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The photodegradation of benzo[e]pyrene (BeP), a ubiquitous polycyclic aromatic hydrocarbon (PAH) contaminant, was investigated in solution and adsorbed on surfaces modeling the atmospheric particulate matter to provide fundamental information that could help to clarify its fate in the atmosphere. Diones, diols, and hydroxy derivatives were identified as the major photoproducts of BeP irradiated under simulated atmospheric conditions. The relative distribution of the products and the photodegradation rates of BeP were affected by the average pore size of the surface. Major photoproducts characterized in samples adsorbed on silica gel and alumina surfaces were not observed in irradiated solutions of BeP in hexane. In acetonitrile, the photodegradation rate was faster than in hexane, and one of the diones was observed. Different photoreaction pathways seem to take place in polar versus nonpolar microenvironments.

Benzo[e]pyrene (BeP) is a widespread polycyclic aromatic hydrocarbon found principally in highly polluted areas. The study of its photochemistry is important because of its possible toxic nature and its potential for phototransformation into biologically active products. We studied the primary photophysical and photochemical degradation processes of BeP, both in solution and adsorbed on silica gel and alumina, acting as models for the atmospheric particulate matter. The radical cation of BeP was characterized as an intermediate species during the photodegradation of BeP in polar solvents and adsorbed on the surfaces. The photoionization process was monophotonic, and once the radical cation was formed, it could react with water or oxygen to yield mainly diones, alcohols, and diols. In alumina, the radical yield was small, in accordance with the low photoreactivity observed on this surface. Two triplet-triplet absorption bands at 350 and 560 nm were observed in the time-resolved spectra of adsorbed BeP and in solution under nitrogen atmosphere. The BeP's triplet state, however, did not play an important role in its photoreaction pathway. The surface's pore size and the coadsorbed water affected the yields and kinetics of the intermediates but not the photodegradation mechanism.

The global minimum-energy and other significant structures of HCl(NH3)n (n=1–4) clusters have been identified by ab initio Monte Carlo simulated annealing. Geometries of the isomers were refined in density functional theoretical and Hartree–Fock plus second-order Møller–Plesset perturbation theoretical calculations. The energy orderings were confirmed in single-point higher-level calculations. While for HCl(NH3) only one hydrogen-bonded structure was found, for the larger clusters both ionic and molecular structures exist. Stabilization by hydrogen bonding is found to be less important in the ammonia clusters than in water clusters of similar size.

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