Abstract

In this work, computational studies of clusters were performed to understand solvent effects on chemical reactivity (Part A), and assess their potential use as micro-reactors to catalyze reactions not usually possible (Part B). In Part A, a comparative investigation of the structural and energetic properties of ions and ion pairs in polar solvent clusters of acetonitrile, ammonia and water is undertaken by means of Monte Carlo simulations with custom-built model potentials. Quantum chemistry calculations demonstrate the presence of nonlinear hydrogen-bonded I-(CH3CN)2 isomers, which leads to a reinterpretation of previous experimental work. In addition, our work has some implications for the strong solvent selectivity observed experimentally for the NaI photoionization in polar clusters. Equilibrium constant calculations of the NaI ion pair in clusters suggest that the lack of large ionized product fragments observed experimentally in acetonitrile is not due to solvent-induced charge separation (as suggested previously in analogy with water clusters), but could be attributed either to differential solvation effects or to solvent evaporation on the ionized state. On the other hand, the lack of large product signal in NaI(NH3)n multi-photon ionization experiments might be connected to the low evaporation temperature of ammonia, which may prevent production of large parent ground-state NaI(NH3)n clusters, and result in massive solvent evaporation on the excited states. In Part B, the dynamics of energy transfer in (O2)n cluster-surface scattering is characterized by means of classical molecular dynamics simulations, providing insights for several experiments, such as the cluster-catalyzed oxidation of a silicon surface. Simulations of the cluster scattering process reveals that the oxidation mechanism cannot occur through molecular dissociation nor by direct molecular reaction. Molecular dynamics simulations of (O2)n cluster-surface scattering, along with high-level quantum chemistry calculations of the (O2) 2 cluster model, suggested a novel 'ladder climbing' mechanism, involving curve-crossing and spin orbit coupling, for the efficient mechanically-induced formation of highly reactive singlet O2 molecules. Such a process may also be responsible for the presence of a 'dark channel' in the enhanced vibrational relaxation of highly excited O2 molecules, and shows that the Born-Oppenheimer approximation breaks down in the 'chemistry with a hammer'