Benchmarking Bimolecular Behavior and Beyond

For small molecular systems, advanced quantum chemical methods can be applied to deliverhighly accurate energies and provide further understanding for future studies. The first project in this dissertation investigates the hydration behavior of the tetrafluoroborate anion with up to four water molecules (BF₄⁻(H₂O)_n=1,2,3,4). The study identifies new hydration motifs and highlights the impact of these interactions on vibrational frequency shifts and dissociation energies, providing insights into the behavior of ionic liquids. The second project explores the hydrogen abstraction reactions of the ethynyl radical, C₂H, with various hydrogen donors, including HNCO, cis/trans-HONO, C₂H₄, and CH₃OH using state-of-the-art computational methods. The study refines reaction enthalpies and barrier heights to subchemical accuracy. The computed rate constants over a broad temperature range offer valuable benchmarks for future experimental and theoretical investigations. The third project further bolsters the benchmarks of the Concordant Mode Approach (CMA), a promising new method that increases the feasibility of larger system sizes and higher levels of theory in quantum chemical computations of molecular vibrational frequencies. Computations targeting CCSD(T)/aug-cc-pVTZ (coupled cluster singles and doubles with perturbative triples using an augmented correlation-consistent polarized-valence triple-ζ basis set) were performed on several molecules from the S22 test set.