The oxidation of organics adsorbed on surfaces by ozone is of fundamental chemical interest and potentially important in the lower atmosphere. Studies of the oxidation of the three-carbon and eight-carbon vinylterminated self-assembled monolayers (SAMs, C3d and C8d) on a silicon ATR (attenuated total reflectance) crystal by gas-phase O 3 at 296 K are reported. Oxidation of the SAMs was followed in real time by ATR-FTIR using ozone concentrations that spanned 5 orders of magnitude, from ∼10 11 to 10 16 molecules cm -3 . For comparison, some studies of the saturated C8 SAM were also carried out. The films were also characterized by atomic force microscopy and water contact angle measurements. The loss of CdC and the formation of CdO were measured in real time and shown to be consistent with a Langmuir-Hinshelwood mechanism in which O 3 is rapidly adsorbed on the surface and then reacts more slowly with the alkene moiety. This is supported by molecular dynamics (MD) calculations which show that O 3 does not simply undergo elastic collisions but has a significant residence time on the surface. However, the kinetics measurements indicate a much longer residence time than the MD calculations, suggesting a chemisorption of O 3 . Formaldehyde was observed as a gas-phase product by infrared cavity ring down spectroscopy. Possible mechanisms of the ozonolysis and its atmospheric implications are discussed.

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.

The uptake of gas phase ozone and the collision rate between ozone and double bonds at three different unsaturated organic interfaces with vapor are studied using classical molecular dynamics computer simulations. The organic systems are a self-assembled monolayer of 1-octenethiolate molecules adsorbed on a gold surface, liquid 1-tetradecene, and a monolayer of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine molecules adsorbed at the water liquid/vapor interface. The structural features of the neat organic systems are characterized and correlated with the dynamics in the presence of gas phase ozone molecules. The collision rate between ozone and a double bond is sensitive to several factors, including the extent of localization of the double bonds in the system and the distance that ozone diffuses into the organic phase. However, the average lifetime of a collision between ozone and a double bond is independent of the organic system. A comparison of the simulation results with experimental results from these systems shows good agreement. The results are discussed in the context of the oxidative processing of organic aerosols in the atmosphere.

The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an ∼4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 µm in diameter), however, the interchain excited state exhibits a large (∼9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

Molecular dynamics computer simulations are used to study hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers (CAFSAMs). Water-water, water-CAFSAM, and internal CAFSAM hydrogen bonds are examined. Roughly half of all adjacent carboxylic acid-terminated hydrocarbon chains are hydrogen-bonded to one another. This is consistent with experimental results reflecting two pKa values for CAFSAMs. Hydrogen-bond dynamics are expressed in terms of hydrogen-bond population autocorrelation functions and are found to be nonexponential. The water-water hydrogen-bond dynamics are slower at the interface than in the bulk, which is similar to what was found at the interface between water and weakly polar liquids such as nitrobenzene. The water-CAFSAM hydrogen bonds are found to be long-lived, on the order of tens of picoseconds. Internal CAFSAM chain-chain hydrogen bonds show almost no relaxation on the simulation time scale.

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Viscosity dependence and solvent effects in the photoisomerization of cis-stilbene: Insight from a molecular dynamics study with an ab initio potential-energy functionThe photodissociation and photoisomerization of ICN in water is studied using molecular dynamics simulations. A water-ICN potential energy function that takes into account the different ground and excited state charges and their shift as a function of the reaction coordinate is developed. The calculations include nonadiabatic transitions between the different electronic states and allow for a complete description of the photodissociation leading to ground-state and excited-state iodine and to recombination producing ICN and INC. The calculated UV absorption spectrum, the cage escape probability, the quantum yield of ICN and INC, and the subsequent vibrational relaxation rate of ICN and INC are in reasonable agreement with recent experiments. The trajectories provide a detailed microscopic picture of the early events. For example, it is shown that most recombination events on the ground state involve nonadiabatic transitions before the molecule has a chance to completely dissociate on the excited state, and that the quantum yield for photoisomerization to form INC is statistically determined very early in the photodissociation process.