Spectroscopic studies of solvation in aqueous and alcoholic media.
thesisposted on 19.11.2015, 08:47 by Howard Lloyd. Robinson
This work is concerned with a study into hydrogen bond formation in alcohol and water systems using Infrared spectroscopy and NMR. In the former case, the first overtone region is investigated for the alcoholic systems and the second overtone region used in the aqueous work. Additional information is gleaned from fundamental studies, however emphasis is attached to the overtone work which amplifies changes in weakly hydrogen bonded entities and non-bonded OH groups, i.e. (OH)free groups and (LP)free groups in the methanol study. The loss of water/alcohol (OH)free groups due to added base is determined by using computer graphics to curve analyse the overtone spectra. It was found that the originally labelled " (OH)free" peak is only about 50% (OH)free, the remainder being weakly hydrogen bonded species. A good correlation is found between experimental and a theoretical consideration of loss of (OH)free groups, using a Law of Mass Action approach. This allows an accurate classification of solvation numbers for solutes studied. The two-dimensional structure and bifunctionality of methanol presents a simpler picture to interpret than does water. The methanol work can ultimately be related to the aqueous results which clearly shows that water brings out the maximum solvation numbers for solutes compared to methanol, t-BuOH and fluoroalcohols. A rise in absorbance is observed in the methanol overtone spectrum in the (LP)free region on adding base or increasing temperature. Curve analysis has been used to monitor changes in [(LP)free] which leads to a determination of solute basicities in methanol. The NMR work examines the change in hydrogen bonded environments for water and also t-BuOH on adding solutes or cosolvents. Aqueous solution shifts are explained using a proposed model which accounts for anion and cation hydration terms and changes in [(OH) free [(LP)free], all of which contribute to the time averaged water resonance. The model makes use of a novel NMR/IR correlation plot which allows IR shifts to be converted into previously unknown NMR shifts for H-bonded species. A similar plot has been constructed in the t-BuOH study.