The Journal of Physical Chemistry A最新文献

Dialkyl carbonates (DACs) have emerged as renewable alternative fuels, attracting considerable interest from researchers in their combustion characteristics. Compared with short-chain DACs, longer-chain DACs have a higher lower-heating value and are more reactive at low temperatures, making them more promising alternatives to diesel. Although extensive studies have been conducted on short-chain DACs, research on longer-chain DACs remains limited. This study conducted the first experimental and modeling investigation into the oxidation chemistry of the longer-chain dibutyl (DBC) and dipentyl (DPeC) carbonates using a jet-stirred reactor (JSR). The mole fractions of the reactants and oxidation products were measured at three equivalence ratios of 0.5, 1.0, and 2.0 within a temperature range of 500–1100 K. Notably, the results revealed that both DBC and DPeC exhibited a pronounced negative temperature coefficient (NTC) behavior, which was absent in short-chain DACs. A detailed kinetic mechanism was developed and validated against the experimental data. Furthermore, a comprehensive analysis of reaction pathways and sensitivity was performed based on the newly developed mechanism. The difference in the oxidation reactivity of DACs with a changed carbon chain length is also illustrated in detail. Analysis of the reaction path reveals that at low temperatures, the fuel molecules are primarily consumed through H-abstraction reactions, generating fuel radicals. A considerable amount of ketohydroperoxides (KHPs) undergo decomposition reactions to form alkyl radicals. The subsequent chain-branching pathways of both the primary fuel radicals and the alkyl radicals contribute to the pronounced low-temperature oxidation reactivity observed for DBC and DPeC. The sensitivity analysis indicates that H-abstraction reactions by OH radicals exert the most significant promoting effect at low temperatures, while the chain-terminating reactions of the key ROO species in both fuel and alkane oxidation chemistry exhibit notable inhibiting effects.

A systematic computational study was conducted on 12 unique 2,6-disubstituted norbornane derivatives to assess the ability of various acceptor groups (A = F, Cl, Br, OH, OCH₃, SH, SCH₃, NHCH₃, N(CH₃)₂, PH₂, PHCH₃, and P(CH₃)₂) to engage in intramolecular hydrogen bonding with a proximal OH donor. 32 unique minima have been characterized with M06-2X and df-MP2 geometry optimizations as well as PNO-LCCSD(T)-F12 single point energy computations. Conformations with the OH donor oriented toward the acceptor to form an OH···A contact (+HB) consistently had lower electronic energies (by approximately 2 to 8 kcal mol^–1) than their counterparts with the OH directed away from the acceptor (−HB). Additional M06-2X computations revealed that these intramolecular contacts also induce significant shifts to lower energy in the OH stretching frequencies (usually by −50 to −100 cm^–1, but more than −200 cm^–1 for OH···N interactions) and notable deshielding of the hydrogen atom (with shifts in the isotropic nuclear magnetic resonance (NMR) chemical shielding constants ranging from −1 to −5 ppm). Quantum theory of atoms in molecules (QTAIM) analysis confirms the presence of bond critical points with electron densities (ca. 0.02 to 0.03 e bohr^–3) that are within the typical range of values for hydrogen bonds. These findings demonstrate the capacity of not only N and O, but also P and S, acceptor groups, to establish appreciable attractive intramolecular interactions with a proximal OH group on a rather rigid molecular scaffold.

Organic acids are important atmospheric compounds that affect the aerosol physicochemical properties and the formation of secondary organic aerosols (SOA) with implications for air quality and climate. Pyruvic acid (PA) is ubiquitous in the atmosphere, biosphere, and hydrosphere. While the pure gas-phase and aqueous-phase chemistry of PA has been extensively studied, its simultaneous interactions with water and ions in the particle phase remains elusive. Here, we present a study on the solvation of PA and its structurally similar analogs─lactic acid (LA), propionic acid (ProA), and 2,2-dihydroxypropionic acid (diol)─by probing geometries, solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a refinement of structures in the aqueous phase based on an elaborate configurational sampling scheme in the gas phase, which we have reported previously. The aqueous phase is modeled using explicit microhydration within an implicit polarizable continuum model. We find that the solvated organic acid clusters have a high conservation of geometry when transitioning from the gas phase to the aqueous particle phase, while the solvated ion-containing clusters show significantly larger structural rearrangements. Solvation of organic acids is found to be thermodynamically favorable in the aqueous particle phase─both with and without ions─unlike in the gas phase. Finally, in order to identify weakly bound clusters and guide future experiments, our IR absorption analysis shows that the harmonic frequencies of PA carboxylic O–H stretching of the microhydrated PA clusters are red-shifted in the spectrum in the aqueous phase compared to the gas phase. Conversely, we find no clear trends in the spectrum obtained with our qualitative approach for the O–H frequencies of the microhydrated ion-containing PA clusters.

Excited-state intramolecular hydrogen transfer (ESIHT) is among the fastest chemical reactions and is a key design element in photoprotective molecules and functional chromophores. Despite the apparent simplicity of the symmetric HO–C═C–C═O ESIHT prototype, its multifunctional nature enables competing nonradiative decay channels, including C═C torsional motion. Here, we compare malonaldehyde (MA), the minimal motif, with its methylated derivative acetylacetone (AcAc) to investigate how electronic and inertial effects of methylation shape the ultrafast dynamics initiated on S₂(ππ*). XMS-CASPT2 nonadiabatic dynamics on the singlet manifold reveal bond-length alternation that drives the wavepacket toward the H-transfer intersection seam rather than undergoing torsional motion directly out of the Franck–Condon region. Methylation destabilizes the S₁(nπ*) state, reducing the S₂/S₁-energy gap and enhancing the asymmetry of the H-transfer intersection seam. As a result, S₂/S₁-decay precedes H-transfer, which mostly takes place only after the population arrives on S₁. Moreover, the methyl groups in AcAc introduce an inertial mismatch between the central methine hydrogen and the terminal methyl groups, which gives rise to two distinct behaviors on S₁: (i) an early ballistic rise in ground-state population within ∼75 fs via twist-pyramidalized geometries akin to the behavior of α,β-enones and (ii) a slower repopulation through torsional motion, with the majority of the population remaining near the planar S₁-minimum. In contrast, MA displays no ballistic channel. Our results for AcAc are consistent with recent time-resolved photoelectron spectroscopy, confirming the ultrafast S₂-lifetime. We propose extending such experiments into the X-ray regime, where the evolution of the oxygen 1s binding energies offers direct, site-specific sensitivity to the H-transfer-mediated motion governing the early decay.

A series of Fischer-type carbenoids of groups 10 and 11, bearing diverse electron-donating and electron-withdrawing substituents, were systematically analyzed using a descriptor-based framework grounded in Conceptual Density Functional Theory (CDFT), assessing whether shared electronic descriptors can rationalize the reaction profiles and reactivity in Ni(II) and Cu(I) complexes in a series of C–H activation reactions. The activation barriers for carbenoid insertion reactions were computed and correlated with reactivity indexes, including the Dual Descriptor and its Grand Canonical extensions based on softness and hardness (SGCDD and PGCDD). Although Ni carbenoids display slightly higher activation barriers than their Cu analogues, both metals exhibit parallel qualitative trends. The PGCDD descriptor showed the strongest predictive capability, yielding high correlations with computed barriers, particularly when refined through excited-state corrections using the Specific State Dual Descriptor (SSDD). Substituent effects especially mesomeric contributions from halogen groups were critically evaluated to establish a coherent electronic interpretation of reactivity. This unified descriptor-based framework provides a transferable methodology for rationalizing C–H activation mechanisms across transition metals, offering valuable insights for the predictive design of metal-carbenoid catalysts.

Dimethyl sulfide (DMS; CH₃SCH₃) is a gas produced by phytoplankton in the ocean and emitted into the atmosphere. DMS emission is the largest source of atmospheric sulfur. Hydroperoxymethyl thioformate (HPMTF) is an oxidation product of DMS in the marine atmosphere. While the formation pathways of HPMTF are well established, the atmospheric removal processes have yet to be fully characterized. Here, we study the photochemistry of HPMTF using computational methods. Our results indicate that HPMTF photolysis is efficient (high quantum yield, ϕ = 0.67), primarily proceeding via S–C bond cleavage in the thioformate (−SCHO) group. However, it is limited by the weak absorption of UV–vis solar radiation, resulting in a long photolytic lifetime (τ ≈ 30 h). Therefore, photolysis is expected to represent a minor sink for atmospheric HPMTF.

Bromine and iodine radicals oxidize gaseous mercury, influencing its lifetime and deposition. Using CCSD(T) and CASPT2 calculations combined with variational transition-state theory, we compare the thermochemistry and kinetics of key reactions forming and destroying Hg–halide species. We investigate the atmospheric oxidation of mercury Hg(0) by Br and I to yield the corresponding Hg(I) halides and the subsequent oxidation reactions yielding Hg(II) compounds (or Hg(0)) via I, Br, BrO, ClO, IO, NO₂, and HO₂. The rate coefficient for ·HgI + I· → HgI₂ (4.2 × 10^–13 cm³ molecule^–1 s^–1) is about half that for the bromine analogue, and ·HgI + IO· → IHgOI is roughly seven times slower than ·HgBr + BrO· → BrHgOBr. The combined electronic-structure and kinetic analysis demonstrates that the employed methods reproduce periodic halogen trends within chemical accuracy, supporting the conclusion that bromine remains the dominant oxidant of atmospheric mercury under current conditions.

This study re-examines the bond cleavage mechanisms of fluoromethane cations (CH₃F⁺) in their low-lying electronic states (X²E, A²A₁, B²E) using a combination of threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging and quantum chemical calculations. The C–H bond cleavage from the X²E state is confirmed to proceed via a statistical, thermodynamically controlled mechanism, as evidenced by a Boltzmann kinetic energy release distribution (KERD). In contrast, the C–F bond cleavage from the excited A²A₁ and B²E states exhibits nonstatistical dynamics. For the A²A₁ state, the dissociation is direct and rapid, characterized by a Gaussian-type KERD and a negative anisotropy parameter (β ≈ −0.5). Crucially, for the B²E state, our results contradict the previously proposed mechanism of internal conversion to the X²E state followed by statistical dissociation. Instead, we provide compelling evidence that CH₃F⁺(B²E) undergoes internal conversion to the A²A₁ state, which then dissociates directly. This revised pathway is supported by RRKM calculations, the near absence of CH₂F⁺ fragments, and the similarities in KERD and β parameters between the B²E and A²A₁ states.

Using an infrared laser to control a molecule’s reactivity by targeting specific vibrational modes has long been of interest to chemists. Rapid intramolecular vibrational energy redistribution (IVR) in molecules poses significant challenges to achieving this, as it quickly transfers the pumped energy to other molecular degrees of freedom (τ_IVR ∼ 1 ps). With recent advances in femtosecond pulsed laser capabilities, however, infrared-laser-driven vibrationally assisted reactivity is worth revisiting. In this theoretical work, we quantify the contributions of both mode-specific assistance and laser-induced heating to reaction rate enhancements of laser-driven molecules. Notably, reactions with lower activation barriers exhibit smaller relative rate enhancements. Furthermore, local heating contributions dominate for low-barrier reactions while the vibrationally assisted component is more prominent for high-barrier reactions (precise statements of what low- and high- barriers depend on thermal diffusivity of the solvent). We obtain approximate bounds for these rate enhancements. While pulsed laser driving yields rate enhancements several orders of magnitude greater than continuous-wave driving for the same absorbed power, the overall increments remain modest under typical experimental conditions, except for low-frequency modes, where they can be substantial.

Predicting the relative stability of energetic isomers is challenging when they exhibit similar reaction energy barriers, as traditional energy-based metrics fail to distinguish them. This study develops the sudden vector projection (SVP) model to resolve these ambiguities by revealing the atom-level vibrational mode specificity that qualitatively illuminates the role of vibrational modes in the decomposition reactivity of five cyclobutane nitric ester isomers. The SVP analysis suggests that O–N stretching serves as a dominant decomposition mode and indicates that certain vibrational modes may play a key role in governing the isomers’ relative instability trends. Isomer 1 exhibits product-driven instability due to a vibrationally excited fragment, while isomer 3 possesses high intrinsic reactivity masked by transition-state stabilization. These insights establish the relative stability order as 1 (least stable) < 3 < 2 < 4 < 5 (most stable). Furthermore, coupling SVP with Boltzmann analysis reveals that during the initial decomposition, there is a temperature at which the relative stability of different isomers changes. This work establishes SVP as a powerful tool that provides atomic-level dynamical insights for predicting stability in energetic isomers.