Chiral intermolecular interactions in solution, in cold rare gas matrices and at interfaces: chiroptical and interfacial force studies
Creators: Yang, Yanqing
Publisher: name: University of Alberta Library
Year: 2022

<p>My PhD thesis centers on characterization of non-covalent interactions involving chiral molecules in solution, in cold rare gas matrices and at organic|water interfaces using a combined experimental and theoretical approach. In order to extract structural information and conformation distributions of different chiral molecules under different environments, several vibrational and vibrational optical activity spectroscopic tools, i.e., Infrared (IR), Raman, vibrational circular dichroism (VCD) and Raman optical activity (ROA), are utilized. To explain the experimental spectroscopic observations, first, systematic conformational searches of these molecular systems were performed using a recently developed conformational searching tool with the inclusion of polarizable continuum model of the appropriate environment, such as argon, and water and methanol solvents. Then the geometry optimizations and spectral simulations of the conformers identified were carried out at the B3LYP-D3BJ/def2-TZVP levels. <br>
In Chapter 3, the conformational distribution and self-aggregation behavior of tetrahydro-2-furoic acid (THFA) were studied by using matrix isolation (MI)-VCD. The well-resolved experimental MI-IR and MI-VCD features in an argon matrix at 10 K, 24 K and 30 K allow one to identify the dominant monomeric and binary conformations. Interestingly, the main binary THFA structures observed consist of two trans-COOH THFA subunits where the hydroxyl and carbonyl groups of COOH are at opposite sides, in contrast to the usual double hydrogen-bonded ring binary structure which contains two cis-COOH subunits. The work showcases the power of MI-VCD spectroscopy in revealing unusual structures formed in a cold rare gas matrix. <br>
Another focus of my thesis research is on solvent effects in chiroptical measurements. In Chapter 4, I studied THFA in water under different pH conditions using VCD spectroscopy. The “clusters-in-a-liquid” solvation model which takes into account of both explicit and implicit solute-solvent interactions was applied to interpret the experimental IR and VCD spectra of THFA under acidic condition satisfactorily. The possible causes for the poorer performance of the above solvation model for THFA under basic condition were discussed. In Chapter 5, IR, Raman, VCD and ROA spectra of N-acetyl-L-cysteine in water and in methanol were measured. The performance of the “clusters-in-a-liquid” solvation approach was compared with that of the ab initio molecular dynamics (AIMD) approach. While both provide satisfactory agreements with the experimental data, the AIMD simulations perform better at the lower wavenumber region, likely because it includes some anharmonic effects whereas the static DFT approach is based on harmonic frequency calculations. <br>
In Chapter 6, I explored a new research direction, i.e., chirality discrimination at a binary toluene (organic)/water(aqueous) interface. By using a combination of interfacial tension measurements and molecular dynamic simulations, Chirality discrimination at a binary toluene (organic)/water(aqueous) interface between R- or S-Tol-BINAP (2,2&#39;-Bis(di-p-tolylphosphino)-1,1&#39;-binaphthyl) molecules and the water-soluble serine chiral species is examined for the first time. The molecular dynamics simulations show that differences in the interfacial tension values arise when a homochiral versus a heterochiral enantiomeric pairs are introduced at the interface, in agreement with the experimental results. Such observations prove that preferential interactions exist between different pairs of enantiomers at the binary interfaces and can be captured directly by using interfacial tension measurements.</p>