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.

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.

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.

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.

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.

Ion reactions in atmospherically-relevant clusters: mechanisms, dynamics and spectroscopic signatures.

Reactions of nitrogen oxides with seawater are of major atmospheric importance, but microscopic understanding of these processes is still largely unavailable. In this paper we explore models of reactions of N2O4 with ions in water in order to provide molecular-level understanding of the processes. Presented here are studies of N2O4 interacting with two ions, SO42- and Cl-, in small water clusters. Reactions of the asymmetric conformer of N2O4 with SO42- ions in water clusters are studied via ab initio molecular dynamics (AIMD) simulations in order to unravel the microscopic mechanism of the processes and predict the timescales of different steps. Spectroscopic signatures of the reaction are proposed. The mechanisms of chloride substitution and hydrolysis of symmetric and asymmetric N2O4 are explored via intrinsic reaction coordinate (IRC) calculations. Spectroscopic calculations for relevant species suggest possible experimental signatures for the processes. The results of these model ion-N2O4 reactions in water throw light on the molecular-level mechanisms of the reactions of nitrogen oxides with seawater.

Time Dependent Density Functional Theory Study of Magnetic Circular Dichroism Spectra of Gold Clusters Au9(PH3)83+ and Au9(PPh3)83.

Magnetic circular dichroism (MCD) spectroscopy is a source of important data about the electronic structure and optical properties of different chemical systems. Theoretical simulation of the MCD spectra can be used to assist in the understanding of empirically measured MCD spectra. In the present paper, a theoretical investigation of electronic and optical properties of phosphine-protected gold clusters with a Au93+ core with D2h symmetry was performed with time-dependent density functional theory. The influence of ligands on the optical properties of the gold core was investigated. Simulations of the optical absorption and MCD spectra were performed for the bare gold Au93+ cluster as well as for ligand-protected Au9(PH3)83+ and Au9(PPh3)83+ species. MCD spectra were calculated at a temperature of 298 K and a magnetic field of 7 T. A comparative analysis of theoretical and experimental data was also performed. The obtained results show that the theoretically simulated MCD spectrum for the Au9(PPh3)83+ ion in gas phase exhibits a reasonable agreement with experimental results for the [Au9(PPh3)8](NO3)3 system, although with a red shift of up to 0.5 µm-1. Overall, MCD provides significant additional details about the electronic structure of the considered systems compared to the absorption spectra.

Time-Dependent Density Functional Theory Investigation of the Electronic Structure and Chiroptical Properties of Curved and Helical Silver Nanowires.

Time-dependent density functional theory methods are employed to examine the evolution of the absorption and circular dichroism (CD) spectra of neutral bare silver helical nanostructures as a function of their geometrical parameters. Calculations of excited states to determine optical absorption and CD spectra were performed using the SAOP/TZP level of theory. In our model, the geometry of the helical silver chain is dependent on the Ag-Ag-Ag bond angle and the Ag-Ag-Ag-Ag dihedral angle. The influence of different geometrical structures on the optical absorption and CD spectra were studied for helical and planar Ag8. Silver nanowires Agn (n = 4, 6, 8, 10, 12) were examined to determine the effect of the helical chain length on the electronic properties. The results show that when the metal atomic chain loses planarity, strong CD signals arise; the intensities of the CD peaks for these structures are strongly affected by the shape and length of the silver nanowires. The theoretically predicted CD spectra of the nonplanar Ag4 and Ag6 model systems show good agreement in spectral shapes and reasonable agreement in peak locations compared to experimental data for silver-DNA clusters. However, the theoretical and experimental results for the longer Ag12 wire show larger differences in the peak locations, which could potentially be caused by effects such as the presence of DNA and cationic silver atoms in the experimental system.

Chiral electronic transitions in fluorescent silver clusters stabilized by DNA.

Fluorescent, DNA-stabilized silver clusters are receiving much attention for sequence-selected colors and high quantum yields. However, limited knowledge of cluster structure is constraining further development of these "AgN-DNA" nanomaterials. We report the structurally sensitive, chiroptical activity of four pure AgN-DNA with wide ranging colors. Ubiquitous features in circular dichroism (CD) spectra include a positive dichroic peak overlying the lowest energy absorbance peak and highly anisotropic, negative dichroic peaks at energies well below DNA transitions. Quantum chemical calculations for bare chains of silver atoms with nonplanar curvature also exhibit these striking features, indicating electron flow along a chiral, filamentary metallic path as the origin for low-energy AgN-DNA transitions. Relative to the bare DNA, marked UV changes in CD spectra of AgN-DNA and silver cation-DNA solutions indicate that ionic silver content constrains nucleobase conformation. Changes in solvent composition alone can reorganize cluster structure, reconfiguring chiroptical properties and fluorescence.