The spectroscopic verification of cooperative C-H &cdots; O interactions in carbon dioxide solutions: Towards the development of carbohydrate-based carbon dioxide processing methods.

摘要

Recently, peracetylated carbohydrates were shown to exhibit unparalleled solubility in liquid and supercritical CO₂. Acetate functionalities serve as the primary solute-solvent interaction sites and may be responsible for enhancing the CO₂-solubility of these compounds, suggesting that acetylation may be used as a method for designing CO₂-philic molecules. Ab initio calculations of model CO₂-carbonyl complexes predicted a previously unknown, weak C-H···O interaction that acts cooperatively with the predominant Lewis acid-Lewis base interaction. This interaction is predicted to induce a bond contraction and a resultant blue-shift in the C-H vibrational mode. The main objectives of this dissertation are to spectroscopically verify the presence of these C-H···O interactions and develop CO₂-processing applications for the acetylated sugars.; Since experimental methods involved pressurized gases, the design and validation of a simple high-pressure Raman and NMR spectroscopic cell were carried out. Using this cell, methyl acetate was shown to swell significantly and preferentially absorb CO₂ over other non-interacting gases, conclusively demonstrating a CO₂-acetate specific interaction. Once a specific interaction was verified, the nature of this interaction was then spectroscopically explored in greater detail. In agreement with ab initio predictions, FTIR and Raman results showed blue-shifting of some C-H vibrational modes in CO₂, which is consistent with the presence of C-H···O interactions. Isodensity temperature studies further demonstrated these shifts at lower temperatures as the weak interaction became more significant with respect to kT. Predicted shifts were also observed in supercritical CO₂, highlighting the relevance of C-H···O interactions under all practical phase conditions. NMR studies demonstrated significant chemical shift density dependence in CO₂ over He and N₂, which is consistent with the directional nature of the interaction-induced electron density redistribution in C-H···O interactions.; Finally, possible applications of peracetylated carbohydrates were examined. Since these materials deliquesce in CO₂ and exist as melts at 25.0°C, CO₂-processing was investigated as an encapsulation technique for the controlled-release of pharmaceuticals, specifically ibuprofen. Several systems (sucrose octaacetate, β-glucose pentaacetate, and acetylated β-cyclodextrin) showed extended release times that were comparable or longer to those achieved using other techniques. This method, however, involves simpler processing and eaiser product recovery using inexpensive, renewable resources.