Adsorption of glyoxal molecules on atmospheric water ice nanoparticles

Ice nanoparticles play an important role in physics and chemistry of the Earth atmosphere. Knowledge about the uptake and incorporation of atmospheric trace gases in ice particles as well as their interactions with water molecules is very important for the understanding of processes at the air/ice interface. The interaction of the atmospheric trace gases with atmospheric nanoparticles is also an important issue for the development of modern physicochemical models. Usually, the interactions between trace gases and small particles considered theoretically apply small-size model complexes or the surface models representing only fragments of the ideal surface. In this study we used modern quantum chemical methods to study the interaction of glyoxal molecules (HCOCHO) with the full-size particles of crystalline water ice of nanoscale size. Glyoxal, the simplest a-dicarbonyl, is an atmospheric relevant carbonyl compound and is formed as product in the photooxidation of simple volatile organic compounds in air in the presence of NOx. The ice particles consisting of 48, 72, and 216 water molecules with a distorted structure of hexagonal water ice Ih were studied using the new SCC-DFTBA method combining well the advantages of the DFT theory and semiempirical methods of quantum chemistry. Typical sizes of the ice particles were in the range 1.5-2.6 nm. The glyoxal molecules were coordinated on different sites of the nanoparticles corresponding to different ice Ih crystal planes: (0001), (10-10), (11-20). The structure of coordination complexes, their vibrational frequencies, the corresponding adsorption energies and thermodynamic parameters (the enthalpy and the Gibbs free energy of adsorption) were evaluated using the full optimization followed by the frequency calculations. Additionally, the different modes of incorporation of the glyoxal molecules into the ice particles were considered and the corresponding structural and energetic parameters were evaluated. The transition states for the possible glyoxal hydration were located and the influence of the water cluster surrounding on the barrier heights was studied. At the different levels of theory, the energy of glyoxal adsorption varies in the range of 38-44 kJ mol-1 which is a little less than the corresponding value for methylhydroperoxide CH3OOH (44-54 kJ mol-1) estimated recently with a similar approach (Ignatov S.K. et al. J. Phys. Chem. C 2009, 118, 9081). The thermodynamic parameters of adsorption, incorporation and hydration show that the surface adsorption is the preferable mode for the interaction of glyoxal at ice nanoparticles under the typical conditions of the upper troposphere. The adsorption of the s-cis conformer of glyoxal is more favorable than the s-trans form, in contrast to the predominance of the second conformer in the gas phase. Financial support by the Russian Foundation for basic Research, project No. 11-03-00085 and German Academic Exchange Service (DAAD) within the Eastpartnership program is greatly acknowledged.