Date: August 25, Monday
Time: 15:00
Place: Chemistry Department Meeting Room
Sena Nur Aykut
Advisor: Asst. Prof. Halil İbrahim Okur
Co-Advisor: Asst. Prof. Fahri Alkan
Title: The Delocalization of Lone Pair Electrons on Oxygen Dictates Solubility In Aqueous Media
Abstract: The solution behavior of polyethers exhibits substantial differences, especially in their solubility in aqueous solution.
polyethylene glycol (PEG) is readily soluble and that makes it a widely utilized polymer in modern technologies, as surface coating, surfactant and coagulating agent. Whereas, polyoxymethylene (POM) is insoluble in aqueous medium, although it is structurally similar. Although some experimental studies and molecular simulations have provided insight into the thermodynamics and structural properties of aqueous polyether solutions, the underlying mechanism behind this surprising phenomenon remains elusive.
In this thesis, small molecule analogs of polyethylene glycol (PEG) and polyoxymethylene (POM) have been investigated for their hydration shell structures. The vibrational spectra of the hydration shell around model small molecules were acquired by employing the recently developed hydration shell spectroscopy method. Interestingly, the small molecules reveal a significant solubility difference. PEG-like monomers, i.e.
1,4-dioxane and dimethoxyethane are very soluble or miscible with water, whereas POM-like ones such as 1,3,5-trioxane and dimethoxymethane have limited aqueous solubility. Their hydration structures are also quite distinct. The hydration shell water molecules of PEG-like molecules are in a more-ordered tetrahedral-coordination configuration. In contrast, POM-like ones have hydrations shells that are significantly altered and are dominated by more-disordered water molecules (~3500 cm-1). Hydration shell spectra demonstrated a clear connection between hydration and their solubility behaviors. Additionally, solute–water intermolecular hydrogen bonding interactions were investigated via ATR-FTIR spectroscopy. The C–O–C stretch band position reports the strength of H-bond interaction. A more significant red shift was observed for PEG-like molecules when the solvent was changed from non-hydrogen bond donor solvent (dichloromethane) to hydrogen bond donor and acceptor ones (H2O and D2O) as high as 17 cm-1. For POM-like ones the same shift can go down to negligible values, as low as 1 cm-1. Complementary computational studies were then employed. Conformational searches and DFT calculations with explicit hydration structures enabled the evaluation of hydrogen-bond distances and interaction energies. All the results revealed that the delocalization of oxygen lone pair electrons into adjacent σC–H and σC–O antibonding orbitals decreases the availability of lone pairs for hydrogen bonding in for both PEG-like and POM-like molecules. This effect is more dominant and almost blocks the H-bonding interaction for 1,3,5-trioxane by double delocalization and conjugation-like molecular orbital formation.