Unimolecular Kinetics of Stabilized CH3CHOO Criegee Intermediates: syn-CH3CHOO Decomposition and anti-CH3CHOO Isomerization

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH3CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH3CHI2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol-1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150-81+176 s-1 when tunneling effects are included but only 5-2+3 s-1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH3CHOO to methyldioxirane, indicating a rate coefficient of 54-21+34 s-1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH3CHOO are provided for use in atmospheric models, and atmospheric implications are discussed.

Computational analyses of the vibrational spectra of fentanyl, carfentanil and remifentanil

The infrared (IR) spectra of fentanyl, carfentanil and remifentanil, and protonated salts, are computed using quantum chemistry methods. New experimental FTIR spectra are also reported and compared to the calculations. The accuracy of two density functional theory methods, B3LYP and M06-2X, are tested against higher level theories (MP2) and the experimental data. Gas phase IR spectra are calculated for both the neutral and protonated molecules in order to compare with the experimental data measured for various salts of fentanyl and its analogues. Key vibrational modes are selected and studied in detail using a vibrational mode locality calculation. The main contributing atomic movements in these vibrational modes are identified.

Unimolecular and water reactions of oxygenated and unsaturated Criegee intermediates under atmospheric conditions

Ozonolysis of unsaturated hydrocarbons (VOCs) is one of the main oxidation processes in the atmosphere. The stabilized Criegee intermediates (SCI) formed are highly reactive oxygenated species that potentially influence the...

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

Calculations on the unimolecular decomposition of the nerve agent VX

It is very difficult to perform experiments on the physical parameters for the thermal decomposition of chemical nerve agents such as VX and computations, therefore, are useful. The reaction dynamics of the gas-phase pericyclic hydrogen transfer of the nerve agent VX is studied computationally. The geometries of the stationary structures are calculated at M06-2X/jul-cc-pVTZ level of theory. Single point energy calculations are carried out at the CBS/QB3 level to correct the energy barriers. Canonical reaction rate constants are calculated as a function of temperature. The one-dimensional semiclassical transition state theory is used to analyse the quantum tunneling effects. A reduced-dimensional hindered rotor model is proposed, tested, and applied to calculate the vibrational partition functions. It is found that the ester (O-side) and thioester (S-side) side chains of VX undergo pericyclic H-transfer reactions that result in decomposition of the molecule. The S-side reaction is favoured both kinetically and thermodynamically and dominates the pyrolysis over the temperature range from 600 K to 1000 K. It is predicted that VX completely decomposes in 2 s at temperatures above 750 K.

Hydrogen tunnelling in the rearrangements of carbenes: the role of dynamical calculations

A tunnelling controlled reaction is studied with semiclassical transition state theory, rationalising the results of experiment.

Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics

The rate constants of H₂ formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg₄Si₄O₁₂)(H₂O)_N, were computed over a wide temperature range [180-2000 K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculating reaction energies and barrier heights, and we computed the minimum energy H + H → H₂ reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semiclassical approaches that take into account tunneling and nonclassical reflection effects by means of the zero curvature tunneling (ZCT), the small curvature tunneling (SCT), and the one-dimensional semiclassical transition state theory (SCTST) methods, which all provided comparable results. Our investigations show that the H₂ formation process following the Langmuir-Hinshelwood (LH) mechanism is more efficient on the hydroxylated (N = 1-4) nanoclusters than on the bare (N = 0) one due to relatively higher reaction barrier height on the latter. H₂ formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg₄Si₄O₁₂)(H₂O)₂ nanocluster for all nine considered methods. Overall, we conclude that all the considered nanoclusters are very efficient catalyzing grains for H₂ formation in the physical conditions of the interstellar medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

Unimolecular Reaction Rate Measurement of syn-CH3CHOO

The unimolecular reactions of Criegee intermediates (CIs) are thought to be one of the significant sources of atmospheric OH radicals. However, stark discrepancies exist in the unimolecular reaction rate of the methyl-substituted CI CH₃CHOO, typically from ozonolysis of alkenes such as trans-2-butene, between the results of ozonolysis of alkene experiments and the up-to-date theoretical calculations. That no further progress has been made since the method that directly produces CIs in the laboratory was developed is mostly attributed to the existence of two conformers, syn- and anti-CH₃CHOO, and the methodological limitations of sensitive conformer-specific detection. We report a conformer-specific measurement of the unimolecular reaction rate of syn-CH₃CHOO by using a high-repetition-rate laser-induced fluorescence method. At 298 K, the observed value of 182 ± 66 s^-1 is in good agreement with recent theoretical calculations.