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.     

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.     

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.     

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.     

Theoretical Study of the Reaction of Carbonyl Oxide with Nitrogen Dioxide: CH2OO + NO2

The reaction mechanism of the reaction of the Criegee intermediate CH₂OO with NO₂ was investigated using quantum chemical and theoretical kinetic methodologies. The reaction shows a rich chemistry, though the number of channels that effectively contribute at room temperature is limited. The theoretical characterization of the entrance transition states was hampered by strongly multireference wave functions. The predicted rate coefficient k (298 K) = 4.4 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ thus carries a large uncertainty, but is in agreement with literature data. We find that the CH₂OO + NO₂ reaction reacts by adduct formation, near‐exclusively forming nitro‐peroxy radicals, ^•OOCH₂NO₂. These will react as other alkylperoxy radicals in the atmosphere, ultimately generating CH₂O and regenerating NO₂ in most reaction conditions. The product predictions contrast with earlier experimental work showing NO₃ formation, but support other observations of adduct products.     

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.     

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.     

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.     

Organic peroxide production in the Cl2-ethane-air photoreaction system

HPLC along with FT-IR technique was used to study the formation of organic peroxides in the Cl₂-ethane-air photoreaction system. Ethyl hydroperoxide (CH₃CH₂₁OOH, EHP) and peroxyacetic acid ( CH₃C(O)OOH, PAA) were conformed to be the peroxide product in the reaction system. In addition, methyl hydroperoxide (CH₃OOH, hydroxymethyl hydroperoxide (HOCH₂OOH, HMHP) and two unidentified organic peroxides were detected for the first time. EHP and were the dominant peroxide products. The identification of HMHP showed that Criegee biradical CH₂OO may be formed as an intermediate in the oxidation of ethane. Simulation results showed that photooxidation of ethane may make substantial contribution to source of organic peroxides in the atmosphere.     

Organic peroxide production in the Cl2-ethane-air photoreaction system

HPLC along with FT-IR technique was used to study the formation of organic peroxides in the Cl₂-ethane-air photoreaction system. Ethyl hydroperoxide (CH₃CH₂₁OOH, EHP) and peroxyacetic acid ( CH₃C(O)OOH, PAA) were conformed to be the peroxide product in the reaction system. In addition, methyl hydroperoxide (CH₃OOH, hydroxymethyl hydroperoxide (HOCH₂OOH, HMHP) and two unidentified organic peroxides were detected for the first time. EHP and were the dominant peroxide products. The identification of HMHP showed that Criegee biradical CH₂OO may be formed as an intermediate in the oxidation of ethane. Simulation results showed that photooxidation of ethane may make substantial contribution to source of organic peroxides in the atmosphere.     

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.     

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.     

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.     

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.     

Formation of organic peroxides in the photooxidation of CH4

The formation of organic peroxides in the Cl-atom-initiated photooxidation of CH₄ in O₂-N₂ mixtures at 101 325 Pa and 298 K was studied with HPLC and FT-IR methods. Four peroxidic products were detected, which were H₂O₂, hydroxymethyl hydroperoxide (HOCH₂OOH; HMHP), methyl peroxide (CH300H; MHP) and dimethyl peroxide (CH₃00CH₃). A chromatogram peak at retention time of 8.08 min was assigned tentatively to peroxyformic acid (HC(O)OOH). The identification of HMHP in the reaction system showed that one of the reaction paths for the self-reaction of CH₃00. led to producing Criegee intermediate CH₂OO. The formation mechanism of organic peroxide in the photooxidation of CH₄ is more complicated than it was assumed before. Photooxidation of CH₄ is probably an important source of organic peroxides in the troposphere. Project supported by the State Scientific and Technological Commission of China (Grant No. E96-05)     

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH₂OO, CH₃CHOO, (CH₃)₂COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH₂OO and (CH₃)₂COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing ¹ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O–O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.     

An Estimation of the Levels of Stabilized Criegee Intermediates in the UK Urban and Rural Atmosphere Using the Steady‐State Approximation and the Potential Effects of These Intermediates on Tropospheric Oxidation Cycles

Levels of the stabilized Criegee Intermediate (sCI), produced via the ozonolysis of unsaturated volatile organic compounds (VOCs), were estimated at two London urban sites (Marylebone Road and Eltham) and one rural site (Harwell) in the UK over the period of 1998–2012. The steady‐state approximation was applied to data obtained from the NETCEN (National Environmental Technology Centre) database, and the levels of annual average sCI were estimated to be in the range of 30–3000 molecules cm⁻³ for UK sites. A consistent diurnal cycle of sCI concentration is estimated for the UK sites with increasing levels during daylight hours, peaking just after midday. The seasonal pattern of sCI shows higher levels in spring with peaks around May due to the higher levels of O₃. The ozone weekend effect resulted in higher sCI in UK urban areas during weekend. The sCI data were modeled using the information provided by the Air Quality Improvement Research Program (AQIRP) and found that the modeled production was five‐ to six‐fold higher than our estimated data, and therefore the estimated sCI concentrations in this study are thought to be lower estimates only. Compared with nighttime, 1.3‐ to 1.8‐fold higher sCI exists under daytime conditions. Using the levels of sCI estimated at Marylebone Road, globally the oxidation rates of NO₂ + sCI (22.4 Gg/yr) and SO₂ + sCI (37.6 Gg/yr) in urban areas can increase their effect in the troposphere and potentially further alter the oxidizing capacity of the troposphere. Further investigations of modeled sCI show that CH₃CHOO (64%) and CH₂OO (13%) are dominant among all contributing sCI at the UK sites.     

Direct Ab Initio Dynamics Study on the Reaction of CH3CHF2 (HFC‐152a) with the Cl Atom

A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction CH₃CHF₂+Cl. One transition state is located for α‐H abstraction, and two are identified for β‐H abstraction. The potential‐energy surface (PES) is obtained at the G3(MP2)//MP2/6‐311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition‐state theory (CVT) with small‐curvature tunneling (SCT) contributions over a wide temperature range of 200–2500 K. The dynamic calculations show that the reaction proceeds mainly by α‐H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.     

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.     

Photodissociation of CH2I2 and Subsequent Electron Transfer in Solution

We studied photoinduced reactions of diiodomethane (CH₂I₂) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond–nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340–740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay (≈500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7–50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH₂I radical by direct dissociation of the photoexcited CH₂I₂ and its disappearance by subsequent primary geminate recombination. The concentration‐dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time‐dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge‐transfer (CT) complex, CH₂I₂^δ+???I^δ?, formed by a photofragment I atom and the solute CH₂I₂ molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH₂I₂⁺???I^? at 385 nm on the picosecond timescale. On the nanosecond scale, I₃^? was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time‐resolved absorption spectra and the reaction rate equations. We ascribed concentration‐dependent dynamics to the CT‐complex formation in pre‐existing aggregates of CH₂I₂ and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH₂I₂ molecules that can participate in CT‐complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH₂I₂. Such conditions were achieved at lower concentrations in acetonitrile than in hexane.