for designing stimuli-responsive luminescent materials.

Electrochemical reduction of nitrate to ammonia is not only an effective strategy for dealing with nitrate pollution but also presents a promising alternative in the field of ammonia synthesis at low temperatures. Nevertheless, current research on nitrate reduction reactions (NO₃RR) has predominantly centered on metal catalyst systems, and owing to an incomplete understanding of the catalytic mechanism, the development of this field still faces significant challenges. This research employs density functional theory (DFT) computations to systematically explore the catalytic capability of single-atom nonmetallic catalysts (NM/g-C₂N) embedded in nitrogen-doped porous graphene (g-C₂N) for the reduction of nitrate to ammonia (NO₃RR-to-NH₃). Findings reveal that Si/g-C₂N and As/g-C₂N systems showcase remarkable electrocatalytic activity for NO₃RR, with limiting potentials for ammonia synthesis as low as −0.23 V and −0.31 V, respectively. Crucially, higher energy barriers effectively inhibit the formation of byproducts (NO₂^–, NO, N₂), thereby boosting ammonia synthesis selectivity. Additionally, the hydrogen evolution reaction (HER) is thermodynamically suppressed due to weak hydrogen adsorption on the catalyst surface. This study not only discovers a new type of NO₃RR catalyst but also provides new ideas and methods for designing novel NO₃RR catalysts.

5-Hydroxyflavone (5HF) is a naturally occurring flavonol with a hydroxyl group at the C5 position and shows unusual proton-transfer properties with a very low fluorescence quantum yield, which justifies its role as a natural UV filter. Using the CASPT2//CASSCF method to study the mechanistic photophysics of its two-water hydrogen-bonded complex 5HF-2H₂O (referred to as 5HF-2W), we have identified four competitive S₂(ππ*) radiationless relaxation channels from the Franck–Condon (FC) point. The first is barrierless excited-state intramolecular proton transfer (ESIPT) to generate the ¹ππ*-T tautomer, which further evolves toward the nearby ¹ππ*/S₀-T conical intersection and then deactivates back to the S₀ state, followed by favorable reverse ground-state proton transfer. The second is indirect ¹ππ*→³ππ* intersystem crossing (ISC) mediated by the dark ¹nπ* state. In this route, the ¹ππ* state first hops to the ¹nπ* state via the ¹ππ*/¹nπ*-N conical intersection, followed by ¹nπ*→³ππ* ISC at the ¹nπ*/³ππ*/³nπ*-N intersection structure to reach the ³ππ* state, enhanced by the CASPT2-computed large ¹nπ*/³ππ* spin–orbit coupling (SOC) of 30.2 cm^–1. The generated ³ππ* state undergoes ESIPT by overcoming a 3.5 kcal/mol energy barrier to yield the ³ππ*-T tautomer, which subsequently runs into the nearby ³ππ*/S₀-T crossing point and hops to the S₀ state. The third is similar to the second one, but its ISC is relayed by the ³nπ* state. At the ¹nπ*/³ππ*/³nπ*-N intersection structure, it first transfers to the ³nπ* state (¹nπ*/³nπ* SOC: 17.4 cm^–1) and then hops to the ³ππ* state through ³nπ*→³ππ* internal conversion (IC) at the ³nπ*/³ππ*-N conical intersection, which is followed by direct ISC from T₁ to S₀ via the ³ππ*/S₀-N crossing point. The last one is direct ¹ππ*→S₀ IC from the FC region through the ¹ππ*/S₀-N conical intersection. This work contributes to the understanding of the photophysics of 5HF-based flavonoids and their analogues.

While CO adsorption on transition metal clusters holds promise for environmental remediation, how silver cluster properties─such as size-dependent atomic packing, molecular orbitals, and superatomic character─govern site-specific binding remains unclear. Here, we combine time-of-flight mass spectrometry (TOF-MS) and density functional theory (DFT) to investigate gas-phase CO reactions with Ag_n^- (n = 10-25) clusters. Charge effects are assessed via CO adsorption on Ag_n^+,0 (n = 14, 16, 18, 20) clusters. We find enhanced CO reactivity below 18 valence electrons, showing odd-even oscillations in adsorption energy, a decline from 18 to 22 electrons, and a rebound beyond 23. Key factors include (1) the stabilization of high-coordination Ag* sites by electron donation; (2) orbital interactions involving σ donation, π back-donation, and newly identified p back-donation requiring matched HOMO/LUMO symmetry; (3) hybridization between C-Ag* σ bonds and adjacent d orbitals favoring icosahedral or oblate over cage structures; and (4) jellium-based electron counting, where open shells enhance reactivity relative to closed ones. These results provide critical atomic-scale insights into CO adsorption mechanisms on silver clusters.

The structures and energetics of the binuclear cyclooctatetraene uranium carbonyls (C₈H₈)₂U₂(CO)_n (n = 2, 3, 4, 5) have been studied by density functional theory. The most interesting observation from this work is the prediction of low-energy structures in the tetracarbonyl system of the type (C₈H₈)₂U₂(η⁴-μ-C₄O₄), in which the four CO groups couple to form a bridging C₄O₄ squarate unit. Such a tetramerization of carbon monoxide to give a squarate unit by organouranium compounds has been observed experimentally by Cloke and co-workers in sandwich compounds of the type (η⁵-Me₅C₅)U(η⁸-C₈H₆{SiR₃}₂) containing both five-membered and eight-membered rings. However, tetramerizations of CO groups to squarate were not predicted in theoretical studies of related (C₈H₈)₂Th₂(CO)₄ or (C₅H₅)₂M₂(CO)₄ systems (M = Th, U). These bridging squarate (C₈H₈)₂U₂(η⁴-μ-C₄O₄) structures found in this work are thermochemically favored to the extent that the lowest energy structure of the tricarbonyl (C₈H₈)₂U₂(CO)₃ is disfavored relative to disproportionation into such a bridging squarate tetracarbonyl structure and the lowest energy structure of the dicarbonyl (C₈H₈)₂U₂(CO)₂. In the remaining low-energy (C₈H₈)₂U₂(CO)_n (n = 2, 3, 4, 5) structures, the carbonyl groups are all isolated, either as terminal CO groups similar to those bonding to d-block metals or as bridging η²-μ-CO groups bonded to uranium through both their carbon and oxygen atoms. The viability of formal uranium oxidation states from +3 to +6, as found experimentally in diverse stable molecules, leads to a variety of spin states and uranium–uranium bonding modes in the low-energy (C₈H₈)₂U₂(CO)_n (n = 2, 3, 4, 5) structures. This contrasts with the previously studied thorium systems (C₈H₈)₂Th₂(CO)_n (n = 2, 3, 4, 5)⁶, where the maximum viable formal thorium oxidation state of +4 limits the range of accessible structure types, metal–metal bonding modes, and spin states.