NMR studies and rotational isomerism in fluoroacetones I. Trifluoroacetones

Abstract

The compounds 1,1,1-trifluoroacetone,1,1,1,3-tetrafluoroacetone,1,1,1-trifluoro-3-chloroacetone, 1,1,1-trifluoro-3-bromoacetone, 1,1,1-trifluoro-3,3-dibromoacetone, 1-fluoro-l-chloroacetone, 1-fluoro-3-chloroacetone, and 3-fluorobutan-2-one were prepared and solvent effects on their NMR parameters, as well as those of pentafluoroacetone, were investigated. The NMR parameters discussed are the proton and fluorine chemical shifts and the 2J HF , 3J HH , 3J HF , 4J HF , and 4 J FF, where available. It was found that the proton chemical shift and the 2 J HF and 4 J HF give the most valuable structural information, at least qualitatively. Fluorine chemical shifts, because of the importance of the paramagnetic term in their shift Hamiltonian, failed to give easily understood correlations with conformation. A variable temperature study of the proton chemical shifts of all but the first of the compounds listed above was carried out to ascertain the relative populations of the stable conformers of these molecules and to obtain the chemical shifts of the isolated conformers along with the thermodynamic parameters of the interconversion. From the obtained populations, the 4 J HF of the isolated conformers were obtained.