Akira Matsugi

Secondary fragmentation of substituted benzylpyridinium “thermometer ions” has been observed previously, but they have not been used as quantitative probes for internal energy calibration. Here, we demonstrate that the secondary fragmentation of 4-trifluoromethyl-substituted benzylpyridiniums provides a high-energy survival-yield probe that extends the measurable internal energy range in collision-induced dissociation (CID). CID of 4-trifluoromethyl-substituted benzylpyridiniums proceeds by N–C bond cleavage to form the corresponding benzyl cation, followed by secondary CF2 loss. This secondary fragmentation can be incorporated into internal energy calibration, provided that the effective dissociation energy for the secondary fragmentation is assigned by considering how excess energy is partitioned during the primary N–C bond cleavage. Using the secondary-fragmentation survival yield extended the CID calibration range from 30 to 60 eV, revealing that internal energy distributions shift toward higher energies and broaden with increasing collision energy. This approach also provide a quantitative probe of high-energy regions inaccessible through primary fragmentation alone.

Criegee intermediates (CIs) formed via alkene ozonolysis play an important role in tropospheric oxidation chemistry. Methyl vinyl ketone oxide (MVKO), an isoprene-derived CI, is expected to have a significant impact in regions with high biogenic emissions; however, quantitative kinetic data for reactions with organic acids remain limited. Here, the reaction kinetics of MVKO with acetic acid (CH3COOH) and propionic acid (CH3CH2COOH) were investigated using time-resolved cavity ring-down spectroscopy (TR-CRDS) at 298 K under low pressure conditions. The measured bimolecular rate coefficients are (2.7 ± 0.1) × 10–10 and (3.6 ± 0.1) × 10–10 cm3 molecule–1 s–1 for CH3COOH and CH3CH2COOH, respectively, indicating near-collision-limited reactivity. Complementary quantum chemical calculations combined with variational transition state theory identify barrierless reaction pathways for MVKO reactions with carboxylic acids, driven by the formation of strong hydrogen-bonding interactions. The calculated high-pressure-limit rate coefficients are in good agreement with the experimental values, with small deviations attributable to falloff effects. Incorporating the derived kinetics into a chemical transport model indicates that reactions with organic acids provide an efficient sink for MVKO in isoprene-rich regions, leading to a maximum reduction of approximately 3% in the carboxylic acid concentration. These results provide robust kinetic constraints for MVKO–acid reactions and improve the representation of isoprene-derived CI chemistry in atmospheric models.

This paper describes a spectroscopic approach for studying the kinetics of the reaction H + O + M → OH(A2Σ+) + M (R1). Vibrational distributions of OH(A) produced during high-temperature reactions of H2/O2 mixtures diluted with Ar have been measured by observing the emission spectra of the OH A2Σ+–X2Π transition. The experiments were performed behind reflected shock waves at temperatures of 1600–2200 K and pressures of 1.4–5.2 bar in a high-repetition-rate shock tube. The observed emissions are attributed to the chemiluminescence of OH(A) formed in (R1). The recorded spectra were fitted with the simulated ones to determine the vibrational distributions of OH(A) for vibrational levels of υ = 0–4. The distributions were significantly nonthermal due to competition between vibrational energy transfer and other involved processes. Then, in order to model the kinetics of (R1), a master equation for the vibrational states of OH(A) is constructed based on the obtained vibrational distributions, as well as available data on predissociation, collisional quenching, spontaneous emission, and vibrational energy transfer of OH(A). The rate constants for (R1) predicted for certain mixtures are found to be fairly consistent with those reported previously based on OH(A) concentration measurements. Meanwhile, substantially smaller rate constants are predicted for highly diluted mixtures. This result is supported by the variation seen in the relative emission rates observed for different mixtures. It is suggested that the rate constant for (R1) depends significantly on third-body molecules due to the dependency of the vibrational energy transfer rate on collision partners.

Significant discrepancies have been observed in the product branching fractions of highly excited ethylene (C₂H₄) generated either by photoexcitation or by chemical activation through the self-reaction of methylene (CH₂) radicals. In this study, to investigate possible nonstatistical effects, classical trajectory calculations were performed on the ground-state potential energy surface for vibrationally excited and chemically activated C₂H₄. The calculated branching fractions for the H atom producing channels, ϕ_b, were in the range of 0.6–0.7 at excitation energies corresponding to ethylene photodissociation and methylene recombination. These values are close to those reported in previous photodissociation experiments as well as predictions by statistical rate theory but are significantly higher than those observed in shock tube experiments on the CH₂ + CH₂ reaction. Selective excitation of certain vibrational modes of C₂H₄ suggested moderate nonstatistical behavior, though insufficient to account for the observed discrepancies. Additionally, new shock tube experiments were conducted to measure the concentration profiles of H atoms produced in CH₂ + CH₂, with CH₂ radicals generated from the thermal decomposition of 3–5 ppm of CH₂I₂ in an Ar bath at a pressure of 2 bar. The rate constant for the CH₂ + CH₂ reaction and the branching fraction for the C₂H₃ + H channel were determined to be k = (1.57 ± 0.39) × 10^–10 cm³ molecule^–1 s^–1 and ϕ_b = 0.15 ± 0.03, respectively, both temperature-independent over 1700–2000 K. This branching fraction is consistent with a previous shock tube study, but is substantially smaller than statistical and trajectory predictions, as well as photodissociation results. The origin of these discrepancies in the product branching fractions remains unsolved. Potential roles of coupling between the ground- and excited-state dynamics in the product branching are discussed.

The reactions H + N₂O → N₂ + OH* (R1) and NH + NO → N₂ + OH* (R2) have been proposed as sources of the electronically excited OH(A²Σ⁺) radicals, denoted as OH*, during combustion of nitrogen-containing compounds. The present study investigates the kinetics of these reactions through OH* emission experiments using a shock tube. Time profiles of OH* concentration measured for several shock-heated mixtures are analyzed with the help of kinetic simulations. The rate constants for R1 are determined using the profiles obtained from H₂/N₂O/Ar, NH₃/N₂O/Ar, and C₂H₅I/N₂O/Ar mixtures and can be expressed as k₁ = (8.4 ± 3.8) × 10^-11 exp(-22900 K/T) cm³ molecule^-1 s^-1 over the temperature range of T = 1600-2200 K. For R2, the rate constants are obtained from the measurements for NH₃/NO/Ar and NH₃/N₂O/NO/Ar mixtures and are represented as k₂ = (1.6 ± 0.8) × 10^-13 exp(-10040 K/T) cm³ molecule^-1 s^-1 over 2000-2600 K. Possible mechanisms for the formation of OH* are discussed based on a comparison of the rate constants.