The Journal of Physical Chemistry A最新文献

The dimensionless Marcus coefficients, β and γ, derived from the Marcus equation for activation free energy, are fundamentally connected to both reaction free energy and intrinsic activation free energy. Their introduction establishes a novel metric for classifying chemical reactions by defining reaction spaces that rationalize reactions based on the underlying energetic characteristics. This study extends beyond mere classification, introducing a comprehensive framework for representing chemical reactions within a parametric {β, γ} space, uniquely accommodating any reaction within a normalized range [0,1]. Through analysis of a data set comprising 5,269 dipolar [3 + 2] cycloaddition reactions, we demonstrate the framework's applicability, uncovering novel patterns and behaviors. These findings enhance the understanding of linear and quadratic free energy relationships, providing chemists with a powerful tool for reaction categorization and design, thereby advancing computational chemistry and reaction design methodologies.

The dissociation behavior of 1-methylpyrene (MP) was investigated by using the SWEET experimental system under electron-induced collisions to elucidate the fragmentation dynamics of polycyclic aromatic hydrocarbon (PAH) ions. Molecular interaction with 200 eV electrons initiated multiple ionization and fragmentation pathways, resulting in molecular cation production. The unfragmented monocation MP⁺ is identified as the most abundant one, the intact dication MP²⁺, the de-ethylated dication, and a stable trication MP³⁺ were also observed, demonstrating the formation and stability (>ms) of highly charged molecular ions. Among the smaller observed hydrocarbon fragments, neutral C₂H_x⁰ and cationic C₂H_x⁺ species were the predominant dissociation products. Experimental findings were compared with density functional theory-based tight-binding molecular dynamics simulations. Measurements of the cation signals as a function of the incident electron energy (17-35 eV) provided appearance energies for the dications and correlated cations. The higher experimental appearance energy values relative to simulated vertical ionization energies suggest the involvement of autoionization processes from initially populated electronically exited states of the molecule.

Photogeneration of reactive oxygen species (ROS) by supramolecular assemblies based on hydrogen bonds is of considerable interest as an alternative to metal-containing photocatalysts. In this work, the mechanisms of ROS and radical generation in melamine barbiturate (MB) crystals are investigated. Using EPR spectroscopy, it was shown that UV irradiation (360 nm) induces the formation of singlet oxygen (¹O₂) with a steady-state concentration of (1.2 ± 0.3) × 10^-14 M, while radical formation in the visible region did not exceed the experimental background within the detection limits. Quantum chemical modeling confirmed that ¹O₂ reacts with barbituric acid to form a hydroperoxyl radical (HOO^·) and a carbon-centered radical. A new approach was developed to detect short-lived radicals generated inside the crystal lattice: a supramolecular MB assembly encapsulating the TEMPO-ol nitroxyl probe was synthesized. X-ray diffraction and EPR data confirmed the successful incorporation of the probe into the crystal structure without a change in its morphology. Kinetic experiments and calculations of thermodynamic parameters demonstrated that TEMPO-ol selectively captures mobile HOO^· radicals in situ through exchange interaction, followed by thermodynamically favorable radical termination. Thus, this work not only reveals the mechanism of ROS photogeneration in MB assemblies but also proposes a new strategy for radical detection in heterogeneous environments with limited availability.

Molecular spin qubits based on transition-metal complexes offer scalable alternatives to spin-defect qubits with performance governed by zero-field splitting (ZFS) and singlet-triplet gaps (ΔE_ST). Here, we leverage the conjugation length of polyacene ligands by tuning the number of fused rings to tailor the ZFS of chromium-acene complexes. Multireference ab initio calculations show that the axial ZFS parameter (D) increases with an increase in the number of fused rings in the ligands, while the transverse component (E) remains negligible. This enhancement in D values originates from spin-spin correlations and amplified spin-orbit coupling, driven by larger effective spin-orbit coupling constants (ζ_eff), reduced ligand-field splitting (Δ), and diminished metal-ligand covalency. All complexes exhibit |D| < 9 GHz and ΔE_ST values (1.09-1.29 eV) in the near-IR range, ensuring that these molecular qubits are both optically addressable and compatible with the X-band frequencies employed in EPR spectroscopy for qubit operations. Analysis of d-d transition energies and simulated absorption spectra further reveals that all complexes possess a triplet ground state and well-separated excited singlet and triplet manifolds, enabling a robust spin-optical interface. Overall, our results demonstrate that the ligand conjugation length can be used as a molecular design strategy for tuning the magnetic anisotropy in molecular spin qubits.

Halogen bond (XB) catalysis provides a sustainable, metal-free strategy for organic synthesis, yet precise control over high enantioselectivity remains a key challenge. Elucidating the microscopic mechanism of XB catalyst selectivity is one of the most significant approaches for the development of superior XB catalysts. Herein, we investigate three iodine(III)-based and four bidentate iodine(I)-based XB donors in the Pictet-Spengler reaction of benzaldehyde and N-benzyltryptamine (for chiral tetrahydro-β-carbolines, THβCs) via density functional theory (DFT) calculations, coupled with complementary static and thermodynamic analysis. The catalytic cycle consists of six key steps, with XB catalysts activating the carbonyl group of benzaldehyde through I···O XB. The bidentate iodine(III)-based catalyst, which integrates a bidentate structure with an iodine(III) center, demonstrated superior activity and selectivity among all seven evaluated catalysts. Reaction energy barriers exhibit a linear correlation not only with the electron density at the bond critical point (BCP) of the I···O interaction (as determined by the quantum theory of atoms in molecules, QTAIM) but also with the integral of carbonyl oxygen charge rearrangement. Electrostatic and polarization interactions dominate the attractive forces between catalysts and substrates. Thermodynamic analysis reveals excellent enantioselectivity ((S)-P: (R)-P > 90:10) among the tested catalysts, the iodine(III)-pyrazolium-based (cat-1) and bidentate iodine(III)-based (cat-3) XB donors exhibit the highest enantiomeric excess ((S)-P: (R)-P ≥ 99:1), attributed to the configuration transformation from (R)-P to (S)-P. Notably, thermodynamic results demonstrate that the steric hindrance arising from the energy barrier difference among various reaction pathways is not the sole determinant of enantioselectivity; the interconversion between different enantiomers also plays a crucial role. This work clarifies the XB catalytic mechanism of the Pictet-Spengler reaction, compares the catalytic performance of iodine(III)-based and bidentate iodine(I)-based XB catalysts, reveals the multifactor-controlled mechanism in regulating the enantioselectivity, and provides theoretical guidance for the rational design of efficient enantioselective XB catalysts.

To gain molecular-level insights into how weak intramolecular noncovalent interactions govern the conformational preference of large semivolatile organic compounds, benzyl benzoate (BnBz), an ester with terminal benzyl and phenyl groups, was investigated using chirped-pulse Fourier transform microwave spectroscopy and quantum chemical calculations. Systematic conformational searches followed by DFT calculations identified four BnBz conformers: two low-energy species with a planar benzoate (C₆H₅-COO) motif and two significantly higher-energy conformers with a nonplanar benzoate moiety. The rotational spectra of the two most stable conformers, BnBz-g and BnBz-t, along with 16 of their ¹³C isotopologues, were observed and assigned. The DFT-predicted stability ordering of these two conformers is reversed when based on zero-point-corrected energies and free energies. Experimental results confirm BnBz-g as the global minimum, resolving the theoretical ambiguity and underscoring the importance of benchmarking computational predictions with conformer-specific data. The conformational conversion barrier was investigated both experimentally using helium, neon, and argon as carrier gases, and computationally, revealing a low barrier height of less than 5 kJ mol^-1. Noncovalent interactions analyses indicate that multiple CH···O hydrogen bonds govern the structural preferences of the low-energy conformers, while additional model calculations show that the π-π stacking motif becomes more prominent with the addition of further bridging methylene groups.

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.

It is now well established that the quantum states in symmetric clusters are grouped into shells and that clusters with filled shells exhibit enhanced stability. While the stability with filled shells is established, the corresponding stability associated with half-filled shells and its role in properties is largely unexplored. In this work, we first examine the stability due to half-filled shells by considering a variety of clusters and show that such fillings indeed enhance the energetic stability. We then demonstrate that such a possibility enables the formation of magnetic species via stable subshells belonging to different quantum numbers. The formation of stable magnetic units with filled subshells opens the door to creating magnetic nanoassemblies with tunable coupling and magnetic anisotropy.