Molecular Dynamics of Photoinduced Reactions of Acrylic Acid: Products, Mechanisms, and Comparison with Experiment

The photochemistry of acrylic acid is of considerable atmospheric importance. However, the mechanisms and the time scales of the reactions involved are unknown. In this work, the products, yields, and reaction pathways of acrylic acid photochemistry are investigated theoretically by molecular dynamics simulations on the ππ* excited state. Two methods were used to describe the excited state: the semiempirical OM2/MRCI and the ab initio ADC(2). Over 100 trajectories were computed with each method. A rich variety of reaction channels including mechanisms, time scales, and yields are predicted for the single potential energy surface used. Main findings include: (1) Products predicted by the calculations are in good agreement with experiments; (2) ADC(2) seems to validate OM2/MRCI predictions on main aspects of mechanisms, but not on time scales. It is concluded that both semiempirical and ab initio molecular dynamics simulations have useful advantages for the description of photochemical dynamics of carboxylic acids

Approximate Quantum Dynamics using Ab Initio Classical Separable Potentials: Spectroscopic Applications

Algorithms for quantum molecular dynamics simulations that directly use ab initio methods have many potential applications. In this article, the ab initio classical separable potentials (AICSP) method is proposed as the basis for approximate algorithms of this type. The AICSP method assumes separability of the total time-dependent wave function of the nuclei and employs mean-field potentials that govern the dynamics of each degree of freedom. In the proposed approach, the mean-field potentials are determined by classical ab initio molecular dynamics simulations. The nuclear wave function can thus be propagated in time using the effective potentials generated “on the fly”. As a test of the method for realistic systems, calculations of the stationary anharmonic frequencies of hydrogen stretching modes were carried out for several polyatomic systems, including three amino acids and the guanine–cytosine pair of nucleobases. Good agreement with experiments was found. The method scales very favorably with the number of vibrational modes and should be applicable for very large molecules, e.g., peptides. The method should also be applicable for properties such as vibrational line widths and line shapes. Work in these directions is underway

Autocorrelation of electronic wave-functions: a new approach for describing the evolution of electronic structure in the course of dynamics

<p>We introduce a new approach for analysing changes in electronic structure in the course of <i>ab initio</i> molecular dynamics simulations. The analysis is based on the time autocorrelation function of the many-body electronic wave-function. The approach facilitates the interpretation of dynamical events that may not be easily revealed by consideration of nuclear configurations alone. We apply the method to several illustrative examples: the shared proton vibration in the F⁻(H₂O) complex, representing changes in strength of non-covalent interactions; proton transfer in the water dimer cation, as an example for chemical reactions in weakly bound systems; and the intramolecular proton transfer in malonaldehyde. In all cases, we observe distinct features in the time autocorrelation function when chemical changes occur. The autocorrelation function serves as an effective reaction coordinate, incorporating all degrees of freedom, including electronic ones. The method is also sensitive to changes in the electronic wave-function not accompanied by significant nuclear motions.</p

Photochemical Reactions of Cyclohexanone: Mechanisms and Dynamics

Photochemistry of carbonyl compounds is of major importance in atmospheric and organic chemistry. The photochemistry of cyclohexanone is studied here using on-the-fly molecular dynamics simulations on a semiempirical multireference configuration interaction potential-energy surface to predict the distribution of photoproducts and time scales for their formation. Rich photochemistry is predicted to occur on a picosecond time scale following the photoexcitation of cyclohexanone to the first singlet excited state. The main findings include: (1) Reaction channels found experimentally are confirmed by the theoretical simulations, and a new reaction channel is predicted. (2) The majority (87%) of the reactive trajectories start with a ring opening via C–C_α bond cleavage, supporting observations of previous studies. (3) Mechanistic details, time scales, and yields are predicted for all reaction channels. These benchmark results shed light on the photochemistry of isolated carbonyl compounds in the atmosphere and can be extended in the future to photochemistry of more complex atmospherically relevant carbonyl compounds in both gaseous and condensed-phase environments

Proton Transfer in Mixed Clusters of Methanesulfonic Acid, Methylamine, and Oxalic Acid: Implications for Atmospheric Particle Formation

Understanding the properties of atmospheric particles made of several components is a very challenging problem. In this paper, we perform quantum chemical calculations for small multicomponent clusters of atmospheric relevance that incorporate methanesulfonic acid (MSA), methylamine (MA), oxalic acid (OxA), and water (H₂O). Potential correlations between theoretical predictions of proton transfer in the small clusters and findings of recent experiments on formation of particles of detectable sizes (>2 nm) from the same components are studied. It is proposed that proton transfer from the acid to the amine in the 1:1 clusters correlates with experiments on particle formation in systems, such as MSA-MA and MSA-MA-OxA. In the case of OxA + MA, which has been observed to give few particles, proton transfer does not occur for the 1:1 cluster but does for the 2:2 cluster. Adding H₂O to OxA-MA promotes the occurrence of proton transfer, and corresponding particles are slightly enhanced. The partial charge on the MA component increases by adding OxA or H₂O to MSA-MA, which is correlated with enhanced particle formation compared to MSA-MA alone. Ab initio molecular dynamics simulations show that proton transfer at room temperature (<i>T</i> = 298 K) and high temperature (<i>T</i> = 500 K) is little affected compared with the equilibrium structure (<i>T</i> = 0 K). These results suggest that small cluster calculations may be useful in predicting the formation of multicomponent particles in the atmosphere

Monosaccharide-Water Complexes: Vibrational Spectroscopy and Anharmonic Potentials

Ab initio vibrational self-consistent field (VSCF) calculations are used to predict the vibrational spectra of an extended series of monosaccharide·D₂O complexes, including glucose, galactose, mannose, xylose, and fucose in their α and β anomeric forms, and compared with recently published experimental data for their (phenyl-tagged) complexes. Anharmonic VSCF-PT2 frequencies are calculated directly, using ab initio hybrid HF/MP2 potentials, to assess their accuracy in reproducing the vibrational anharmonicities and provide a more rigorous basis for vibrational and structural assignments. The average discrepancies between the calculated and experimental frequencies are ∼1.0–1.5%, and the first-principles spectroscopic calculations, free of any empirical scaling, yield results of high accuracy. They encourage confidence in their future application to the assignment of other carbohydrate systems, both free and complexed, and an improved understanding of their intra- and intermolecular carbohydrate interactions

The Role of Oxalic Acid in New Particle Formation from Methanesulfonic Acid, Methylamine, and Water

Atmospheric particles are notorious for their effects on human health and visibility and are known to influence climate. Though sulfuric acid and ammonia/amines are recognized as main contributors to new particle formation (NPF), models and observations have indicated that other species may be involved. It has been shown that nucleation from methanesulfonic acid (MSA) and amines, which is enhanced with added water, can also contribute to NPF. While organics are ubiquitous in air and likely to be involved in NPF by stabilizing small clusters for further growth, their effects on the MSA–amine system are not known. This work investigates the effect of oxalic acid (OxA) on NPF from the reaction of MSA and methylamine (MA) at 1 atm and 294 K in the presence and absence of water vapor using an aerosol flow reactor. OxA and MA do not efficiently form particles even in the presence of water, but NPF is enhanced when adding MSA to OxA-MA with and without water. The addition of OxA to MSA-MA mixtures yields a modest NPF enhancement, whereas the addition of OxA to MSA-MA-H₂O has no effect. Possible reasons for these effects are discussed

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

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

NO_<i>x</i> Reactions on Aqueous Surfaces with Gaseous HCl: Formation of a Potential Precursor to Atmospheric Cl Atoms

Chlorine atoms are highly reactive free radicals known to catalyze ozone depletion in the stratosphere and organic oxidation in the troposphere. They are readily produced photolytically upon irradiation of some stable Cl containing species, for instance, nitrosyl chloride, ClNO. We predict the formation of ClNO using ab initio molecular dynamics (AIMD) simulations of an NO₂ dimer on the surface of a thin film of water upon which gaseous HCl impinges. The reactant is chloride ion formed when HCl ionizes on the water film. The same mechanism for ClNO production may occur in humid environments when ONONO₂ (the asymmetric NO₂ dimer examined here) comes in contact with either HCl or sea salt. The film of water serves to (1) stabilize ONONO₂ on the film surface so that it is localized and physically accessible for reaction, (2) provide the medium to ionize HCl, and (3) activate ONONO₂ making it more susceptible to nucleophilic attack by chloride. This substitution/elimination mechanism is new for NO_<i>x</i> chemistry on thin water films and could not be derived from studies on small clusters