Spectroscopic studies of solvation.
thesisposted on 19.11.2015, 08:47 by V. K. Thomas
The solvating mechanisn of methanol is similar to that of water. Methanol has "been used extensively in this work in preference to water because the two dimensional structure and bifunctionality of methanol present a picture which is much simpler to interpret than does water. Four main types of methanol molecule are defined. These are the monomer, the fully bonded 'bulk' molecule, (OH)free, which is bonded only via its lone pair and (LP)free, which, is bonded only via its hydrogen atom. A consideration of the relative concentrations of these species in any system, together with any hydrogen bonded units formed with other molecules present, can be used to explain or predict NMR or IR frequency shifts. The self association of methanol, in several solvents, is examined. A monomer-tetramer model is used to explain polymer growth in carbon tetrachloride. Cyclic polymers are thought to be the preferred structures, particularly at low temperatures. NMR and IR spectral evidence, for the existence of (LP) free in methanol, and in support of the structures defined, is found in the presence of strong bases, such as triethylamine, and with magnesium perchlorate solutions at low temperatures. In a comparison of NMR and IR solvation shifts, induced by both bases and anions, no distinction can be drawn between solvating mechanisms with either type of acceptor, indicating that hydrogen bonding is responsible for the shifts in both cases and that the electric field of the anion need not be considered. An NMR study of aqueous solutions of the borohydride anion show it to be symmetrically solvated in water, forming hydrogen bonds comparable in strength to the bromide anion. The hydrogen bonds are believed to be directed towards the faces of the borohydride tetrahedron rather than towards the borohydride protons.