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.