The Journal of Physical Chemistry A最新文献

We have measuredthe rate coefficient for the reactionbetweenthe 2-methylpropan-1-yl radical and molecular oxygen as a functionof temperature and pressure in both He and N₂ bath gasesusing photoionization mass spectrometry. Similar measurements wereperformed for the 2-methylpropan-2-yl radical. It was found that 2-methylpropan-1-ylreacts much slower with O₂ than 2-methylpropan-2-yl bothin the falloff region and near the high-pressure limit. We complementedthe experimental work by constructing master-equation models for bothreaction systems to simulate their kinetics under conditions not coveredby experiments. By optimizing the value of few key parameters againstmeasured data, the models were able to reproduce the observed temperatureand pressure dependencies of both rate coefficients. To facilitatethe use of present results in modeling, PLOG and Chebyshev rate-coefficientparametrizations are provided. Although 2-methylpropan-1-yl reactsmuch slower with O₂ than 2-methylpropan-2-yl, the formerreaction generates more products that lead to chain-propagation and-branching than the latter, which almost exclusively forms 2-methylpropeneand hydroperoxyl and is thus chain-terminating.

The accurate determination of Raman cross sections ischallengingfor multiple reasons, the major one being the incomplete intensitycalibration of experimental setups. In this work, the absolute Ramancross sections of CO₂, a molecule important in environmentalscience and chemical engineering, are determined to high accuracy.To achieve this, we utilized a Raman instrument calibrated for preciseintensity measurements using the rotational bands of H₂. Supported by rigorous quantum mechanical data on H₂ anda careful analysis of uncertainties introduced by various experimentalfactors, we reduced the uncertainty in the determined cross sectionsfor the Fermi doublet of CO₂ to 6% (3σ), with theprimary sources of experimental uncertainty stemming from the detectors’digitization error and thermal fluctuations. This report marks themost precise determination of Raman cross-section values to date,paving the way for improved quantitative Raman analysis of a widerange of samples and the retrieval of experimental transition polarizabilitiesfor benchmarking ab initio methods.

Iridium-based photosensitizershave attracted significantattentionin photodynamic therapy (PDT) due to their exceptional photophysicalproperties and chemical stability, as well as tunable phosphorescenceemission spectrum and high triplet state production yields. Photosensitizerswith large two-photon absorption (TPA) and mitochondrial targetingcapabilities are particularly promising for clinical PDT, as theyenable deeper tissue penetration and reduced damage to normal cells.In this study, we theoretically studied photophysical and photodynamicproperties and the photosensitization reaction mechanism of a seriesof iridium-based photosensitizers with modified C^N and N^N ligands(a2–a6, b1/b1-r, and b2/b2-r) by TDDFT/DFT methods. The photophysicalproperties, including one- and two-photon absorption spectra, frontiermolecular orbitals, and singlet and triplet excitation energies, werecalculated. Additionally, rate constants for intersystem crossing,fluorescence, and phosphorescence, along with water solubility andlipophilicity metrics (logP), were determined to assess both efficacyand biocompatibility. The results elucidate the modulation roles ofthe chelated ligands and ancillary ligands in TP-PDT efficiency, indicatingthat the asymmetric iso-fused-benzene ring modification of the N^Nligand is a robust design strategy for comprehensively enhancing photosensitizationperformance. Complexes a2, b2, and b1-r show greater promise as candidates for two-photon PDTphotosensitizers owing to their large TPA cross sections, extendedtriplet state lifetimes, and balanced water solubility and lipophilicity.Notably, the b1-r complex can undergo both Type I andType II PDT photosensitization mechanisms, which will help addressthe issue of drug resistance arising from the hypoxic environmentin deep-seated tumors.

N⁶-Hydroxymethyladenosine(hm⁶A)and N⁶-formyladenosine (f⁶A) are two importantepigeneticRNA nucleosides, which have shown important biological functions inthe demethylation process of N⁶-methyladenosine (m⁶A). However, their excited-state decay mechanisms after ultraviolet(UV) radiation are unclear. Herein, we have investigated the excited-stateproperties and relaxation pathways of hm⁶A and f⁶A by using the CASPT2//CASSCF method. For hm⁶A, the photoexcited ¹ππ* state first bifurcates to reach the ¹ππ* minimum and ¹ππ*_CT minimum, which can either internally convert to the S₀ state via the ¹ππ*/S₀ and ¹ππ*_CT/S₀ conical intersections(CIs), or undergo intersystem crossing (ISC) to the ³ππ*state via the ¹ππ*/³ππ*and ¹ππ*_CT/³ππ*singlet–triplet crossing points (STCPs), respectively. Finally,the populated ³ππ* state can further evolveinto the nearby ³ππ*/S₀ STCP tohop to the S₀ state. Differently, for f⁶A, thephotoexcited ¹ππ* state first relaxes to its ¹ππ* minimum, from which the system can internallyconvert to the ¹nπ* state directly or to the S₀ state via the nearby ¹ππ*/S₀ CI. Additionally, the ¹ππ* → ³nπ* and ¹nπ* → ³nπ*ISC processes can occur at the ¹ππ* minimumand ¹nπ* minimum, respectively, which are followedby ISC to the S₀ state via the ³nπ*/S₀ STCP or internal conversion (IC) to the ³ππ*state via the ³nπ*/³ππ* CI.Subsequently, the ³ππ* state deactivates tothe S₀ state via the ³ππ*/S₀ and ³ππ*/S₀-I STCPs. Thiswork rationalizes the ultrafast excited-state relaxation and tripletformation of hm⁶A and f⁶A and could contributeto understanding the photophysical and photochemical mechanisms ofsimilar epigenetic RNA nucleosides and their derivatives.

Hydrogenated carbon clusters represent structural intermediatesconveniently placed between isolated molecular hydrocarbons and extendedcarbon materials in which hydrogen content and chemical functionalizationcritically determine both structure and optical response. In thiswork, we present a computational investigation of medium-sized carbonclusters (ranging from 20 to 35 carbon atoms) with different degreesof hydrogenation, nitrogen doping, and amino-functionalization. Afairly large ensemble of structures was generated using ab initiomethods showing that hydrogenation induces a significant structuralreorganization, progressively disrupting π-conjugation and increasingsp³ character. Time-dependent density functional theorycalculations were carried out for the C₃₀ family of clusters,showing that increasing hydrogenation induces a systematic blue shiftof the absorption spectra and a depletion of low-energy transitions,consistent with the reduction of π-conjugated domains. In contrast,nitrogen doping and amino-functionalization partially counteract thiseffect by inducing the presence of localized electronic states thatproduce absorption in the visible range. Partially hydrogenated C30clusters exhibit large Stokes shifts, further enhanced by amino functionalization,while N-doping promotes visible emission and excited-state localization.We conclude that hydrogenation and nitrogen doping generally act astwo complementary driving forces on the optical properties of carbonclusters and disordered carbon-based systems.

Quantum-chemical calculations were performed to elucidatethe electrophilicand nucleophilic properties of first-row transition-metal centersin M(acac)₂ (M = Co²⁺, Ni²⁺, Cu²⁺; Hacac = acetylacetone), as well as the nature of the resultantM···N/I interactions with pyrazine and molecular iodine.The Co and Ni centers exhibit electrophilic–nucleophilic dualism:they can act as electrophiles to form semicoordination bonds, andalso function as nucleophiles to form halogen bonds via their d_z² orbitals. By contrast, the Cucenter behaves exclusively as an electrophile and yields stable π-holecomplexes. The M···N and M···I_{electron belt} bonds in M(acac)₂···C₄H₄N₂ and M(acac)₂···I₂(A) are semicoordinate interactions dominated by noncovalentcharacter. The I_σ-hole···Mmetal-involving halogen bond within M(acac)₂···I₂(B) exhibits partial covalent character, while the bifurcatedI···(O,O) halogen bonds in M(acac)₂···I₂(C) correspond to typical closed-shell noncovalent interactions.The primary stabilizing factors of these complexes are determinedby the nature of the metal centers. London dispersion forces playa pivotal role in the nucleophilic complexes M(acac)₂···I₂(B). For the electrophilic complexes M(acac)₂···C₄H₄N₂/I₂(A), the stabilityof Co-containing complexes is governed by electrostatic interactions,while Ni-containing complexes are stabilized by dispersion forces.Both electrostatic and dispersion forces contribute to stabilizingCu-based complexes.

Current noisy intermediate-scale quantum (NISQ) devicesremainlimited in their ability to perform accurate quantum chemistry simulationsbecause of restricted numbers of high-fidelity qubits and short coherencetimes. To overcome these challenges, we introduce a reduced-density-matrix(RDM)-based perturbative variational quantum eigensolver (VQE) frameworkthat augments active-space VQE with perturbation theory to recoverelectron correlation beyond the active space without increasing thequbit count or variational circuit depth. We formulated a fully coupledapproach (VQE-PTs) and a diagonal approximation (VQE-PT). The formerretains couplings among orthonormalized perturbers, whereas the latterneglects these couplings to simplify the classical postprocessing.Numerical simulations of HF, N₂, and F₂ showthat VQE-PTs provides a robust formulation across different molecularsystems, while VQE-PT offers an efficient approximation. We furtherexperimentally implement VQE-PT on the Quafu superconducting quantumprocessor for F₂, achieving a mean absolute error of 1.2mHa along the potential energy surface after error mitigation. Theseresults demonstrate perturbative VQE as a practical framework forincorporating dynamic correlations in quantum chemistry simulations.

The dispersive opticalactivity of conformationally flexible S-2-methylaziridine(S-2-MeAz) has beeninvestigated experimentally under isolated and solvated conditionsto elucidate the impacts of large-amplitude (conformational) and small-amplitude(vibrational) nuclear degrees of freedom on intrinsic chiroptical response and the extrinsic perturbationscaused by seven distinct solvents. All solution-phase measurementsof optical rotatory dispersion (ORD) revealed plain-looking curvesof uniformly negative polarity in the near-ultraviolet/visible regions,while their solvent-free counterpart showed evidence of a zero-crossingthat would lead to positive rotatory powers for wavelengths ≤350nm. A variety of quantum-chemical paradigms built upon density-functional(dispersion-corrected B3LYP and cam-B3LYP) and coupled cluster (CCSD)levels of theory were enlisted to explore the structural and responsecharacteristics of stationary points within the ground electronicstate of S-2-MeAz. Two stable conformations possessingdistinct orientations of the amino (N–H) moiety with respectto the methyl substituent on the aziridine ring were identified, withthe C1 (trans or anti) isomer, whichis the global-minimum of energy in all media, predicted to have wavelength-resolvedpositive specific rotations that were 2–3 times smaller inmagnitude than the corresponding negative values of the higher-lyingC2 (cis or syn) form. The antagonisticchiroptical signatures of these species were merged under the independent-conformeransatz by using ambient (T = 300 K) population fractionsderived from various relative internal-energy (without and with vibrationalzero-point offsets) and free-energy metrics. For isolated (vapor-phase) S-2-MeAz molecules, quantitative agreement between theoryand experiment was attained by combining ensemble-averaged CCSD calculationsof purely electronic optical activity (EOA) evoked from the staticequilibrium configurations of C1 and C2 with commensurate harmonicand anharmonic nuclear contributions to vibronically-/vibrationally-inducedoptical activity (V2OA). Detailed analyses found essentially all 27modes of each conformer to participate actively without any particularmotions of the molecular framework dominating. In contrast, attemptsto model condensed-phase behavior based on the conformer structures,energies, and response-properties (viz., EOA) emerging from PCM andSMD implicit-solvation treatments failed to reproduce the overallshapes of observed ORD profiles, thereby suggesting that other neglectedfactors (e.g., explicit solute–solvent interactions) may beat play.

Thetopology of cyclic conjugated molecules plays a fundamentalrole in determining how electronic excitation evolves following photoexcitation.Connecting cyclic π-conjugated systems to linear conjugatedextensions provides an effective strategy for tuning their optoelectronicand excited-state dynamical properties through molecular connectivity.Here, we employ nonadiabatic excited-state molecular dynamics simulationsto investigate the relaxation mechanisms of cycloparaphenylenes connectedthrough linear conjugated extensions, creating linear and disjointarchitectures through ortho/meta-substitution patterns. These twotopologies exhibit very similar absorption spectra and ultrafast relaxationto the lowest excited state, while displaying markedly different patternsof excited-state reorganization. In both systems, the initially delocalizedexcitation evolves toward a localized excited state. However, analysisof the time-dependent transition density reveals that the disjointarchitecture preferentially channels excitation toward the ortho-associatedregion of the nanohoop, whereas the linear analogue maintains a morebalanced participation of ortho- and meta-associated regions. Fluxanalysis further shows that the buildup of transition density in thefinal localized state is predominantly fed through ortho-related pathwaysin the disjoint topology, while both pathways contribute more evenlyin the linear architecture. These results show that the differentconnectivity patterns can bias the spatial redistribution of electronicexcitation without substantially altering global relaxation dynamics.For the two architectures studied here, the different ortho/meta-connectivitypatterns therefore influence the preferred spatial localization ofthe excited state, providing a mechanistic basis for tuning photoinducedexcitation redistribution through molecular connectivity.

Ammonia/hydrogen mixture fuel isrecognized as a promising carbon-neutralfuel, and as such, accurate prediction of its ignition propertiesis essential. In this study, we present an explicitly analytical theoryof the ignition delay times (IDTs) for an ammonia/hydrogen binaryfuel. Different from the fitted formulas of IDTs, the current theoryadopts the eigenvalue analysis of the Jacobian matrix with a detailedammonia–hydrogen kinetic model, which retains the essentialspecies and elementary reactions. Comparison with experimental dataand computational results from detailed kinetics shows that the theorygives accurate predictions of IDTs over wide ranges of temperatures,pressures, and compositions. The current approach provides a new methodologyfor fast evaluation of ammonia–hydrogen binary fuel IDTs whilemaintaining the underlying kinetic information.