An NMR study on solvent effects on chemical exchange of a push-pull ethylene

Abstract

Solvent effects on the chemical exchange of a push pull Ethylenes <p> Chemical exchange rates provide a sensitive probe of solvent effects in solution. The push-pull ethylene methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has three exchange processes E-Z (the same as E' -Z'), E-E', and Z-Z', and we have measured these rates in acetone-d6, chloroform-d, tetrahydrofuran-d8, toluene-d8, methanol-d4, acetonitrile-d3, and methylene chloride-d2 through 1D 1 H.NMR at different temperatures. From this we obtain ΔG+, ΔHt+, ΔS+ of activation for each solvent by employing an Eyring plot. As the solvent changes, ΔH+ is approximately constant whereas ΔS+ follows the solvent polarity. </p> <p> Since the equilibrium constant is less than one, we would expect it to increase with temperature, but it decreases. This is seen in the Van't Hoff plot where the slope is positive indicating that the process of going from the major site to the minor site is exothermic, so not only is the rate governed by entropy effects, but also the equilibrium constant between E and Z. </p> <p> The rotation between the C-N and the C=C bond occurs through a non-concerted mechanism. Two reasons why this occurred was apparent through EXSY, since the intensity of the off diagonal peak varies with different mixing time and second, the rates from C=C rotation (E-Z) and C-N rotations (E-E' and Z-Z') are different at the same temperature </p>