The Journal of Physical Chemistry A最新文献

The electrification of chemical processes using plasma generates an increasing demand for sensors, monitoring concentrations of plasma-activated species such as radicals. Radical probes are a low-cost in situ method for spatially resolved quantification of the radical density in a plasma afterglow using the heat from the exothermal recombination of radicals on a catalytic surface. However, distinguishing recombination heating from other heat fluxes in the system is challenging. In this study, we present a heat flux analysis based on probe measurements inside the reactor, with simultaneous IR imaging monitoring of the temperature of the reactor wall. The impact of radiation heat on a single thermocouple as well as the advantage of a dual thermocouple setup (using a catalytic unit together with a reference thermocouple) is shown. We add a heat sink with a monitored temperature to the dual thermocouple setup, allowing the determination of conductive and radiative heat fluxes. The heat sink gives more information on the measurement and reduces ambiguities in the evaluation used by others. The probe was tested by mapping N atom densities throughout the plasma afterglow of our reactor, enabling evaluation of the recombination kinetics of the radicals in the gas phase. Three-body recombination was shown to be the main pathway of recombination, with a recombination rate of k_rec = (2.0 ± 0.9)·10^-44 m⁶/s, which is in line with the known literature findings, demonstrating that the measured species are N radicals and the probe did not influence the plasma or recombination reactions in the afterglow.

Gas-phase reactions of [OsB_x]⁺ (x = 1-4) with methane at ambient temperature have been studied by using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. The [OsB_x]⁺ (x = 1-4) cluster ions can undergo dehydrogenation reactions with methane. Comprehensive analysis of the [OsB_x]⁺/CH₄ (x = 1-4) system with Os-complexes ([OsC_y]⁺ (y = 1-3) and [OsO_z]⁺ (z = 1-3)) shows that the large polarity of the cluster and the high sum of the pair energies between Os and the ligand in the ETS-NOCV combine to promote the ability of the cluster to activate methane. Cluster polarity may induce heterolytic cleavage of the C-H bond, and the sum of the pair energies of the fragments may reduce the cluster orbital energy to match the methane orbital and improve the cluster stability. The synergistic interplay of these two factors may offer a viable approach for the activation of methane in the condensed phase, which involves modulating the coordination environment of the active sites to enhance the stability and facilitate C-H bond cleavage and the degree of matching with methane orbitals. A nonlinear function is used to extract second-order characteristic features that have a significant impact on the energy difference based on the limited energy difference data of the OsB_mC_nO_lH_k units. A neural network model is next designed to predict the reaction barrier for methane conversion by OsM₄⁺ (M = C, N, O, Al, Si, or P) with high accuracy.

Push-pull fluorophores with donor-π-acceptor architectures are attractive scaffolds for the design of probes and labels for two-photon microscopy. Such fluorophores undergo a significant charge-delocalization in the excited state, which is essential for achieving a large two-photon absorption cross-section and brightness. The polarized excited state may, however, also facilitate excited-state proton transfer (ESPT) pathways that can interfere with the probe response. Herein, we employed steady-state and time-resolved spectroscopic studies to elucidate whether ESPT is responsible for the pH-dependent emission response of the Zn(II)-selective fluorescent probe chromis-1. Composed of a push-pull architecture with a pyridine ring as the acceptor, the chromis-1 fluorophore core acts as a photobase that promotes ESPT upon acidification. Although the pK_a of the pyridine acceptor increases more than six orders of magnitude upon excitation, the photobasicity is not sufficient to deprotonate solvent water molecules under neutral conditions. Rather, the pH-dependent emission response is caused by the pendant bis-isonicotinic acid chelating group which upon protonation facilitates an excited-state intramolecular proton transfer to the pyridine acceptor. A simple permutation of the core pyridine nitrogen from the para- to the ortho-position relative to the thiazole substituent was sufficient to reduce the excited-state basicity by two orders of magnitude without compromising the two-photon excited brightness. These results highlight the importance of choosing the appropriate fluorophore core and chelating moiety for minimizing pH-dependent responses in the design of fluorescent probes for biological imaging.

The dicarbon molecule, C₂, is one of the most important diatomic species in various gaseous environments. Despite extensive spectroscopic studies in the last two centuries, the radiative and photodissociative properties of C₂ in its highly excited electronic states are still largely unexplored, particularly in the short vacuum ultraviolet (VUV) region. In this study, the lifetimes of C₂ for rotational levels in the recently identified 1³Σ_g⁺ state up to the vibrational level ν' = 4 and in the f³Σ_g^- state up to ν' = 2 are measured for the first time with a VUV-pump-UV-probe photoionization scheme. It is found that all rovibrational levels in the f³Σ_g^- state have lifetimes shorter than the dynamical limit of ∼4.7 ns of the present method, and the lifetimes in the 1³Σ_g⁺ state show obvious and complex dependence on the rotational and vibrational quantum levels. Generally, the lifetime in the 1³Σ_g⁺ state becomes shorter at higher rotational and vibrational levels, i.e., the longest lifetime measured in the ν' = 0 level is ∼18 ns, while all rotational levels in the ν' = 4 level are found to have shorter lifetimes than ∼4.7 ns. This implies that predissociation might occur and become more prominent at the higher energy levels in the 1³Σ_g⁺ state. The prominent dependence of the lifetime on the rotational, vibrational and electronic level indicates complex dynamical processes in the highly excited states of C₂.

Halogenated volatile organic compounds (HVOCs) pose significant bioaccumulative and toxicological risks, necessitating effective strategies for their removal. Here, we show, through a computational study employing density functional theory and coupled cluster methods, the detailed mechanism and kinetic properties of Cl-initiated degradation reactions of 2-chloropropane (2-CP, (CH₃)₂CHCl) and 2-methylpropanoyl halide ((CH₃)₂CHCOX, X = Cl, Br, F). The reaction rate constants of all the channels were calculated by the canonical variational transition state theory (CVT) with the correction of the small curvature tunneling effect (SCT) at 200-1000 K. The subsequent transformation pathways of the major radical products of (CH₃)₂CHCl and (CH₃)₂CHCOCl in the presence of O₂, NO, and HO₂ radical were investigated. The results elucidate the reaction pathways and rate constants, which are in excellent agreement with the experimental data at 296 K. We further explore the atmospheric implications of these reactions by assessing the atmospheric lifetime (τ) and ozone depletion potential (ODP). Additionally, we delve into the aquatic toxicity and bioaccumulation potential of the reactants and their transformation products. This study not only advances our knowledge of the atmospheric fate of halogenated hydrocarbons but also underscores the importance of considering the environmental and toxicological impacts in the development of HVOC mitigation strategies.

The thermal unimolecular decay of ethoxy is important in high-temperature combustion environments where the ethoxy radical is a key reactive intermediate. Two dissociation pathways of ethoxy, including the β-C-C scission to yield CH₃ + CH₂O and the H-elimination to make H + CH₃CHO, were characterized using a high-level coupled-cluster-based composite quantum chemical method (mHEAT-345(Q_Λ)). The former route is found to be dominant while the latter is insignificant, in agreement with previous experimental and theoretical studies. Thermal rate coefficients are calculated for P = 0.001-658 atm (of air) and T = 300-2500 K using semiclassical transition state theory (SCTST) in combination with a pragmatic two-dimensional E,J-resolved master equation (2DME). The effects of tunneling and anharmonicity on the calculated rate constants are also examined. The tunneling factor is found to be inversely dependent on pressure, contrary to previous observations of pressure-dependent tunneling in entrance channels.

We introduce an approach to describe the long-time dynamics of multiatomic molecules by modulating the free energy landscape (FEL) to capture dominant features of the energy-barrier crossing dynamics of the all-atom (AA) system. Notably, we establish that the self-diffusion coefficient of coarse-grained (CG) systems can be accurately delineated by enhancing conservative force fields with high-frequency perturbations. Using theoretical arguments, we show that these perturbations do not alter the lower-order distribution functions, thereby preserving the structure of the AA system after coarse-graining. We demonstrate the utility of this approach using molecular dynamics simulations of simple molecules in bulk with distinct dynamical characteristics with and without time scale separations as well as for inhomogeneous systems where a fluid is confined in a slit-like nanochannel. Additionally, we also apply our approach to more powerful many-body potentials optimized by using machine learning (ML).