Methyl Hydroperoxide Research Articles

Thermochemistry of the smallest QOOH radical from the roaming fragmentation of energy selected methyl hydroperoxide ions.

The dissociative photoionization processes of methyl hydroperoxide (CH3OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH3OOH+ ions dissociate into CH2OOH+, HCO+, CH3+, and H3O+ ions in the 11.4-14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest "QOOH" radical, CH2OOH+. An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH2OOH+ ion, and a 74.2 ± 2.6 kJ mol-1 mixed experimental-theoretical 0 K heat of formation for the CH2OOH radical. The proton affinity of the Criegee intermediate, CH2OO, was also obtained from the heat of formation of CH2OOH+ (792.8 ± 0.9 kJ mol-1) to be 847.7 ± 1.1 kJ mol-1, reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes was used to model the higher-energy fragmentation. Supported by Born-Oppenheimer molecular dynamics simulations, we found that the HCO+ fragment ion is produced through a roaming transition state followed by a low barrier. H3O+ is formed in a consecutive process from the CH2OOH+ fragment ion, while direct C-O fission of the molecular ion leads to the methyl cation.

Role of Torsion-Vibration Coupling in the Overtone Spectrum and Vibrationally Mediated Photochemistry of CH3OOH and HOOH

The yield of vibrationally excited OH fragments resulting from the vibrationally mediated photodissociation of methyl hydroperoxide (CH3OOH) excited in the vicinity of its 2νOH and 3νOH stretching overtones is compared with that resulting from excitation of the molecule to states with three quanta in the CH stretches and to the state with two quanta in the OH stretch and one in the OOH bend (2νOH + νOOH). We find that the OH fragment vibrational state distribution depends strongly on the vibrational state of CH3OOH prior to photodissociation. Specifically, dissociation from the CH stretch overtones and the stretch/bend combination band involving the OH stretch and OOH bend produced significantly less vibrationally excited OH fragments compared to that produced following excitation of CH3OOH to an overtone in the OH stretch. While the absence of vibrationally excited OH photoproducts following excitation of the CH overtone is not surprising, the lack of vibrationally excited OH following excitation to the 2νOH+νOOH combination band is unexpected given that photodissociation following excitation to the lower-energy 2νOH state produces OH products in v = 1 as well as in its ground state. This trend persists even when the electronic photodissociation wavelength is changed from 532 to 355 nm and thus suggests that the observed disparity arises from differences in the nature of the initially populated vibrational states. This lack of vibrationally excited OH products likely reflects the enhanced intramolecular vibrational energy redistribution associated with the stretch/bend combination level compared to the pure OH stretch overtone. Consistent with this hypothesis, photodissociation from the stretch/bend combination level of the smaller HOOH molecule produces more vibrationally excited OH fragments compared to that resulting from the corresponding state of CH3OOH. These results are investigated using second-order vibrational perturbation theory based on an internal coordinate representation of the normal modes. Consistent with the observations, the first-order correction to the wave function shows stronger coupling of the 2νOH+νOOH state to states with torsion excitation compared to the other bands considered in this study.

Formaldehyde in the Tropical Western Pacific: Chemical sources and sinks, convective transport, and representation in CAM-Chem and the CCMI models.

Formaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HOx. In remote marine environments, such as the Tropical Western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here, we have used observations from the CONTRAST field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 pptv near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM-Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry-Climate Model Initiative (free-running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NOx emissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere.

Atmospheric hydroperoxides measured over a rural site in central Japan during spring: helicopter-borne measurements

Measurements of the concentrations of H2O2 and methyl hydroperoxide (MHP), O3, and SO2 over Imizu City, Toyama Prefecture, Japan were performed in March using a helicopter. H2O2 concentrations were higher at an altitude of approximately 2,400 m (8,000 ft). The H2O2 concentrations (< 0.8 ppb) in the spring were much lower than those observed during the summer observations. MHP was also higher in the high-altitude atmosphere. Lower concentrations of H2O2 were observed when high air pollutants were actively transported from Asian continent. The concentrations of H2O2 were mostly lower than those of SO2; this condition is called oxidant limitation. If H2O2 concentration rises in cold months, the acidification of cloud water may be accelerated at high elevations in central Japan where air pollution is actively transported.

On the Formation of the C2H6O Isomers Ethanol (C2H5OH) and Dimethyl Ether (CH3OCH3) in Star-forming Regions

Abstract The structural isomers ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3) were detected in several low-, intermediate-, and high-mass star-forming regions, including Sgr B2, Orion, and W33A, with the relative abundance ratios of ethanol/dimethyl ether varying from about 0.03 to 3.4. Until now, no experimental data regarding the formation mechanisms and branching ratios of these two species in laboratory simulation experiments could be provided. Here, we exploit tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) to detect and analyze the production of complex organic molecules (COMs) resulting from the exposure of water/methane (H2O/CH4) ices to energetic electrons. The main goal is to understand the formation mechanisms in star-forming regions of two C2H6O isomers: ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3). The results show that the experimental branching ratios favor the synthesis of ethanol versus dimethyl ether (31 ± 11:1). This finding diverges from the abundances observed toward most star-forming regions, suggesting that production routes on interstellar grains to form dimethyl ether might be missing; alternatively, ethanol can be overproduced in the present simulation experiments, such as via radical–radical recombination pathways involving ethyl and hydroxyl radicals. Finally, the PI-ReTOF-MS data suggest the formation of methylacetylene (C3H4), ketene (CH2CO), propene (C3H6), vinyl alcohol (CH2CHOH), acetaldehyde (CH3CHO), and methyl hydroperoxide (CH3OOH), in addition to ethane (C2H6), methanol (CH3OH), and CO2 detected from infrared spectroscopy. The yield of all the confirmed species is also determined.

Selective Oxidation of Methane to Methanol over ZSM‐5 Catalysts in Aqueous Hydrogen Peroxide: Role of Formaldehyde

AbstractThe selective oxidation of methane in aqueous hydrogen peroxide over ZSM‐5 catalysts with different Si/Al ratios has been investigated. Methyl hydroperoxide was confirmed as the initial product of methane oxidation by a study of product distribution versus reaction time in agreement with previous work. Formaldehyde was identified as an intermediate in the oxidation reaction pathway from MeOOH to formic acid and ultimately CO2. Hydrogen evolution during the oxidation of methane is reported for the first time. Oxidation of the HCHO intermediate to formic acid was demonstrated to be the source of the evolved hydrogen. HCHO appeared to be the source of low levels of soluble polyoxomethylene. Higher productivity of liquid oxygenated products is reported compared to previous work.

Measurement of hydrogen peroxide and organic hydroperoxide concentrations during autumn in Beijing, China

Gaseous peroxides play important roles in atmospheric chemistry. To understand the pathways of the formation and removal of peroxides, atmospheric peroxide concentrations and their controlling factors were measured from 7:00 to 20:00 in September, October, and November 2013 at a heavily trafficked residential site in Beijing, China, with average concentrations of hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP) at 0.55ppb and 0.063ppb, respectively. H2O2 concentrations were higher in the afternoon and lower in the morning and evening, while MHP concentrations did not exhibit a regular diurnal pattern. Both H2O2 and MHP concentrations increased at dusk in most cases. Both peroxides displayed monthly variations with higher concentrations in September. These results suggested that photochemical activity was the main controlling factor on variations of H2O2 concentrations during the measurement period. Increasing concentrations of volatile organic compounds emitted by motor vehicles were important contributors to H2O2 and MHP enrichment. High levels of H2O2 and MHP concentrations which occurred during the measurement period probably resulted from the transport of a polluted air mass with high water vapor content passing over the Bohai Bay, China.

Observation of atmospheric peroxides during Wangdu Campaign 2014 at a rural site in the North China Plain

Abstract. Measurements of atmospheric peroxides were made during Wangdu Campaign 2014 at Wangdu, a rural site in the North China Plain (NCP) in summer 2014. The predominant peroxides were detected to be hydrogen peroxide (H2O2), methyl hydroperoxide (MHP) and peroxyacetic acid (PAA). The observed H2O2 reached up to 11.3 ppbv, which was the highest value compared with previous observations in China at summer time. A box model simulation based on the Master Chemical Mechanism and constrained by the simultaneous observations of physical parameters and chemical species was performed to explore the chemical budget of atmospheric peroxides. Photochemical oxidation of alkenes was found to be the major secondary formation pathway of atmospheric peroxides, while contributions from alkanes and aromatics were of minor importance. The comparison of modeled and measured peroxide concentrations revealed an underestimation during biomass burning events and an overestimation on haze days, which were ascribed to the direct production of peroxides from biomass burning and the heterogeneous uptake of peroxides by aerosols, respectively. The strengths of the primary emissions from biomass burning were on the same order of the known secondary production rates of atmospheric peroxides during the biomass burning events. The heterogeneous process on aerosol particles was suggested to be the predominant sink for atmospheric peroxides. The atmospheric lifetime of peroxides on haze days in summer in the NCP was about 2–3 h, which is in good agreement with the laboratory studies. Further comprehensive investigations are necessary to better understand the impact of biomass burning and heterogeneous uptake on the concentration of peroxides in the atmosphere.

Thermochemical Properties and Bond Dissociation Energies for Fluorinated Methanol, CH3-xFxOH, and Fluorinated Methyl Hydroperoxides, CH3-xFxOOH: Group Additivity.

Oxygenated fluorocarbons are routinely found in sampling of environmental soils and waters as a result of the widespread use of fluoro and chlorofluoro carbons as heat transfer fluids, inert materials, polymers, fire retardants and solvents; the influence of these chemicals on the environment is a growing concern. The thermochemical properties of these species are needed for understanding their stability and reactions in the environment and in thermal process. Structures and thermochemical properties on the mono- to trifluoromethanol, CH3-xFxOH, and fluoromethyl hydroperoxide, CH3-xFxOOH (1 ≤ x ≤ 3), are determined by CBS-QB3, CBS-APNO, and G4 calculations. Entropy, S°298, and heat capacities, Cp(T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31+G(d,p) density functional method. Potential barriers for the internal rotations are also calculated from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potentials. Standard enthalpies of formation, ΔfH°298 (units in kcal mol(-1)) are CH2FOOH (-83.7), CHF2OOH (-138.1), CF3OOH (-193.6), CH2FOO(•) (-44.9), CHF2OO(•) (-99.6), CF3OO(•) (-153.8), CH2FOH (-101.9), CHF2OH (-161.6), CF3OH (-218.1), CH2FO(•) (-49.1), CHF2O(•) (-97.8), CF3O(•) (-150.5), CH2F(•) (-7.6), CHF2(•) (-58.8), and CF3(•) (-112.6). Bond dissociation energies for the R-OOH, RO-OH, ROO-H, R-OO(•), RO-O(•), R-OH, RO-H, R-O(•), and R-H bonds are determined and compared with methyl hydroperoxide to observe the trends from added fluoro substitutions. Enthalpy of formation for the fluoro-hydrocarbon oxygen groups C/F/H2/O, C/F2/H/O, C/F3/O, are derived from the above fluorinated methanol and fluorinated hydroperoxide species for use in Benson's Group Additivity. It was determined that fluorinated peroxides require interaction terms O/CH2F/O, O/CHF2/O, and O/CF3/O, as opposed to the common (O/C/O) group in hydrocarbons, resulting from interactions of the peroxide oxygen with the fluorines. Hydrogen bond dissociation increment (HBI) groups are also developed.

Methane Activation by Selective Oxidation

The selective activation of methane by oxidation is a topic that has fascinated scientists and engineers for over a century. In this paper some of the recent approaches will be described and discussed. In particular, the use of FeZSM5 as a catalyst for methane oxidation with aqueous hydrogen peroxide will be described. With this catalyst the primary product is methyl hydroperoxide which decomposes to give methanol. This catalyst is highly active at 50 °C but yields formic acid which is formed by the sequential oxidation of methanol. Addition of Cu either in solution or within the catalyst matrix switches off the sequential oxidation and methanol is obtained with very high selectivity. Comments are made concerning the prospects for the use of molecular oxygen as the terminal oxidant which is the preferred route.

Divergent reactivity of platinum(ii) and palladium(ii) methylperoxo complexes and the formation of an unusual hemi-aminal complex.

The 6,6''-diaminoterpyridine palladium(ii) methylperoxo complex eliminates methyl hydroperoxide and reacts with acetone to form a novel hemi-aminal palladium complex, whereas the analogous platinum(ii) complex generates formaldehyde and a platinum(ii) hydroxo complex.

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether.

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + ĊH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OĊHOOH intermediate, which predominantly leads to the HPMF.

Theoretical spectroscopic characterization at low temperatures of methyl hydroperoxide and three S-analogs.

The low temperature spectra of the detectable species methyl hydroperoxide (CH3OOH) and three sulfur analogs, the two isomers of methanesulfenic acid (CH3SOH and CH3OSH) and the methyl hydrogen disulfide (CH3SSH), are predicted from highly correlated ab initio methods (CCSD(T) and CCSD(T)-F12). Rotational parameters, anharmonic frequencies, torsional energy barriers, torsional energy levels, and their splittings are provided. Our computed parameters should help for the characterization and the identification of these organic compounds in laboratory and in the interstellar medium.

Computational studies of atmospherically-relevant chemical reactions in water clusters and on liquid water and ice surfaces.

CONSPECTUS: Reactions on water and ice surfaces and in other aqueous media are ubiquitous in the atmosphere, but the microscopic mechanisms of most of these processes are as yet unknown. This Account examines recent progress in atomistic simulations of such reactions and the insights provided into mechanisms and interpretation of experiments. Illustrative examples are discussed. The main computational approaches employed are classical trajectory simulations using interaction potentials derived from quantum chemical methods. This comprises both ab initio molecular dynamics (AIMD) and semiempirical molecular dynamics (SEMD), the latter referring to semiempirical quantum chemical methods. Presented examples are as follows: (i) Reaction of the (NO(+))(NO3(-)) ion pair with a water cluster to produce the atmospherically important HONO and HNO3. The simulations show that a cluster with four water molecules describes the reaction. This provides a hydrogen-bonding network supporting the transition state. The reaction is triggered by thermal structural fluctuations, and ultrafast changes in atomic partial charges play a key role. This is an example where a reaction in a small cluster can provide a model for a corresponding bulk process. The results support the proposed mechanism for production of HONO by hydrolysis of NO2 (N2O4). (ii) The reactions of gaseous HCl with N2O4 and N2O5 on liquid water surfaces. Ionization of HCl at the water/air interface is followed by nucleophilic attack of Cl(-) on N2O4 or N2O5. Both reactions proceed by an SN2 mechanism. The products are ClNO and ClNO2, precursors of atmospheric atomic chlorine. Because this mechanism cannot result from a cluster too small for HCl ionization, an extended water film model was simulated. The results explain ClNO formation experiments. Predicted ClNO2 formation is less efficient. (iii) Ionization of acids at ice surfaces. No ionization is found on ideal crystalline surfaces, but the process is efficient on isolated defects where it involves formation of H3O(+)-acid anion contact ion pairs. This behavior is found in simulations of a model of the ice quasi-liquid layer corresponding to large defect concentrations in crystalline ice. The results are in accord with experiments. (iv) Ionization of acids on wet quartz. A monolayer of water on hydroxylated silica is ordered even at room temperature, but the surface lattice constant differs significantly from that of crystalline ice. The ionization processes of HCl and H2SO4 are of high yield and occur in a few picoseconds. The results are in accord with experimental spectroscopy. (v) Photochemical reactions on water and ice. These simulations require excited state quantum chemical methods. The electronic absorption spectrum of methyl hydroperoxide adsorbed on a large ice cluster is strongly blue-shifted relative to the isolated molecule. The measured and calculated adsorption band low-frequency tails are in agreement. A simple model of photodynamics assumes prompt electronic relaxation of the excited peroxide due to the ice surface. SEMD simulations support this, with the important finding that the photochemistry takes place mainly on the ground state. In conclusion, dynamics simulations using quantum chemical potentials are a useful tool in atmospheric chemistry of water media, capable of comparison with experiment.

Diagnostic Analysis of the Sulfate Aerosol Pollution in Spring over Pearl River Delta, China

The study of sulfate aerosols has important research significance to atmospheric chemistry and climate change. A diagnostic study on generation mechanism of sulfate aerosol over the PRD region was conducted using sulfate tracking method. Three pollution episodes occurred in March 2012, in which the first two episodes were mostly caused by fine particulate matters and their secondary aerosols were mainly inorganic sulfates, resulting in low visibility. In contrast, the third pollution episode was dominated by coarse particles with relatively greater visibility. The results of sulfate tracking showed that in addition to emission sources and contributions of initial and boundary conditions, sulfate-generating reaction processes were closely correlated with relative humidity. The first two episodes occurred under the influence of southerly warm and wet air flow system with relatively high humidity, in which the conditions were favorable for aqueous reaction processes. The primary type of liquid-phase oxidation was with hydrogen peroxide (H2O2), followed by catalytic oxidation with Fe and Mn and oxidation with ozone (O3), while peroxyacetic acid (PAA) and methylhydroperoxide (MHP) had very small contributions. Meanwhile, the third pollution episode happened during a warming period following passage of a cold front and was influenced by northerly wind system. Consequently, humidity was relatively low, and therefore, contribution of liquid-phase oxidation reactions was weaker while contributions by gas-phase oxidation between S (IV) and hydroxyl radical (OH) became more prominent. In addition, the simulated vertical distribution results showed that the mass concentration of sulfate aerosols decreased with increasing height. For near-surface layers, sulfates were mainly contributed by emission sources, boundary and initial conditions, liquid-phase oxidations with H_2O_2 and catalytic conversions with Fe and Mn; while for upper layers, sulfates were contributed by, in the order of significance, liquid-phase oxidations with H_2O_2, catalytic conversions with Fe and Mn, emission sources and gas-phase oxidations.

Hydrogen peroxide enhances the oxidation of oxygenated volatile organic compounds on mineral dust particles: a case study of methacrolein.

Heterogeneous oxidation of oxygenated volatile organic compounds (OVOCs) serves as an important sink of OVOCs as well as a source of secondary organic material. However, the roles of gas phase oxidants in these reactions are poorly understood. In this work, we present the first laboratory study of the heterogeneous reactions of methacrolein (MACR) on various mineral dust particles in the presence of gaseous H2O2. It is found that the presence of gaseous H2O2 significantly promotes both the uptake and oxidation of MACR on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, but not on CaCO3. The oxidation of MACR produces organic acids as its major low-molecular-weight product, whose yields are enhanced by a factor of 2-6 in the presence of H2O2. In addition, organic peroxides such as methyl hydroperoxide, peroxyformic acid, and peroxyacetic acid are only formed in the presence of H2O2, and the formation of methyl hydroperoxide indicates that MACR oxidation on the surface involves reaction with OH radicals. A probe reaction using salicylic acid verifies the production of OH radicals from H2O2 decomposition on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, which rationalizes the enhanced MACR oxidation observed on these particles. The uptake coefficients of MACR on kaolinite, α-Fe2O3, and TiO2 in the presence of H2O2 are on the order of 10(-5)-10(-4). Our results provide new insights into the formation and chemical evolution of organic species in the atmosphere.

Kinetic nature of blue flames in the autoignition of methane

Kinetic calculations within the framework of detailed reaction mechanisms showed that the blue flames observed during autoignition of methane in an internal combustion engine are combined cool-blue flames. In the course of methane oxidation, methyl hydroperoxide CH3O2H and hydrogen peroxide H2O2 appear and accumulate almost simultaneously. The partial decomposition of these species yields a hydroxyl OH concentration peak, thereby giving rise to a local acceleration of the oxidation reaction, accompanied by luminescence. Kinetic curves for the main products were obtained and the boundaries of cool-blue flames were determined, which turned out to be in satisfactory agreement with the experimental data.

Selective oxidation of hydrocarbons on supported Au catalysts

The importance of proximity of isolated TiOx cluster to Au surface for the formation of acetone in the selective oxidation of propane was investigated using a model system of Au particles decorated with mononuclear TiOx units dispersed in silica clusters. The results showed that no acetone was formed without TiOx, and its formation was more prominent on a sample containing a higher density of TiOx. Covering a Au/TiO2 catalyst with a layer of TiOx-containing silica changed the product selectivity from predominently propene to acetone. The data strongly supported the importance of isolated TiOx clusters near the Au surface in acetone formation, and suggested the relevance of Au-TiOx interface sites. Such interface sites might also be important in selective propene epoxidation that was coupled to CO oxidation in a H2O-methanol mixture. Using oxygen isotope labeling technique, it was found that the proximal source of oxygen in propene oxide was not from H2O but from O2. The observation was consistent with a methyl hydroperoxide intermediate, which could be formed by oxidation of a Ti methoxy at the Au-support interface.

On the transferability of atomic contributions to the optical rotatory power of hydrogen peroxide, methyl hydroperoxide and dimethyl peroxide

The chirality of molecules expresses itself, for example, in the fact that a solution of a chiral molecule rotates the plane of linear polarised light. The underlying molecular property is the optical rotatory power (ORP) tensor, which according to time-dependent perturbation theory can be calculated as mixed linear response functions of the electric and magnetic dipole moment operators. Applying a canonical transformation of the Hamiltonian, which reformulates the magnetic dipole moment operator in terms of the operator for the torque acting on the electrons, the ORP of a molecule can be partitioned into atomic and group contributions. In the present work, we investigate the transferability of such individual contributions in a series of small, chiral molecules: hydrogen peroxide, methyl hydroperoxide and dimethyl peroxide. The isotropic atomic or group contributions have been evaluated for the hydrogen, oxygen and carbon atoms as well as for the methyl group at the level of time-dependent density functional theory with the B3LYP exchange-correlation functional employing a large Gaussian basis set. We find that the atomic or group contributions are not transferable among these three molecules.

Investigating the photo-oxidative and heterogeneous chemical production of HCHO in the snowpack at the South Pole, Antarctica

Environmental context Snowpacks present a surprisingly active environment for photochemistry, leading to sunlight-induced oxidation of deposited organic matter and the subsequent emission of a variety of photochemically active trace gases. We seek to address questions regarding the ultimate fate of organic matter deposited onto snow in the remote regions of the world. The work is relevant to atmospheric composition and climate change. Abstract We investigate snowpack fluxes of formaldehyde (HCHO) into the South Pole boundary layer using steady-state photochemical models. We study two chemical sources of HCHO within the snowpack. First, we study chemical production of HCHO from the processing of methyl hydroperoxide (CH3OOH): photolysis, reaction with the hydroxyl radical (OH•), and by an acid catalysed rearrangement. Assuming surface layer concentration effects for acidic solutes, we show that the acid catalysed production of HCHO within ice could contribute a non-negligible source to the snowpack HCHO budget. This novel source of HCHO complements existing explanations of HCHO fluxes based on physical emission of HCHO from snow. Secondly, we investigate HCHO production from the oxidation of organic matter (OM) by OH• within snow to explain observed fluxes of photochemical origin from the South Pole snowpack. This work shows that laboratory-derived photochemical production rates of HCHO and our standard model are inconsistent with field observations, which has implications for the distribution of OM relative to oxidants within ice particles. We resolve this inconsistency using new laboratory measurements of the molecular dynamics of the OH• photofragment from hydrogen peroxide (H2O2) and nitrate (NO3–) photolysis, which show that only OH produced in the outermost monolayers can contribute to gas phase and surface layer chemistry. Using these new measurements in conjunction with realistic treatments of ice grain size, H2O2 and NO3– distribution within ice grains, diffusion of gas species within solid ice, and observed OM particle size distributions yields snowpack HCHO photochemical production rates more consistent with observations.