Synthesis and NMR spectra of tetrahydrofuran-2-13C
An original method was developed for the synthesis of THF labeled selectively with the 13C isotope at the α-carbon atom. The effective reduction of symmetry brought about by the insertion of the isotopic label removes the excess degeneracy of the spin systems describing the multiplet structure of the 1H and 13C NMR spectra, making it possible to undertake a detailed analysis. Exact values for the 1H–1H spin-spin coupling constants through four bonds and also the 13C–1H and 13C–13C spin-spin coupling constants involving the α-carbon atom were determined for the first time. The isotopic chemical shifts of the protons and 13C nuclei caused by replacement of the 12C nucleus by 13C were determined. These data can be used to construct a quantitative model of the conformational behavior of THF as a molecular system in which pseudorotation in terms of vibrations with large amplitude is realized.