Studies of solution thermodynamics by NMR spectroscopy
Abstract
The charge-transfer equilibrium of the acceptor 1,2-diiodotetra-fluoroethane with tris(4,4,4-trifluorobutyl)amine (1) and a set of model amines in decane was observed by $\sp{19}$F-NMR. This charge-transfer effect is proposed as a monitor of steric hindrance to approach of the nitrogen of the donor amine. In addition to 1, a set of model compounds were prepared: butyl derivatives of 1, N,N-bis(4,4,4-trifluorobutyl)butyl-amine (2) and N-(4,4,4-trifluorobutyl)dibatylamine (3); and the substituted pyrrolidines, 1-butyl-cis-3,4-bis(2,2,2-trifluoroethyl)pyrrolidine (4) and 1-(4,4,4-trifluorobutyl)-cis-3,4-bis(2,2,2-trifluoroethyl)pyrrolidine (5). The limitations of the charge-transfer equilibrium model for the determination of small K's were explored. It is found that the charge-transfer effects for tributylamine (6) and 4 are large compared to 1, 2, 3, 5, and the non-nucleophilic N,N-diisopropylethylamine (7). The charge-transfer effect is ordered: ${\it 6} > {\it 4} \gg {\it 7} > {\it 3} > {\it 5} > {\it 2} > {\it 1.\ 1}$ is not nucleophilic enough to quaternize on treatment with 4,4,4-trifluorobutyl bromide. This is in conflict with conventional wisdom on reactivity of remotely trifluoromethylated compounds, but is consistent with the lack of charge-transfer. The state of water solubilized by the reversed micelles of the surfactants cetyltrimethylammonium bromide (CTAB) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in organic solvents was examined by a novel NMR method. The dependence of the $\sp1$H chemical shift of solubilized water and the $\sp1$H and $\sp{13}$C shifts of the surfactant on the deuterium content of solubilized water were used to calculate the isotope fractionation factor, $\phi.$ The micelle-solubilized water does not seem to coexist in "layers" of different degrees of structure. A preliminary study of the $\sp1$H-NMR chemical shift of water in aqueous alcoholic (C$\sb1$-C$\sb4$ alcohols) solutions shows that $\rm{d\over dT}({d(\Delta\delta)\over dm})$ is proportional to $\rm\triangle C\sbsp{p2}{\circ},$ as suggested by a recent thermodynamic model of hydrophobic hydration of hydrocarbons (N. Muller, J. Solution Chem., 17, 661 (1988).) The value of the proportionality constant predicted by the model is smaller by a factor of ${\sim}{2.5}$ from the experimentally determined value. A review of variable temperature NMR provides a practical guide for spectroscopists.
Degree
Ph.D.
Advisors
Muller, Purdue University.
Subject Area
Chemistry
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