Methyl and Pentyl Chloride in a Microhydrated Environment and at the Liquid Water-Vapor Interface: A Theoretical Study

The solvation properties of methyl and pentyl chloride were studied in a microhydrated environment with up to 10 explicit water molecules and at the liquid water vapor interface. Geometry optimizations were performed in the former case using the density functional based tight binding (DFTB), DFTB-D, and Moller-Plesset perturbation theory (MP2) levels of theory. The microhydrated alkyl chloride complexes were characterized in terms of hydrogen bonding and energetic stability. The DFTB and DFTB-D results were verified by comparison with those obtained by MP2. Good agreement between the MP2 and DFTB-D results is found. Complexes where the alkyl chloride molecule is attached to an edge of the water cluster are found to be most stable. Pronounced stability is also observed for cubic arrangements of the alkyl chloride water complexes. Molecular dynamics simulations based on the DFTB and DFTB-D methods were used to study the adsorption process of the alkyl chloride molecules to a water surface. The dynamics simulations show that the methyl chloride molecule is located at the water surface preferentially with the methyl group oriented toward the water surface, while for pentyl chloride, owing to the longer nonpolar hydrocarbon chain, a parallel alignment at the water surface is found with the hydrocarbon chain pointing slightly to the gas phase. Despite some quantitative differences, the present work provides confirmation of the somewhat surprising preferential orientation of the methyl chloride molecule at the water vapor interface predicted in a recent study using molecular dynamics simulations based on an empirical force field (Harper et al., J. Plays. Chem. A 2009, 113, 2015-2024). The observed difference in preferred alignment at the aqueous surface between the methyl chloride and the longer-chain alkyl chloride is likely to have consequences for the chemistry of alkyl halides adsorbed on the surface of aqueous and ice particles in the atmosphere.