Formation of Chlorine in the Atmosphere by Reaction of Hypochlorous Acid with Seawater.

The highly reactive dihalogens play a significant role in the oxidative chemistry of the troposphere. One of the main reservoirs of these halogens is hypohalous acids, HOX, which produce dihalogens in the presence of halides (Y-), where X, Y = Cl, Br, I. These reactions occur in and on aerosol particles and seawater surfaces and have been studied experimentally and by field observations. However, the mechanisms of these atmospheric reactions are still unknown. Here, we establish the atomistic mechanism of HOCl + Cl- → Cl2 + OH- at the surface of the water slab by performing ab initio molecular dynamics (AIMD) simulations. Main findings are (1) This reaction proceeds by halogen-bonded complexes of (HOCl)···(Cl-)aq surrounded with the neighboring water molecules. (2) The halogen bonded (HOCl)···(Cl-)aq complexes undergo charge transfer from Cl- to OH- to form transient Cl2 at neutral pH. (3) The addition of a proton to one proximal water greatly facilitates the Cl2 formation, which explains the enhanced rate at low pH.

pH Dependence of the speciation and optical properties of 4-benzoylbenzoic acid.

Organic chromophores initiate much of daytime aqueous phase chemistry in the environment. Thus, studying the absorption spectra of commonly used organic photosensitizers is paramount to fully understand their relevance in environmental processes. In this work, we combined UV-Vis spectroscopy, 1H-NMR spectroscopy, quantum chemical calculations, and molecular dynamics simulations to investigate the absorption spectra of 4-benzoyl benzoic acid (4BBA), a widely used photosensitizer and a common proxy of environmentally relevant chromophores. Solutions of 4BBA at different pH values show that protonated and deprotonated species have an effect on its absorbance spectra. Theoretical calculations of these species in water clusters provide physical and chemical insights into the spectra. Quantum chemical calculations were conducted to analyze the UV-Vis absorbance spectra of 4BBA species using various cluster sizes, such as C6H5COC6H4COOH·(H2O)n, where n = 8 for relatively small clusters and n = 30 for larger clusters. While relatively small clusters have been successfully used for smaller chromophores, our results indicate that simulations of protonated species of 4BBA require relatively larger clusters of n = 30. A comparison between the experimental and theoretical results shows good agreement in the pH-dependent spectral shift between the hydrated cluster model and the experimental data. Overall, the theoretical and empirical results indicate that the experimental optical spectra of aqueous phase 4BBA can be represented by the acid-base equilibrium of the keto-forms, with a spectroscopically measured pKa of 3.41 ± 0.04. The results summarized here contribute to a molecular-level understanding of solvated organic molecules through calculations restricted to cluster models, and thereby, broader insight into environmentally relevant chromophores.

Effects of Microhydration on the Mechanisms of Hydrolysis and Cl- Substitution in Reactions of N2 O5 and Seawater.

The reaction of N2 O5 at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N2 O5 reacts preferentially with Cl- to form ClNO2 /NO3 - (Cl- substitution), but can also react with H2 O to form 2HNO3 (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N2 O5 /Cl- /nH2 O (n=2-5), resulting in the identification of three reaction motifs. First, we uncovered an SN 2-type Cl- substitution reaction of N2 O5 that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO2 intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H2 O attacks N2 O5 (one-step hydrolysis). We find that Cl- substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO2 intermediate) likely only when concentrations of Cl- are high.

Electronic and mechanical anharmonicities in the vibrational spectra of the H-bonded, cryogenically cooled X- · HOCl (X=Cl, Br, I) complexes: Characterization of the strong anionic H-bond to an acidic OH group.

We report vibrational spectra of the H2-tagged, cryogenically cooled X- · HOCl (X = Cl, Br, and I) ion-molecule complexes and analyze the resulting band patterns with electronic structure calculations and an anharmonic theoretical treatment of nuclear motions on extended potential energy surfaces. The complexes are formed by "ligand exchange" reactions of X- · (H2O)n clusters with HOCl molecules at low pressure (∼10-2 mbar) in a radio frequency ion guide. The spectra generally feature many bands in addition to the fundamentals expected at the double harmonic level. These "extra bands" appear in patterns that are similar to those displayed by the X- · HOD analogs, where they are assigned to excitations of nominally IR forbidden overtones and combination bands. The interactions driving these features include mechanical and electronic anharmonicities. Particularly intense bands are observed for the v = 0 → 2 transitions of the out-of-plane bending soft modes of the HOCl molecule relative to the ions. These involve displacements that act to break the strong H-bond to the ion, which give rise to large quadratic dependences of the electric dipoles (electronic anharmonicities) that drive the transition moments for the overtone bands. On the other hand, overtone bands arising from the intramolecular OH bending modes of HOCl are traced to mechanical anharmonic coupling with the v = 1 level of the OH stretch (Fermi resonances). These interactions are similar in strength to those reported earlier for the X- · HOD complexes.

Energy transfer, pre-reactive complex formation and recombination reactions during the collision of peroxy radicals.

In this paper we study collisions between polyatomic radicals - an important process in fields ranging from biology to combustion. Energy transfer, formation of intermediate complexes and recombination reactions are treated, with applications to peroxy radicals in atmospheric chemistry. Multi-reference perturbation theory, supplemented by coupled-cluster calculations, describes the potential energy surfaces with high accuracy, including the interaction of singlet and triplet spin states during radical recombination. Our multi-reference molecular dynamics (MD) trajectories on methyl peroxy radicals confirm the reaction mechanism postulated in earlier studies. Specifically, they show that if suitable pre-reactive complexes are formed, they will rapidly lead to the formation and subsequent decomposition of tetroxide intermediates. However, generating multi-reference MD trajectories is exceedingly computationally demanding, and we cannot adequately sample the whole conformational space. To answer this challenge, we promote the use of a novel simplified semi-empirical MD methodology. It assumes the collision is governed by two states, a singlet (S0) and a triplet (T1) state. The method predicts differences between collisions on S0 and T1 surfaces, and qualitatively includes not only pre-reactive complex formation, but also recombination processes such as tetroxide formation. Finally, classical MD simulations using force-fields for non-reactive collisions are employed to generate thousands of collision trajectories, to verify that the semi-empirical method is sampling collisions adequately, and to carry out preliminary investigations of larger systems. For systems with low activation energies, the experimental rate coefficient is surprisingly well reproduced by simply multiplying the gas-kinetic collision rate by the simulated probability for long-lived complex formation.

Preparation and Characterization of the Halogen-Bonding Motif in the Isolated Cl-·IOH Complex with Cryogenic Ion Vibrational Spectroscopy.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon formation of the ion-molecule complexes [XHOY]- (X, Y = Cl, Br, I): a hydrogen (HB) bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen. Here we isolate the X-bonded Cl-·IOH ion-molecule complex by collisions of I-·(H2O)n clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H2-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O-I bond (by 168 cm-1 and 0.13 Å, respectively, relative to isolated HOI). The frequency of the O-I stretch (estimated 436 cm-1) is also encoded in the spectrum by the weak v = 0 → 2 overtone transition at 872 cm-1.

Absorption Spectra and the Electronic Structure of Gallic Acid in Water at Different pH: Experimental Data and Theoretical Cluster Models.

Gallic acid (GA) has been characterized in terms of its optical properties in aqueous solutions at varying pH in experiments and in theoretical calculations by analyzing the protonated and deprotonated forms of GA. This work is part of a series of studies of the optical properties of different carboxylic acids in aqueous media. The experimental electronic spectra of GA exhibit two strong well-separated absorption peaks (B- and C-bands), which agree with previous studies. However, in the current study, an additional well-defined low-energy shoulder band (A-band) in the optical spectra of GA was identified. It is likely that the A-band occurs for other carboxylic acids in solution, but because it can overlap with the B-band, it is difficult to discern. The theoretical calculations based on density functional theory were used to simulate the optical absorption spectra of GA in water at different pH to prove the existence of this newly found shoulder band and to describe and characterize the full experimental optical spectra of GA. Different cluster models were tested: (i) all water molecules are coordinated near the carboxy-group and (ii) additional water molecules near the hydroxy-groups of the phenyl ring were included. In this study, we found that both the polarizable continuum model (dielectric property of a medium) and neighboring water molecules (hydrogen-bonding) play significant roles in the optical spectrum. The results showed that only an extended cluster model with water molecules near carboxy- and hydroxy-groups together with the polarizable continuum model allowed us to fully reproduce the experimental data and capture all three absorption bands (A, B, and C). The oscillator strengths of the absorption bands were obtained from the experimental data and compared with theoretical results. Additionally, our work provides a detailed interpretation of the pH effects observed in the experimental absorption spectra.

Toward a microscopic model of light absorbing dissolved organic compounds in aqueous environments: theoretical and experimental study.

Water systems often contain complex macromolecular systems that absorb light. In marine environments, these light absorbing components are often at the air-water interface and can participate in the chemistry of the atmosphere in ways that are poorly understood. Understanding the photochemistry and photophysics of these systems represents a major challenge since their composition and structures are not unique. In this study, we present a successful microscopic model of this light absorbing macromolecular species termed "marine derived chromophoric dissolved organic matter" or "m-CDOM" in water. The approach taken involves molecular dynamics simulations in the ground state using on the fly Density Functional Tight-Binding (DFTB) electronic structure theory; Time Dependent DFTB (TD-DFTB) calculations of excited states, and experimental measurements of the optical absorption spectra in aqueous solution. The theoretical hydrated model shows key features seen in the experimental data for a collected m-CDOM sample. As will be discussed, insights from the model are: (i) the low-energy A-band (at 410 nm) is due to the carbon chains combined with the diol- and the oxy-groups present in the structure; (ii) the weak B-band (at 320-360 nm) appears due to the contribution of the ionized speciated form of m-CDOM; and (iii) the higher-energy C-band (at 280 nm) is due to the two fused ring system. Thus, this is a two-speciated formed model. Although a relatively simple system, these calculations represent an important step in understanding light absorbing compounds found in nature and the search for other microscopic models of related materials remains of major interest.

Size-Dependent Onset of Nitric Acid Dissociation in Cs+·(HNO3)(H2O)n=0-11 Clusters at 20 K.

We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs+·(HNO3)(H2O)n=0-11 clusters at 20 K. Dramatic spectral changes are observed in the n = 7-9 range that are traced to the formation of many isomeric structures associated with intermediate transfer of the acidic proton to the water network. This transfer is complete by n = 10, which exhibits much simpler vibrational band patterns consistent with those expected for a tricoordinated hydronium ion (the Eigen motif) along with the NO stretching bands predicted for a hydrated NO3- anion that is directly complexed to the Cs+ cation. Theoretical analysis of the n = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs+ and H3O+ cations flank the NO3- anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO3-/H3O+ ion pair by the proximal metal ion.

Dual Basis Approach for Ab Initio Anharmonic Calculations of Vibrational Spectroscopy: Application to Microsolvated Biomolecules.

A dual electronic basis set approach is introduced for more efficient but accurate calculations of the anharmonic vibrational spectra in the framework of the vibrational self-consistent field (VSCF) theory. In this approach, an accurate basis set is used to compute the vibrational spectra at the harmonic level. The results are used to scale the potential surface from a more modest but much more efficient basis set. The scaling is such that at the harmonic level the new, scaled potential agrees with one of the accurate basis sets. The approach is tested in the application of the microsolvated, protected amino acid Ac-Phe-OMe, using the scaled anharmonic hybrid potential in the VSCF and VSCF-PT2 algorithms. The hybrid potential method yields results that are in good accord with the experiment and very close to those obtained in calculations with the high-level, very costly potential from the large basis set. At the same time, the hybrid potential calculations are considerably less expensive. The results of the hybrid calculations are much more accurate than those computed from the potential surface corresponding to the modest basis set. The results are very encouraging for using the hybrid potential method for inexpensive yet sufficiently accurate anharmonic calculations for the spectra of large biomolecules.

Impact of pH and NaCl and CaCl2 Salts on the Speciation and Photochemistry of Pyruvic Acid in the Aqueous Phase.

Recent studies have shown that pyruvic acid can produce higher-molecular-weight compounds upon irradiation in the aqueous phase. These compounds can contribute to the formation of secondary organic aerosols. There have been several previous studies on the effect of ionic strength on the photochemistry of pyruvic acid; however, few of them investigated the effects of marine relevant salts such as NaCl and CaCl2. In this study, we examine the effect of NaCl and CaCl2, namely, containing the coordinating cations Na+ and Ca2+, on the speciation, absorption properties, and photoreactivity of pyruvic acid in aqueous solutions of varying pH. NMR shows that both Ca2+ and Na+ further deprotonate pyruvic acid and decrease the diol to ketone ratio of pyruvic acid than in pure water at the same pH, especially at more acidic pH (pH less than 4). The absorption spectrum shows a strong red shift in the peak maxima for the n → π* transition of pyruvic acid in NaCl/CaCl2 solutions. This dependence is much more pronounced for divalent cations (Ca2+) compared to monovalent cations (Na+). Vertical excitation energy calculations of the anionic ketone form of pyruvic acid confirm the same red shift on the n → π* transition peak in the presence of Ca2+. In addition, the presence of NaCl/CaCl2 suppresses the photolysis rate of pyruvic acid, which could be due to the deprotonation of pyruvic acid by the cations and the lower photochemical reactivity for pyruvate, the deprotonated form.

Absorption spectra of pyruvic acid in water: insights from calculations for small hydrates and comparison to experiment.

Pyruvic acid is abundant in the atmosphere and in seawater, being a decay product of living organisms. Although very small in size (10 atoms), pyruvic acid exhibits conformational complexity in the gas phase and in solution, which is reflected in the UV spectrum. The gas phase UV spectrum of pyruvic acid differs from the spectrum of pyruvic acid in water. The main atmospherically relevant absorption peak in the gas phase is blue shifted by about 0.43 eV (40 nm difference in the peak location) in water. The origin of the blue shift has not been established thus far. This paper aims at a microscopic understanding of the absorption spectrum of pyruvic acid in aqueous media by a combined experimental and theoretical approach. 1H NMR experiments were performed to reveal the contribution of the different conformers in solution as a function of pH. Computationally, hydrates of sizes up to 5 water molecules using two different species of pyruvic acid, the neutral acid and the anionic form were considered. Vertical excitation energies using the ADC(2) method (algebraic-diagrammatic construction through second order) of these structures provide insights into the blue shift of the atmospherically relevant absorption peak. Additionally, molecular dynamics simulation on MP2 (Møller-Plesset perturbation theory) ground state of small clusters of pyruvic acid with four water molecules were calculated and used in computing the vertical excitation spectrum along the dynamics. This is found to describe very accurately the experimental spectrum. Overall, the results show that small hydrate models including the roles of both neutral and deprotonated speciated forms provide a good quantitative description and a microscopic interpretation of the experimental spectrum of pyruvic acid in aqueous solution.

Correction: Isomer-specific cryogenic ion vibrational spectroscopy of the D2 tagged Cs+(HNO3)(H2O)n=0-2 complexes: ion-driven enhancement of the acidic H-bond to water.

Correction for 'Isomer-specific cryogenic ion vibrational spectroscopy of the D2 tagged Cs+(HNO3)(H2O)n=0-2 complexes: ion-driven enhancement of the acidic H-bond to water' by Sayoni Mitra et al., Phys. Chem. Chem. Phys., 2020, DOI: 10.1039/c9cp06689f.

Absorption spectra of benzoic acid in water at different pH and in the presence of salts: insights from the integration of experimental data and theoretical cluster models.

The absorption spectra of molecular organic chromophores in aqueous media are of considerable importance in environmental chemistry. In this work, the UV-vis spectra of benzoic acid (BA), the simplest aromatic carboxylic acid, in aqueous solutions at varying pH and in the presence of salts are measured experimentally. The solutions of different pH provide insights into the contributions from both the non-dissociated acid molecule and the deprotonated anionic species. The microscopic interpretation of these spectra is then provided by quantum chemical calculations for small cluster models of benzoic species (benzoic acid and benzoate anion) with water molecules. Calculations of the UV-vis absorbance spectra are then carried out for different clusters such as C6H5COOH·(H2O)n and C6H5COO-·(H2O)n, where n = 0-8. The following main conclusions from these calculations and the comparison to experimental results can be made: (i) the small water cluster yields good quantitative agreement with observed solution experiments; (ii) the main peak position is found to be very similar at different levels of theory and is in excellent agreement with the experimental value, however, a weaker feature about 1 eV to lower energy (red shift) of the main peak is correctly reproduced only by using high level of theory, such as Algebraic Diagrammatic Construction (ADC); (iii) dissociation of the BA into ions is found to occur with a minimum of water molecules of n = 8; (iv) the deprotonation of BA has an influence on the computed spectrum and the energetics of the lowest energy electronic transitions; (v) the effect of the water on the spectra is much larger for the deprotonated species than for the non-dissociated acid. It was found that to reproduce experimental spectrum at pH 8.0, additional continuum representation for the extended solvent environment must be included in combination with explicit solvent molecules (n ≥ 3); (vi) salts (NaCl and CaCl2) have minimal effect on the absorption spectrum and; (vii) experimental results showed that B-band of neutral BA is not sensitive to the solvent effects whereas the effect of the water on the C-band is significant. The water effects blue-shift this band up to ∼0.2 eV. Overall, the results demonstrate the ability to further our understanding of the microscopic interpretation of the electronic structure and absorption spectra of BA in aqueous media through calculations restricted to small cluster models.

Isomer-specific cryogenic ion vibrational spectroscopy of the D2 tagged Cs+(HNO3)(H2O)n=0-2 complexes: ion-driven enhancement of the acidic H-bond to water.

We report how the binary HNO3(H2O) interaction is modified upon complexation with a nearby Cs+ ion. Isomer-selective IR photodissociation spectra of the D2-tagged, ternary Cs+(HNO3)H2O cation confirms that two structural isomers are generated in the cryogenic ion source. In one of these, both HNO3 and H2O are directly coordinated to the ion, while in the other, the water molecule is attached to the OH group of the acid, which in turn binds to Cs+ with its -NO2 group. The acidic OH stretching fundamental in the latter isomer displays a ∼300 cm-1 red-shift relative to that in the neutral H-bonded van der Waals complex, HNO3(H2O). This behavior is analyzed with the aid of electronic structure calculations and discussed in the context of the increased effective acidity of HNO3 in the presence of the cation.

Integrated experimental and theoretical approach to probe the synergistic effect of ammonia in methanesulfonic acid reactions with small alkylamines.

While new particle formation events have been observed worldwide, our fundamental understanding of the precursors remains uncertain. It has been previously shown that small alkylamines and ammonia (NH3) are key actors in sub-3 nm particle formation through reactions with acids such as sulfuric acid (H2SO4) and methanesulfonic acid (CH3S(O)(O)OH, MSA), and that water also plays a role. Because NH3 and amines co-exist in air, we carried out combined experimental and theoretical studies examining the influence of the addition of NH3 on particle formation from the reactions of MSA with methylamine (MA) and trimethylamine (TMA). Experiments were performed in a 1 m flow reactor at 1 atm and 296 K. Measurements using an ultrafine condensation particle counter (CPC) and a scanning mobility particle sizer (SMPS) show that new particle formation was systematically enhanced upon simultaneous addition of NH3 to the MSA + amine binary system, with the magnitude depending on the amine investigated. For the MSA + TMA reaction system, the addition of NH3 at ppb concentrations produced a much greater effect (i.e. order of magnitude more particles) than the addition of ∼12 000 ppm water (corresponding to ∼45-50% relative humidity). The effect of NH3 on the MSA + MA system, which is already very efficient in forming particles on its own, was present but modest. Calculations of energies, partial charges and structures of small cluster models of the multi-component particles likewise suggest synergistic effects due to NH3 in the presence of MSA and amine. The local minimum structures and the interactions involved suggest mechanisms for this effect.

Microscopic Mechanisms of N2O5 Hydrolysis on the Surface of Water Droplets.

Reactions of N2O5, in particular heterogeneous hydrolysis, play a vital role in determining the chemistry of the atmosphere. The N2O5 heterogeneous hydrolysis reaction has been the subject of extensive research for decades, yet the physicochemical details of the mechanism have not been established. In this study, we show that this reaction can occur on the surface of a pure water droplet. We compute a relevant transition state for a nano-size model system and follow its evolution in time by means of ab initio molecular dynamics. This transition state, where N2O5 has a strong ion-pair character, leads directly to HNO3. Both electrophilic and nucleophilic mechanisms take place. It is suggested that corresponding simulations for hydrolysis in the bulk are desirable.

SN2 Reactions of N2O5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products.

Reactions of dinitrogen pentoxide (N2O5) greatly affect the concentrations of NO3, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N2O5 with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl-, SO42-, ClO4-, and RCOO- (R = H, CH3, C2H5). The following main results are obtained: (i) all the reactions take place by an SN2-type mechanism, with a transition state that involves a contact ion pair (NO2+NO3-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO2+) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO2 formation from N2O5 reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO4NO2-) and RCOONO2, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.

Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes: Anharmonic Ab Initio Classical Separable Potential Calculations.

The anharmonic frequencies of O-H, C-H, and N-H stretching modes of hydrogen-bonded glycine-H2O complexes are calculated using ab initio classical separable potential approximation. In this approach, ab initio molecular dynamic simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Three complex structures are studied, which differ in the location of the water molecule on the amino acid. Significant differences are found between the spectra of the three structures, and signatures of individual complexes are established. It is demonstrated that anharmonic effects are essential in the discrimination between different structures, while frequency differences at the harmonic level are much smaller. Intensities are also computed and found to carry information on differences between structures, but the role of anharmonicity in this is small.

Structures, Stability, and Decomposition Dynamics of the Polynitrogen Molecule N5+B(N3)4- and Its Dimer [N5+]2[B(N3)4-]2.

There is much current interest in materials that are made entirely or mostly of nitrogen atoms. Such materials, polynitrogens, may reveal new aspects of nitrogen chemistry, and are believed to provide a possible basis for novel energetic substances. An interesting family of such materials, in which the N5+ group appears as a cation, was prepared by K. O. Christe and co-workers. Little is known as yet on the microscopic properties of these materials. In this paper, we report theoretical calculations to predict the structure, energetic stability and decomposition dynamics of the polynitrogen molecule N5+B(N3)4-, the building block of a solid prepared by Christe, and of the dimer of this molecule. The structures are computed at the B3LYP-D3 level of DFT. ab initio molecular dynamics simulations are used to explore the thermal stability of the species and the decomposition mechanism. It is found that the N5+B(N3)4- ion-pair decomposes on a picosecond time scale at T = 200 K, with an ultrafast release of four N2 molecules, which is very exothermic. The species B(N3)3 is a product. The dimer is considerably more stable. Sensitivity of the process to temperature and to an applied force is reported. Possible applications of this material are briefly discussed.