Bimolecular reactions on sticky and slippery clusters: Electron-induced reactions of hydrogen peroxide.

Nanoparticles can serve as an efficient reaction environment for bimolecular reactions as the reactants concentrate either inside the nanoparticle or on the surface of the nanoparticle. The reaction rate is then controlled by the rate of formation of the reaction pairs. We demonstrate this concept on the example of electron-induced reactions in hydrogen peroxide. We consider two types of nanoparticle environments: solid argon particles, only weakly interacting with the hydrogen peroxide reactant, and ice particles with a much stronger interaction. The formation of hydrogen peroxide dimers is investigated via classical molecular dynamics (MD) simulations on a microsecond timescale. With a modified force field for hydrogen peroxide, we found out a fast formation and stabilization of the hydrogen peroxide dimer for argon nanoparticles, while the reaction pair was formed reversibly at a much slower rate on the water nanoparticles. We have further investigated the electron-induced reactions using non-adiabatic ab initio MD simulations, identifying the possible reaction products upon the ionization or electron attachment. The major reaction path in all cases corresponded to a proton transfer. The computational findings are supported by mass spectrometry experiments, where large ArM and (H2O)M nanoparticles are generated, and several hydrogen peroxide molecules are embedded on these nanoparticles in a pickup process. Subsequently, the nanoparticles are ionized either positively by 70 eV electrons or negatively by electron attachment at electron energies below 5 eV. The recorded mass spectra demonstrate the efficient coagulation of H2O2 on ArM, while it is quite limited on (H2O)M.

Proton transfer from pinene stabilizes water clusters.

We ionize small mixed pinene-water clusters by electron impact or by using photons after sodium doping and analyze the products by mass spectrometry. Electron ionization results in the formation of pure pinene, mixed pinene-water and protonated water cluster cations. The "fragmentation free" photoionization after sodium doping results into the formation of only water-Na+ clusters with a mean cluster size below that observed after electron ionization. We show that protonated water clusters are formed both directly and indirectly via pinene ionziation. The latter pathway is detailed by ab intio calculations, demonstrating the feasibility of proton transfer from pinene for larger water clusters. In small clusters, the proton transfer reaction is controlled by proton solvation energy and we can thus estimate its value for finite size clusters. The observed stabilization mechanism of water clusters may contribute to the formation of cloud condensation nuclei in the atmosphere.