Reactions of Cl-, Br-, and I- ions in seawater with incoming molecules from the gas phase are of major atmospheric importance, but their mechanisms are mostly unknown. In this study, using ab initio molecular dynamics (AIMD) simulations, the microscopic mechanism of the halogen exchange reaction in water, HOCl + I- → HOI + Cl-, is unraveled. The main findings are as follows: (1) The reaction proceeds through a halogen-bonded isomer of the complex of HOCl with I-, which is present in water and has a significant lifetime. The hydrogen-bonded isomer of the complex seems to play no role in the reaction. (2) Several water molecules act to catalyze the reaction through a Grotthuss-like mechanism that is totally different from that of halogen exchange in the gas phase. These results may have important implications for the chemistry of seawater, in particular for other reactions involving halogenated species.

Peroxy radicals are key intermediates in many atmospheric processes. Reactions between such radicals are of particular interest as they can lead to accretion products capable of participating in new particle formation (NPF). These reactions proceed through a tetroxide intermediate, which then decomposes to a complex of two alkoxy radicals and O2, with spin conservation dictating that the complex must be formed in the triplet state. The alkoxy complex can follow different pathways e.g. hydrogen(H)-shift reactions, dissociation reactions etc., but the details of the full processes are not yet fully understood. This paper establishes the microscopic mechanisms of the H-shift and other associated pathways in the context of a self-reaction between methoxy radicals, with focus on the roles of the singlet and triplet states involved. Dynamics in time is explored by two methods: the multireference XMS-CASPT2 and very recently developed mixed reference spin-flip TDDFT (MRSF-TDDFT). The metadynamics method is used to compute energetics. The XMS-CASPT2 and the MRSF-TDDFT dynamics simulations yield similar results. This would be very encouraging for future simulations for large radicals, since MRSF-TDDFT simulations enjoy the advantages of linear response theory. Our calculations demonstrate that the reaction between methoxy radicals, though initiated on the triplet state, leads to products predominantly on the singlet surface, following efficient intersystem crossing (ISC). The computed branching ratio between H-shift and dissociation channels agrees well with experiment.

Benzothiazoles are in widespread use as components of, or precursors to, a variety of consumer and industrial products. This class of compounds encompasses the simplest molecule benzothiazole (BTH) in which a benzene ring is fused to a thiazole ring, as well as a series of derivatives which are commonly functionalized at the C2 position of the thiazole ring. The addition of groups at this position modifies the reactivity in ways that are not well-known. While the reactions of benzothiazoles in water have been the subject of investigation, in part for wastewater treatment applications, much less is known about their atmospheric reactions where gas phase oxidation by the OH radical is expected to dominate. We report here studies of the kinetics, products, and mechanism of reaction of 2-methylbenzothiazole (MeBTH) with OH in the gas phase using a combination of experiments and theory. Comparison to previous studies of the OH oxidation of BTH highlights the impact of substitution of a methyl group at the 2-position on the products and reactivity. Specifically, the rate constant at 298 K and 1 atm pressure for the MeBTH-OH reaction is (3.0 ± 0.4) × 10-12 cm3 molecule-1 s-1 (1σ), about 50% faster than that of BTH. In addition, attack of OH on the -CH3 group at the 2-position of the thiazole ring to form the aldehyde as the stable product becomes important, accounting for ∼ 33% of the overall reaction. Formation of the phenol-type products from attack on the benzene ring accounts for the remainder, with the experimental relative yields consistent with theoretical predictions based on energies of formation of the prereaction MeBTH···OH complex. The formation of the aldehyde product (2-CHO-BTH) involves a sequence of five distinct stages involving two oxygen molecules and one NO. Both processes involve a spin flip of unpaired electrons, which enables a transition between electronic states that is essential for the reaction to proceed. Using the room temperature rate constant, the estimated lifetimes of MeBTH in air range from about 9 h to 4 days over OH concentrations of 107 - 106 cm-3. Thus, this reaction represents a significant loss process for MeBTH in air both outdoors and indoors, and exposures and toxicity of both the parent MeBTH and its oxidation products need to be taken into account in assessments of its environmental fates.

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- -> Cl-2 + 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 Cl-2 at neutral pH. (3) The addition of a proton to one proximal water greatly facilitates the Cl-2 formation, which explains the enhanced rate at low pH.

Benzothiazole (BTH) and its derivatives are amongst a group of emerging contaminants that are widely distributed in the environment due to their extensive use in many different consumer products. In air, reaction with the hydroxyl radical (OH) is expected to be a major loss process for BTH in the gas phase, but the kinetics and mechanisms are unknown. Here, we report a combination of experiments and theory to determine both the rate constant and products of the reaction of OH with the smallest member of the series, benzothiazole, in the gas phase. The mechanism first involves an attack by OH on BTH to produce several OHBTH intermediates. This is followed by O2 reactions with OHBTH, leading to several stable products successfully predicted by theory. Relative rate studies at 1 atm in air and 298 K using benzene as a reference gave a rate constant for the BTH + OH reaction of 2.1 +/- 0.1 x 10-12 (1 sigma) cm3 per molecule per s, which translates to a lifetime in air of 5.5 days at 1 x 106 OH cm-3. Four hydroxybenzothiazole products reflecting attack on different carbon atoms of the benzene ring were measured (n-OHBTH, where n = 4, 5, 6, 7), with the relative product yields well predicted by the calculated formation energies of the pre-reaction OH & ctdot;BTH complex. Attack of OH on the -CH of the thiazole ring leads to the formation of 2-OHBTH, representing a smaller fraction of the overall reaction, and is shown to proceed through a more complex mechanism than attack on the benzene ring. A theoretical approach to predicting chromatographic retention times of the products based on solvation free energies (Delta Gsolv) was successful for most of the products. These studies illustrate how the powerful combination of experiment and theory can be used to predict products of atmospheric oxidation of emerging contaminants and ultimately used to assess their impacts on the environment.

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.

Photochemistry of nitroaromatics in organic solvents can vary significantly from the photochemistry in aqueous solutions.

The Cover Feature shows the nighttime reactions of dinitrogen pentoxide (N2O5) with chloride (Cl−) and water at the surface of sea spray aerosols. In this work we identify 3 reaction mechanisms in the competition between Cl− substitution and hydrolysis of N2O5 via quantum chemical and dynamics studies of model clusters. The competition of these reactions at aerosol interfaces significantly affects atmospheric levels of O3, NOx, and OH. Cover design by Jami Butler. More information can be found in the Research Article by Laura M. McCaslin, Benny Gerber and co-workers.

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.

The impact of the stereochemistry of the glycosyl cation species upon its dynamic properties is examined together with their vibrational spectra in order to gain insight into the effects of configurational isomerism on conformer dynamics and proton mobility. Ab initio molecular dynamics (AIMD) simulations and infrared multiple photon dissociation (IRMPD) spectroscopy explore the conformational and reactive dynamics of two pairs of glycosyl cation isomers: (1) protonated α- and β- anomers of methyl-D-galactopyranoside and (2) the oxocarbenium ions of the D-aldohexose C2 epimers galactose and talose. Analysis of these simulations together with experimental spectroscopy, interpreted by anharmonic calculations, points to the key role played by the intramolecular hydrogen bonds which are present in a unique pattern and extent in each isomer. We find that the reactivity of galactoside stereoisomers toward acid-catalyzed nucleophilic substitution, as gauged by the ability to form free oxocarbenium ions, differs markedly in a way that agrees with experimental measurements in the condensed phase. Other properties such as conformer stability and vibrational transitions were also found to reflect the characteristic hydrogen bonding interactions present in each isomer. In both systems, the stereochemistry is shown to determine the strength of intramolecular hydrogen bonding as well as between which substituents proton transfer is possible. We expect that the critical impact of non-covalent interactions on stereoisomer selectivity may be a widely found phenomenon whose effects should be further investigated.

The lifetimes of pre-reactive complexes, although implicitly part of the equations used to model many gas-phase bimolecular reactions, have seldom been included in quantitative calculations of rate coefficients. Here, we demonstrate the application of empirical molecular dynamics simulations of collisions between peroxy radicals to model association lifetimes. With the exception of the methyl peroxy-acetyl peroxy system, measurements of the lifetimes based on a phenomenological model are shown to correlate well with available experimental data for recombination reactions of peroxy radicals in cases where the rate -limiting transition state lies below the reactants in energy. Further, we predict reaction rates for larger alpha-pinene-derived peroxy radicals, and we interpret our results in tandem with available experimental data on these systems, which are of great relevance to improve our understanding of atmospheric aerosol formation.

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.

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.

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.

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.

This is the story of a career in theoretical chemistry during a time of dramatic changes in the field due to phenomenal growth in the availability of computational power. It is likewise the story of the highly gifted graduate students and postdoctoral fellows that I was fortunate to mentor throughout my career. It includes reminiscences of the great mentors that I had and of the exciting collaborations with both experimentalists and theorists on which I built much of my research. This is an account of the developments of exciting scientific disciplines in which I was involved: vibrational spectroscopy, molecular reaction mechanisms and dynamics, e.g., in atmospheric chemistry, and the prediction of new, exotic molecules, in particular noble gas molecules. From my very first project to my current work, my career in science has brought me the excitement and fascination of research. What a wonderful pursuit!

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.

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.

Oxidation of ammonia is an important atmospheric process, but detailed mechanisms are for the most part yet unknown. Here the later steps in a mechanism forming nitric oxide and water from NH2 and O2 are studied. The transition states for isomerisation of NH2OO to HNOOH and for dissociation of HNOOH to HNO and OH are identified and the subsequent formation of nitric oxide and water is discussed. This process is studied in small water clusters to model the effect of the nearest neighbouring water molecules in larger clusters or at extended surfaces, demonstrating the effect of solvation in lowering the barrier height for NH2OO to HNOOH isomerisation. Proton transfer through the surrounding water provides a feasible reaction path in this system. As a result, it is shown that the high barrier for isomerisation between intermediates in the gas phase is not likely to prevent ammonia oxidation in small water clusters or aerosols.