Evidence that Criegee intermediates drive autoxidation in unsaturated lipids.

Autoxidation is an autocatalytic free-radical chain reaction responsible for the oxidative destruction of organic molecules in biological cells, foods, plastics, petrochemicals, fuels, and the environment. In cellular membranes, lipid autoxidation (peroxidation) is linked with oxidative stress, age-related diseases, and cancers. The established mechanism of autoxidation proceeds via H-atom abstraction through a cyclic network of peroxy-hydroperoxide-mediated free-radical chain reactions. For a series of model unsaturated lipids, we present evidence for an autoxidation mechanism, initiated by hydroxyl radical (OH) addition to C=C bonds and propagated by chain reactions involving Criegee intermediates (CIs). This mechanism leads to unexpectedly rapid autoxidation even in the presence of water, implying that as reactive intermediates, CI could play a much more prominent role in chemistries beyond the atmosphere.

Heterogeneous Ozonolysis of Squalene: Gas-Phase Products Depend on Water Vapor Concentration.

Previous work examining the condensed-phase products of squalene particle ozonolysis found that an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and smaller particle diameter, suggesting that water changes the fate of the Criegee intermediate. To determine if this volume loss corresponds to an increase in gas-phase products, we measured gas-phase volatile organic compound (VOC) concentrations via proton-transfer-reaction time-of-flight mass spectrometry. Studies were conducted in a flow-tube reactor at atmospherically relevant ozone (O3) exposure levels (5-30 ppb h) with pure squalene particles. An increase in water vapor concentration led to strong enhancement of gas-phase oxidation products at all tested O3 exposures. An increase in water vapor from near zero to 70% relative humidity (RH) at high O3 exposure increased the total mass concentration of gas-phase VOCs by a factor of 3. The observed fraction of carbon in the gas-phase correlates with the fraction of particle volume lost. Experiments involving O3 oxidation of shirts soiled with skin oil confirms that the RH dependence of gas-phase reaction product generation occurs similarly on surfaces containing skin oil under realistic conditions. Similar behavior is expected for O3 reactions with other surface-bound organics containing unsaturated carbon bonds.

Multiphase Mechanism for the Production of Sulfuric Acid from SO2 by Criegee Intermediates Formed During the Heterogeneous Reaction of Ozone with Squalene.

Here we report a new multiphase reaction mechanism by which Criegee intermediates (CIs), formed by ozone reactions at an alkene surface, convert SO2 to SO3 to produce sulfuric acid, a precursor for new particle formation (NPF). During the heterogeneous ozone reaction, in the presence of 220 ppb SO2, an unsaturated aerosol (squalene) undergoes rapid chemical erosion, which is accompanied by NPF. A kinetic model predicts that the mechanism for chemical erosion and NPF originate from a common elementary step (CI + SO2) that produces both gas phase SO3 and small ketones. At low relative humidity (RH = 5%), 20% of the aerosol mass is lost, with 17% of the ozone-surface reactions producing SO3. At RH = 60%, the aerosol shrinks by 30%, and the yield of SO3 is <5%. This multiphase formation mechanism of H2SO4 by CIs is discussed in the context of indoor air quality and atmospheric chemistry.

Connecting the Elementary Reaction Pathways of Criegee Intermediates to the Chemical Erosion of Squalene Interfaces during Ozonolysis.

Criegee intermediates (CI), formed in alkene ozonolysis, are central for controlling the multiphase chemistry of organic molecules in both indoor and outdoor environments. Here, we examine the heterogeneous ozonolysis of squalene, a key species in indoor air chemistry. Aerosol mass spectrometry is used to investigate how the ozone (O3) concentration, relative humidity (RH), and particle size control reaction rates and mechanisms. Although the reaction rate is found to be independent of RH, the reaction products and particle size depend upon H2O. Under dry conditions (RH = 3%) the reaction produces high-molecular-weight secondary ozonides (SOZ), which are known skin irritants, and a modest change in particle size. Increasing the RH reduces the aerosol size by 30%, while producing mainly volatile aldehyde products, increases potential respiratory exposure. Chemical kinetics simulations link the elementary reactions steps of CI to the observed kinetics, product distributions, and changes in particle size. The simulations reveal that ozonolysis occurs near the surface and is O3-transport limited. The observed secondary ozonides are consistent with the formation of mainly secondary CI, in contrast to gas-phase ozonolysis mechanisms.

Probing the Heterogeneous Ozonolysis of Squalene Nanoparticles by Photoemission.

The heterogeneous reaction of ozone (O3) with 200 nm squalene nanoparticles is studied using near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet (UPS) and X-ray photoelectron spectroscopy (XPS). Photoelectrons are detected from free nanoparticle beams using a velocity map imaging (VMI) spectrometer capable of detecting photoelectrons with up to 40 eV of kinetic energy. Heterogeneous kinetics are quantified using changes in the UPS, XPS, and NEXAFS spectrum, yielding uptake coefficients for the decay of the double bonds in squalene of (3.1 ± 0.7) × 10-4, (2.6 ± 0.6) × 10-4, and (2.9 ± 0.7) × 10-4, respectively. When comparing these values with the uptake coefficient, (1.0 ± 0.2) × 10-3, determined by the molecular decay of squalene measured with aerosol mass spectrometry, it is found that on average 1.6 ± 0.2 double bonds are removed for each ozone-squalene reactive collision, suggesting the importance of evaporation of small molecular weight reaction products from the aerosol. From further analysis of the nanoparticle XPS spectrum, it is found that ozonolysis increases the oxygen-to-carbon (O:C) ratio of the aerosol to 0.43 ± 0.03 and produces 16 ± 4% and 84 ± 4% secondary ozonides and carbonyls, respectively. The methods developed here show how aerosol photoemission can be used to quantify heterogeneous reaction on free nanoparticles.

Gas phase vibrational spectroscopy of the protonated water pentamer: the role of isomers and nuclear quantum effects.

We use cryogenic ion trap vibrational spectroscopy to study the structure of the protonated water pentamer, H+(H2O)5, and its fully deuterated isotopologue, D+(D2O)5, over nearly the complete infrared spectral range (220-4000 cm-1) in combination with harmonic and anharmonic electronic structure calculations as well as RRKM modelling. Isomer-selective IR-IR double-resonance measurements on the H+(H2O)5 isotopologue establish that the spectrum is due to a single constitutional isomer, thus discounting the recent analysis of the band pattern in the context of two isomers based on AIMD simulations 〈W. Kulig and N. Agmon, Phys. Chem. Chem. Phys., 2014, 16, 4933-4941〉. The evolution of the persistent bands in the D+(D2O)5 cluster allows the assignment of the fundamentals in the spectra of both isotopologues, and the simpler pattern displayed by the heavier isotopologue is consistent with the calculated spectrum for the branched, Eigen-based structure originally proposed 〈J.-C. Jiang, et al., J. Am. Chem. Soc., 2000, 122, 1398-1410〉. This pattern persists in the vibrational spectra of H+(H2O)5 in the temperature range from 13 K up to 250 K. The present study also underscores the importance of considering nuclear quantum effects in predicting the kinetic stability of these isomers at low temperatures.

Probing the microsolvation of a quaternary ion complex: gas phase vibrational spectroscopy of (NaSO4(-))2(H2O)(n=0-6, 8).

We report infrared multiple photon dissociation spectra of cryogenically-cooled (NaSO4(-))2(H2O)n dianions (n = 0-6, 8) in the spectral range of the sulfate stretching and bending modes (580-1750 cm(-1)). Characteristic absorption bands and structural trends are identified based on a comparison to harmonic spectra of minimum-energy structures. The bare quarternary complex (NaSO4(-))2 exhibits a C2h structure containing two fourfold-coordinated sodium cations in-between the two chelating sulfate dianions. Its stepwise solvation is driven by an interplay of SO4(2-)-H2O and Na(+)-H2O interactions. The first water binds in a tridentate intersulfate-bridging fashion. The second and third water molecules bind to the sulfate groups at either end of the complex, which is followed by the onset of water hydrogen-bond network formation. In contrast to the binary ion pair, NaSO4(-), no clear evidence for the disruption of the quaternary ion complex upon microhydration is found up to n = 8, underlining its remarkable stability and suggesting that the formation of quaternary ion complexes plays a central role in the initial stages of prenucleation in aqueous Na2SO4 solutions.

Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer.

We use infrared/infrared double-resonance population labeling (IR(2)MS(2)) spectroscopy in the spectral region of the free and hydrogen-bonded OH stretching fundamentals (2880-3850 cm(-1)) to identify the number and to isolate the vibrational signatures of individual isomers contributing to the gas-phase IR spectra of the cryogenically cooled protonated water clusters H(+)(H2O)n·H2/D2 with n = 7-10. For n = 7, four isomers are identified and assigned. Surprisingly, the IR(2)MS(2) spectra of the protonated water octa-, nona-, and decamer show no evidence for multiple isomers. The present spectra support the prediction that the quasi-2D to 3D structural transition occurs in between n = 8 and 9 in the cold cluster regime. However, the same models have difficulty explaining the remarkable size dependence of the isomer population reported here.

Microhydrated dihydrogen phosphate clusters probed by gas phase vibrational spectroscopy and first principles calculations.

We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H2PO4(-)(H2O)n anions (n = 2-12) in the spectral range of the stretching and bending modes of the solute anion (600-1800 cm(-1)). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogen bonding to the two terminal P[double bond, length as m-dash]O groups, these are hydrated before the two P-OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H2PO4(-)(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H2PO4(-), provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.

Thermodynamics of water dimer dissociation in the primary hydration shell of the iodide ion with temperature-dependent vibrational predissociation spectroscopy.

The strong temperature dependence of the I(-)·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van't Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I(-)·(H2O)2 system sample the extended potential energy surface with increasing temperature.

Site-specific vibrational spectral signatures of water molecules in the magic H3O+ (H2O)20 and Cs+ (H2O)20 clusters.

Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, along with isotopic substitution, to quench water clusters attached to the H3O(+) and Cs(+) ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm(-1) range. The magic H3O(+)(H2O)20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.

Spectroscopic identification of a bidentate binding motif in the anionic magnesium-CO2 complex ([ClMgCO2 ](-)).

A magnesium complex incorporating a novel metal-CO2 binding motif is spectroscopically identified. Here we show with the help of infrared photodissociation spectroscopy that the complex exists solely in the [ClMg(η(2) -O2 C)](-) form. This bidentate double oxygen metal-CO2 coordination has previously not been observed in neutral nor in charged unimetallic complexes. The antisymmetric CO2 stretching mode in [ClMg(η(2) -O2 C)](-) is found at 1128 cm(-1) , which is considerably redshifted from the corresponding mode in bare CO2 at 2349 cm(-1) , suggesting that the CO2 moiety has a considerable negative charge (∼1.8 e(-) ). We also employed electronic structure calculations and kinetic analysis to support the interpretation of the experimental results.

Vibrational spectroscopy of the water-nitrate complex in the O-H stretching region.

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.

Infrared photodissociation spectroscopy of microhydrated nitrate-nitric acid clusters NO3(-)(HNO3)(m)(H2O)(n).

Infrared multiple photon dissociation (IRMPD) spectra of NO3(-)(HNO3)m(H2O)n(H2)z with m = 1-3, up to n = 8 and z ≥ 1, are measured in the fingerprint region (550-1880 cm(-1)), directly probing the NO-stretching modes, as well as bending and other lower frequency modes. The assignment of the spectra is aided by electronic structure calculations. The IRMPD spectrum of the m = 1, n = 0 cluster is distinctly different from all the other measured spectra as a result of strong hydrogen bonding, leading to an equally shared proton in between two nitrate moieties (O2NO(-)···H(+)···ONO2(-)). It exhibits a strong absorption at 877 cm(-1) and lacks the characteristic NO2-antisymmetric stretching/NOH-bending mode absorption close to 1650 cm(-1). Addition of at least one more nitric acid molecule or two more water molecules weakens the hydrogen bond network, breaking the symmetry of this arrangement and leading to localization of the proton near one of the nitrate cores, effectively forming HNO3 hydrogen-bonded to NO3(-). Not all IR active modes are observed in the IRMPD spectra of the bare nitrate-nitric acid clusters. Addition of a water or a hydrogen molecule lowers the dissociation limit of the complexes and relaxes (H2O) or lifts (H2) this IRMPD transparency.

Large amplitude motion in cold monohydrated dihydrogen phosphate anions H2PO4(-)(H2O): infrared photodissociation spectroscopy combined with ab initio molecular dynamics simulations.

The vibrational spectroscopy of monohydrated dihydrogen phosphate anions, H2PO4(-)(H2O), is studied in the O-H stretching (2700-3900 cm(-1)) and the fingerprint regions (600-1800 cm(-1)). Assignment of the experimental infrared multiple photon photodissociation spectra based on the predicted harmonic spectra of energetically low-lying 0 K structures is not conclusive. Ab initio molecular dynamics simulations reveal that the water molecule undergoes large amplitude motion, even at low internal temperatures, and that the dipole time correlation function qualitatively captures the anharmonic effects of the low-barrier isomerization reaction on the infrared intensities.

Isomer-selective detection of hydrogen-bond vibrations in the protonated water hexamer.

The properties of hydrogen ions in aqueous solution are governed by the ability of water to incorporate ions in a dynamical hydrogen bond network, characterized by a structural variability that has complicated the development of a consistent molecular level description of H(+)(aq). Isolated protonated water clusters, H(+)(H2O)n, serve as finite model systems for H(+)(aq), which are amenable to highly sensitive and selective gas phase spectroscopic techniques. Here, we isolate and assign the infrared (IR) signatures of the Zundel-type and Eigen-type isomers of H(+)(H2O)6, the smallest protonated water cluster for which both of these characteristic binding motifs coexist, down into the terahertz spectral region. We use isomer-selective double-resonance population labeling spectroscopy on messenger-tagged H(+)(H2O)6·H2 complexes from 260 to 3900 cm(-1). Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers and allow attributing the increased width of IR bands associated with H-bonded moieties to anharmonicities rather than excited state lifetime broadening. Characteristic hydrogen-bond stretching bands are observed below 400 cm(-1).

Vibrational spectroscopy of bisulfate/sulfuric acid/water clusters: structure, stability, and infrared multiple-photon dissociation intensities.

The structure and stability of mass-selected bisulfate, sulfuric acid, and water cluster anions, HSO4(-)(H2SO4)m(H2O)n, are studied by infrared photodissociation spectroscopy aided by electronic structure calculations. The triply hydrogen-bound HSO4(-)(H2SO4) configuration appears as a recurring motif in the bare clusters, while incorporation of water disrupts this stable motif for clusters with m > 1. Infrared-active vibrations predominantly involving distortions of the hydrogen-bound network are notably missing from the infrared multiple-photon dissociation (IRMPD) spectra of these ions but are fully recovered by messenger-tagging the clusters with H2. A simple model is used to explain the observed "IRMPD transparency".

Communication: Vibrational spectroscopy of atmospherically relevant acid cluster anions: bisulfate versus nitrate core structures.

Infrared multiple photon dissociation spectra for the smallest atmospherically relevant anions of sulfuric and nitric acid allow us to characterize structures and distinguish between clusters with a bisulfate or a nitrate core. We find that bisulfate is the main charge carrier for HSO(4)(-)·H(2)SO(4)·HNO(3) but not for NO(3)(-)·H(2)SO(4)·HNO(3). For the mixed dimer anion, we find evidence for the presence of two isomers: HSO(4)(-)·HNO(3) and NO(3)(-)·H(2)SO(4). Density functional calculations accompany the experimental results and provide support for these observations.

Time-resolved photoelectron spectroscopy of liquids.

We present a novel setup for the investigation of ultrafast dynamic processes in a liquid jet using time-resolved photoelectron spectroscopy. A magnetic-bottle type spectrometer with a high collection efficiency allows the very sensitive detection of photoelectrons emitted from a 10 µm thick liquid jet. This translates into good signal/noise ratio and rapid data acquisition making femtosecond time-resolved experiments feasible. We describe the experimental setup, a detailed spectrometer characterization based on the spectroscopy of nitric oxide in the gas phase, and results from femtosecond time-resolved experiments on sodium iodide solutions. The latter experiments reveal the formation and evolution of the solvated electron and we characterize two distinct spectral components corresponding to initially thermalized and unthermalized solvated electrons. The absence of dark states in photoionization, the direct measurement of electron binding energies, and the ability to resolve dynamic processes on the femtosecond time scale make time-resolved photoelectron spectroscopy from the liquid jet a very promising method for the characterization of photochemical processes in liquids.

Time-resolved photoelectron spectroscopy of solvated electrons in aqueous NaI solution.

Time-resolved photoelectron spectroscopy was used to study the energetics and dynamics of solvated electrons in aqueous solution. Solvated electrons are generated by ultrafast photodetachment in a 100 mM aqueous NaI solution. Initially, an ensemble of strongly bound ("cold") solvated electrons and an ensemble of weakly bound ("hot") electrons in an unequilibrated solvent environment are observed. We report an ultrafast recombination channel for the "hot" electrons with a rate of (800 fs)(-1) which is in competition with thermalization occurring with a rate of (1.1 ps)(-1). The thermalized electrons recombine with the iodide radical with a rate of (22 ps)(-1). About 35% of the thermalized electrons escape geminate recombination and form free, solvated electrons. The vertical detachment energy for the solvated electron is determined to be 3.40 eV. No indication for a surface-bound electron at lower binding energies was observed.