Insights into the Ultrafast Photodissociation Dynamics of Isoprene-Derived Criegee Intermediates

Isoprene is the most abundant nonmethane volatile organic compound emitted into the troposphere by terrestrial vegetation. Reaction with ozone represents an important isoprene removal process from the troposphere and is a well-known source of Criegee intermediates (CIs), which are reactive carbonyl oxides. Three CIs, formaldehyde oxide (CH₂OO), methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide) are formed during isoprene ozonolysis. All three CIs contain strongly absorbing ππ* states, electronic excitation, which leads to dissociation to form aldehyde/ketone + oxygen products. Here, we compare the excited state chemistry of CH₂OO, MVK-oxide and MACR-oxide in order to ascertain how increasing molecular complexity affects their photodynamics. In CH₂OO, vertical excitation to the S₂ state leads to prompt O-O bond fission with a unity quantum yield. Branching into both the O (¹D) + H₂CO (S₀) and O (³P) + H₂CO (T₁) product channels is predicted, with 80% of trajectories dissociating to form the former product pair. Analogous vertical excitation of the lowest energy conformers of MVK-oxide and MACR-oxide also undergoes O-O bond fission to form O + MVK/MACR products—albeit with a nonunity quantum yield. In the latter case, ca. 10% and 25% of trajectories remain as the parent MVK-oxide and MACR-oxide molecules, respectively. Additionally, at most only 5% of the dissociating trajectories form O (³P) + MVK/MACR (T₁) products, with a greater fraction forming O (¹D) + MVK/MACR (S₀) products (cf. CH₂OO). This latter observation coupled with the greater fraction of undissociated trajectories aligns with the bathochromic shift in the electronic absorption of the MACR-oxide and MVK-oxide (cf. CH₂OO). We discuss the implications of the results in a broader context, including those that are relevant to the atmosphere.     

Spectroscopy of the Simplest Criegee Intermediate CH2OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra

CH₂OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas‐phase studies on CH₂OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH₂OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH₂OO. In order to encourage more direct studies on CH₂OO and other Criegee intermediates, the electronic and photoelectron spectra of CH₂OO have been simulated using high level electronic structure calculations and Franck–Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH₂OO were calculated with TDDFT, EOM‐CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low‐lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH₂OO corresponds to the ${{\rm{\tilde B}}}$ ¹A′ ← ${{\rm{\tilde X}}}$ ¹A′ absorption. It is a broad band in the region 250–450 nm showing extensive structure in vibrational modes involving O–O stretching and C‐O‐O bending. Evidence is presented to show that the electronic absorption spectrum of CH₂OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH₂OO was prepared in this earlier work from the reaction of CH₂I with O₂ and that the assignment of the observed spectrum solely to CH₂IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH₂OO consists of two overlapping vibrationally structured bands corresponding to one‐electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.     

Reactions between Criegee Intermediates and the Inorganic Acids HCl and HNO3: Kinetics and Atmospheric Implications

Criegee intermediates (CIs) are a class of reactive radicals that are thought to play a key role in atmospheric chemistry through reactions with trace species that can lead to aerosol particle formation. Recent work has suggested that water vapor is likely to be the dominant sink for some CIs, although reactions with trace species that are sufficiently rapid can be locally competitive. Herein, we use broadband transient absorption spectroscopy to measure rate constants for the reactions of the simplest CI, CH₂OO, with two inorganic acids, HCl and HNO₃, both of which are present in polluted urban atmospheres. Both reactions are fast; at 295 K, the reactions of CH₂OO with HCl and HNO₃ have rate constants of 4.6×10^?11 cm³ s^?1 and 5.4×10^?10 cm³ s^?1, respectively. Complementary quantum‐chemical calculations show that these reactions form substituted hydroperoxides with no energy barrier. The results suggest that reactions of CIs with HNO₃ in particular are likely to be competitive with those with water vapor in polluted urban areas under conditions of modest relative humidity.     

Rate Coefficients of C1 and C2 Criegee Intermediate Reactions with Formic and Acetic Acid Near the Collision Limit: Direct Kinetics Measurements and Atmospheric Implications

Rate coefficients are directly determined for the reactions of the Criegee intermediates (CI) CH₂OO and CH₃CHOO with the two simplest carboxylic acids, formic acid (HCOOH) and acetic acid (CH₃COOH), employing two complementary techniques: multiplexed photoionization mass spectrometry and cavity‐enhanced broadband ultraviolet absorption spectroscopy. The measured rate coefficients are in excess of 1×10^?10 cm³ s^?1, several orders of magnitude larger than those suggested from many previous alkene ozonolysis experiments and assumed in atmospheric modeling studies. These results suggest that the reaction with carboxylic acids is a substantially more important loss process for CIs than is presently assumed. Implementing these rate coefficients in global atmospheric models shows that reactions between CI and organic acids make a substantial contribution to removal of these acids in terrestrial equatorial areas and in other regions where high CI concentrations occur such as high northern latitudes, and implies that sources of acids in these areas are larger than previously recognized.     

Solvent effects on the absorption spectrum and recombination kinetics of diphenylcarbonyl oxide

The absorption spectra and rate constants of diphenylcarbonyl oxide recombination in a series of solvents and their binary mixtures were determined by flash photolysis. An increase in the solvent polarity causes hypsochromic shift of the maximum in the absorption spectrum of Ph₂COO. The analysis of the solvent effect on the recombination rate constant in terms of the four-parameter Koppel—Palm equation shows that the reactivity of carbonyl oxide depends on both specific and non-specific solvations. Quantum chemical B3LYP/6-31G(d) calculations of H₂COO and PhHCOO carbonyl oxides as well as the complexes of H₂COO with acetonitrile and ethylene in different media were performed using a polarized continuum model.     

Classical trajectory study of three-body direct photofragmentation of Cd(CH3)2. Comparison of sampling methods

The classical trajectory method has been used to investigate product-state symmetry in the three-body direct photofragmentation of Cd(CH₃)₂. Photon absorption was assumed to satisfy the Franck principle and relative weights for trajectory initial conditions were assigned by either the Wigner or the classical (300 K) distribution function. In qualitative agreement with earlier “quasi-classical” sampling studies of Kellman et al. for photolysis of the Cd(CH₃)₂ vibrational ground state, product-state asymmetry is found to increase with increasing photolysis wavelength. Contrary to the “quasi-classical” sampling results, however, sampling with the classical distribution function indicates that product-state symmetry is unaffected by vibrational excitation of Cd(CH₃)₂ prior to photolysis. For sampling with the Wigner distribution function, in agreement with the “quasi-classical” sampling studies, excitation of the Cd(CH₃)₂ asymmetric stretch prior to photolysis is found to increase product-state asymmetry.     

Surprising Stability of Larger Criegee Intermediates on Aqueous Interfaces

Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born–Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn ‐ and anti ‐CH₃CHOO at the air–water interface. They show that unlike the simplest Criegee intermediate (CH₂OO), both syn ‐ and anti ‐CH₃CHOO remain inert towards reaction with water. The unexpected high stability of C₂ Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre‐reaction complex for the Criegee–water reaction. The simulation of the larger Criegee intermediates, (CH₃)₂COO, syn ‐ and anti ‐CH₂C(CH₃)C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air–water interface.     

Gas‐Phase Structure Determination of Dihydroxycarbene,One of the Smallest Stable Singlet Carbenes

Carbenes are reactive molecules of the form R¹ C̈ R² that play a role in topics ranging from organic synthesis to gas‐phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HO C̈ OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high‐level coupled‐cluster calculations. The semi‐experimental equilibrium structure derived from five isotopic variants of HO C̈ OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HO C̈ OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH₂O₂ chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH₂OO.     

Emission spectrum of CH3S radical

The emission spectrum of the CH₃S radical is obtained in the photolysis of CH₃SCH₃ by using a xenon resonance lamp. The emission is also obtained in the photolysis of CH₃SSCH₃ at ca. 200 nm irradiation as well as at 147.0 nm irradiation. The excitation threshold to produce CH₃S fluorescence in the photolysis of CH₃SSCH₃ is determined to be 202 ± 3 nm or 6.14 ± 0.09 eV. The ratios of electronic quenching rate to fluorescence rate of CH₃S^* with N₂, H₂, D₂ and CH₄ are determined.     

Gas‐Phase Structure Determination of Dihydroxycarbene,One of the Smallest Stable Singlet Carbenes

Carbenes are reactive molecules of the form R¹? C?? R² that play a role in topics ranging from organic synthesis to gas‐phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HO? C?? OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high‐level coupled‐cluster calculations. The semi‐experimental equilibrium structure derived from five isotopic variants of HO? C?? OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HO? C?? OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH₂O₂ chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH₂OO.     

UV Spectrum of the Simplest Deuterated Criegee Intermediate CD2OO

The ultraviolet (UV ) absorption spectrum of the simplest deuterated Criegee intermediate CD₂OO has been measured and compared with that of CH₂OO . While the UV spectra of CH₂OO and CD₂OO are similar in the overall shape, distinctive oscillatory structures at the long wavelength side of the absorption band show clear effect of isotopic substitution. The average spacing between the vibrational peaks decreases from 606 cm⁻¹ for CH₂OO to 528 cm⁻¹ for CD₂OO . This large isotope effect cannot be explained by one‐dimensional model along the dissociative O−O bond. Instead, vibrational modes involving motions of the H‐atoms are expected to be responsible for the observed oscillatory structure. This isotope effect offers a stringent test for theoretical investigations on the absorption spectrum and excited‐state dynamics of the simplest Criegee intermediate.     

Electrospray ionization mass spectrometry of organogermanium compounds

A detailed study of the solution chemistry and mass spectrometry of six carboxylato-organogermanium compounds in aqueous solution has been carried out using electrospray ionization and MSⁿ techniques. The different types of hydrolysis products and their probable structures, which include the oligomers and their fragment ions plus water adduct ions formed by ion-molecule reactions, are presented, e.g., HO-cyclic-(-Ge(O⁻)CH₂CH₂COO) A, HO-cyclic-(-Ge(O-cyclic-(Ge(O⁻)CH₂CH₂COO)CH₂CH₂COO) B, ⁻OGeO-cyclic-(-Ge(OH)CH₂CH₂COO) C, and ⁻CH=CHGeO-cyclic-(-Ge(OH)CH₂CH₂COO) D, etc. The proposed cyclic structures are confirmed by theoretical calculations. Copyright © 2001 John Wiley & Sons, Ltd.     

Temperature‐Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

The rate coefficients for gas‐phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH₂OO + CF₃COOH)=(3.4±0.3)×10⁻¹⁰ cm³ s⁻¹ and k((CH₃)₂COO + CF₃COOH)=(6.1±0.2)×10⁻¹⁰ cm³ s⁻¹ at 294 K exceed estimates for collision‐limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre‐reactive complex. Fits to a model incorporating this complex formation give k [cm³ s⁻¹]=(3.8±2.6)×10⁻¹⁸ T² exp((1620±180)/T) + 2.5×10⁻¹⁰ and k [cm³ s⁻¹]=(4.9±4.1)×10⁻¹⁸ T² exp((1620±230)/T) + 5.2×10⁻¹⁰ for the CH₂OO + CF₃COOH and (CH₃)₂COO + CF₃COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.     

Temperature-Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

The rate coefficients for gas-phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH₂OO + CF₃COOH)=(3.4±0.3)×10⁻¹⁰ cm³ s⁻¹ and k((CH₃)₂COO + CF₃COOH)=(6.1±0.2)×10⁻¹⁰ cm³ s⁻¹ at 294 K exceed estimates for collision-limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre-reactive complex. Fits to a model incorporating this complex formation give k [cm³ s⁻¹]=(3.8±2.6)×10⁻¹⁸ T² exp((1620±180)/T) + 2.5×10⁻¹⁰ and k [cm³ s⁻¹]=(4.9±4.1)×10⁻¹⁸ T² exp((1620±230)/T) + 5.2×10⁻¹⁰ for the CH₂OO + CF₃COOH and (CH₃)₂COO + CF₃COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.     

Temperature-dependent kinetics of the simplest Criegee intermediate reaction with dimethyl sulfoxide

Criegee intermediates are thought to play roles in atmospheric chemistry, including OH radical formation, oxidation of SO₂, NO₂, etc. CH₂OO is the simplest Criegee intermediate, of which the reactivity has been a hot topic. Here we investigated the kinetics of CH₂OO reaction with dimethyl sulfoxide (DMSO) under 278–349 K and 10–150 Torr. DMSO is an important species formed in the oxidation of dimethyl sulfide in the biogenic sulfur cycle. The concentration of CH₂OO was monitored in real-time via its mid-infrared absorption band at about 1,286 cm⁻¹ (Q branch of the ν₄ band) with a high-resolution quantum cascade laser spectrometer. The 298 K bimolecular rate coefficient was determined to be k₂₉₈ = (2.3 ± 0.3) × 10⁻¹² cm³/s at 30 Torr with an Arrhenius activation energy of −3.9 ± 0.2 kcal/mol and a weak pressure dependence for pressures higher than 30 Torr (k₂₉₈ = (2.8 ± 0.3) × 10⁻¹² cm³/s at 100 Torr). The reaction is speculated to undergo a five-membered ring intermediate, analogous to that of CH₂OO with SO₂. The negative activation energy indicates that the rate-determining transition state is submerged. The magnitude of the reaction rate coefficient lies in between those of CH₂OO reactions with (CH₃)₂CO and with SO₂.     

Temperature‐ and Pressure‐Dependent Kinetics of CH2OO + CH3COCH3 and CH2OO + CH3CHO: Direct Measurements and Theoretical Analysis

The rate coefficients of the gas‐phase reactions CH₂OO + CH₃COCH₃ and CH₂OO + CH₃CHO have been experimentally determined from 298–500 K and 4–50 Torr using pulsed laser photolysis with multiple‐pass UV absorption at 375 nm, and products were detected using photoionization mass spectrometry at 10.5 eV. The CH₂OO + CH₃CHO reaction's rate coefficient is ～4 times faster over the temperature 298–500 K range studied here. Both reactions have negative temperature dependence. The T dependence of both reactions was captured in simple Arrhenius expressions: The rate of the reactions of CH₂OO with carbonyl compounds at room temperature is two orders of magnitude higher than that reported previously for the reaction with alkenes, but the A factors are of the same order of magnitude. Theoretical analysis of the entrance channel reveals that the inner 1,3‐cycloaddition transition state is rate limiting at normal temperatures. Predicted rate‐coefficients (RCCSD(T)‐F12a/cc‐pVTZ‐F12//B3LYP/MG3S level of theory) in the low‐pressure limit accurately reproduce the experimentally observed temperature dependence. The calculations only qualitatively reproduce the A factors and the relative reactivity between CH₃CHO and CH₃COCH₃. The rate coefficients are weakly pressure dependent, within the uncertainties of the current measurements. The predicted major products are not detectable with our photoionization source, but heavier species yielding ions with masses m/z = 104 and 89 are observed as products from the reaction of CH₂OO with CH₃COCH₃. The yield of m/z = 89 exhibits positive pressure dependence that appears to have already reached a high‐pressure limit by 25 Torr.     

Carbon kinetic isotope effect in the oxidation of methane by hydroxyl

The carbon kinetic isotope effect in the reaction of CH₄ with OH has an experimentally measured value of 1.003. The measurement was performed using a static system in which the source of OH was the gas-phase photolysis of H₂O₂ with ultraviolet light produced by a high-pressure mercury arc lamp. Implications for the tropospheric cycle of CH₄ are considered briefly.     

Photolysis and desorption of adsorbed halomethane molecules under long-wavelength radiation of excimer lasers

The photolysis and desorption of CH₂I₂, CH₃I, CCl₄, CHCl₃, CH₂Cl₂, CF₂Cl₂, and CHF₂Cl molecules adsorbed on fused silica under the action of XeF and XeCl laser radiation absorbed by these molecules have been studied. The desorption of molecules that occurs due to expansion to the long-wavelength region of the absorption spectrum of molecules in the adsorbed state, compared to the gas phase, predominates. The laser desorption is characterized by a strong nonlinear dependence on the density of radiation energy. Depending on the relationship between the laser radiation wavelength and the spectrum of electronic states of molecules, photolysis is observed upon absorption of either one or two photons. At an increased fluence of laser radiation energy, the one-photon detachment of the primary CH₂I fragment from the CH₂I₂ molecule changes into the three-photon process. A similar behavior is revealed for the desorption of CH₃I molecules from clusters formed on the surface in multilayer adsorption coverages.     

Biological Activity of Flavonoids Copper Complexes

Three flavonoid copper(II) complexes Cu₂(quercetin)(CH₃COO)₃(CH₃OH) ( 1 ), Cu(anthrarufin)(CH₃COO)·1/2H₂O ( 2 ) and Cu(naringin)(OCH₃)(CH₃OH)₂ ( 3 ) have been synthesized and characterized by elemental analysis, IR, electronic absorption and EPR (X‐band) spectroscopy. The complexes have a strong protective action over the Δsod1 mutant of S. cerevisiae against reactive oxygen radicals generated by an external source of free radicals (H₂O₂ or the superoxide‐generating, menadione). On the other hand, the complexes cleave DNA efficiently even in the absence of reducing agents. The main reactive oxygen species responsible for the DNA strand cleavage have been determined using radical scavengers. A probably mechanism of the DNA damage is proposed.     

Isomerization and Decomposition of a Criegee Intermediate in the Ozonolysis of Alkenes: Dynamics Using a Multireference Potential

The isomerization and decomposition dynamics of the simplest Criegee intermediate CH₂OO have been studied by classical trajectory simulations using the multireference ab initio MR‐PT2 potential on the fly. A new, accelerated algorithm for dynamics with MR‐PT2 was used. For an initial temperature of 300 K, starting from the transition state from CH₂OO→CH₂O₂ , the system reaches the dioxirane structure in around 50 fs, then isomerizes to formic acid (in ca. 2800 fs), and decomposes into CO+H₂O at around 2900 fs. The contributions of different configurations to the multiconfigurational total electronic wave function vary dramatically along the trajectory, with diradical contributions being important for transition states corresponding to H‐atom transfers, while being only moderately significant for CH₂OO. The implications for reactions of Criegee intermediates are discussed.