Physicochemical Properties of Hydrated Complexes in the Earth's Atmosphere

Clusters in a Saturated Vapor: Pressure-Based Temperature Enhancement of the Cluster Fraction

Recent computational findings of temperature increase of clustering degree in different saturated vapors are analyzed. A thermodynamic proof is given and a simple criterion suggested. Illustrations are presented with argon, water, and carbon vapor. Relationships to the fullerene stability island are discussed, too.

Weakly bound molecules in the atmosphere: a case study of HOOO

Weakly bound molecules--particularly hydrated complexes of abundant atmospheric species--have long been postulated to play an important role in atmospherically relevant reactions. For example, such complexes could seed cloud formation and alter the global radiation budget. In this Account, we initially describe the current data on weakly bound species produced in association reactions of the hydroxyl radical (OH) with molecular partners, particularly oxygen (O(2)), nitric acid (HONO(2)), and nitrogen dioxide (NO(2)). Researchers have identified weakly bound association products of these reactions as the hydrogen trioxy (HOOO) radical, the doubly hydrogen-bonded OH-HONO(2) complex, and peroxynitrous acid (HOONO), respectively. In each case, previous kinetic studies of the reaction or OH vibrational relaxation processes have indicated unusual, non-Arrhenius behavior. Under the temperature-pressure conditions of the Earth's lower atmosphere, these processes exhibit a negative temperature dependence, indicative of an attractive interaction, or a pressure dependence. Researchers have subsequently carried out extensive theoretical studies of the properties of these weakly bound molecules, but the theoretical studies have lacked experimental validation. Next, we describe experimental studies to determine the vibrational frequencies and stability of HOOO as a prototypical example of these weakly bound molecules. We then use these data to assess its importance in the atmosphere. We discuss the efficient production of the HOOO radical from OH and O(2) under laboratory conditions and its subsequent detection using infrared action spectroscopy, a highly sensitive and selective double resonance technique. Using excitation of OH stretch and combination bands comprising OH stretch with lower frequency modes, we obtain detailed spectroscopic information on the vibrational modes of the two conformers of HOOO. In addition, we infer fundamental information about the dissociation dynamics from the OH product state distribution, which provides insight into the chemical bonding in HOOO. Perhaps most importantly, we utilize a simple conservation of energy relationship based on the highest energetically open OH product state to derive a rigorous upper limit for the stability of HOOO relative to the OH + O(2) asymptote of 5.3 kcal mol(-1). When combined with previous experimental rotational constants that reflect the structure of the HOOO radical, our laboratory characterization of its stability and vibrational frequencies provides critical information to assess its thermochemical properties. Using standard statistical mechanics approaches, we can calculate the likely atmospheric abundance of HOOO. We estimate that up to 25% of the OH radicals in the vicinity of the tropopause may be associated with O(2) as a weakly bound molecule.

High-Resolution Microwave Studies of Ring-Structured Complexes between Trifluoroacetic Acid and Water

The rotational spectra of the complexes between one trifluoroacetic acid molecule and up to three water molecules have been recorded using a pulsed nozzle Fourier transform microwave spectrometer. The unambiguous assignments of them are made on the basis of the agreement between the experimentally determined rotational constants and the theoretical predictions from ab initio calculations using MP2/6-311++G(2df,2pd). All the complexes exhibit hydrogen-bonded ring structures. The fine splittings observed in some of the a-type transitions of the trifluoroacetic acid-H2O dimer were analyzed in terms of the likely tunneling motions of the hydrogens in the H2O molecule. Further calculations of the equilibrium constants for these three hydrated complexes of trifluoroacetic acid were also made to evaluate their fractions against the trifluoroacetic acid monomer in the atmosphere.

Stability of the Hydrogen Trioxy Radical via Infrared Action Spectroscopy

The hydrogen trioxy radical (HO3) has been proposed as an intermediate in several important chemical reactions and relaxation processes involving OH in the atmosphere. In this work, the gas-phase infrared action spectrum of HO3 is obtained in the OH overtone region, along with the product state distribution of the OH fragment following dissociation. The highest observed OH product channel sets an upper limit for the HO-O2 binding energy of 6.12 kcal mol(-1). The experimental stability of HO3 and derived equilibrium constant imply that up to 66% of atmospheric OH may be converted into HO3 in the tropopause region.

Hydrogen bonded OH-stretching vibration in the water dimer

We have calculated the frequencies and intensities of the hydrogen-bonded OH-stretching transitions in the water dimer complex. The potential-energy curve and dipole-moment function are calculated ab initio at the coupled cluster with singles, doubles, and perturbative triples level of theory with correlation-consistent Dunning basis sets. The vibrational frequencies and wavefunctions are found from a numerical solution to a one-dimensional Schrödinger equation. The corresponding transition intensities are found from numerical integration of these vibrational wavefunctions with the ab initio calculated dipole moment function. We investigate the effect of counterpoise correcting both the potential-energy surface and dipole-moment function. We find that the effect of using a numeric potential is significant for higher overtones and that inclusion of a counterpoise correction for basis set superposition error is important.

In search of CS2(H2O)n=1–4 clusters

Gaussian-3 and MP2/aug-cc-pVnZ methods have been used to calculate geometries and thermochemistry of CS(2)(H2O)n, where n=1-4. An extensive molecular dynamics search followed by optimization using these two methods located two dimers, six trimers, six tetramers, and two pentamers. The MP2/aug-cc-pVDZ structure matched best with the experimental result for the CS(2)(H2O) dimer, showing that diffuse functions are necessary to model the interactions found in this complex. For larger CS(2)(H2O)n clusters, the MP2/aug-cc-pVDZ minima are significantly different from the MP2(full)6-31G* structures, revealing that the G3 model chemistry is not suitable for investigation of sulfur containing van der Waals complexes. Based on the MP2/aug-cc-pVTZ free energies, the concentration of saturated water in the atmosphere and the average amount of CS(2) in the atmosphere, the concentrations of these clusters are predicted to be on the order of 10(5) CS(2)(H2O) clusters.cm(-3) and 10(2) CS(2)(H2O)(2) clusters.cm(-3) at 298.15 K. The MP2/aug-cc-pVDZ scaled harmonic and anharmonic frequencies of the most abundant dimer cluster at 298 K are presented, along with the MP2/aug-cc-pVDZ scaled harmonic frequencies for the CS(2)(H(2)O)(n) structures predicted to be present in a low-temperature molecular beam experiment.

Infrared Spectroscopy of Ozone−Water Complex in a Neon Matrix

Matrix isolation infrared spectroscopy has been applied to study an ozone-water complex of atmospheric interest. The complex was identified in the spectral region of three normal modes of ozone and water. Ab initio calculation at MP4(SDQ), QCISD, and CCSD(T) levels indicates the existence of only one stable conformer, which accords with the present experimental result. This conformer belongs to the Cs symmetry group where two molecular planes of ozone and water are perpendicular to the Cs symmetry plane. The binding energy was calculated to be 1.89 kcal/mol at the CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p) level of theory. The formation constant and atmospheric abundance of the ozone-water complex are estimated using the thermodynamic and spectroscopic data obtained.

Diffusivity fractionations of H2(16)O/H2(17)O and H2(16)O/H2(18)O in air and their implications for isotope hydrology

We have determined the isotope effects of (17)O and (18)O substitution of (16)O in H(2)O on molecular diffusivities of water vapor in air by the use of evaporation experiments. The derived diffusion fractionation coefficients (17)alpha(diff) and (18)alpha(diff) are 1.0146 +/- 0.0002 and 1.0283 +/- 0.0003, respectively. We also determined, for the first time, the ratio ln((17)alpha(diff))/ln((18)alpha(diff)) as 0.5185 +/- 0.0002. This ratio, which is in excellent agreement with the theoretical value of 0.5184, is significantly smaller than the ratio in vapor-liquid equilibrium (0.529). We show how this new experimental information gives rise to (17)O excess in meteoric water, and how it can be applied in isotope hydrology.

Influence of Intramolecular Hydrogen Bond Strength on OH-Stretching Overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.

Molecular complexes in close and far away

In this review, gas-phase chemistry of interstellar media and some planetary atmospheres is extended to include molecular complexes. Although the composition, density, and temperature of the environments discussed are very different, molecular complexes have recently been considered as potential contributors to chemistry. The complexes reviewed include strongly bound aggregates of molecules with ions, intermediate-strength hydrogen bonded complexes (primarily hydrates), and weakly bonded van der Waals molecules. In low-density, low-temperature environments characteristic of giant molecular clouds, molecular synthesis, known to involve gas-phase ion-molecule reactions and chemistry at the surface of dust and ice grains is extended here to involve molecular ionic clusters. At the high density and high temperatures found on planetary atmospheres, molecular complexes contribute to both atmospheric chemistry and climate. Using the observational, laboratory, and theoretical database, the role of molecular complexes in close and far away is discussed.

Prediction of accurate anharmonic experimental vibrational frequencies for water clusters, (H2O)n, n=2-5

Accurate anharmonic experimental vibrational frequencies for water clusters consisting of 2-5 water molecules have been predicted on the basis of comparing different methods with MP2/aug-cc-pVTZ calculated and experimental anharmonic frequencies. The combination of using HF/6-31G* scaled frequencies for intramolecular modes and anharmonic frequencies for intermolecular modes gives excellent agreement with experiment for the water dimer and trimer and are as good as the expensive anharmonic MP2 calculations. The water trimer, the cyclic Ci and S4 tetramers, and the cyclic pentamer all have unique peaks in the infrared spectrum between 500 and 800 cm-1 and between 3400 and 3700 cm-1. Under the right experimental conditions these different clusters can be uniquely identified using high-resolution IR spectroscopy.

Ozonolysis and photolysis of alkene-terminated self-assembled monolayers on quartz nanoparticles: implications for photochemical aging of organic aerosol particles

Photolysis of alkene-terminated self assembled monolayers (SAM) deposited on Degussa SiO(2) nanoparticles is studied following oxidation of SAM with a gaseous ozone/oxygen mixture. Infrared cavity ring-down spectroscopy is used to observe gas-phase products generated during ozonolysis and subsequent photolysis of SAM in real time. Reactions taking place during ozonolysis transform alkene-terminated SAM into a photochemically active state capable of photolysis in the tropospheric actinic window (lambda > 295 nm). Formaldehyde and formic acid are the observed photolysis products. Photodissociation action spectra of oxidized SAM and the observed pattern of gas-phase products are consistent with the well-established Criegee mechanism of ozonolysis of terminal alkenes. There is strong evidence for the presence of secondary ozonides (1,3,4-trioxalones) and other peroxides on the oxidized SAM surface. The data imply that photolysis plays a role in atmospheric aging of primary and secondary organic aerosol particles.

Global Search for Minimum Energy (H2O)n Clusters, n = 3−5

The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere.

Overtone spectroscopy of H2O clusters in the vOH=2 manifold: Infrared-ultraviolet vibrationally mediated dissociation studies

Spectroscopy and predissociation dynamics of (H2O)2 and Ar-H2O are investigated with vibrationally mediated dissociation (VMD) techniques, wherein upsilon(OH) = 2 overtones of the complexes are selectively prepared with direct infrared pumping, followed by 193 nm photolysis of the excited H2O molecules. As a function of relative laser timing, the photolysis breaks H2O into OH and H fragments either (i) directly inside the complex or (ii) after the complex undergoes vibrational predissociation, with the nascent quantum state distribution of the OH photofragment probed via laser-induced fluorescence. This capability provides the first rotationally resolved spectroscopic analysis of (H2O)2 in the first overtone region and vibrational predissociation dynamics of water dimer and Ar-water clusters. The sensitivity of the VMD approach permits several upsilon(OH) = 2 overtone bands to be observed, the spectroscopic assignment of which is discussed in the context of recent anharmonic theoretical calculations.

Infrared Identification of Matrix Isolated H2O·O2

Theoretical studies of the H2O.O2 complex have been carried out over the past decade, but the complex has not previously been experimentally identified. We have assigned IR vibrations from an H2O.O2 complex in an inert rare gas matrix. This identification is based upon theoretical calculations and concentration dependent behavior of absorption bands observed upon co-deposition of H2O and O2 in argon matrixes at 11.5 +/- 0.5 K. To aid assignment, we have used a harmonically coupled anharmonic oscillator local mode model with an ab initio calculated dipole moment function to calculate the OH-stretching and HOH-bending frequencies and intensities in the complex. The high abundance of H2O and O2 makes the H2O.O2 complex likely to be significant in atmospheric and astrophysical chemistry.

Complexes and Clusters of Water Relevant to Atmospheric Chemistry: H2O Complexes with Oxidants

Experimental observations and data from quantum chemical calculations on complexes between water molecules and small, oxygen-containing inorganic species that play an important role as oxidants in the atmosphere (O(1D), O(3P), O2(X3sigmag), O2(b1sigmag+), O3, HO, HOO, HOOO, and H2O2) are reviewed, with emphasis on their structure, hydrogen bonding, interaction energies, thermodynamic parameters, and infrared spectra. In recent years, weakly bound complexes containing water have increasingly attracted scientific attention. Water in all its phases is a major player in the absorption of solar and terrestrial radiation. Thus, complexes between water and other atmospheric species may have a perceivable influence on the radiative balance and contribute to the greenhouse effect, even though their concentrations are low. In addition, they can play an important role in the chemistry of the Earth's atmosphere, particularly in the oxidation of trace gases. Apart from gas-phase complexes, the interactions of oxidants with ice surfaces have also received considerable advertency lately due to their importance in the chemistry of snow, ice clouds, and ice surfaces (e.g., ice shields in polar regions). In paleoclimate--respectively paleoenvironmental--studies, it is essential to understand the transfer processes from the atmosphere to the ice surface. Consequently, special attention is being paid here to the intercomparison of the properties of binary complexes and the complexes and clusters of more complicated compositions, including oxidants adsorbed on ice surfaces, where ice is considered a kind of large water cluster. Various facts concerning the chemistry of the Earth's atmosphere (concentration profiles and possible influence on radical reactions in the atmosphere) are discussed.

Theoretical study of O2-H2O: potential energy surface, molecular vibrations, and equilibrium constant at atmospheric temperatures

The intermolecular potential energy surface of O(2)-H(2)O was investigated at ab initio MP2 and MRSDCI levels using the aug-cc-pVTZ basis set. The vibrational levels were evaluated by numerically solving the Schrödinger equations for the nuclear motions with the ab initio potential functions using one- to three-dimensional finite-element methods. On the basis of the calculated partition functions, the equilibrium constant of the complex, K(p), was studied. The K(p) values at atmospheric temperatures of 200-300 K were found to be 1-2 orders of magnitude less than previous estimates from the harmonic oscillator approximation.

Overtone-Induced Decarboxylation: A Potential Sink for Atmospheric Diacids

Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO(2)NO(2)) and inducing intramolecular rearrangement followed by reaction (in H(2)SO(4)). Here, we propose another potentially important process: the decarboxylation of organic acids. To demonstrate this possibility, we have calculated the decarboxylation pathways for malonic acid and its monohydrate. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results. The transition state is a six-membered ring structure which is accessed via concerted O-H and C-C stretches; excitation of v(OH) > or = 3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low-energy isomer of the malonic acid-water complex forms an eight-membered, multiply hydrogen bonded structure, bound by 3-6 kcal/mol, somewhat less stable than the lowest energy, six-membered ring isomer. Decarboxylation of such complexes uses water as a catalyst; the water accepts an acidic proton from one malonic acid group and transfers a proton to the carbonyl of the other acid group. The barrier for this process is 20-22 kcal/mol, suggesting that complexes excited to v(OH) > or = 2 possess sufficient energy to react. Using estimated absorption cross sections for the OH overtone transitions, we suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas-phase reaction with OH for removal of the acid.

Thermodynamics of Forming Water Clusters at Various Temperatures and Pressures by Gaussian-2, Gaussian-3, Complete Basis Set-QB3, and Complete Basis Set-APNO Model Chemistries; Implications for Atmospheric Chemistry

The Gaussian-2, Gaussian-3, complete basis set- (CBS-) QB3, and CBS-APNO methods have been used to calculate Delta H degrees and Delta G degrees values for neutral clusters of water, (H(2)O)(n), where n = 2-6. The structures are similar to those determined from experiment and from previous high-level calculations. The thermodynamic calculations by the G2, G3, and CBS-APNO methods compare well against the estimated MP2(CBS) limit. The cyclic pentamer and hexamer structures release the most heat per hydrogen bond formed of any of the clusters. While the cage and prism forms of the hexamer are the lowest energy structures at very low temperatures, as temperature is increased the cyclic structure is favored. The free energies of cluster formation at different temperatures reveal interesting insights, the most striking being that the cyclic trimer, cyclic tetramer, and cyclic pentamer, like the dimer, should be detectable in the lower troposphere. We predict water dimer concentrations of 9 x 10(14) molecules/cm(3), water trimer concentrations of 2.6 x 10(12) molecules/cm(3), tetramer concentrations of approximately 5.8 x 10(11) molecules/cm(3), and pentamer concentrations of approximately 3.5 x 10(10) molecules/cm(3) in saturated air at 298 K. These results have important implications for understanding the gas-phase chemistry of the lower troposphere.

An ab initio correlated study of the potential energy surface for the HOBr.H[sub 2]O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order Møller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.

Geometrical and vibrational DFT studies of HOBr·(H2O)n clusters (n = 1–4)

The geometrical structures and the vibrational spectra of the HOBr·(H2O)n clusters (n = 1–4) have been calculated at the DFT level of theory, using the pBP method and the DN* and DN** numerical basis sets. The results showed that the interaction involving the H of the HOBr and the O of the water molecule represent the preferred arrangements for these hydrated compounds. Both HOBr·H2O and HOBr·(H2O)2 clusters presented stable structures with syn and anti conformations, the syn being the most stable. The HOBr·(H2O)3 and the HOBr·(H2O)4 clusters have presented stable cyclic structures. In the HOBr·H2O and HOBr·(H2O)2 clusters, low-frequency stretching values could be assigned to hydrogen bonds, but the same could not be done so clearly for the HOBr·(H2O)3 and the HOBr·(H2O)4 cyclic clusters. The binding energies were also determinated for these HOBr hydrated clusters, showing that the addition of a water molecule to the HOBr·H2O and HOBr·(H2O)2 clusters increases the binding energy by approximately 4 kcal mol–1, while the addition of a water molecule to the HOBr·(H2O)3 cluster decreases this value by 4 kcal mol–1.Key words: DFT, numerical basis, HOBr·(H2O)n, clusters.

Atmospheric detection of water dimers via near-infrared absorption

Weakly bonded pairs of water molecules (H2O)2, or water dimers (WDs), may play an important role in photochemistry and climate, but the overlap of most of its spectral features with the water monomer (WM) has made detection difficult. We report on WD absorption measurements by means of atmospheric long-path (18.34 kilometers) differential optical absorption spectroscopy of the near-infrared OH stretching mode 0>f 4>b overtone transition predicted to be located near 746 nanometers. Our observation is in reasonable agreement with the known thermochemistry, calculated and measured structure, and spectroscopy (band strength, shape, and width) of the WD. The observation implies that the WD 0>f 4>b band is located at 749.5 nanometers, with a full width at half maximum of approximately 19.4 wave numbers, and that its band strength ranges between 1.23 x 10(-22) and 5.25 x 10(-22) centimeters per molecule.