Computational studies on chemical ionization and reaction rates of atmospherically relevant oxidized multifunctional compounds

High pressure chemical ionization has recently been used with mass spectrometers to measure atmospheric molecules and molecule clusters. In anion chemical ionization, negatively charged reagent ions ionize the neutral sample molecules (or clusters) mainly by forming ion-molecule clusters. The detection of neutral molecules is highly dependent on how effective the chemical ionization process-es are, since the mass spectrometers can only detect charged molecules or clusters. This causes uncertainties in the measurements of most atmospheric trace gas molecules. In addition, mass spectrometers are able to detect only the molecular mass of the sample molecules, which means that other methods are needed to find the molecular structures of the detected compounds. In this work, we take a look at how quantum chemistry can be used to model different chemical ionization processes in a typical chemical ionization instrument, and to calculate reaction rates for unimolecular gas-phase reactions. With our computations, we were able to explain some of the experimental observations regarding the differences in the detection efficiencies of some reagent ions. The cause for the low detection efficiency of some sample molecules was found to be less stable ion-molecule clusters. Our calculations showed an increasing cluster stability for each of the studied reagent anions with the increase of the number of oxygen atoms in the sample molecule. This means that less oxygenated molecules generally have lower detection efficiencies than the more oxygenated ones. In addition, the computed reaction rate coefficients of two different unimolecular HO2 loss reaction mechanisms showed that, due to collisional stabilization, this reaction is too slow to compete with bimolecular reactions under atmospheric conditions, especially if the reactant is an oxygenated organic molecule.Maan ilmakehä sisältää typen, hapen ja hiilidioksidin lisäksi muun muassa suuren määrän erilaisia orgaanisia yhdisteitä. Aerosolihiukkasten muodostumisen kannalta erityisen mielenkiintoisia yhdisteitä ovat paljon happea sisältävät, alhaisen haihtuvuuden omaavat molekyylit. Nämä yhdisteet muodostuvat helposti haituvien molekyylien hapettumisreaktioiden tuotteina kaasufaasissa. Tuotteita voidaan mitata suoraan kaasufaasista käyttämällä massaspektrometriaa, mutta neutraalit molekyylit on varattava, ennen kuin niitä pystytään mittaamaan. Väitöskirjassa käydään läpi, kuinka laskennallisia menetelmiä on käytetty sekä hapettumisreaktioiden tutkimiseen että molekyylien varausmekanismien mallintamiseen

Solubility and Activity Coefficients of Atmospheric Surfactants in Aqueous Solution Evaluated Using COSMOtherm

Fatty acids (CH3(CH2)(n-2)COOH) and their salts are an important class of atmospheric surfactants. Here, we use COSMOtherm to predict solubility and activity coefficients for C-2-C-12 fatty acids with even number of carbon atoms and their sodium salts in binary water solutions and also in ternary water-inorganic salt solutions. COSMOtherm is a continuum solvent model implementation which can calculate properties of complex systems using quantum chemistry and thermodynamics. Calculated solubility values of the organic acids in pure water are in good agreement with reported experimental values. The comparison of the COSMOtherm-derived Setschenow constants for ternary solutions comprising NaCl with the corresponding experimental values from the literature shows that COSMOtherm overpredicts the salting out effect in all cases except for the solutions of acetic acid. The calculated activity and mean activity coefficients of fatty acids and fatty acid sodium salts, respectively, show deviation of the systems from ideal solution. The computed mean activity coefficients of the fatty acid salts in binary systems are in better agreement with experimentally derived values for the organic salts with longer aliphatic chain (C-8-C-10). The deviation of the solutions from ideality could lead to biased estimations of cloud condensation nuclei number concentrations if not considered in Kohler calculations and cloud microphysics.Peer reviewe

NH4+ Association and Proton Transfer Reactions With a Series of Organic Molecules

In this study, we present reactions of NH4+ with a series of analytes (A): acetone (C3H6O), methyl vinyl ketone (C4H6O), methyl ethyl ketone (C4H8O) and eight monoterpene isomers (C10H16) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 meV and 80 meV. The ketones, having a substantially lower proton affinity than NH3, produce only cluster ions NH4+(A) in detectable amounts at 55 meV. At 80 meV, no cluster ions were detected meaning that these adduct ions are formed by strongly temperature dependent association reactions. Bond energies of cluster ions and proton affinities for most monoterpenes are not known and were estimated by high level quantum chemical calculations. The calculations reveal monoterpene proton affinities, which range from slightly smaller to substantially higher than the proton affinity of NH3. Proton affinities and cluster bond energies allow to group the monoterpenes as a function of the enthalpy for the dissociation reaction . We find that this enthalpy can be used to predict the NH4+-A cluster ion yield. The present study explains product ion formation involving NH4+ ion chemistry. This is of importance for chemical ionization mass spectrometry (CIMS) utilizing NH4+ as well as NH4+(H2O) as reagent ions to quantitatively detect atmospherically important organic compounds in real-time.Peer reviewe

Technical note : Estimating aqueous solubilities and activity coefficients of mono- and alpha,omega-dicarboxylic acids using COSMOtherm

We have used the COSMOtherm program to estimate activity coefficients and solubilities of mono- and alpha, omega-dicarboxylic acids and water in binary acid-water systems. The deviation from ideality was found to be larger in the systems containing larger acids than in the systems containing smaller acids. COnductor-like Screening MOdel for Real Solvents (COSMO-RS) underestimates experimental monocarboxylic acid activity coefficients by less than a factor of 2, but experimental water activity coefficients are underestimated more especially at high acid mole fractions. We found a better agreement between COSMOtherm-estimated and experimental activity coefficients of monocarboxylic acids when the water clustering with a carboxylic acid and itself was taken into account using the dimerization, aggregation, and reaction extension (COSMO-RS-DARE) of COSMOtherm. COSMO-RS-DARE is not fully predictive, but fit parameters found here for water-water and acid-water clustering interactions can be used to estimate thermodynamic properties of monocarboxylic acids in other aqueous solvents, such as salt solutions. For the dicarboxylic acids, COSMO-RS is sufficient for predicting aqueous solubility and activity coefficients, and no fitting to experimental values is needed. This is highly beneficial for applications to atmospheric systems, as these data are typically not available for a wide range of mixing states realized in the atmosphere, due to a lack of either feasibility of the experiments or sample availability. Based on effective equilibrium constants of different clustering reactions in the binary solutions, acid dimer formation is more dominant in systems containing larger dicarboxylic acids (C-5-C-8), while for monocarboxylic acids (C-1-C-6) and smaller dicarboxylic acids (C-2-C-4), hydrate formation is more favorable, especially in dilute solutions.Peer reviewe

Computational Comparison of Acetate and Nitrate Chemical Ionization of Highly Oxidized Cyclohexene Ozonolysis Intermediates and Products

During the past few years nitrate chemical ionization has been used to detect highly oxidized products from OH-and O-3-initiated alkene autoxidation. These have been speculated to play a significant role in atmospheric aerosol formation. As less oxidized autoxidation products have not been detected using nitrate chemical ionization, and the absolute concentrations of the highly oxidized species are as yet unknown, other reagent ions, such as acetate, are needed both to verify the detection efficiency of nitrate chemical ionization and to measure the less oxidized compounds. Here we compare the formation free energies of the acetate and nitrate clusters of several atmospherically relevant RO2 intermediates and products derived from cyclohexene ozonolysis, calculated at the omega B97xD/aug-cc-pVTZ level of theory. We found that, for the molecules with one hydrogen bonding peroxy acid group, the binding with nitrate is on average 7.5 kcal/mol weaker than with acetate and the binding is on average 10.5 kcal/mol weaker for molecules with two hydrogen bonding peroxy acid groups. We also calculated the deprotonation energies of the RO2 intermediates and the closed-shell products and found that acetate is able to deprotonate almost all of these molecules, while deprotonation with nitrate is (as expected for the conjugate base of a strong acid) not favorable.Peer reviewe

First oxidation products from the reaction of hydroxyl radicals with isoprene for pristine environmental conditions

Isoprene, C5H8, inserts about half of the non-methane carbon flux of biogenic origin into the atmosphere. Its degradation is primarily initiated by the reaction with hydroxyl radicals. Here we show experimentally the formation of reactive intermediates and corresponding closedshell products from the reaction of hydroxyl radicals with isoprene for low nitric oxide and low hydroperoxy radical conditions. Detailed product analysis is achieved by mass spectrometric techniques. Quantum chemical calculations support the usefulness of applied ionization schemes. Observed peroxy radicals are the isomeric HO-C5H8O2 radicals and their isomerization products HO-C5H8(O-2)O-2, bearing most likely an additional hydroperoxy group, and in traces HO-C5H8(O-2)(2)O-2 with two hydroperoxy groups. Main closed-shell products from unimolecular peroxy radical reactions are hydroperoxy aldehydes, C5H8O3, and smaller yield products with the composition C5H8O4 and C4H8O5. Detected signals of C10H18O4, C10H18O6, and C5H10O2 stand for products arising from peroxy radical self- and cross-reactions.Peer reviewe

Modeling the Charging of Highly Oxidized Cyclohexene Ozonolysis Products Using Nitrate-Based Chemical Ionization

Several extremely low volatility organic compounds (ELVOCs) formed in the ozonolysis of endocyclic alkenes have recently been detected in laboratory and field studies. These experiments have been carried out with chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOP) with nitrate ions as reagent ions. The nitrate ion binds to the detected species through hydrogen bonds, but it also binds very strongly to one or two neutral nitric acid molecules. This makes the measurement highly selective when there is an excess amount of neutral nitric acid in the instrument. In this work, we used quantum-chemical methods to calculate the binding energies between a nitrate ion and several highly oxidized ozonolysis products of cydohexene. These were then compared with the binding energies of nitrate ion nitric acid clusters. Systematic configurational sampling of the molecules and clusters was carried out at the B3LYP/6-31+G* and omega B97xD/aug-cc-pVTZ levels, and the final single-point energies were calculated with DLPNO-CCSD(T)/def2-QZVPP. The binding energies were used in a kinetic simulation of the measurement system to determine the relative ratios of the detected signals. Our results indicate that at least two hydrogen bond donor functional groups (in this case, hydroperoxide, OOH) are needed for an ELVOC molecule to be detected in a nitrate ion CI-APi-TOP. Also, a double bond in the carbon backbone makes the nitrate cluster formation less favorable.Peer reviewe

Temporal and Spatial Variation in Scots Pine Resin Pressure and Composition

Resin is a first-line defense in pine trees, but important questions regarding its temporal and spatial variation remain unsolved. Resin pressure varies according to water potential in dry conditions, but in moist conditions, it follows temperature dynamics for a yet unknown reason. Relations between resin composition, resin pressure, and shoot monoterpene emissions are also unquantified. To gain mechanistic understanding on the resin dynamics in a boreal forest, we measured temperature and water potential dependency of Scots pine resin pressure. We attempted to quantify the temperature dependency of resin pressure in terms of three contributions: 1) saturation vapor pressure, 2) thermal expansion, and 3) N2, O2, and CO2 solubility. We also analyzed monoterpene composition in the resin and the shoot emissions of 16 pines with gas chromatography mass spectrometry to study their interrelations. We show that in moist conditions, resin pressure is driven by temperature at a diurnal scale, but also affected by soil water potential at a day-to-day scale. Diurnal temperature dependency was explained by thermal expansion of resin and changes in bubble volume due to changes in gas solubility in resin with temperature. Resin pressures correlated also with total monoterpene and α-pinene content in resin and with total monoterpene and ∆3-carene and terpinolene emissions from shoots.Resin is a first-line defense in pine trees, but important questions regarding its temporal and spatial variation remain unsolved. Resin pressure varies according to water potential in dry conditions, but in moist conditions, it follows temperature dynamics for a yet unknown reason. Relations between resin composition, resin pressure, and shoot monoterpene emissions are also unquantified. To gain mechanistic understanding on the resin dynamics in a boreal forest, we measured temperature and water potential dependency of Scots pine resin pressure. We attempted to quantify the temperature dependency of resin pressure in terms of three contributions: (1) saturation vapor pressure, (2) thermal expansion, and (3) N2, O2, and CO2 solubility. We also analyzed monoterpene composition in the resin and the shoot emissions of 16 pines with gas chromatography mass spectrometry to study their interrelations. We show that in moist conditions, resin pressure is driven by temperature at a diurnal scale, but also affected by soil water potential at a day-to-day scale. Diurnal temperature dependency was explained by thermal expansion of resin and changes in bubble volume due to changes in gas solubility in resin with temperature. Resin pressures correlated also with total monoterpene and α-pinene content in resin and with total monoterpene and Δ3-carene and terpinolene emissions from shoots.Peer reviewe

Gas-to-Particle Partitioning of Cyclohexene- and alpha-Pinene-Derived Highly Oxygenated Dimers Evaluated Using COSMOtherm

Oxidized organic compounds are expected to contribute to secondary organic aerosol (SOA) if they have sufficiently low volatilities. We estimated saturation vapor pressures and activity coefficients (at infinite dilution in water and a model water-insoluble organic phase) of cyclohexene- and alpha-pinene-derived accretion products, "dimers", using the COSMOtherm19 program. We found that these two property estimates correlate with the number of hydrogen bond-donating functional groups and oxygen atoms in the compound. In contrast, when the number of H-bond donors is fixed, no clear differences are seen either between functional group types (e.g., OH or OOH as H-bond donors) or the formation mechanisms (e.g., gas-phase radical recombination vs liquid-phase closed-shell esterification). For the cyclohexene-derived dimers studied here, COSMOtherm19 predicts lower vapor pressures than the SIMPOL.1 group-contribution method in contrast to previous COSMOtherm estimates using older parameterizations and nonsystematic conformer sampling. The studied dimers can be classified as low, extremely low, or ultra-low-volatility organic compounds based on their estimated saturation mass concentrations. In the presence of aqueous and organic aerosol particles, all of the studied dimers are likely to partition into the particle phase and thereby contribute to SOA formation.Peer reviewe

Computational and Experimental Investigation of the Detection of HO2 Radical and the Products of Its Reaction with Cyclohexene Ozonolysis Derived RO2 Radicals by an Iodide-Based Chemical Ionization Mass Spectrometer

The HO2 radical is an important atmospheric molecule that can potentially influence the termination of autoxidation processes of volatile organic compounds (VOCs) that lead to the formation of highly oxygenated multifunctional compounds (HOMs). In this work, we demonstrate the direct detection of the HO2 radical using an iodide-based chemical ionization mass spectrometer (iodide-CIMS). Expanding on the previously established correlation between molecule-iodide binding enthalpy and iodide-CIMS instrument sensitivity, the experimental detection of the HO2 radical was preceded by the quantum chemical calculation of the HO2*I- cluster (PBE/aug-cc-pVTZ-PP level), which showed a reasonably strong binding enthalpy of 21.60 kcal/mol. Cyclohexene ozonolysis intermediates and closed-shell products were next detected by the iodide-CIMS. The ozone-initiated cyclohexene oxidation mechanism was perturbed by the introduction of the HO2 radical, leading to the formation of closed-shell hydroperoxides. The experimental investigation once again followed the initial computational molecule-iodide binding enthalpy calculations. The quantum chemical calculations were performed at the PBE/aug-cc-pVTZ-PP level for radicals and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP level for the closed-shell products. A comparison between the iodide-CIMS and nitrate-CIMS spectra with identical measurement steps revealed that the iodide-CIMS was able to detect the low-oxidized (O/C ratio 0.5 and 0.66) cyclohexene ozonolysis monomer products more efficiently than nitrate-CIMS. Higher-oxidized monomers (O/C ratio 1 to 1.5) were detected equally well by both methods. An investigation of dimers showed that both iodide- and nitrate-CIMS were able to detect the dimer compositions possibly formed from reactions between the peroxy radical monomers considered in this study. Additionally, iodide-CIMS detected organic ions that were formed by a previously suggested mechanism of dehydroxylation of peroxy acids (and deoxygenation of acyl peroxy radicals) by H2O*I- clusters. These mechanisms were computationally verified.Peer reviewe