Coupling constants provide important information on the conformation of molecules in solution. At present, four of the ten possible coupling constants n J HH have been experimentally determined for tetrahydrofuran (THF) and tetrahydrothiophene (THT) [1]. We have used these data to evaluate the parameters of the dynamic structure of the THF and TTF molecules in a new scheme for conformational analysis involving the quantum-chemical description of the dynamic systems in terms of large-amplitude oscillations. X = O, S Fig. 1. Conformational space of THF and THT. __________________________________________________________________________________________ 1

An original method was developed for the synthesis of THF labeled selectively with the 13 C 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 1 H and 13 C NMR spectra, making it possible to undertake a detailed analysis. Exact values for the 1 H-1 H spinspin coupling constants through four bonds and also the 13 C-1 H and 13 C-13 C spin-spin coupling constants involving the α-carbon atom were determined for the first time. The isotopic chemical shifts of the protons and 13 C nuclei caused by replacement of the 12 C nucleus by 13 C 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.Five-membered saturated heterocycles, including tetrahydrofuran (THF, 1), are important fragments of natural compounds -sugars, nucleic acids, etc. The presence of these flexible structure fragments makes the skeleton of nucleic acids, for example, sufficiently mobile to ensure rapid and reliable recognition of the substrates. To understand these processes at the molecular level it is necessary to construct a quantitative model of the conformational behavior of five-membered rings [1]. The first attempt to explain the specific character of the conformation of five-membered rings and, in particular, the anomalously high entropy of cyclopentane formation was made by Kilpatrick et al. [2] in terms of pseudorotation -a synchronous cyclic process, accompanied by the successive going out from the basal plane of the ring from one to four structural elements forming this ring, with ten pairs of canonical conformations of the "envelope" and "twist" types. In papers by Pople and co-workers [3], a convenient and sufficiently illustrative scheme was proposed for the parametrization of this process using the pseudorotation phase angle (φ) and the folding amplitude (Q).Among five-membered heterocycles, a special place is occupied by THF. Many attempts have been made to characterize its structure with the use of practically all the principal physical methods: X-ray structural analysis at -125°C [4]; electron diffraction [5]; microwave spectroscopy [6-9]; NMR spectroscopy both in an isotropic liquid phase [10-13] and in nematic liquid-crystalline solutions [14,15]; photoelectron spectroscopy [16]. There have been numerous attempts to calculate the pseudorotation parameters in tetrahydrofuran by