The Journal of Physical Chemistry A最新文献

We explore the ionization of isoprene and the unimolecular dissociation chemistry of its radical cation employing imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The origin band of the threshold photoelectron spectrum (TPES) indicates an ionization energy of 8.87 ± 0.01 eV in agreement with the calculated CBS-QB3 values. Franck–Condon (FC) simulations rationalize the vibrational structure of the ground-state band in the TPES, and Outer Valence Green’s Function calculations for the excited electronic states are compared with the excited-state bands in the TPES. Four major fragment ions were observed at photon energies above 10.5 eV: C₅H₇⁺ (m/z 67, loss of H), C₄H₅⁺ (m/z 53, loss of CH₃), C₃H₆⁺ (m/z 42, loss of C₂H₂), and C₃H₄⁺ (m/z 40, loss of C₂H₄). Dissociative ionization paths were calculated at the CBS-QB3//B3LYP/6-311+G(d,p) level of theory, The literature mechanisms for CH₃ and C₂H₄ losses cannot be reconciled with the experimental results and the reaction paths and energetics are revisited. Methyl radical loss is found to form the methylcyclopropenyl cation, while ethylene loss results in the allene radical cation rather than ionized propyne.

Aromaticity and antiaromaticity are foundational concepts in chemistry, yet their precise classification, differentiation, and quantification remain the subject of ongoing debate in the literature. In this work, we systematically investigate aromaticity and antiaromaticity patterns for a series of substituted fulvene derivatives in both lowest singlet and triplet states and then cross-correlate their numerical results from the information-theoretic approach (ITA), energetic information, topological analysis, and molecular properties (e.g., atomic polarizability, C₆ dispersion coefficient, Hirshfeld charge, and electron density) with different aromaticity indexes (e.g., NICS(0), NICS(1), FLU, HOMA, and HOMER). Our cross-correlation results reveal that aromatic and antiaromatic systems exhibit completely opposite patterns in each of the spin states. In addition to ITA quantities previously identified that can be employed to distinguish aromaticity from antiaromaticity, newly introduced energetic information, and the topological analysis of ITA quantities also verify this regularity. Notably, the same opposite behavior between aromaticity and antiaromaticity is also observed for atomic polarizability, C₆ dispersion coefficient, Hirshfeld charge, and electron density, uncovering the intrinsic connections between electron delocalization and molecular response properties. Furthermore, the four properties demonstrate strong linear correlations with the ITA and energetic information quantities. This study should have provided new qualitative and quantitative perspectives and insights into understanding aromatic and antiaromatic propensities of molecular systems.

Phosphate-containing molecules are ubiquitous in nature, where they play crucial roles in biochemical processes. Further, they are of technical importance, for example, in certain batteries and in fuel cells, where a unique property of phosphoric acid is exploited─its exceptionally high proton conductivity. Proton transport in phosphoric acid is known to involve proton shuttling; however, the elementary steps involved are not clear. To elucidate the hydrogen bonding preferences of phosphoric acid, we investigate the dihydrogen phosphate anion as well as the deprotonated dimer of phosphoric acid (H₃PO₄·H₂PO₄^-) in the gas phase using infrared action spectroscopy in helium nanodroplets and infrared D₂-tagging photodissociation spectroscopy, and the experimental spectra are compared to theoretical ones. Theory finds for H₃PO₄·H₂PO₄^- two different structures that are predicted to be nearly isoenergetic. The comparison to the experimental spectra, however, allows for a clear assignment and structure identification. The resulting structure has an interesting binding motif, which might be of relevance to interactions of phosphoric acid in the condensed phase and which can serve as a benchmark for quantum chemical calculations.

Gaining mechanistic insight into singlet oxygen sensitization, binding, and release is critical to the rational design of triggered delivery. Here, the intermediates and transition states for photosensitization, binding, and release of molecular oxygen with three N-substituted bisphenalenyl molecules (1–3) bearing substituents with varying electronic effects (CF₃, H, and OCH₃) were calculated with density functional theory. Previously, it was seen that more electron donation speeds up the binding of oxygen, and both electron-donating and electron-withdrawing groups slow down the oxygen release. All of the transition states found were concerted additions and releases of the molecular oxygen. Computationally, there is a strong driving force for the bisphenalenyl molecules to form precomplexes with ground-state molecular oxygen. This allows for fast photosensitization and direct binding to form endoperoxides. Thus, the measured binding rate seems to be governed by complex formation rather than by the energy transfer or binding activation energy. Singlet oxygen is subsequently released from the substituted endoperoxides (1-EPO and 3-EPO) through a concerted mechanism. The unsubstituted 2-EPO proceeds through nonconcerted mechanisms for both binding and release, which single-reference calculations were unable to elucidate. These findings provide valuable insights into the design of efficient oxygen-sensitive systems for the storage and controlled delivery of singlet oxygen.

In our work with a Au thiolate nanocluster (Au₂₀(SG)₁₆, where SG is the tripeptide glutathione), we noticed that it underwent a self-photooxidation reaction in the presence of white light and oxygen. We now report on mechanistic studies using photophysical, photochemical, theoretical, and indirect trapping methods. We find rapid total quenching of singlet oxygen (¹O₂) by ground-state Au₂₀(SG)₁₆, with evidence of dioxygen insertion into the nanocluster. Supported by analyses with IR, ESI-MS, and density functional theory, we propose the formation of Au–O–O–SG bonds in the Au nanocluster. The expansion of the staple motif from dioxygen insertion is attributed to heightened lability and blebbing (a protrusion) arising from the O–O group. We then demonstrated that the self-photooxidized Au₂₀(SG)₁₆ undergoes oxygen-atom transfer to a phosphine trap in the dark.

Inverted singlet-triplet (INVEST) emitters are emerging as attractive alternatives to conventional TADF systems, yet their practical use is hindered by poor radiative efficiency. A key limitation arises from the vanishing oscillator strength (f_osc) of the lowest excited singlet state (S₁), where the S₀ → S₁ transition is optically inactive due to strong charge-transfer character and negligible HOMO-LUMO overlap. Here, we investigate a series of cyclazine derivatives, including two newly designed chromophores, and demonstrate that dimerization provides an effective rule to overcome the intrinsic oscillator strength bottleneck. Electronic structure calculations at the double-hybrid functional-based LR-TDDFT and domain-based local pair natural orbital (DLPNO) similarity-transformed EOM-CCSD (STEOM-CCSD) level of theory reveal that J-aggregate dimers substantially enhance f_osc compared to H-aggregate dimers and isolated monomers. We have systematically computed the excited-state properties of the J-aggregate, H-aggregate, and monomer of the studied systems. It is found that the S₁ state of the J-aggregate dimer has a mixed locally excited state, which is absent in the case of the H-aggregate. This aggregation-induced enhancement of the oscillator strength establishes a general design principle for bright INVEST emitters, opening pathways toward efficient organic optoelectronic materials.

The manganese(I)-pincer catalyzed α-alkylation of sulfones with alcohols offers an efficient and sustainable C–C coupling strategy, though its mechanistic details and selectivity remain poorly understood. In this study, we employ density functional theory (DFT) to elucidate the full catalytic cycle, which comprises four key steps: (i) base-assisted activation of the precatalyst, (ii) alcohol dehydrogenation through a kinetically favorable double hydrogen transfer (barrier: 13.2 kcal/mol), (iii) low-barrier condensation between the resulting aldehyde and sulfone to form an α,β-unsaturated intermediate, and (iv) Mn–H mediated hydrogenation via an intermolecular hydrogen transfer mechanism. The rate-determining step exhibits a barrier of 27.0 kcal/mol, consistent with the requirement for reaction temperatures of 150 °C. Selectivity is governed by both kinetic and thermodynamic factors: the α-alkylation pathway is favored over competing Julia-type olefination and α-alkenylation routes, which are hindered by high endothermicity (43.8 kcal/mol) and substantial kinetic barriers (up to 57.9 kcal/mol). These insights provide a mechanistic foundation for the design of more efficient and selective earth-abundant base-metal catalysts for sustainable synthesis.

We report experimental observations of extremely high levels of above-threshold ionization (ATI) and extensive delayed ionization of photoelectrons from argon clusters in moderately intense nanosecond laser fields at 532 nm. We have successfully projected the cluster explosion and expansion process onto the time-of-flight axis of the photoelectrons. The photoelectron spectra can be separated into three groups: a fast group that reaches as high as 3500 times the ponderomotive energy (U_p) of the laser field, an ATI group that shows the addition of up to 8 photons (over 200U_p), and a delayed group that are ionized ∼100 ns after laser excitation ∼1 mm downstream from the excitation spot. The delayed electrons are tentatively attributed to field ionization of near-threshold electrons contained in the expanding nanoplasma after an initial Coulomb explosion; hence, the delay times of these electrons demonstrate a dependence on the strength of the extraction field. These surprising discoveries demonstrate that the intermediate intensity regime is not a simple extrapolation of strong fields, and new phenomena warrant detailed investigation.