Reaction Kinetics of CH2OO and syn-CH3CHOO Criegee Intermediates with Acetaldehyde.

Criegee intermediates exert a crucial influence on atmospheric chemistry, functioning as powerful oxidants that facilitate the degradation of pollutants, and understanding their reaction kinetics is essential for accurate atmospheric modeling. In this study, the kinetics of CH2OO and syn-CH3CHOO reactions with acetaldehyde (CH3CHO) were investigated using a flash photolysis reaction tube coupled with the OH laser-induced fluorescence (LIF) method. The experimental results indicate that the reaction of syn-CH3CHOO with CH3CHO is independent of pressure in the range of 5-50 Torr when using Ar as the bath gas. However, the rate coefficient for the reaction between CH2OO and CH3CHO at 5.5 Torr was found to be lower compared to the near-constant values observed between 10 and 100 Torr. Furthermore, the reaction of syn-CH3CHOO with CH3CHO demonstrated positive temperature dependence from 283 to 330 K, with a rate coefficient of (2.11 ± 0.45) × 10-13 cm3 molecule-1 s-1 at 298 K. The activation energy and pre-exponential factor derived from the Arrhenius plot for this reaction were determined to be 2.32 ± 0.49 kcal mol-1 and (1.66 ± 0.61) × 10-11 cm3 molecule-1 s-1, respectively. In comparison, the reaction of CH2OO with CH3CHO exhibited negative temperature dependence, with a rate coefficient of (2.16 ± 0.39) × 10-12 cm3 molecule-1 s-1 at 100 Torr and 298 K and an activation energy and a pre-exponential factor of -1.73 ± 0.31 kcal mol-1 and (1.15 ± 0.21) × 10-13 cm3 molecule-1 s-1, respectively, over the temperature range of 280-333 K.

Water-participated mild oxidation of ethane to acetaldehyde.

The direct conversion of low alkane such as ethane into high-value-added chemicals has remained a great challenge since the development of natural gas utilization. Herein, we achieve an efficient one-step conversion of ethane to C2 oxygenates on a Rh1/AC-SNI catalyst under a mild condition, which delivers a turnover frequency as high as 158.5 h-1. 18O isotope-GC-MS shows that the formation of ethanol and acetaldehyde follows two distinct pathways, where oxygen and water directly participate in the formation of ethanol and acetaldehyde, respectively. In situ formed intermediate species of oxygen radicals, hydroxyl radicals, vinyl groups, and ethyl groups are captured by laser desorption ionization/time of flight mass spectrometer. Density functional theory calculation shows that the activation barrier of the rate-determining step for acetaldehyde formation is much lower than that of ethanol, leading to the higher selectivity of acetaldehyde in all the products.

The simplest Criegee intermediate CH2OO reaction with dimethylamine and trimethylamine: kinetics and atmospheric implications.

We have used the OH laser-induced fluorescence (LIF) method to measure the kinetics of the simplest Criegee intermediate (CH2OO) reacting with two abundant amines in the atmosphere: dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Our experiments were conducted under pseudo-first-order approximation conditions. The rate coefficients we report are (2.15 ± 0.28) × 10-11 cm3 molecule-1 s-1 for (CH3)2NH at 298 K and 10 Torr, and (1.56 ± 0.23) × 10-12 cm3 molecule-1 s-1 for (CH3)3N at 298 K and 25 Torr with Ar as the bath gas. Both reactions exhibit a negative temperature dependence. The activation energy and pre-exponential factors derived from the Arrhenius equation were (-2.03 ± 0.26) kcal mol-1 and (6.89 ± 0.90) × 10-13 cm3 molecule-1 s-1 for (CH3)2NH, and (-1.60 ± 0.24) kcal mol-1 and (1.06 ± 0.16) × 10-13 cm3 molecule-1 s-1 for (CH3)3N. We propose that the electronegativity of the atom in the co-reactant attached to the C atom of CH2OO, in addition to the dissociation energy of the fragile covalent bonds with H atoms (H-X bond), plays an important role in the 1,2-insertion reactions. Under certain circumstances, the title reactions can contribute to the sink of amines and Criegee intermediates and to the formation of secondary organic aerosol (SOA).

Full-dimensional neural network potential energy surface and dynamics of the CH2OO + H2O reaction.

An accurate global full-dimensional machine learning-based potential energy surface (PES) of the simplest Criegee intermediate (CH2OO) reaction with water monomer was developed based on the high level of extensive CCSD(T)-F12a/aug-cc-pVTZ calculations. This analytical global PES not only covers the regions of reactants to hydroxymethyl hydroperoxide (HMHP) intermediates, but also different end product channels, which facilities both the reliable and efficient kinetics and dynamics calculations. The rate coefficients calculated by the transition state theory with the interface to the full-dimensional PES agree well with the experimental results, indicating the accuracy of the current PES. Extensive quasi-classical trajectory (QCT) calculations were performed both from the bimolecular reaction CH2OO + H2O and from HMHP intermediate on the new PES. The product branching ratios of hydroxymethoxy radical (HOCH2O, HMO) + OH radical, formaldehyde (CH2O) + H2O2 and formic acid (HCOOH) + H2O were calculated. The reaction yields dominantly HMO + OH, because of the barrierless pathway from HMHP to this channel. The computed dynamical results for this product channel show the total available energy was deposited into the internal rovibrational excitation of HMO, and the energy release in OH and translational energy is limited. The large amount of OH radical found in the current study implies that the CH2OO + H2O reaction can provide crucially OH yield in Earth's atmosphere.

Sulfur Poisoning and Self-Recovery of Single-Site Rh1 /Porous Organic Polymer Catalysts for Olefin Hydroformylation.

Sulfur poisoning and regeneration are global challenges for metal catalysts even at the ppm level. The sulfur poisoning of single-metal-site catalysts and their regeneration is worthy of further study. Herein, sulfur poisoning and self-recovery are first presented on an industrialized single-Rh-site catalyst (Rh1 /POPs). A decreased turnover frequency of Rh1 /POPs from 4317 h-1 to 318 h-1 was observed in a 1000 ppm H2 S co-feed for ethylene hydroformylation, but it self-recovered to 4527 h-1 after withdrawal of H2 S, whereas the rhodium nanoparticles demonstrated poor activity and self-recovery ability. H2 S reduced the charge density of the single Rh atom and lowered its Gibbs free energy with the formation of inactive (SH)Rh(CO)(PPh3 -frame)2 , which could be regenerated to active HRh(CO)(PPh3 -frame)2 after withdrawing H2 S. The mechanism and the sulfur-related structure-activity relationship were highlighted. This work provides an understanding of heterogeneous ethylene hydroformylation and sulfur-poisoned regeneration in the science of single-atom catalysts.

Kinetics of the Simplest Criegee Intermediate CH2OO Reaction with tert-Butylamine.

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with tert-butylamine ((CH3)3CNH2) was studied under pseudo-first-order conditions with the OH laser-induced fluorescence (LIF) method at the temperature range of 283-318 K and the pressure range of 5-75 Torr. Our pressure-dependent measurement showed that at 5 Torr─the lowest pressure measured in the current experiment─this reaction was under the high-pressure limit condition. At 298 K, the reaction rate coefficient was measured to be (4.95 ± 0.64) × 10-12 cm3 molecule-1 s-1. The title reaction was observed to be negative temperature-dependent; the activation energy of (-2.82 ± 0.37) kcal mol-1 and the pre-exponential factor of (4.21 ± 0.55) × 10-14 cm3 molecule-1 s-1 were derived from the Arrhenius equation. The rate coefficient of the title reaction is slightly larger than (4.3 ± 0.5) × 10-12 cm3 molecule-1 s-1 of the CH2OO reaction with methylamine; the electron inductive effect and the steric hindrance effect might play a role in contributing to such difference.

A molecular beam-surface apparatus for quantum state-resolved adsorption studies.

Understanding the microscopic mechanism of molecule-surface interaction is of great importance in the study of chemical dynamics. Yet, it remains challenging to experimentally acquire quantum state resolved results, particularly the results related to different degrees of freedom of the reactants. Here, we report the design and performance of a new apparatus for molecule-surface dynamics studies, which enable the measurement of quantum state-resolved adsorption. A continuous narrow-linewidth infrared laser source and molecular beam unit were developed and employed in this new apparatus to achieve independent control on different degrees of freedom (translation, vibration, and rotation) of the molecule. Preliminary results on hydrogen and hydrogen chloride adsorption on the Cu (111) surface were also presented.

Chloride-Mediated Peroxide-Free Photochemical Oxidation of Proteins (PPOP) in Mass Spectrometry-Based Structural Analysis.

Ultraviolet (UV) laser photolysis of hydrogen peroxide (H2O2) for the in situ generation of hydroxyl radicals (•OH) is a widely utilized strategy in the oxidation footprinting of native proteins and mass spectrometry (MS)-based structural analysis. However, it remains challenging to realize peroxide-free photochemical oxidation footprinting. Herein, we describe the footprinting of native proteins by chloride-mediated peroxide-free photochemical oxidation of proteins (PPOP). The protein samples are prepared within biocompatible phosphate-buffered saline (PBS) containing 10 mM Gln as radical scavengers and oxidized in a capillary flow reactor directly under a single-pulse (10 ns) irradiation of a 193 nm ArF UV laser. The main oxidized protein residues are CMYWFHLI. We demonstrate that the PPOP-MS strategy is highly sensitive to the protein high-order structures and can be applied to monitor the protein-drug interfaces, which provides a promising footprinting alternative for protein structure-function explorations.

Photochemical bromination and iodination of peptides and proteins by photoexcitation of aqueous halides.

Although halogen atoms can greatly improve the stability, selectivity, and bioactivity of proteins, direct halogenation of proteins or peptides by chemical strategy has been never achieved. Herein, we describe the developments of direct photochemical bromination and iodization of unprotected proteins and peptides in the direct irradiation device and the single-pulsed irradiation capillary reactor with biocompatible aqueous halides Br- and I-, respectively. These novel photochemical modifications are triggered by 193 nm laser photoexcitation of commonly photo-inert halide ions to form active radical species. High protein modification efficiency (>90%) can be achieved under just 10 ns ultra-short irradiation of a single pulse of laser shot while the compact native protein structure could be largely retained. The specifically modified residues are Tyr, His, Trp for bromination and Tyr, His for iodization. The photochemical halogenation sites and rates are highly selective to protein native structures, providing dynamic insights into protein structure alterations and protein-drug interfaces in human serum protein (HSA)-warfarin interaction. This novel 193 nm photochemical strategy opens new opportunities for the protein structure-function explorations.

Experimental and Computational Studies of Criegee Intermediate syn-CH3CHOO Reaction with Hydrogen Chloride.

Hydrogen chloride (HCl) contributes substantially to the atmospheric Cl; both species could affect the composition of Earth's atmosphere and the fate of pollutants. Here, we present the kinetics study for syn-CH3CHOO reaction with HCl using experimental measurement and theoretical calculations. The experiment was conducted in a flow tube reactor at a pressure of 10 Torr and temperatures ranging from 283 to 318 K by using the OH laser-induced fluorescence (LIF) method. Transition-state theory and quantum chemistry calculations with QCISD(T) were used to calculate the rate coefficients. Weak negative temperature dependence was observed with a measured activation energy of -(2.98 ± 0.12) kcal mol-1 and a calculated zero-point-corrected barrier energy of -3.29 kcal mol-1. At 298 K, the rate coefficient was measured to be (4.77 ± 0.95) × 10-11 cm3 s-1, which was in reasonable agreement with 2.2 × 10-11 cm3 s-1 from the theoretical calculation.

Kinetics of CH2OO and syn-CH3CHOO reaction with acrolein.

The kinetics for the reactions of CH2OO and syn-CH3CHOO with acrolein, a typical unsaturated aldehyde in the atmosphere, were studied in a flash photolysis flow reactor using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 281 to 318 K, and pressures ranging from 5 to 200 Torr. No obvious dependence of the rate coefficients on pressure was observed under the current experimental conditions. Both reactions exhibit negative temperature-dependence, with an activation energy of (-1.70 ± 0.19) and (-1.47 ± 0.24) kcal mol-1 for CH2OO and syn-CH3CHOO reacting with acrolein, derived from the Arrhenius equation. At 298 K, the measured rate coefficients for CH2OO/syn-CH3CHOO + acrolein reactions are (1.63 ± 0.19) × 10-12 cm3 s-1 and (1.17 ± 0.16) × 10-13 cm3 s-1, respectively. The rate coefficient of the former reaction is in reasonable agreement with a recent theoretical result.

Environmentally friendly, durable and transparent anti-fouling coatings applicable onto various substrates.

Anti-fouling coatings are of great interest because of their unique wettability and self-cleaning property, but their widespread applications are seriously hindered by low stability, heavy usage of fluorinated compounds and low transparency, etc. Here, we report a new kind of smooth anti-fouling coatings based on methyltrimethoxysilane. The coatings were fabricated by preparing a stock solution via hydrolytic condensation of methyltrimethoxysilane in isopropanol, followed by wiping the glass slide with the non-woven fabric that sucked the stock solution. The transparent anti-fouling coatings have excellent anti-fouling properties against various fluids such as water, n-hexadecane, diiodomethane, daily encountered liquids (e.g., milk, coffee, red wine, soy sauce and cooking oil), mark seals, artificial fingerprint liquids and paints (both water-based and oil-based), etc. The fluids can easily roll off from the 4-30° titled coatings. Furthermore, the coatings have good mechanical (200 cycles of friction, scratching and bending), chemical (saline, acidic and basic solutions) and thermal stability (boiling and 300 °C heating) regarding the easy sliding behavior of the probing liquids. In addition, the anti-fouling coatings are applicable onto various substrates via the same procedure. The smooth anti-fouling coatings have huge potential applications, owing to the excellent anti-fouling properties, high stability as well as the non-fluorinated and simple preparation method.

Temperature- and pressure-dependent rate coefficient measurement for the reaction of CH2OO with CH3CH2CHO.

Propionaldehyde is one of the most abundant aldehydes, which are an important class of volatile organic compounds. In this work, the rate coefficient of the reaction of the simplest Criegee intermediate CH2OO with propionaldehyde (CH3CH2CHO) was measured for the first time in a flash photolysis reaction tube by using the OH laser-induced fluorescence (LIF) method at temperature and pressure in the range of 283 to 318 K and 5 to 200 Torr. This reaction is observed to be pressure- and temperature-dependent. The measured rate coefficient at 50 Torr is in the vicinity of the high-pressure limit value of (3.23 ± 0.49) × 10-12 cm3 s-1 at 298 K, which is in agreement with a previously reported theoretical result of 2.44 × 10-12 cm3 s-1. The Arrhenius plot of the temperature-dependent rate coefficients yields an activation energy of (-1.99 ± 0.23) kcal mol-1.

Kinetic Studies for the Reaction of syn-CH3CHOO with CF3CH═CH2.

Hydrofluoroolefins (HFOs, CxF2x+1CH═CH2) have great potential to replace hydrofluorocarbons (HFCs) as refrigerants. Here the kinetics for the reaction of syn-CH3CHOO with CF3CH═CH2 (HFO-1243zf), the simplest of HFOs, have been studied in a flash photolysis flow reactor at a total pressure of 50 Torr, by using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 283 to 318 K. A weak positive temperature dependence was observed, with an activation energy of 1.41 ± 0.12 kcal mol-1. At 298 K, the measured rate coefficient was (2.42 ± 0.51) × 10-14 cm3 s-1, in the vicinity of the previously reported upper limit value for the reaction of CH2OO with CF3CH═CH2.

In situ formation of mononuclear complexes by reaction-induced atomic dispersion of supported noble metal nanoparticles.

Supported noble metal nanoclusters and single-metal-site catalysts are inclined to aggregate into particles, driven by the high surface-to-volume ratio. Herein, we report a general method to atomically disperse noble metal nanoparticles. The activated carbon supported nanoparticles of Ru, Rh, Pd, Ag, Ir and Pt metals with loading up to 5 wt. % are completely dispersed by reacting with CH3I and CO mixture. The dispersive process of the Rh nanoparticle is investigated in depth as an example. The in-situ detected I• radicals and CO molecules are identified to promote the breakage of Rh-Rh bonds and the formation of mononuclear complexes. The isolated Rh mononuclear complexes are immobilized by the oxygen-containing functional groups based on the effective atomic number rule. The method also provides a general strategy for the development of single-metal-site catalysts for other applications.

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.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory.

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of -0.53 kcal mol-1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10-14 cm3 molecule-1 s-1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10-14 cm3 molecule-1 s-1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time.

A kinetic study of the CH2OO Criegee intermediate reaction with SO2, (H2O)2, CH2I2 and I atoms using OH laser induced fluorescence.

The OH laser induced fluorescence method was used to study the kinetics of CH2OO reacting with SO2, (H2O)2, CH2I2 and I atoms. Decay of CH2OO is not strictly first-order since its self-reaction is rapid. With this consideration, we derived the rate coefficient of CH2OO + SO2/(H2O)2/CH2I2/I taking into account the contribution of the CH2OO self-reaction. For the CH2OO + SO2 reaction, the rate coefficient is measured to be (3.88 ± 0.13) × 10-11 cm3 molecule-1 s-1 at 10 Torr, which agrees very well with a previously reported value obtained by directly monitoring CH2OO using the UV absorption method with the CH2OO self-reaction considered. We did not observe obvious evidence for SO2 catalysed CH2OO isomerization or the intersystem crossing effect in this reaction. CH2OO + (H2O)2 is supposed to account for the major sink of CH2OO in the atmosphere, but previous rate coefficient measurements were not in good agreement. We have revisited this reaction including the self-reaction of CH2OO and obtained the rate coefficient to be (7.53 ± 0.38) × 10-12 cm3 molecule-1 s-1 at 60 Torr and 300 K. The rate coefficients of CH2OO + CH2I2 and CH2OO + I were measured to be (5.2 ± 2.6) × 10-14 and (2.2 ± 1.1) × 10-12 cm3 molecule-1 s-1 respectively.

Influence of carboxymethyl cellulose and sodium alginate on sweetness intensity of Aspartame.

Sensory evaluation of Aspartame in the presence of sodium carboxymethyl cellulose (CMC-L) and sodium alginate (SA) revealed that only CMC-L showed a suppression effect, while SA did not. By using an artificial taste receptor model, we found that the presence of SA or CMC-L resulted in a decrease in association constants. Further investigation of CMC-L solution revealed that the decrease in water mobility and diffusion also contribute to the suppression effect. In the case of SA, the decreased viscosity and comparatively higher amount of free water facilitated the diffusion of sweetener, which might compensate for the decreased binding constant between Aspartame and receptor. This may suppress the impact of SA on sweetness intensity. The results suggest that exploring the binding affinity of taste molecules with the receptor, along with water mobility and diffusion in hydrocolloidal structures, provide sufficient information for understanding the mechanism behind the effect of macromolecular hydrocolloids on taste.

Coherent superposition of M-states in a single rovibrational level of H2 by Stark-induced adiabatic Raman passage.

We prepare an ensemble of isolated rovibrationally excited (v = 1, J = 2) H2 molecules in a phase-locked superposition of magnetic sublevels M using Stark-induced adiabatic Raman passage with linearly polarized single-mode pump (at 532 nm, ∼6 ns pulse duration, 200 mJ/pulse) and Stokes (699 nm, ∼4 ns pulse duration, 20 mJ/pulse) laser excitation. A biaxial superposition state, given by [line]ψ(t)⟩ = 1/√(2)[[line]ν = 1, J = 2, M = -2⟩ - [line]ν = 1, J = 2, M = +2⟩], is prepared with linearly but cross-polarized pump and Stokes laser pulses copropagating along the quantization z-axis. The degree of phase coherence is measured by using the O(2) line of the H2 E,F-X (0,1) band via 2 + 1 resonance enhanced multiphoton ionization (REMPI) at 210.8 nm by recording interference fringes in the REMPI signal in a time-of-flight mass spectrometer as the direction of the UV laser polarization is rotated using a half-wave plate. Nearly 60% population transfer from H2 (v = 0, J = 0) ground state to the superposition state in H2 (v = 1, J = 2) is measured from the depletion of the Q(0) line of the E,F-X (0,0) band as the Stokes frequency is tuned across the (v = 0, J = 0) → (v = 1, J = 2) Raman resonance.