High-Level Coupled-Cluster Characterization of Ethynyl Radical Reactions and Other Small Challenging Molecules

For small molecular systems, convergent quantum chemical methods may be employed to obtain results rivaling experimental accuracy. The first project in this dissertation presents an in-depth methodological study of the gas-phase methyl anion (CH3-) with its peculiar basis set dependence. The next project characterizes various constitutional isomers of Ca2H2 and Ca2H4 with highly accurate coupled-cluster methods and second order vibrational perturbation theory (VPT2). The third project examines the mechanism of the reaction between the ubiquitous ethynyl radical (C2H) and molecular oxygen (O2), employing both coupled-cluster methods as well as multi-reference configuration interaction. The fourth project likewise examines reactions with ethnynl radical, but with a focus on hydrogen-abstractions that may occur with various H-atom donors instead. The final chapter summarizes key observations from these computational studies.