The Journal of Physical Chemistry A最新文献

The vacuum ultraviolet (VUV) photochemistry of trifluoromethanesulfonyl fluoride (CF₃SO₂F), a promising eco-friendly insulating gas to replace sulfur hexafluoride (SF₆), was investigated in the 11-14 eV energy range by using synchrotron radiation photoionization mass spectrometry and high-level theoretical calculations. It was found that the CF₃SO₂F⁺ parent ion formed from VUV photoionization is not stable and dissociates into CF₃⁺, SO₂⁺, and SOF⁺ fragment ions. The vertical ionization energy (VIE) of the parent molecule to the repulsive X²A' cationic state was calculated to be 13.46 eV, while the adiabatic ionization energy (AIE) to the bound A²A″ state was determined to be 12.65 eV. Notably, the experimental appearance energies of the CF₃⁺ and SO₂⁺ fragments coincide with the AIE of the first excited state (A²A″), rather than the lower appearance energies predicted from ground-state potential energy surfaces. In addition, the dissociation mechanisms to produce CF₃⁺ and SO₂⁺ fragment ions were discussed in detail. For CF₃⁺, photoexcitation populates the bound A²A″ state, which then crosses via a conical intersection to the repulsive X²A' state, leading to direct dissociation into CF₃⁺ + FSO₂. This predissociation pathway governs the experimental onset of the m/z = 69 signal. The m/z = 64 signal, corresponding to the SO₂⁺ fragment, is less intense than the m/z = 69 signal because its formation involves a more complex, multistep pathway. This pathway needs isomerization and crossing another conical intersection, making it less favorable than the direct fragmentation that produces the CF₃⁺ fragment.

Furanoids are relevant atmospheric volatile organic compounds originating from sources like the degradation of conjugated dienes and increasing biomass burning. This study is focused on the atmospheric fate of products derived from the ozonolysis of 2,5-dihydrofuran (2,5-DHF). Considering the Criegee mechanism as a principal reaction pathway, two main stabilized Criegee Intermediates (sCI) are formed: the sCI syn and anti conformers. First, the global ozonolysis reaction rate was theoretically calculated, yielding a value of 1.7 × 10^-17 cm³ molecule^-1 s^-1 at the F12-CCSDT/cc-pVDZ-F12//M06-2X/6-311++G(3df,3pd) level of theory. This result shows excellent agreement with experimental data. The subsequent reactions of these sCI were investigated, including their unimolecular decompositions and bimolecular reactions with atmospheric water vapor. The main products formed in the presence of water are vinyl-hydroperoxide (VHP) and hydroxy-hydroperoxide (HHP) compounds. These products are atmospheric sources of OH radicals, H₂O₂, and different alkoxy radicals. Consequently, it can be concluded that the final products of ozonolysis of 2,5-DHF are potential sources of reactive species in the atmosphere.

The mechanisms for the self- and cross-reactions of anti-CH₃CHOO and syn-CH₃CHOO conformers have been investigated by ab initio quantum-chemical and statistical-theory calculations. The results of the study indicate that at 298 K under 5-Torr He pressure, the self-reaction of anti-CH₃CHOO is the fastest with k_aa = 4.90 × 10^-10 cm³ molecule^-1 s^-1, the anti-syn cross-reaction with k_as = 1.82 × 10^-10 cm³ molecule^-1 s^-1, and the self-reaction of syn-CH₃CHOO with k_ss = 1.28 × 10^-10 cm³ molecule^-1 s^-1. The theoretical results, including the deactivation of internally excited dimers formed by initial bimolecular association reactions accounting for more than 50% of the predicted rates, agree with the recent experimental data within reported errors measured at 298 K and 2-10 Torr He pressure.

High-performance liquid chromatography (HPLC) is a key component of analytical chemistry workflows. In HPLC, analytes are separated by retention times (RTs), which depend on analyte partitioning between a column stationary phase and a solvent mobile phase. Measured RT depends on the details of the chromatographic method: solvent and stationary phase composition, solvent gradient, pH, temperature, and more. Predicting RT remains a challenge across different chromatographic conditions. We present a pilot study using quantum chemistry and continuum solvent models to predict analyte transfer between stationary and mobile phases. Simulations of a set of oxysterols suggest that computed solvation free energies can complement machine-learned models and potentially advance de novo prediction of RT.

This computational study describes the mechanisms behind some interesting features of an acyclic nitrone obtained from the oxidation of 5-methylaminomethyl uridine (mnm⁵U), a pyrimidine ribonucleoside found at the wobble anticodon position of tRNA. The ground state geometry of this nitrone is characterized by an H-bond (1.9 Å) between the N-O and the H-O (at the 5' position of the ribofuranose ring) bonds. The S₀-S₂ and S₀-S₃ transitions at this geometry are strongly allowed. These vertical excitations are followed by relaxation passages through consecutive conical intersection (CI) channels (CI_S3/S2 → CI_S2/S1 → CI_S0/S1). The CNO moiety becomes upside or downside twisted along these pathways with a continuous decrease in the C-O bond distance, eventually leading to their respective oxaziridines. The reverse thermal pathway of oxaziridine → nitrone conversion requires overcoming a barrier of 27 kcal/mol, while the oxaziridine → amide conversion through a [1,2]-H shift requires more energy (40-47 kcal/mol). Investigations on the spin-trapping ability of this ribonucleoside-derived nitrone have indicated its possible efficiency in this field. The ΔG_rxn,_aq values of the spin-adduct formations of this nitrone with biologically important free radicals are in line with those of the best known spin-traps. These results open up a so far unexplored possibility of a new class of spin-trapping nitrones formed on tRNA oxidation.

Few kinetic studies are conducted in aqueous solution without the addition of an inert salt to maintain constant ionic strength. The immediate reason is to maintain constant activity coefficients, which are strongly ionic strength dependent. Rate constants are subsequently reported at a given ionic strength, or in some cases, a study of ionic strength dependence is used for extrapolation to determine the rate constants, k₀, at zero ionic strength, at infinite dilution. This is in line with the more commonly determined thermodynamic equilibrium constant, K₀. In both instances, the goal is to determine the intrinsic rate or equilibrium constant of a molecular chemical interaction without interference from other compounds in the solution. The addition of inert salts has several disadvantages and limitations: inert salts are usually not completely inert and can interfere with the process under investigation; they can be expensive; and, most importantly, they never maintain constant ionic strength. In fact, they do not reduce the changes in absolute ionic strength at all; they only reduce relative ionic strength changes. Here, we present a new method that avoids these constraints by incorporating ionic strength and activity coefficient changes directly into the numerical analysis of the measured data. Using the Ni²⁺-oxalate reaction as a case study, we apply activity coefficient corrections (Debye-Hückel, Davies, SIT) within numerical integration of the rate equations, enabling k₀ to be determined from a single experiment at minimal ionic strength. Replicate measurements yield consistent results, log$\left(k_0^{+}\right)$ = 5.20 ± 0.02. This approach eliminates inert salt addition, simplifies experimental procedures, and provides a generalizable framework for obtaining physically meaningful rate constants.

This work investigates the gas-phase reactivity of fluorinated ethers CF₃CHFOCF₃ and CF₃CH₂OCF₃ toward ^•OH radicals, corresponding to the reactions CF₃CHFOCF₃ + ^•OH → CF₃C^•FOCF₃ + H₂O (1) and CF₃CH₂OCF₃ + ^•OH → CF₃C^•HOCF₃ + H₂O (2). Geometry optimizations and harmonic vibrational frequency calculations were performed at the M06-2X/6-311++G(3df,3pd) level of theory. More accurate energy estimates were obtained via single-point calculations using the CBS-QB3//M06-2X/6-311++G(3df,3pd) composite method. The potential energy profiles derived at 0 K indicate that, for both ethers, H-abstraction reactions proceed through transition states involving the formation of pre- and postreactive complexes. Rate constants were calculated over the temperature range of 200-1000 K employing canonical transition state theory (CTST), incorporating tunneling corrections via the Eckart method. The high-pressure limit Arrhenius equations derived at the CBS-QB3//M06-2X/6-311++G(3df,3pd) level can be represented by k = C exp[-(D₁ - (D₂/T))/T], where C = (5.9 ± 1.7) × 10^-12 cm³ molecule^-1 s^-1, D₁ = (4032 ± 1327) K, and D₂ = (4.7 ± 1.1) × 10⁵ K² for reaction (1), and C = (1.2 ± 0.2) × 10^-11 cm³ molecule^-1 s^-1, D₁ = (2921 ± 1168) K, and D₂ = (2.9 ± 0.8) × 10⁵ K² for reaction (2). Additionally, atmospheric lifetimes of the studied ethers were estimated and discussed.

Excited state intramolecular proton transfer (ESIPT) has been investigated in two prototypical systems─salicylaldehyde azine (SAA) and 1,5-dihydroxyanthraquinone (DHAQ)─using transient absorption spectroscopy upon ultraviolet excitation into the less studied higher excited (S_n) manifold. Excitation with sub-30 fs pulses and broadband visible probing has allowed for direct measurement of the ESIPT rate. In conjunction with steady-state measurements and TD-DFT calculations, a complete delineation of the ultrafast photophysics has been carried out. In SAA, ESIPT remains ultrafast (∼30 fs), consistent with previous S₁ excitation studies. Coherent vibrational beats maps reveal significant wavelength dependence, however. Theoretical analysis suggests that the observed modes and their intensities in coherent vibrational spectra are modulated by the nature of the electronically excited state. In DHAQ, the first direct observation of ESIPT presents a time-constant of ∼85 fs, and a slower component of 9 ps, akin to previous reports on double-proton transfer systems. Collectively, the results suggest that while the ultrafast ESIPT rate remains largely invariant vis-à-vis the excitation energy, the product yield, as well as accompanying coherent oscillations, may be substantively altered, owing to the existence of alternative decay pathways.

We studied the X-ray-induced fragmentation of isothiocyanic acid, HNCS, following core ionization and excitation at the N1s, C1s, and S2p edges by Auger electron-ion coincidence spectroscopy. Mostly similar fragmentation products were identified at the different edges for normal and resonant Auger-Meitner decay, respectively. Upon normal Auger-Meitner decay, the dication was found to dissociate predominantly by C-S bond cleavage into HNC⁺ + S⁺ and CN⁺ + S⁺ ion pairs. The higher yield of undissociated HNCS²⁺ after S2p ionization is explained by a propensity for the population of low-binding energy final states at this edge. Following resonant core excitation into the 13a' or 4a″ orbital, S⁺ was found to be the main fragment, followed by HNC⁺. Fragments that require isomerization were observed with low yields after both core ionization and excitation. A comparison to isocyanic acid, HNCO, revealed significant differences in the fragmentation pattern of the two molecules.