Role of solvent on chemical reaction dynamics of small molecules

Abstract

For this thesis work, equilibrium (EMD) and non-equilibrium (NEMD) molecular dynamics simulations have been used to determine the underlying structural and dynamical processes in solution phase reactions involving diatomic solutes. This characterization is necessary to predict solvent effects on process involving larger molecules. The impulsive photodissociation of mercuric iodide in ethanol, HgI2 → HgI + I, produces highly vibrationally excited HgI with narrow bond length distributions. This has been used as a prototype to study wavepacket motion, vibrational relaxation and rotational dynamics. The vibrational energy decay rate of HgI obtained from the EMD simulations is in quantitative agreement with the experimentally determined value. Energy dissipation is mainly facilitated by the Lennard-Jones force fluctuations. The NEMD simulations support the experimental observation of the vibrational wavepacket motion on the rotational dynamics of the HgI which strongly depends on the variations in the partial charges of HgI with internuclear distance. Another system considered in this thesis work, the aqueous hypochlorite ion, undergoes light induced charge shifting from the negatively charged O atom (ClO−) to the Cl atom (OCl−). The charge shifting event, ClO− → OCl− , is used as a prototype to study solvation dynamics. The NEMD simulations predict that solvent response occurs on two timescales which are close to those measured by experiments on the same system. The rapid and slower time components have been associated with the destruction of solvent structure around the O atom and the creation of a new structure around the Cl atom, respectively. The applicability of linear response to this process is also examined with an eye toward possible future experiments. The curvatures of the harmonic free energy surfaces of ClO− and OCl − are not the same indicating that there is nonlinearity in the solvation dynamics caused by the asymmetry of the system: The average solvent structures around the two forms are not interchangeable. This study has shown the importance of incorporating the change in effective atomic radii during an electronic transition, if necessary with a corresponding modification of Lennard-Jones potential parameters