Long range deuterium isotope effects on 13C NMR chemical shifts of 2-alkanones in CD3OD solutions of imidazolium acetate ionic liquids

Abstract

Deuterium isotope substitution in one part of a molecule could produce a significant effect on chemical shifts of neighbouring nuclei as well as on nuclei, located far from the site of replacement. To estimate how far this influence could extend the reaction of proton–deuterium exchange of several 2-alkanones in deuterated methanol solutions of 1-methyl 3-ethyl imidazolium acetate ionic liquid (IL) was studied in detail using ¹³C NMR spectroscopy. Deuteration occurs in alkyl groups of 2-alkanones neighboring the ketonic group via keto–enol tautomerization catalyzed by IL. In the course of the reaction, various isotopomers with various deuteration levels are formed, among which a dynamic equilibrium is established. The number of substituted deuterons affects not only the multiplicity and chemical shifts of directly bonded carbon, but carbons in the groups further along the alkyl chain. Moreover, the latter groups better indicate the level and site of substitution.