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.
